Application of Active Soil Gas Screening for the Identification of Groundwater Contamination with Chlorinated Hydrocarbons at an Industrial Area—A Case Study of the Former Refrigerator Manufacturer Calex (City of Zlaté Moravce, Western Slovakia)

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript presents a case study on using active soil gas screening to identify and delineate groundwater contamination with chlorinated hydrocarbons (CLHCs) in an industrial area of Western Slovakia.

 

1.    The introduction can include discussions of more recent technological developments in soil gas screening and how this method compares to other approaches like direct groundwater sampling or geophysical techniques.

2.    The paper can discuss seasonal variations or soil moisture might affect gas concentrations and VOC reading.

3.    The results section can include a comparison with international guidelines or limits for safe groundwater use.

4.    Authors can include more details on the uncertainties associated with the risk assessment

 

5.    Figure captions can be more detailed to provide a clearer explanation of the data. 

Author Response

Dear Reviewer #1,

Thank you very much for your support of this manuscript. We appreciate it very much. We have accepted all your comments and the detailed response to them and the changes in the manuscript are provided below.

comment 1 – The introduction can include discussions of more recent technological developments in soil gas screening and how this method compares to other approaches like direct groundwater sampling or geophysical techniques.

Response: Thanks for the comment. Soil gas surveys can provide relatively rapid and cost-effective site data that can help direct more cost and invasive techniques. To demonstrate and highlight this and to justify the methodology used in the study, we included a separate paragraph in the introduction about conventional groundwater monitoring techniques, brief description about their advantages and limitations and how can soil gas measurements, but also other already verified and more widely used pre-screening methods (i.e. phytoscreening, geophysical methods, or direct-push methods), help to overcome the limitations of conventional methods (please, see also Page 3, Lines 95-112 in the revised manuscript):

“For the investigation and delineation of VOCs in groundwater and soil, conventional methods are performed, including analysis of groundwater and/or soil samples from soil borings and monitoring wells. Samples from these investigations provide reliable information about VOCs in groundwater and/or soil, both qualitatively and quantitatively [23, 24]. However, these methods have some limitations – they are expensive, invasive, time-consuming, labour-intensive, one-dimensional and characterised by serious issues of spatial under-sampling in the space [25-28], especially in the densely build-up, still active industrial facilities, with buried installations or hazards such as cables. This entails an enhanced risk of overlooking single contamination sources or even high-risk areas, which in consequence can dramatically hinder risk assessment and effective remediation [29-31]. Therefore, characterisation with sufficient spatial resolution is one of the main concerns and still open areas of research. Today, there are multiple cost-effective, reliable and fast screening methods, e.g. phytoscreening [32-34], geophysical methods [31, 35, 36], direct-push with membrane interface probe (MIP) and laser-induced fluorescence (LIF) sensors [37-39], and soil gas measurements and sampling [40-43]. These, if applied correctly, minimise the risk of missing single contamination sources or high-risk areas without increasing the financial burden by directing and focusing the subsequent, more precise but also more expensive methods [44-47].”

comment 2 – The paper can discuss seasonal variations or soil moisture might affect gas concentrations and VOC reading.

Response: Admittedly, there are limitations and uncertainties that are associated with soil gas measurements, even though the methodology is constantly developed and optimized. We summarized the most important ones in the Conclusions to balance the findings and emphasize the need of future research in this area, in particular in the coupling of the data from conventional monitoring with non- or low-invasive screening methods (soil gas measurements, geophysics, direct-push, etc.) (please, see also the revised manuscript, Page 21, Lines 750-766):

“There are some study’s limitations and uncertainties, which result mainly from the size of the study area. They can be reduced by methodology and optimisation only to a certain extent, and can bias the results and conclusions. Such uncertainties are the possible presence of preferential pathways (i.e. soil cracks, utilities, conductive zones) in a densely build-up area with still active industrial facilities, ambient air intrusion, possible distortion of concentration gradient between the groundwater table and ground surface in soil gas by hydrologic and geologic variables, such as varying moisture content, perched water or impermeable layers, heterogeneity in microbial activity and the content of organic matter and clay that affects the adsorption of hydrocarbons in soils and limits partitioning of contaminants into the vapour phase, and other factors. Despite this, the application of rapid screening soil gas measurement led to the identification of before undetected groundwater contamination sources, confirmed by subsequent groundwater sampling by new designed monitoring wells. Without the application of screening soil gas measurements, these contamination sources would remain undetected, leading to a difficult and risky decision about site management. This emphasises the advantage of applying soil gas measurements and sampling as one of the inexpensive screening methods that are able to cover wide areas with reasonable efforts.”

comment 3 – The results section can include a comparison with international guidelines or limits for safe groundwater use.

Response: We have extended comparison of measured concentrations of chlorinated hydrocarbons in groundwater to the well-recognised drinking water standards according to WHO, USEPA and EU. For this purpose, we have revised also Table S3 in Supplementary material, showing the drinking water limit values of WHO, USEPA and EU for the investigated chlorinated hydrocarbons and percentage number of exceedances of all wells. The comparison is described as follows (please, see also the revised version of the manuscript, Page 13, Lines 455-470; section 3.1. Occurrence of chlorinated hydrocarbons and their spatial distribution in groundwater):

“Although groundwater from this area is not used for drinking purposes, comparison to drinking water standards could be taken as an indicator of the level of groundwater contamination with CLHCs. The concentration data were compared to the drinking water standards of the EU, USEPA and WHO (Table S3). The comparative results confirmed that PCE and TCE were of primary interest, when more than 60% of sampling sites exceeded the EU limit for the sum of PCE and TCE (10 μg/L). Both WHO and USEPA list drinking water standards separately for PCE and TCE. In any case, 50% of all wells exceeded the USEPA standard for PCE (5.0 μg/L), but with a significant decrease to 14% considering the less stringent WHO limit (100 μg/L). TCE concentrations were higher in 43% of sampling sites compared to the WHO and USEPA criteria of 8.0 μg/L and 5.0 μg/L, respectively. Cis- and trans-DCE, VC and TCA showed a lower frequency of exceeding the respective drinking water standards; 11%, 5% and 5% of all wells, respectively. The severity of groundwater contamination was also indicated by comparison to the IT criteria (Table S3). Up to 42% of all wells showed PCE above the IT criterion (20 μg/L), TCE exceeded the IT value of 50 μg/L in 28% of the wells, but cis- + trans-DCE were above the IT (50 μg/L) in 11% of all wells.”

comment 4 – Authors can include more details on the uncertainties associated with the risk assessment.

Response: We are thankful for this comment. We have added the discussion about the potential uncertainties in the risk assessment. This reads as follows (please, read also the revised manuscript, Pages 18-19, Lines 662-681):

“In this study, the uncertainty of the human health risk assessment could relate primarily to the concentrations of CLHCs in groundwater and the resulting uncertainties in the model evaluation of their outdoor and indoor air concentrations. Health risk were calculated from the maximum concentrations of CLHCs in groundwater, determined in one sampling campaign, which corresponded to the worst-case scenario. However, it is known that the concentrations of CLHCs in groundwater fluctuate seasonally, spatially and vertically, making an impact on the exposure concentration [8, 134]. As stated above, a significant exposure route was indoor breathing with a modelled indoor air concentration of individual CLHCs using the Johnson-Ettinger transport model incorporated in the RISC software. Research has documented large spatial variability in soil gas concentrations of VOCs under buildings, which is less observable in indoor air concentrations due to efficient air mixing [50]. However, on the other hand, there are significant temporal fluctuations of indoor air concentrations of VOCs over time periods of hours to months [135]. These temporal fluctuations are dependent on several factors, e.g. building differential pressure, temporal changes of soil moisture, temperature and groundwater table, building ventilation, meteorological conditions, etc. [136]. Therefore, it is almost impossible to consider a single value of the indoor air concentration of VOCs when assessing their intake via the inhalation route. It should be also emphasised that the values of some toxicity parameters for cis-DCE are not available, so its contribution to the overall health risk might be underestimated.”

comment 5 - Figure captions can be more detailed to provide a clearer explanation of the data.

Response: Dear reviewer, we have little changed the “Figure captions” to be clearer. Please, see the revised manuscript.

Thank you very much again for your expert comments.

Sincerely,

Edgar Hiller (Corresponding author).

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Comments to the authors:

The manuscript introduces an innovative methodology for sampling and analyzing chlorinated hydrocarbons, allowing faster and more cost-effective determinations. It demonstrates strong robustness in the discussion and presentation of results, showcasing high scientific quality overall. From my perspective, the following comments and observations could enhance the overall presentation of the manuscript:

 

1.           In the abstract section, Please check the abbreviations used for tetrachloroethylene (PCE) and 28 trichloroethylene (PCE).

2.           The abstract section does not clearly describe the study's objective, which is not explicitly stated at the beginning. In addition, a brief explanation of the methods used is not indicated. Authors should revise this section and indicate the main objective and methods in this part of the manuscript. Methods must include chemical analysis techniques.

3.           The abstract should briefly mention the HHRA to enhance its scope and underscore the significance of evaluating potential health risks.

4.           In the Introduction section, the authors start directly by describing contaminated areas in a particular area (Slovakia). It would be more desirable to begin with a general background on CLHCs contamination and its global problems (using specific examples and statistics ) and then describe the local problems in their study area. A simple literature review yields several references that can be used to address this issue.

5.           In the Introduction section, it would be desirable to expand the information to include a brief description of the limitations of conventional groundwater monitoring techniques (e.g., high cost, time demand, etc.) and how soil gas analysis helps to overcome these challenges. This will better justify the methodology used in the study and help appreciate its application in the study area and other areas of interest at the global level. This would lead to establishing a clear hypothesis about the study and explicitly stating it in the manuscript. This hypothesis should align with the objectives and explain what the authors hope to find or demonstrate through their research.

6.           Please indicate what the conventional methods are for determining these compounds. The authors must also discuss the limitations of conventional methods for detecting and monitoring CLHCs in groundwater and explain how these limitations can hinder effective remediation and risk assessment (lines 104-105).

7.           One of the study's objectives is to assess the risks to human health. I think the authors should better describe and provide more background in the Introduction section on the risks of population exposure to these compounds, what health risk assessment entails, and their most accepted methods for this type of contaminants. This would emphasize the importance of timely and accurate contamination detection using the gas techniques proposed in this study.

8.           The 1.1 subsection should be relocated to the Materials and Methods section.

9.           The authors indicate that soil conditions were assessed before the in situ soil gas measurements and sampling and assumed to be contamination-free. How did the authors arrive at this assumption? Is there any reference to previously published studies on this topic in the study area? This needs to be clarified in the manuscript, as soil contamination may influence the interpretation of the results, leading to inappropriate conclusions (lines 173-174).

10.       What type of sorbent was used to collect soil gas samples in the tubes?

11.       Please ensure that all abbreviations are well-defined the first time they are used in the manuscript (e.g., BTEX).

12.       Is the absence of VC at the analyzed sites beneficial or detrimental to the study's objectives and the characterization of contamination in the area? It should be considered that VC is more toxic than PCE or TCE; therefore, I think the implications of incomplete dechlorination of the contaminants should be thoroughly discussed in the manuscript (lines 253-255).

13.       The authors found that PCE, TCE, and cis-DCE concentrations in several wells were above the 1:1 concentration line despite a general decrease in their concentrations over time. How do the authors explain these results?

14.       The authors report higher concentrations of VOCs in the residential area and attribute them to additional sources, but they do not provide any basis or data to support this claim (Lines 337-339). Therefore, the contribution of VOCs from these types of sources should be investigated in depth and verified based on bibliographic references.

15.       The legends in Figure 4 are difficult to read. Please ensure they are legible.

16.       Regarding the health risk assessment, although concentrations may vary over time, more detailed discussion could be given on how these fluctuations were addressed and whether measures were taken to average data or consider seasonal variations. This would help contextualize the risk assessment better.

17.       Although the advantages of the sampling and analytical methodology used to measure CLHCs are mentioned in the Conclusions section, a short section on the study's limitations and associated uncertainties could be added, which would strengthen their conclusions. I think this would help provide a more balanced view of the findings and suggest areas for future research.

18.       A few grammatical and spelling errors must be corrected throughout the manuscript.

Author Response

Dear Reviewer #2,

We would like to thank you for accepting to review our manuscript and we appreciate much your positive feedback.

However, you had a number of comments that were very helpful to us. We have incorporated all the comments in the revised version of the manuscript. Your comments and our detailed responses are listed below.

comment 1 – In the abstract section, please check the abbreviations used for tetrachloroethylene (PCE) and 28 trichloroethylene (PCE).

Response: Thank you very much for your detailed review. We have corrected this error.

comment 2 – The abstract section does not clearly describe the study's objective, which is not explicitly stated at the beginning. In addition, a brief explanation of the methods used is not indicated. Authors should revise this section and indicate the main objective and methods in this part of the manuscript. Methods must include chemical analysis techniques.

Response: Thank you very much. The abstract section was completely revised and sub-divided into “Background”, “Methods”, “Results” and “Conclusion” sections according to instructions of authors (please, see also the revised manuscript, Pages 1-2, Lines 27-55):

“Abstract: Background: Groundwater contamination with chlorinated hydrocarbons (CLHCs), particularly with tetrachloroethylene (PCE) and trichloroethylene (TCE), which are used in industry for degreasing and cleaning, can be considered a serious problem concerning the entire world. In addition to conventional groundwater monitoring from a network of wells, several screening methods have been proposed to identify and delineate groundwater contamination with volatile organic compounds (VOCs), such as soil gas measurement, bioindicators, direct-push technologies or geophysical techniques. The main objectives of this study were to confirm the feasibility of active soil gas screening for the characterisation of groundwater contamination with CLHCs under the wider area of the former refrigerator manufacturer (city of Zlaté Moravce, western Slovakia) and to evaluate the human health risks through exposure to CLHCs present in groundwater. Methods: Conventional site investigation based on concentration measurements using gas chromatography-mass spectrometry from monitoring wells and soil gas measurements using a portable photo-ionisation detector device were applied. Results: Chemical analyses showed persistent contamination of groundwater with PCE and TCE and other CLHCs, such as cis-1,2-dichloroethylene (cis-DCE) or 1,1,2-trichloroethane (TCA), being most severe in the zone of the former factory (up to 2690, 83,900, 6020 and 156 µg/L for PCE, TCE, cis-DCE and TCA, respectively) but also extended into the residential zone located 600 m along the groundwater flow line. Soil gas measurements of VOCs and other chemical parameters (methane (CH₄), total petroleum (TP), carbon dioxide (CO₂) and oxygen (O₂)) from a densely designed network of sampling points (n = 300) helped trace the current state of groundwater contamination. Spatial distribution maps of VOCs concentrations in soil gas clearly marked the areas of the highest CLHCs concentrations in groundwater. Principal component analysis (PCA) confirmed a significant correlation of VOCs and CLHCs with the first principal component, PC1, explaining up to 84% of the total variability of the concentration data, suggesting that VOCs in soil gas were a suitable marker of the extent of groundwater contamination with CLHCs. Despite severe groundwater contamination with CLHCs reaching residential areas, local residents were not exposed to non-carcinogenic risks, but a potential carcinogenic risk was present. Conclusion: Based on the results, it could be confirmed that soil gas screening is an efficient and quick tool for identifying the sources of groundwater contamination with CLHCs as well as the level of this contamination.”

comment 3 – The abstract should briefly mention the HHRA to enhance its scope and underscore the significance of evaluating potential health risks.

Response: We agree with you. We have added a mention about human health risk assessment in Abstract section (Page 2, Lines 51-52 in the revised manuscript):

“Despite severe groundwater contamination with CLHCs reaching residential areas, local residents were not exposed to non-carcinogenic risks, but a potential carcinogenic risk was present.”

comment 4 – In the Introduction section, the authors start directly by describing contaminated areas in a particular area (Slovakia). It would be more desirable to begin with a general background on CLHCs contamination and its global problems (using specific examples and statistics) and then describe the local problems in their study area. A simple literature review yields several references that can be used to address this issue.

Response: We have completely re-considered Introduction section. Now, it starts with a general background as follows (please, see also Page 2, Lines 59-82 in the revised manuscript):

“Groundwater in urbanised and industrial areas is highly vulnerable to contamination with a wide range of volatile organic compounds (VOCs). An important position among VOCs is occupied by chlorinated ethenes, which are used in industrial processes as dry-cleaning and degreasing agents [1, 2]. Tetrachloroethylene (PCE) and trichloroethylene (TCE) belong to the best-known and historically most studied chlorinated solvents. The long-term interest in the study of these compounds results from their high toxicity, mobility, persistence and the fact that they often accumulate in aquifers to a state of saturation as dense non-aqueous phase liquids (DNAPLs), representing a challenge for the development of effective groundwater remediation methods [3-5]. The highest concentrations of PCE, TCE and their main anaerobic degradation products, cis-1,2-dichloroethylene (cis-DCE) and vinyl chloride (VC), in groundwater are usually associated with point sources. Groundwater in the close vicinity of a point source, e.g. landfill of chemical waste and various industrial facilities contained up to thousands of ug/L PCE and TCE with significant concentrations of cis-DCE and VC [6-8]. However, CLHCs concentrations of environmental concern may reach further from the source as a result of their transport under favourable conditions limiting natural attenuation [9, 10].

Tetrachloroethylene is classified as probably carcinogenic to humans (Group 2A), while TCE and VC belong to carcinogens to humans (Group 1) [11], highlighting the need for studies of groundwater contamination with CLHCs, especially in residential areas adjacent to industrial sites. In addition, several epidemiological studies have shown that long-term environmental exposure of humans to PCE, TCE and VC causes an increase in the incidence of cancer and other serious diseases [5, 12-14]. The harmfulness of chlorinated ethenes is reflected, for example, in their strict limits in waters intended for drinking purposes [15-17].”

comment 5 – In the Introduction section, it would be desirable to expand the information to include a brief description of the limitations of conventional groundwater monitoring techniques (e.g., high cost, time demand, etc.) and how soil gas analysis helps to overcome these challenges. This will better justify the methodology used in the study and help appreciate its application in the study area and other areas of interest at the global level. This would lead to establishing a clear hypothesis about the study and explicitly stating it in the manuscript. This hypothesis should align with the objectives and explain what the authors hope to find or demonstrate through their research.

Response: Yes, you are right. Soil gas surveys can provide relatively rapid and cost-effective site data that can help direct more cost and invasive techniques. To demonstrate and highlight this and to justify the methodology used in the study, we included a separate paragraph in the introduction about conventional groundwater monitoring techniques, brief description about their advantages and limitations and how can soil gas measurements, but also other already verified and more widely used pre-screening methods (i.e. phytoscreening, geophysical methods, or direct-push methods), help to overcome the limitations of conventional methods. One of the aims of our study was to verify the areal extension of soil-gas contamination especially in the zone of expected groundwater contamination sources and in the adjacent area and thus to identify the potential locations of groundwater contamination sources, especially of the “hidden” ones, which could be overlooked by conventional methods, due to their limited densities and irregular distributions. Adding this information to the introduction (please, see Page 3, Lines 95-112 in the revised manuscript) better highlights the objectives of our research:

“For the investigation and delineation of VOCs in groundwater and soil, conventional methods are performed, including analysis of groundwater and/or soil samples from soil borings and monitoring wells. Samples from these investigations provide reliable information about VOCs in groundwater and/or soil, both qualitatively and quantitatively [23, 24]. However, these methods have some limitations – they are expensive, invasive, time-consuming, labour-intensive, one-dimensional and characterised by serious issues of spatial under-sampling in the space [25-28], especially in the densely build-up, still active industrial facilities, with buried installations or hazards such as cables. This entails an enhanced risk of overlooking single contamination sources or even high-risk areas, which in consequence can dramatically hinder risk assessment and effective remediation [29-31]. Therefore, characterisation with sufficient spatial resolution is one of the main concerns and still open areas of research. Today, there are multiple cost-effective, reliable and fast screening methods, e.g. phytoscreening [32-34], geophysical methods [31, 35, 36], direct-push with membrane interface probe (MIP) and laser-induced fluorescence (LIF) sensors [37-39], and soil gas measurements and sampling [40-43]. These, if applied correctly, minimise the risk of missing single contamination sources or high-risk areas without increasing the financial burden by directing and focusing the subsequent, more precise but also more expensive methods [44-47].”

comment 6 – Please indicate what the conventional methods are for determining these compounds. The authors must also discuss the limitations of conventional methods for detecting and monitoring CLHCs in groundwater and explain how these limitations can hinder effective remediation and risk assessment (lines 104-105).

Response: Due to the extent of the Introduction section, we summarised the information about conventional methods for determination of the contamination and limitations of these methods into one paragraph (discussed above in point no. 5; please, see Page 3, Lines 95-112 in the revised manuscript). Contamination does not only pose a direct risk by exposure to contaminants, it also indirectly restrains economic development and harms the quality of life due to the slow processes of investigation and remediation and the resulting long period of uncertainty. Therefore, there is a need for cost-effective, reliable and fast tools for characterization of pollution, tools for measurements as well as models for risk-evaluation. To accent this, we explained it in the separate paragraph of the Introduction section (please, see Page 3, Lines 123-133 in the revised manuscript):

“In order to minimise the risk of missing single groundwater contamination source and avoid inaccurate reconstruction of the spatial contamination extent, a more thorough pre-screening of the site at a reduced cost with rapid analytical results was carried out using soil gas measurements and sampling, whose effective use, also in combination with phytoscreening or geophysical methods, has already been confirmed [26, 33, 43]. Moreover, results of soil gas measurements can be much more sensitive in some cases than those of soil and water sampling outputs [42]. On the other hand, screening techniques cannot replace conventional in situ sampling. Soil and/or groundwater sampling with chemical analysis should be applied as the last step to confirm and interpret identified contamination and to quantify the contaminant levels to avoid potential misinterpretation of findings [31, 52].”

The information in this paragraph is related to and supplements the information given in the newly added paragraph discussed above in the point. 5 (please, see Page 3, Lines 95-112 in the revised manuscript).

In Conclusion, the mention about limitations of conventional methods, related risks and the potentialities of their overcome by pre-screening methods was also added/modified (please, see the revised manuscript, Page 21, Lines 759-766):

“Despite this, the application of soil gas screening led to the identification of before undetected groundwater contamination sources, confirmed by subsequent groundwater sampling by new designed monitoring wells. Without the application of screening soil gas measurements, these contamination sources would remain undetected, leading to a difficult and risky decision about site management. This emphasises the advantage of applying soil gas measurements and sampling as one of the inexpensive screening methods that are able to cover wide areas with reasonable efforts.”

comment 7 – One of the study's objectives is to assess the risks to human health. I think the authors should better describe and provide more background in the Introduction section on the risks of population exposure to these compounds, what health risk assessment entails, and their most accepted methods for this type of contaminants. This would emphasize the importance of timely and accurate contamination detection using the gas techniques proposed in this study.

Response: We have added more information about health risk assessment. You can find it here as follows (please, see also Page 2, Lines 75-94 in the Introduction section of the revised manuscript):

“Tetrachloroethylene is classified as probably carcinogenic to humans (Group 2A), while TCE and VC belong to carcinogens to humans (Group 1) [11], highlighting the need for studies of groundwater contamination with CLHCs, especially in residential areas adjacent to industrial sites. In addition, several epidemiological studies have shown that long-term environmental exposure of humans to PCE, TCE and VC causes an increase in the incidence of cancer and other serious diseases [5, 12-14]. The harmfulness of chlorinated ethenes is reflected, for example, in their strict limits in waters intended for drinking purposes [15-17].

Human exposure to CLHCs in groundwater occurs through several routes such as oral ingestion, dermal contact with water during irrigation and hygiene or inhalation of CLHCs vapours indoors and outdoors [8, 18]. Human health risk assessment is an important tool for quantifying the contaminant intake of the exposed population from a particular medium through one or more exposure routes. Risk assessment characterises specific health risks, i.e. the existence of an incremental excess lifetime risk of developing cancer or the probability of an adverse effect in the exposed population [19]. It thus helps to decide the need for groundwater remediation and to implement effective remediation measures in the context of their technical complexity, cost and implementation time [20]. According to [18, 19, 21, 22], risk assessment of organic contaminants to human health is also important for ensuring the safety of groundwater, which is used as a source of drinking water.”

comment 8 – The 1.1 subsection should be relocated to the Materials and Methods section.

Response: We have moved the subsection into the Materials and Methods section. Please, see Pages 5, Lines 207-235 in the revised manuscript.

comment 9 – The authors indicate that soil conditions were assessed before the in situ soil gas measurements and sampling and assumed to be contamination-free. How did the authors arrive at this assumption? Is there any reference to previously published studies on this topic in the study area? This needs to be clarified in the manuscript, as soil contamination may influence the interpretation of the results, leading to inappropriate conclusions (lines 173-174).

Response: You are right. Thank you very much for the comment. This was not very well stated. We have change it as follows (please, see also Page 6, Lines 268-274):

“Soil gas measurements were also carried out at two sites outside the study area (marked as SG background 1 (above the industrial area) and SG background 2 (left bank of the local stream of Žitava) in Figure 1), i.e. at sites that are not in contact with source area A. Concentrations of chemical indicators in soil gas at these sites were considered as local background because groundwater contamination with CLHCs was not confirmed after repeated sampling [70].”

comment 10 – What type of sorbent was used to collect soil gas samples in the tubes?

Response: Based on the guidelines and with regard the standard analytical method used by the accredited analytical laboratory, as a sorbent to collect soil gas samples in the tubes was activated carbon. We have included the information in the Materials and Methods section (please, see Page 6, Line 296-298 in the revised manuscript):

“A volume of soil gas equal to three system volumes was vented from each soil probe before the soil gas samples were collected in a glass tube with sorbent (activated carbon).”

comment 11 – Please ensure that all abbreviations are well-defined the first time they are used in the manuscript (e.g., BTEX).

Response: This is now corrected in the revised version of the manuscript. Thank you much for careful reading.

comment 12 – Is the absence of VC at the analyzed sites beneficial or detrimental to the study's objectives and the characterization of contamination in the area? It should be considered that VC is more toxic than PCE or TCE; therefore, I think the implications of incomplete dechlorination of the contaminants should be thoroughly discussed in the manuscript (lines 253-255).

Response: Thanks for the interesting comment. We have tried some discussion about this point (please, see also Page 9, Lines 386-393 in the revised manuscript):

“A large part of the area has oxygen-saturated groundwater (Figure 3), in which VC is unstable and rapidly oxidises aerobically [10, 84]. This could be the reason for the low frequency of its occurrence in groundwater. It was found that cis-DCE is also degraded by aerobic oxidation but to a lesser extent [85], which finally coincides with the higher frequency of occurrence of cis-DCE compared to that of VC (Figures 3 and 4). From the point of view of groundwater contamination and its impact on the environment and human health, this is a favourable situation, since VC is more toxic compared to cis-DCE, PCE and TCE.”

comment 13 – The authors found that PCE, TCE, and cis-DCE concentrations in several wells were above the 1:1 concentration line despite a general decrease in their concentrations over time. How do the authors explain these results?

Response: Thank you much. We have added the following explanation (please, see also Pages 10-11, Lines 421-432 in the revised manuscript):

“On the other hand, high CLHCs concentrations in the source area A (Figure 5), and PCE, TCE and cis-DCE concentrations in several wells above the 1:1 concentration line (that is, a straight line expressing the same concentration in an individual well between the two evaluated sampling years), i.e. higher in the last groundwater sampling than in previous years (Figure S2) were observed. This is evidence of the persistent contamination of the aquifer. These compounds accumulate in the deepest aquifer parts as DNAPLs. The fate of DNAPLs is complicated, they can be divided into smaller units that are trapped in low permeability zones, releasing contaminants for a long time [87, 88]. It has been estimated that DNAPLs can persist for hundreds of years [89]. Also, seepage of chlorinated solvents from still operating smaller manufactures in the source area A, and slow diffusional dissolution from the sorbed state into the pore spaces of the aquifer, a process that may take years [90], cannot be ruled out.”

comment 14 – The authors report higher concentrations of VOCs in the residential area and attribute them to additional sources, but they do not provide any basis or data to support this claim (Lines 337-339). Therefore, the contribution of VOCs from these types of sources should be investigated in depth and verified based on bibliographic references.

Response: You are right. Therefore, we have extended the discussion about these additional sources, also using several references. The discussion is as follows (please, see also Page 14, Lines 499-509 in the revised manuscript):

“On the other hand, higher VOC concentrations could be due to additional sources in residential area, especially road traffic. This was clearly seen from the semi-quantitative soil gas analysis of sample S286 located at the roundabout with VOCs concentration of 7620 μg/m³ and TP of 2032 mg/m³. Other soil probes along the busy road (orange circles at the lower boundary of the study area, Figure 1) had also higher concentrations of VOCs (3205–7480 μg/m³) and TP (13–116 mg/m³), which might be an obstacle for the success of delineating the lateral extent of the CLHCs contamination plume. Studies confirmed that traffic emissions and spills of motor fuels and oils were a significant source of VOCs to soils and air [95-97]. However, our semi-quantitative analyses without knowledge of the occurrence of individual VOCs in soil gas could not confirm contributions to VOCs from other sources.”

comment 15 – The legends in Figure 4 are difficult to read. Please ensure they are legible.

Response: For better readability, we enlarged the font in the legend of Figure 4 (now, it is Figure 5) (please see the edited Figure 5, Page 12 in the revised document). Since a similar figure is presented in Supplementary Material (Figure S3), we adjusted the font size in the legend of this figure as well.

comment 16 – Regarding the health risk assessment, although concentrations may vary over time, more detailed discussion could be given on how these fluctuations were addressed and whether measures were taken to average data or consider seasonal variations. This would help contextualize the risk assessment better.

Response: Dear reviewer. We have used “maximum” concentrations of CLHCs found in areas B and C because we have no time-dependent or seasonal data. The maximum concentrations may eventually represent “the worst-case scenario” in the risk assessment. Despite this, we have discussed these interesting points (please, see also Page 19, Lines 703-722-681 in the revised manuscript):

“In this study, the uncertainty of the human health risk assessment could relate primarily to the concentrations of CLHCs in groundwater and the resulting uncertain-ties in the model evaluation of their outdoor and indoor air concentrations. Health risk were calculated from the maximum concentrations of CLHCs in groundwater, deter-mined in one sampling campaign, which corresponded to the worst-case scenario. However, it is known that the concentrations of CLHCs in groundwater fluctuate seasonally, spatially and vertically, making an impact on the exposure concentration [8, 134]. As stated above, a significant exposure route was indoor breathing with a modelled indoor air concentration of individual CLHCs using the Johnson-Ettinger transport model incorporated in the RISC software. Research has documented large spatial variability in soil gas concentrations of VOCs under buildings, which is less observable in indoor air concentrations due to efficient air mixing [50]. However, on the other hand, there are significant temporal fluctuations of indoor air concentrations of VOCs over time periods of hours to months [135]. These temporal fluctuations are dependent on several factors, e.g. building differential pressure, temporal changes of soil moisture, temperature and groundwater table, building ventilation, meteorological conditions, etc. [136]. Therefore, it is almost impossible to consider a single value of the indoor air concentration of VOCs when assessing their intake via the inhalation route. It should be also emphasised that the values of some toxicity parameters for cis-DCE are not available, so its contribution to the overall health risk might be underestimated.”

comment 17 – Although the advantages of the sampling and analytical methodology used to measure CLHCs are mentioned in the Conclusions section, a short section on the study's limitations and associated uncertainties could be added, which would strengthen their conclusions. I think this would help provide a more balanced view of the findings and suggest areas for future research.

Response: Yes, we fully agree. Even though soil gas measurements and sampling as well as other pre-screening methods are constantly being developed and optimized, admittedly, there are still limitations and uncertainties that were also associated with our study. We summarized the most important ones in the Conclusions to balance the findings and emphasize the need of future research in this area, in particular in the coupling of the data from conventional monitoring with non- or low-invasive screening methods (soil gas measurements, geophysics, direct-push, etc.) (please, see the revised document, Page 21, Lines 750-766):

“There are some study’s limitations and uncertainties, which result mainly from the size of the study area. They can be reduced by methodology and optimisation only to a certain extent, and can bias the results and conclusions. Such uncertainties are the possible presence of preferential pathways (i.e. soil cracks, utilities, conductive zones) in a densely build-up area with still active industrial facilities, ambient air intrusion, possible distortion of concentration gradient between the groundwater table and ground surface in soil gas by hydrologic and geologic variables, such as varying moisture content, perched water or impermeable layers, heterogeneity in microbial activity and the content of organic matter and clay that affects the adsorption of hydrocarbons in soils and limits partitioning of contaminants into the vapour phase, and other factors. Despite this, the application of soil gas screening led to the identification of before undetected groundwater contamination sources, confirmed by subsequent groundwater sampling by new designed monitoring wells. Without the application of screening soil gas measurements, these contamination sources would remain undetected, leading to a difficult and risky decision about site management. This emphasises the advantage of applying soil gas measurements and sampling as one of the inexpensive screening methods that are able to cover wide areas with reasonable efforts.”

comment 18 – A few grammatical and spelling errors must be corrected throughout the manuscript.

Response: We have addressed this as much as possible.

We hope that we were able to sufficiently respond to your very helpful comments and incorporate them into the revised version of the manuscript.

Thank you very much for all your suggestions.

Sincerely Yours,

Edgar Hiller (Corresponding author).

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Dear authors,

I have carefully reviewed your research article titled "Application of Active Soil Gas Screening for the Identification 2 of Groundwater Contamination with Chlorinated Hydrocar- 3 bons at an Industrial Area – a Case Study of the Former Refrig- 4 erator Manufacturer Calex (City of Zlaté Moravce, Western Slo- 5 vakia ." While I acknowledge the scientific value of your study, I have identified several areas that require revision and improvement before considering it for publication. I kindly request that you address the following points:

1. Section 2: Materials and Methods: Did you use chemicals in this work?

2. What specific improvement did you consider regarding the methodology? 

3. Page 8, line 284: What do you mean by ratio 1:1?

4. Page 9, line 330: What is the highest VOC concentration? Explain this sentence (line 330).

5. Section 3.3: How did the Human Health risk assessment links to the agriculturally activity as stated in line 114?

I believe that addressing these points will significantly enhance the quality and clarity of your research article.

Author Response

Dear Reviewer #3,

We are very grateful that you accepted to review our study and that you took the time to read it carefully. Your comments were very helpful to us, and we have tried to include them in the revised manuscript as best as we can.

Responses to your comments and changes made to the manuscript are summarised in detail below.

comment 1 – Section 2: Materials and Methods: Did you use chemicals in this work?

Response: No, we have not used chemicals in this work because all chemical analyses of groundwater samples were carried in accredited laboratories. In addition, groundwater samples were collected into the pre-cleaned bottles and by a sampling procedure that are obtained the laboratory, which performed chemical analysis.

comment 2 – What specific improvement did you consider regarding the methodology?

Response: Admittedly, there are limitations and uncertainties that are associated with soil gas measurements. We summarised the limitations of soil gas measurements, such as seasonal variations, soil moisture and others in the Materials and methods section, where we stated the improvements and optimisations of the methodology, by which we tried to overcome these limitations to the greatest possible extent (please, see also Page 6, Lines 275-294 in the revised manuscript):

“As already known [13, 38, 49, 64, 65, 73-78], there are limitations of soil gas measurement and sampling methods, such as meteorological conditions (rain-fall, low barometric pressure, wind), depth of groundwater, lithology and properties of the overlaying soils (very low permeable or saturated soils, high soil moisture, preferential pathways), microbial activity and biodegradation (resistance of contaminants to biodegradation), etc. To overcome these limitations, prior to soil gas measurements, a detailed historical study of the geology, hydrogeology and operational activity of the industrial area, continuous cored borings in individual parts of the studied area to collect information about lithology and depth of groundwater. Subsequently, all soil gas measurements were carried out following the same procedure, sampling sites spacing and frequency was thickened, and where possible, the measurements were carried out in a regular grid, decontamination procedures were practiced to prevent contaminant gain or loss that results from adsorption onto sampling equipment, blank samples were tested regularly, and pressure, temperature, wind speed, depth of groundwater and rainfall were monitored during the whole soil gas measurement campaign. To limit the effect of soil moisture and seasonal variations on the results of the soil gas measurements to the greatest possible extent, soil gas measurements were realised during longer dry periods without precipitation and in the shortest possible time, while season, moisture content and water-table depth variations among others also affect the biodegradation of VOCs in soil gas available for measurements and sampling [65].”

comment 3 – Page 8, line 284: What do you mean by ratio 1:1?

Response: Thank you much for this comment. We have clarified this point as follows (please, see also the revised manuscript, Page 10, Lines 421-425):

“On the other hand, high CLHCs concentrations in the source area A (Figure 5), and PCE, TCE and cis-DCE concentrations in several wells above the 1:1 concentration line (that is, a straight line expressing the same concentration in an individual well between the two evaluated sampling years), i.e. higher in the last groundwater sampling than in previous years (Figure S2) were observed.”

comment 4 – Page 9, line 330: What is the highest VOC concentration? Explain this sentence (line 330).

Response: We have clarified this as follows (please, see also the revised manuscript, Page 13, Lines 492-496):

“The highest VOCs concentrations within the source area (from 4530 μg/m³ up to the upper detection limit of the instrument, >100,000 μg/m³) were found at the processing and cleaning sites for refrigerator components, the chemical warehouse and the pumping station for chlorinated solvents (site S268).”

comment 5 – Section 3.3: How did the Human Health risk assessment links to the agriculturally activity as stated in line 114?

Response: We apologise for this error. We have deleted this part from the statement. We were focusing on the risk assessment for residents. Now, it reads as follows (please, see also the revised version of the manuscript, Page 5 Lines 203-205):

“This last objective is important because the groundwater contamination with CLHCs is extended to the surrounding residential zones.”

We hope we have met your expectations as much as possible and thank you once again for your highly valuable comments on this study.

Sincerely Yours,

Edgar Hiller (Corresponding author).

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

The article titled "Application of Active Soil Gas Screening for the Identification of Groundwater Contamination with Chlorinated Hydrocarbons at an Industrial Area – a Case Study of the Former Refrigerator Manufacturer Calex (City of Zlaté Moravce, Western Slovakia)" addresses a critical environmental issue—groundwater contamination with chlorinated hydrocarbons (CLHCs)—which is a major concern globally. The relevance of this topic is high, as groundwater contamination is a persistent issue in industrial regions, and the health implications of pollutants like PCE, TCE, and DCE are well-established. The use of active soil gas screening as a semi-quantitative, cost-effective approach for delineating groundwater contamination is innovative. The study convincingly shows that this method is a time-saving alternative to conventional methods, making it highly valuable for large-scale environmental monitoring, particularly in contaminated industrial zones.

The study includes extensive data, using statistical tools like Principal Component Analysis (PCA) to strengthen the correlation between volatile organic compounds (VOCs) in soil gas and groundwater contamination. This statistical rigor enhances the credibility of the findings.

The human health risk assessment (HHRA) adds significant value by quantifying potential carcinogenic and non-carcinogenic risks. This inclusion makes the study not just environmentally relevant but also human-centered, connecting the scientific findings directly to real-world consequences.

By comparing current contaminant levels with historical data (from 2005 and 2015), the study provides a long-term perspective on contamination trends, demonstrating how industrial legacy pollutants behave over time.

 * 

Aera of improvements to consider in the article

 1. Lack of Remediation Detail

 While the study identifies the persistent contamination and mentions the necessity for remediation, it does not provide enough information on the types of remediation strategies that have been or should be, employed. Given the severity of contamination, more discussion on potential remediation approaches (e.g., bioremediation, pump-and-treat, or in-situ methods) would have been beneficial.

 2. Insufficient Exploration of Uncertainties

Although the study mentions the limitations of soil gas screening (e.g., variability in subsurface conditions), it does not delve deeply into the uncertainties associated with the measurement techniques, especially how they might affect the accuracy of contamination delineation. A clearer assessment of these uncertainties would strengthen the argument for the method’s reliability.

3. Limited Focus on Potential Confounding Factors

(Lines 70 - 77/Lines 156 – 160): The study acknowledges potential confounding factors, such as road traffic and smaller industrial operations, that could contribute to VOC levels in residential areas. However, these factors are not explored in depth. A more thorough investigation into how these confounders might influence the results could improve the clarity of the study's conclusions regarding the contamination sources.

4. Narrow Geographic Focus

Lines (117- 123): While the case study is detailed and specific to the Zlaté Moravce region, the article could benefit from a broader contextualization. For instance, the study could discuss how the findings apply to similar industrial areas worldwide, offering a more global perspective on groundwater contamination with CLHCs and the applicability of soil gas screening methods.

The study would benefit from a discussion on how similar regions facing legacy contamination could apply these findings. This would make the study more relevant to a global audience of environmental scientists, policymakers, and remediation experts.

5. Health Risk Quantification Limitations

Lines (196- 235) The HHRA component does not account for all potential contaminants, as the authors acknowledge the absence of data for certain chemicals. Additionally, the study uses default population weights and other assumptions that might not accurately reflect the specific population at risk. Addressing these limitations or proposing a method to refine risk assessment models would add depth to the analysis.

6. Limited Engagement with Stakeholders

The article could be improved by incorporating perspectives from stakeholders, such as local governments or industrial stakeholders involved in the remediation process. Their input could provide insight into the challenges of implementing remediation measures and monitoring efforts in contaminated sites.

7. Policy Recommendations

Adding a section on policy implications would enhance the study's practical utility. Recommendations for regulatory bodies regarding monitoring standards or mandatory remediation processes could make the article more impactful for decision-makers.

8. Future Research Directions

 

While the article identifies key findings, it does not propose concrete future research directions. The authors could have explored how emerging technologies like remote sensing or more advanced chemical sensors could complement soil gas screening methods for groundwater contamination studies.

The article is generally well-written, but some areas could be improved for clarity, depth, and engagement with the audience. The following is advice that may be taken into consideration to enhance the article.

The introduction effectively highlights the global relevance of groundwater contamination with chlorinated hydrocarbons (CLHCs), setting the stage for why this study is important. The environmental and health risks posed by contaminants like PCE and TCE are articulated, which captures the reader’s attention. However, the introduction could benefit from situating the study more within a global context of groundwater contamination in industrial areas. Mentioning other similar cases worldwide or discussing the extent of this issue in industrial regions could give the study a broader appeal. For example, references to global contamination trends or other significant contaminated industrial sites would help readers understand the global implications of the study.

While the contamination problem is well-described, the rationale for using soil gas screening as the chosen method could be better explained. A clearer explanation of why soil gas screening is particularly suitable for this case and how it improves upon traditional groundwater sampling methods would strengthen the introduction. This would help justify the methodology early on, instead of waiting until the discussion.

The introduction could emphasize more clearly what gaps in knowledge this study aims to fill. While it mentions that contamination persists and remediation is required, a more explicit identification of gaps in current monitoring or remediation techniques would help the reader understand the novel contribution of the study.

 

The research objectives are mentioned briefly, but they could be stated more clearly and explicitly toward the end of the introduction. A clear outline of the specific aims would help set expectations for the rest of the paper.

While the sampling process is clear, there is little information on how the instruments (photoionization detectors, infrared detectors) were calibrated before use. Adding details about calibration procedures and quality control measures would improve the transparency and reliability of the methodology.

The depth of 0.8 meters for soil gas sampling is mentioned, but there is no discussion on why this specific depth was chosen. It would be beneficial to explain why this depth was suitable for detecting volatile organic compounds (VOCs) in this context and how deeper or shallower samples might have affected results.

The criteria for selecting new groundwater sampling sites based on soil gas data could be more detailed. Explaining how soil gas measurements directly informed the placement of new wells would improve the logical flow between the methodologies.

The discussion on confounding factors like road traffic and smaller industrial operations affecting VOC concentrations is mentioned but not fully explored. A more thorough discussion of these external influences would help assess the reliability of the soil gas screening data in areas distant from the former factory.

Author Response

Dear Reviewer #4,

We appreciate very much your detailed review of the manuscript and the time devoted to reading it. You have several valuable comments that were different from those written by other four reviewers and some comments are similar to them, especially to Reviewer #2. We tried to include all your comments in the manuscript as best we could, although we were short on time because we received your review from the editor much later than the reviews from the other four reviewers.

Our detailed responses are shown below:

Area of improvements to consider in the article

comment 1. Lack of Remediation Detail - While the study identifies the persistent contamination and mentions the necessity for remediation, it does not provide enough information on the types of remediation strategies that have been or should be, employed. Given the severity of contamination, more discussion on potential remediation approaches (e.g., bioremediation, pump-and-treat, or in-situ methods) would have been beneficial.

Response: OK, thank you much for the comment. This is briefly mentioned in the relation to future directions as follows (please, see also Page 18, Lines 645-648 in the revised manuscript):

“After obtaining results from other conventional monitoring and screening methods, remediation efforts should focus on identified sources of groundwater contamination using a combination of pump-and-treat methods, injection of oxidising or reducing agents, electrochemical degradation, and bioaugmentation.”

comment 2. Insufficient Exploration of Uncertainties - Although the study mentions the limitations of soil gas screening (e.g., variability in subsurface conditions), it does not delve deeply into the uncertainties associated with the measurement techniques, especially how they might affect the accuracy of contamination delineation. A clearer assessment of these uncertainties would strengthen the argument for the method’s reliability.

Response: As we responded to your comment about calibration, the used portable device was calibrated on daily basis. Moreover, as stated in another comment about the selection of soil depth for soil gas sampling, the depth of 0.8 m below the surface for soil gas measurement and sampling was selected to minimise the effect of changes in barometric pressure, temperature, break-through of ambient air from the surface, high saturation, clay, organic matter and presence of anthropogenic sediments, which could limit soil gas transfer at the near surface horizon. Measurement uncertainties were also reduced by the fact that field works were carried out continuously without delay within several days of the same weather conditions. All these aspects are incorporated in the revised manuscript as explained in the comments mentioned below.

comment 3. Limited Focus on Potential Confounding Factors - (Lines 70 - 77/Lines 156 – 160): The study acknowledges potential confounding factors, such as road traffic and smaller industrial operations, that could contribute to VOC levels in residential areas. However, these factors are not explored in depth. A more thorough investigation into how these confounders might influence the results could improve the clarity of the study's conclusions regarding the contamination sources.

The discussion on confounding factors like road traffic and smaller industrial operations affecting VOC concentrations is mentioned but not fully explored. A more thorough discussion of these external influences would help assess the reliability of the soil gas screening data in areas distant from the former factory.

Response: Thank you very much for the comment. We have tried to bring more light into the effects of road traffic and smaller industrial facilities on VOCs readings in the residential zone. This shown as follows (please, see also revised manuscript, Pages 13-14, Lines 496-509):

“The mean VOCs concentration in agricultural land (area B) and residential area C was 1470 and 3750 μg/m³, respectively. The elevated VOC concentrations in the soil gas of the residential area C located approximately 600 m from the source area A clearly indicated the persistence of groundwater contamination over a large area. On the other hand, higher VOC concentrations could be due to additional sources in residential area, especially road traffic. This was clearly seen from the semi-quantitative soil gas analysis of sample S286 located at the roundabout with VOCs concentration of 7620 μg/m³ and TP of 2032 mg/m³. Other soil probes along the busy road (orange circles at the lower boundary of the study area, Figure 1) had also higher concentrations of VOCs (3205–7480 μg/m³) and TP (13–116 mg/m³), which might be an obstacle for the success of delineating the lateral extent of the CLHCs contamination plume. Studies confirmed that traffic emissions and spills of motor fuels and oils were a significant source of VOCs to soils and air [95-97]. However, our semi-quantitative analyses without knowledge of the occurrence of individual VOCs in soil gas could not confirm contributions to VOCs from other sources.”

comment 4. Narrow Geographic Focus - Lines (117-123): While the case study is detailed and specific to the Zlaté Moravce region, the article could benefit from a broader contextualization. For instance, the study could discuss how the findings apply to similar industrial areas worldwide, offering a more global perspective on groundwater contamination with CLHCs and the applicability of soil gas screening methods. The study would benefit from a discussion on how similar regions facing legacy contamination could apply these findings. This would make the study more relevant to a global audience of environmental scientists, policymakers, and remediation experts.

Response: Dear Reviewer, we have extended this at several sites of the manuscript. For example, new statements in the Introduction secton read as (please, see also Page 4, Lines 150-160):

“Soil gas measurement is a rapid, non-invasive screening method, but limited only to VOCs in the unsaturated zone with medium to high permeability, i.e. it is not feasible in low permeable rock environment. The disadvantage is the fact that soil gas sampling captures a limited area, but this can be overcome by thickening the sampling grid [26, 38]. Soil gas measurement and analysis are a constantly evolving method. Considerable research is focused on the determination of greenhouse gases [66, 67], and the latest technological developments in soil gas screening, such as the development of modern spectrometry methods, soil gas sampling technique, involving AI approaches, application of mathematical statistics and regression analysis, or the influence of design features on the accuracy of measurements over time, attentively summarised a recent extensive review [68].”

Further, we state (Pages 16-17, Lines 618-631):

“The success of the application of this screening method for the identification of groundwater contamination could be related to the appropriate timing of field sampling and determination, which was carried out on days of stable weather, limiting the effect of humidity, temperature and pressure on the volatilisation [50]. The permeable unsaturated zone and relatively shallow groundwater of this area increased the feasibility of soil gas screening. The limitation of the application of soil gas measurements was documented in the residential area where engineering networks, built-up zones, road traffic and operations such as car repair shops play a role in VOC readings through changes in the migration pathways of VOCs or their excess sources not related to groundwater. Based on the results of this study, soil gas screening can be recommended as a highly effective methodology for the delineation of groundwater contamination with VOCs for other areas as well but for its successful application, it is necessary to know the site history, geology and hydrogeological conditions, and take into account other local specificities that might affect the migration of VOCs vapours.”

comment 5. Health Risk Quantification Limitations - Lines (196-235) The HHRA component does not account for all potential contaminants, as the authors acknowledge the absence of data for certain chemicals. Additionally, the study uses default population weights and other assumptions that might not accurately reflect the specific population at risk. Addressing these limitations or proposing a method to refine risk assessment models would add depth to the analysis.

Response: Thank you for the comment. Yes, you are right that human health risk quantification is only an approximation of the actual state due to uncertainties in exposure factors a chemical toxicity data for organic compounds. We think that the major uncertainty in human health risk assessment is the vapour concentration, which was not directly measured, only estimated using models incorporated in the RISC software. We have summarised this as (please, see also Page 19, Lines 703-722 in the revised manuscript):

“In this study, the uncertainty of the human health risk assessment could relate primarily to the concentrations of CLHCs in groundwater and the resulting uncertainties in the model evaluation of their outdoor and indoor air concentrations. Health risk were calculated from the maximum concentrations of CLHCs in groundwater, determined in one sampling campaign, which corresponded to the worst-case scenario. However, it is known that the concentrations of CLHCs in groundwater fluctuate seasonally, spatially and vertically, making an impact on the exposure concentration [8, 134]. As stated above, a significant exposure route was indoor breathing with a modelled indoor air concentration of individual CLHCs using the Johnson-Ettinger transport model incorporated in the RISC software. Research has documented large spatial variability in soil gas concentrations of VOCs under buildings, which is less observable in indoor air concentrations due to efficient air mixing [50]. However, on the other hand, there are significant temporal fluctuations of indoor air concentrations of VOCs over time periods of hours to months [135]. These temporal fluctuations are dependent on several factors, e.g. building differential pressure, temporal changes of soil moisture, temperature and groundwater table, building ventilation, meteorological conditions, etc. [136]. Therefore, it is almost impossible to consider a single value of the indoor air concentration of VOCs when assessing their intake via the inhalation route. It should be also emphasised that the values of some toxicity parameters for cis-DCE are not available, so its contribution to the overall health risk might be underestimated.”

comment 6. Limited Engagement with Stakeholders - The article could be improved by incorporating perspectives from stakeholders, such as local governments or industrial stakeholders involved in the remediation process. Their input could provide insight into the challenges of implementing remediation measures and monitoring efforts in contaminated sites.

and comment 7. Policy Recommendations - Adding a section on policy implications would enhance the study's practical utility. Recommendations for regulatory bodies regarding monitoring standards or mandatory remediation processes could make the article more impactful for decision-makers.

Response: Combined response for comments no. 6 and no. 7, which are closely related, is presented here. Due to persistent and very extensive groundwater contamination of the given area and the associated human health risks, remediation is inevitable. Providing a consistent, realistic and accurate model from coupling the data from conventional monitoring with non- or low-invasive screening methods (soil gas measurements, geophysics, direct-push, etc.) and hydro-geochemical modelling, which is a future direction of our research, will serve for choosing an effective remediation approach for the locality. Due to this, we included in the manuscript section about potential remediation strategies, policy implications and perspectives from stakeholders (please, see Page 18, Lines 645-656 of the revised manuscript):

“After obtaining results from other conventional monitoring and screening methods, remediation efforts should focus on identified sources of groundwater contamination using a combination of pump-and-treat methods, injection of oxidising or reducing agents, electrochemical degradation, and bioaugmentation.

The area has been longer listed in the information system of the environmental burdens of the Ministry of the Environment of the Slovak Republic (MoE SR) as a con-firmed environmental burden with a high priority. Exploration works as well as the subsequent remediation are carried out under the supervision of the MoE SR and funded by Structural Fund of the European Union. Due to the identified extent of contamination, the residents of the affected area were informed by the local government about the state of contamination with the recommendation not to use groundwater for drinking, watering gardens and bathing until the area is remediated.”

comment 8. Future Research Directions - While the article identifies key findings, it does not propose concrete future research directions. The authors could have explored how emerging technologies like remote sensing or more advanced chemical sensors could complement soil gas screening methods for groundwater contamination studies.

Response: This is now shown in the revised manuscript; in Results and discussion section (please, see Page 18, Lines 637-645):

“The future research directions should be oriented on coupling the data from conventional monitoring with non- or low-invasive screening methods (soil gas measurements, geophysics, direct-push, etc.) and hydro-geochemical modelling to provide a consistent, realistic and accurate image (conceptual model) integrating the information from different data sources. This could gain the information about the current development and future fate of the extent of contamination, processes of its natural biodegradation, and thus, setting the right remediation approaches. Simultaneously, the method should be applied to similar “mega-sites” in order to compare the results and optimise the approach for the future.”

Then it follows in your review:

The article is generally well-written, but some areas could be improved for clarity, depth, and engagement with the audience. The following is advice that may be taken into consideration to enhance the article.

comment 1a – The introduction effectively highlights the global relevance of groundwater contamination with chlorinated hydrocarbons (CLHCs), setting the stage for why this study is important. The environmental and health risks posed by contaminants like PCE and TCE are articulated, which captures the reader’s attention. However, the introduction could benefit from situating the study more within a global context of groundwater contamination in industrial areas. Mentioning other similar cases worldwide or discussing the extent of this issue in industrial regions could give the study a broader appeal. For example, references to global contamination trends or other significant contaminated industrial sites would help readers understand the global implications of the study.

Response: This comment had also another reviewer. We have completely modified the Introduction section. The Introduction starts with the description of groundwater contamination with CLHCs within the global context. The revised introductory section reads now (please, see also Page 2, Lines 59-94):

“Groundwater in urbanised and industrial areas is highly vulnerable to contamination with a wide range of volatile organic compounds (VOCs). An important position among VOCs is occupied by chlorinated ethenes, which are used in industrial processes as dry-cleaning and degreasing agents [1, 2]. Tetrachloroethylene (PCE) and trichloroethylene (TCE) belong to the best-known and historically most studied chlorinated solvents. The long-term interest in the study of these compounds results from their high toxicity, mobility, persistence and the fact that they often accumulate in aquifers to a state of saturation as dense non-aqueous phase liquids (DNAPLs), representing a challenge for the development of effective groundwater remediation methods [3-5]. The highest concentrations of PCE, TCE and their main anaerobic degradation products, cis-1,2-dichloroethylene (cis-DCE) and vinyl chloride (VC), in groundwater are usually associated with point sources. Groundwater in the close vicinity of a point source, e.g. landfill of chemical waste and various industrial facilities contained up to thousands of ug/L PCE and TCE with significant concentrations of cis-DCE and VC [6-8]. However, CLHCs concentrations of environmental concern may reach further from the source as a result of their transport under favourable conditions limiting natural attenuation [9, 10].

Tetrachloroethylene is classified as probably carcinogenic to humans (Group 2A), while TCE and VC belong to carcinogens to humans (Group 1) [11], highlighting the need for studies of groundwater contamination with CLHCs, especially in residential areas adjacent to industrial sites. In addition, several epidemiological studies have shown that long-term environmental exposure of humans to PCE, TCE and VC causes an increase in the incidence of cancer and other serious diseases [5, 12-14]. The harmfulness of chlorinated ethenes is reflected, for example, in their strict limits in waters intended for drinking purposes [15-17].

Human exposure to CLHCs in groundwater occurs through several routes such as oral ingestion, dermal contact with water during irrigation and hygiene or inhalation of CLHCs vapours indoors and outdoors [8, 18]. Human health risk assessment is an important tool for quantifying the contaminant intake of the exposed population from a particular medium through one or more exposure routes. Risk assessment characterises specific health risks, i.e. the existence of an incremental excess lifetime risk of developing cancer or the probability of an adverse effect in the exposed population [19]. It thus helps to decide the need for groundwater remediation and to implement effective remediation measures in the context of their technical complexity, cost and implementation time [20]. According to [18, 19, 21, 22], risk assessment of organic contaminants to human health is also important for ensuring the safety of groundwater, which is used as a source of drinking water.”

comment 2a – While the contamination problem is well-described, the rationale for using soil gas screening as the chosen method could be better explained. A clearer explanation of why soil gas screening is particularly suitable for this case and how it improves upon traditional groundwater sampling methods would strengthen the introduction. This would help justify the methodology early on, instead of waiting until the discussion.

and comment 3a – The introduction could emphasize more clearly what gaps in knowledge this study aims to fill. While it mentions that contamination persists and remediation is required, a more explicit identification of gaps in current monitoring or remediation techniques would help the reader understand the novel contribution of the study.

and comment 4a – The research objectives are mentioned briefly, but they could be stated more clearly and explicitly toward the end of the introduction. A clear outline of the specific aims would help set expectations for the rest of the paper.

Response: We agree with you. These 3 comments are related to each other, therefore, we give response to them together. A clearer explanation of why soil gas screening is particularly suitable for this case and how it improves upon traditional groundwater sampling methods is indicated in the Introduction section as follows (please, see also Page 3, Lines 95-112 in the revised manuscript):

“For the investigation and delineation of VOCs in groundwater and soil, conventional methods are performed, including analysis of groundwater and/or soil samples from soil borings and monitoring wells. Samples from these investigations provide reliable information about VOCs in groundwater and/or soil, both qualitatively and quantitatively [23, 24]. However, these methods have some limitations – they are expensive, invasive, time-consuming, labour-intensive, one-dimensional and characterised by serious issues of spatial under-sampling in the space [25-28], especially in the densely build-up, still active industrial facilities, with buried installations or hazards such as cables. This entails an enhanced risk of overlooking single contamination sources or even high-risk areas, which in consequence can dramatically hinder risk assessment and effective remediation [29-31]. Therefore, characterisation with sufficient spatial resolution is one of the main concerns and still open areas of research. Today, there are multiple cost-effective, reliable and fast screening methods, e.g. phytoscreening [32-34], geophysical methods [31, 35, 36], direct-push with membrane interface probe (MIP) and laser-induced fluorescence (LIF) sensors [37-39], and soil gas measurements and sampling [40-43]. These, if applied correctly, minimise the risk of missing single contamination sources or high-risk areas without increasing the financial burden by directing and focusing the subsequent, more precise but also more expensive methods [44-47].”

Regarding the comment 3, we state (please, see also Page 3, Lines 123-133 in the revised manuscript):

“In order to minimise the risk of missing single groundwater contamination source and avoid inaccurate reconstruction of the spatial contamination extent, a more thorough pre-screening of the site at a reduced cost with rapid analytical results was carried out using soil gas measurements and sampling, whose effective use, also in combination with phytoscreening or geophysical methods, has already been confirmed [26, 33, 43]. Moreover, results of soil gas measurements can be much more sensitive in some cases than those of soil and water sampling outputs [42]. On the other hand, screening techniques cannot replace conventional in situ sampling. Soil and/or groundwater sampling with chemical analysis should be applied as the last step to confirm and interpret identified contamination and to quantify the contaminant levels to avoid potential misinterpretation of findings [31, 52].”

We have tried to improve the research objective as follows (please, see also Pages 4-5, Lines 175-205 in the revised manuscript):

“To identify these potentially ”hidden” and persistent contamination sources, an extensive active soil gas measurement was carried out. It should be noted that the soil gas measurement was not a substitute for conventional methodology but served as a screening tool, allowing more effective use of conventional methods. Based on the soil gas screening, new groundwater sampling sites were selected for further exploration work to confirm these sources and identify sites intended for remediation. Although many studies and academic reports used soil gas screening to determine subsurface contamination by CLHCs and other VOCs, and to identify its main sources, they were mostly limited to pre-characterised source locations with significant accumulation of contaminants (so-called hot spots) that were identified by conventional groundwater sampling methodology. In this study, soil gas screening was used over a large area and at sites hundreds of meters away from the contamination source. Taking into account the advantages and disadvantages of soil gas screening, this method seems to be suitable for the studied area as the existing data showed that the groundwater contained contrasting concentrations of CLHCs spanning several orders of magnitude, and at the same time, the area has a relatively uniform rock environment with high permeability. In addition, elucidation of the extent of groundwater contamination and its sources is of public interest due to permanent settlement in the south-western part of the study area, where residents are exposed to vapour intrusions from contaminated groundwater. In addition, the results of this study could be verified in other industrial areas with similar type of contamination and geological settings. Therefore, the main objectives of this study could be summarised as follows: (i) to determine the extent of groundwater contamination with CLHCs using conventional methodology and existing historical data at an industrial area (the former refrigerator manufacturer – Calex company, city of Zlaté Moravce in western Slovakia), (ii) to apply soil gas screening to further refine the delineation of groundwater contamination and source identification, and to verify the suitability of its application by combining the results of soil gas screening with results of conventional methodology through spatial distribution map outputs and multivariate statistics, and last but not least, (iii) to assess human health risks from the exposure of residents to CLHCs in groundwater. This last objective is important because the groundwater contamination with CLHCs is extended to the surrounding residential zones.”

comment 5a – While the sampling process is clear, there is little information on how the instruments (photoionization detectors, infrared detectors) were calibrated before use. Adding details about calibration procedures and quality control measures would improve the transparency and reliability of the methodology.

Response: Yes, we definitely agree that information on instruments calibration before use will improve the transparency and reliability of the methodology. Photoionisation detector gives an approximate contamination concentration in equivalent of isobutylene, detecting the whole VOCs present in soil gas. Both IR Methane and IR Total Petroleum channels are calibrated with methane. In the Material and methods section, we have added information about calibration of the sensors and about fast in situ correction of PID detector, that was conducted before each soil gas measurement day. Please, see Page 5, Lines 248-251 of the revised manuscript:

“This device enables a real-time chemical monitoring (contaminant detection) by measuring total VOCs concentration with PID calibrated using isobutylene and separate measurements of CH₄, TP and CO₂ with an infrared (IR) analyser calibrated using methane.”

And Page 6, Lines 262-265 of the revised manuscript:

“The PID detector was calibrated daily with 100 ppm isobutylene standard to provide fast in situ correction of instrument's response that might be changed due to internal dusting or depositing of various particles, etc.”

Information on quality control measures have been already stated in the revised manuscript (please, see Page 6, Lines 223-225 of the revised manuscript):

“Before each measurement, the soil gas installation was purged by extracting three system volumes, and measurement and sampling started only after the stabilisation of the physicochemical parameters monitored by the PID.”

Admittedly, there are limitations and uncertainties that are associated with soil gas measurements. We summarised the limitations of soil gas measurements, such as seasonal variations, soil moisture and others in the Materials and methods section, where we stated the improvements and optimizations of the methodology by which we tried to overcome these limitations to the greatest possible extent (please, see also Page 6, Lines 275-294 in the revised manuscript):

“As already known [13, 38, 49, 64, 65, 73-78], there are limitations of soil gas measurement and sampling methods, such as meteorological conditions (rain-fall, low barometric pressure, wind), depth of groundwater, lithology and properties of the overlaying soils (very low permeable or saturated soils, high soil moisture, preferential pathways), microbial activity and biodegradation (resistance of contaminants to biodegradation), etc. To overcome these limitations, prior to soil gas measurements, a detailed historical study of the geology, hydrogeology and operational activity of the industrial area, continuous cored borings in individual parts of the studied area to collect information about lithology and depth of groundwater. Subsequently, all soil gas measurements were carried out following the same procedure, sampling sites spacing and frequency was thickened, and where possible, the measurements were carried out in a regular grid, decontamination procedures were practiced to prevent contaminant gain or loss that results from adsorption onto sampling equipment, blank samples were tested regularly, and pressure, temperature, wind speed, depth of groundwater and rainfall were monitored during the whole soil gas measurement campaign. To limit the effect of soil moisture and seasonal variations on the results of the soil gas measurements to the greatest possible extent, soil gas measurements were realised during longer dry periods without precipitation and in the shortest possible time, while season, moisture content and water-table depth variations among others also affect the biodegradation of VOCs in soil gas available for measurements and sampling [65].”

comment 6a – The depth of 0.8 meters for soil gas sampling is mentioned, but there is no discussion on why this specific depth was chosen. It would be beneficial to explain why this depth was suitable for detecting volatile organic compounds (VOCs) in this context and how deeper or shallower samples might have affected results.

Response: Prior to soil gas measurements, a detailed historical study of the geology, hydrogeology and operational activity of the industrial area was conducted, vertical profiling was conducted by continuous cored borings in individual parts of the studied area to collect information about lithology and depth of groundwater.

As already stated in the revised manuscript: “…prior to soil gas quality prospection, the pump flow rate (1.0 L/min.), soil gas volume (0.4 L), and measurement duration (25 s) were adjusted to the level of soil contamination and permeability”. Simultaneously the optimal depth of soil gas measurement and sampling (0.8 m) was determined during the soil-air permeability tests in vertical profiles at selected locations. We added this information in the revised manuscript on the Page 6, Lines 252-262:

“Simultaneously with determining the optimal depth of soil gas measurements and sampling (0.8 m), the pump flow rate (1.0 L/min.), soil gas volume (0.4 L), and measurement duration (25 s) were adjusted to the level of soil contamination and soil air permeability tests in vertical profiles at selected sites prior to soil gas screening. The depth of 0.8 m below the surface for soil gas measurement and sampling was selected to minimise the effect of changes in barometric pressure, temperature, break-through of ambient air from the surface, high saturation, clay, organic matter and presence of anthropogenic sediments, which could limit soil gas transfer at the near surface horizon. Carrying out soil gas measurements in this depth should avoid water-table depth variations, depending on the season and the rainfalls, that may also have an impact on the soil gas transfer.”

comment 7a – The criteria for selecting new groundwater sampling sites based on soil gas data could be more detailed. Explaining how soil gas measurements directly informed the placement of new wells would improve the logical flow between the methodologies.

Response: To highlight the logical flow between the methodologies, we highlighted the criteria for selecting new groundwater sampling sites based on soil gas measurements data in the Materials and Methods section (please, see Page 7, Lines 306-311 of the revised manuscript):

“Before the screening, groundwater samples were taken from 25 existing monitoring wells (marked as O1–O25 in Figure 1). Based on the results of the soil gas screening, at sites with anomalously high concentrations of VOCs or other gases (CH₄, CO₂ and O₂) in contrast to their mean concentrations in the study area as a whole, further conventional works were optimised, specifically new monitoring wells for groundwater sampling were situated.”

More detailed information about the spatial distribution of VOC concentrations in soil gas, the mean concentrations of VOCs in the study area as a whole and particularly anomalously high VOC concentrations in the sampling sites, considered as the groundwater contamination sources was already stated in the revised manuscript. Please, see Page 13, Lines 487-489:

“Considering the area as a whole, the mean VOCs concentration was 3090 μg/m³, while the highest mean VOCs concentration (4850 μg/m³) was in the source area A (Figure 5 and Table S5).”

Please, see also Page 14, Lines 510-520:

“A more detailed view of the spatial distribution of VOCs concentrations in soil gas in relation to the concentrations of other gases (CH₄, CO₂ and O₂) and CLHCs in groundwater helped to identify sites that can be considered as a source of groundwater contamination, and to better understand the natural processes responsible for the observed spatial distribution of CLHCs concentrations in groundwater. Particularly high VOCs concentrations (between 8954–>100,000 μg/m³), often associated with high CH₄ concentrations, were found at sampling sites S264, S99, S10, S292 and S268, all from the source area A (Table S5). These sampling sites, considered as the contamination sources, corresponded to high CLHCs concentrations in groundwater (Figure 5) as confirmed by chemical analyses from wells O6, O13, O14, O16 and N18 (concentration range from 111 (PCE), 12 (TCE), 9.24 (cis-DCE), 3.73 (1,1,2-TCA) µg/L up to the limit of aqueous solubility; Table S4).”

These observations were also confirmed by the statistical processing of the data through PCA analysis, as already stated in the revised manuscript (please, see Pages 14-15, Lines 551-559):

“The above-mentioned soil probes, characterised by an anomalous concentration of at least one parameter, were also evidently distinguishable from other soil probes in the bi-plot of the first two main components of observations (Figure 6a). Most of the observations formed one main cluster, situated in the middle of the bi-plot. Soil probe S268 showed VOCs concentrations above the upper detection limit of the instrument, while soil probes S264-265 and S286 were characterised by high TP levels or increased VOCs concentrations. Other soil probes, correlating negatively with PC1 and representing outliers, had high CO₂ concentration (>50,000 mg/m³) compared to the rest.”

Thank you very much again for your impactful comments.

Sincerely,

Edgar Hiller (Corresponding author).

Author Response File: Author Response.pdf

Reviewer 5 Report

Comments and Suggestions for Authors

The study addresses the issue of chlorinated hydrocarbon pollution in groundwater within industrial areas, utilizing soil gas measurement techniques to identify and delineate the extent of contamination. This research holds significant importance for environmental management and pollution remediation. The identification of a notable correlation between VOCs and CLHCs through principal component analysis is a highlight of the study. However, there are several areas that require improvement:

1. The importance and significance of the research are not comprehensively described in the “Introduction” section.

2. The “Conclusions” section lacks a sufficient outlook on future research directions.

3. There are some formatting issues present throughout the manuscript.

Therefore, I recommend major revisions. Specific suggestions include:

1. Page 2, Line 70: The term "Volatile organic compounds" appears repetitively and should be abbreviated. Additionally, the manuscript uses both "VOC" and "VOCs"; please standardize this terminology.

2. Page 2, Lines 60-62: The manuscript mentions pollution by compounds such as PCE and TCE. Please include specific concentration levels and corresponding drinking water limits or concentrations affecting human health to underscore the significance of the study.

3. Page 3, Line 100: The phrase “Despite some minor remedial interventions” should specify which measures are being referred to, along with an explanation of why they were ineffective, leading into the importance of the authors' research.

4. Page 5, Line 164: The term "petroleum hydrocarbons (TP)" is used; however, on Page 6, Line 202, it is referred to as “TP (total petroleum).” Please clarify this inconsistency.

5.Page 20, Line 530: When discussing “significant correlation,” please provide relevant correlation data to enhance the reader’s understanding of the conclusions drawn.

 

6. Although the study provides a detailed description of the pollution status and health risks, the discussion on remediation strategies is somewhat lacking. Given the severe and complex nature of the pollution in the study area, I suggest the authors briefly discuss potential remediation methods and technologies in the conclusions or future research directions to offer more guiding suggestions for practical pollution management.

Author Response

Dear Reviewer #5,

We really appreciate your support and positive response to our study. Thanks so much for your time in evaluating this manuscript. Your comments and recommendations have been taken into account when editing the manuscript. Below is a list of your comments and our response to them.

General comments:

comment 1 – The importance and significance of the research are not comprehensively described in the “Introduction” section.

Response: We have tried to improve these points in Introduction section. We have added some ideas and the statements are the following (please, see the revised manuscript, Pages 4-5, Lines 175-205):

“To identify these potentially ”hidden” and persistent contamination sources, an extensive active soil gas measurement was carried out. It should be noted that the soil gas measurement was not a substitute for conventional methodology but served as a screening tool, allowing more effective use of conventional methods. Based on the soil gas screening, new groundwater sampling sites were selected for further exploration work to confirm these sources and identify sites intended for remediation. Although many studies and academic reports used soil gas screening to determine subsurface contamination by CLHCs and other VOCs, and to identify its main sources, they were mostly limited to pre-characterised source locations with significant accumulation of contaminants (so-called hot spots) that were identified by conventional groundwater sampling methodology. In this study, soil gas screening was used over a large area and at sites hundreds of meters away from the contamination source. Taking into account the advantages and disadvantages of soil gas screening, this method seems to be suitable for the studied area as the existing data showed that the groundwater contained contrasting concentrations of CLHCs spanning several orders of magnitude, and at the same time, the area has a relatively uniform rock environment with high permeability. In addition, elucidation of the extent of groundwater contamination and its sources is of public interest due to permanent settlement in the south-western part of the study area, where residents are exposed to vapour intrusions from contaminated groundwater. In addition, the results of this study could be verified in other industrial areas with similar type of contamination and geological settings. Therefore, the main objectives of this study could be summarised as follows: (i) to determine the extent of groundwater contamination with CLHCs using conventional methodology and existing historical data at an industrial area (the former refrigerator manufacturer – Calex company, city of Zlaté Moravce in western Slovakia), (ii) to apply soil gas screening to further refine the delineation of groundwater contamination and source identification, and to verify the suitability of its application by combining the results of soil gas screening with results of conventional methodology through spatial distribution map outputs and multivariate statistics, and last but not least, (iii) to assess human health risks from the exposure of residents to CLHCs in groundwater. This last objective is important because the groundwater contamination with CLHCs is extended to the surrounding residential zones.”

 

comment 2 – The “Conclusions” section lacks a sufficient outlook on future research directions.

Response: Soil gas surveys can provide relatively rapid and cost-effective site data that can help direct more cost and invasive techniques, which we demonstrated in our study. We realize the need of the characterization of contamination with sufficient spatial resolution, particularly at so called “megasites”. Therefore, we focus our future research directions in coupling of the data from conventional monitoring with non- or low-invasive screening methods (soil gas measurements, geophysics, direct-push, etc.) (please, see also Page 18, Lines 637-645 in the revised manuscript):

“The future research directions should be oriented on coupling the data from conventional monitoring with non- or low-invasive screening methods (soil gas measurements, geophysics, direct-push, etc.) and hydro-geochemical modelling to provide a consistent, realistic and accurate image (conceptual model) integrating the information from different data sources. This could gain the information about the current development and future fate of the extent of contamination, processes of its natural biodegradation, and thus, setting the right remediation approaches. Simultaneously, the method should be applied to similar “mega-sites” in order to compare the results and optimise the approach for the future.”

 

Specific suggestions:

comment 1 –Page 2, Line 70: The term “Volatile organic compounds” appears repetitively and should be abbreviated. Additionally, the manuscript uses both “VOC” and “VOCs”; please standardize this terminology.

Response: Thank you for careful reading. We have fixed it throughout the manuscript and used “VOCs” term, as well as “CLHCs”, etc.

comment 2 – Page 2, Lines 60-62: The manuscript mentions pollution by compounds such as PCE and TCE. Please include specific concentration levels and corresponding drinking water limits or concentrations affecting human health to underscore the significance of the study.

Response: We have changed this section. We have written this as follows (please, see also the revised manuscript, Page 4, Lines 168-171):

“Compared to drinking water standards applicable in the European Union (EU) [15], the maximum concentrations of PCE, TCE and VC in groundwater for 2015 (6482, 85,200 and 5186 μg/L for PCE, TCE and VC, respectively) exceeded the limit values by more than 8500 and 10,000 times, respectively.”

comment 3 – Page 3, Line 100: The phrase “Despite some minor remedial interventions” should specify which measures are being referred to, along with an explanation of why they were ineffective, leading into the importance of the authors' research.

Response: Due to the fact that “minor remedial interventions” were carried out by a private company in the past, information about the extent and effectiveness of these interventions is very limited. We added the basic and only available information about the location and type of these remedial interventions that were carried out between years 2003 and 2005 to the revised manuscript (please, see Page 4, Lines 171-173 of the revised manuscript):

“Despite some minor remedial pump-and-treat interventions in the restricted area of the northern part of the industrial area [71, 72], severe groundwater contamination persists to these days.”

One note else: Due to the continuous extensive groundwater contamination, which was also identified during the groundwater monitoring carried out after these remedial interventions, and the continuous significant groundwater contamination that persists to these days, we assume the “minor remedial interventions” as ineffective.

comment 4 – Page 5, Line 164: The term "petroleum hydrocarbons (TP)" is used; however, on Page 6, Line 202, it is referred to as “TP (total petroleum).” Please clarify this inconsistency.

Response: We apologise for the inconsistency. We have fixed throughout the manuscript as “total petroleum (TP)”.

comment 5 – Page 20, Line 530: When discussing “significant correlation,” please provide relevant correlation data to enhance the reader’s understanding of the conclusions drawn.

Response: Thank you for the comment. We have added in the Conclusion section the following statement (please, see also the revised manuscript, Page 21, Lines 747-749):

“Specifically, both CLHCs and VOCs concentrations highly correlated with the first principal component (PC1) with high values of the component scores (>0.96) and their almost complete overlap.”

comment 6 – Although the study provides a detailed description of the pollution status and health risks, the discussion on remediation strategies is somewhat lacking. Given the severe and complex nature of the pollution in the study area, I suggest the authors briefly discuss potential remediation methods and technologies in the conclusions or future research directions to offer more guiding suggestions for practical pollution management.

Response: Due to persistent and extensive groundwater contamination of the study area and the associated human health risks, remediation is inevitable. Providing a consistent, realistic and accurate model from coupling the data from conventional monitoring with non- or low-invasive screening methods (soil gas measurements, geophysics, direct-push, etc.) and hydro-geochemical modelling, which is a future direction of our research, will serve for choosing an effective remediation approach for the locality. Due to this, we included in the manuscript a potential remediation strategy with a list of effective standard and progressive remediation methods (please, see Page 18, Lines 645-648 of the revised manuscript):

“After obtaining results from other conventional monitoring and screening methods, remediation efforts should focus on identified sources of groundwater contamination using a combination of pump-and-treat methods, injection of oxidising or reducing agents, electrochemical degradation, and bioaugmentation.”

We hope that we have met your expectations and we clarified all the uncertainties.

Thank you very much again for the highly expert revision of the manuscript and comments.

Sincerely Yours,

Edgar Hiller (Corresponding author).

Author Response File: Author Response.pdf

Round 2

Reviewer 5 Report

Comments and Suggestions for Authors

Accept.

 

Comments on the Quality of English Language

The English language quality is good, but it requires some refinement for better clarity and professionalism.