Primary Soil Investigation near a Potentially Contaminated Site in Cluj-Napoca, Romania

Abstract

Considering the need to identify areas contaminated by anthropogenic activities to include them in remediation programs, the main objective of the present study was to investigate the heavy metals content in several soil samples from the proximity of a potentially contaminated site. The total metal concentrations for Cu, Mn, Fe, Cr, Ni, Pb, Zn, and Cd were determined by flame atomic absorption spectrometry (AAS-F) along with the general physico-chemical parameters (pH, redox potential, electrical conductivity, total dissolved solids, and salinity). The results showed that some of the soils classified for sensitive use had high contents of Cu, Cr, Pb, and Zn, exceeding the alert and even intervention threshold. With exception of Zn, the content of heavy metals in the soils with less sensitive usage was relatively low, being within the limits imposed by national legislation. Various fluctuations in the metal content were observed between the soils sampled from 5 cm depth and those from 30 cm depth. The overall results should present interest for the local authorities and be correlated with environmental concerns for the human population living in the area.

1. Introduction

Management of contaminated sites is an environmental challenge from the beginning of the 2000s, when the first attempts to create a Soil Directive for the European Union generated the Thematic Strategy for Soil Protection, including a communication from the commission to the other European Institutions (Comm(2006)231) [1], a proposal for a Soil Framework Directive (Comm(2006)232) [2], and an Impact Assessment (SEC(2006)1165 and SEC(2006)620) [3,4]. The common effort of the EU countries led to a report which revealed that almost 1.2 million potentially contaminated sites have been generated in the EU due to industrial and agricultural processes [5]. However, the report underlines that this number is considered to represent just 45% of the real number of potentially contaminated sites, while 340,000 of the identified sites were framed as contaminated sites. In 2018, a new estimation was presented at European level, and the number of affected areas raised to 2.8 million potentially contaminated sites [6]. In Romania, the environmental regulations recognize 1183 of potentially contaminated sites and 210 contaminated sites [7]. As in the case of the European report, these dates are considered to represent less than the real number of potentially contaminated sites and contaminated sites.

Soil contamination is a serious problem all over the world, and the environmental implications are observed inclusively at social and economic levels [8,9]. The main causes of soil pollution are represented by anthropogenic activities, especially industries such as mining, metallurgy, chemical industry, oil industry, old deposits of pesticides, uncontrolled municipal waste dumps, and transport activities [10]. A wide variety of contaminants can be generated from the specific activities of these industries, such as intense production processes, inappropriate chemical substances handling or disposal, or even human errors. The main contaminants present in soils framed as contaminated sites are heavy metals and petroleum hydrocarbons, assumed to represent 60% of the forms of soil contamination [11,12].

Many studies focused on the identification of chemical substances into soil, in the attempt to assess the pollution level in areas of interest [13]. In the increased demand for urban development, one of the principles is to occupy, after remediation, the former contaminated site (brownfield) within that city instead of using a new green field [14]. This is known as the concept of redevelopment of industrial areas, used for cities which had special areas assigned as industrial platforms, and the main goal is to preserve the environment as much as possible from new anthropogenic impact [15].

The present study focused on the evaluation of soil pollution near an area known as potentially contaminated site with heavy metals. Since urban redevelopment is planned in that area, for the safety of citizens, the assessment was based on a survey in the vicinity of this potentially contaminated site, as access to the site would not have been allowed. The investigation covered the soil found in the range of 100 m from the contaminated area, thus corresponding also with the regulations asked by the Romanian legislation (Law 74/2019) for the management of contaminated sites [16]. Since the framing of a contaminated site is based on the chemical content of the pollutants [16], the total metal concentrations for Cu, Mn, Fe, Cr, Ni, Pb, Zn, and Cd were determined by flame atomic absorption spectrometry (AAS-F). Additionally, the general physico-chemical parameters (pH, redox potential, electrical conductivity, total dissolved solids, and salinity) were evaluated. The results revealed increased metal concentrations, so the assessment was focused on possible environmental risks for the inhabitants. Therefore, the assessment should present interest for the local authorities, because the new national regulations from 2019 to 2021 require decontamination, along with protection measurements for population.

2. Materials and Methods

The study area is located in the former industrial area of Cluj-Napoca city (Romania), and the potentially contaminated site was assumed to be the source of metal contamination for the surroundings (Figure 1). The activity responsible for contamination is shoe manufacturing, with the adhesive warehouse, artificial sole manufacturing, fuel storage, and tannery as sources of contaminant release in the form of heavy metals and petroleum substances (Appendix A). Nearby, other economical operators and neighborhoods with houses are present.

For the field investigation, the sampling points needed to be as close as possible to the source of contamination (not further than 1 km) and placed on all cardinal coordinators. Five sampling points (P₁–P₅) were chosen at distances ranging between 10 and 100 m (P₁ 15 m, P₂ 30 m, P₃ 30 m, P₄ 100m, P₅ 10 m) from the potentially contaminated site, and one reference point (P_m) from 300 m (Figure 1). The existing vegetation was removed from the soil surface, and around 500 g of soil was collected from 5 cm depth (Px-5) and 30 cm depth (Px-30), at each sampling point, according to Ord. 184/1997 [17]. The sampling was performed with a stainless-steel hand auger and transferred to polyethylene bags, followed by transport to laboratory at a constant temperature (4 °C) and absence of light.

Samples from P₁, P_2, and P₃ were framed in the less sensitive soil use class because they were collected from the proximity of the potentially contaminated site where the industrial activity takes place, while samples P₄, P_5, and P_m were collected from residential areas, so they needed to be framed as sensitive soil use.

All the soil samples were oven dried at 105 °C until constant weight, after which the samples were grounded and sieved through a 250 μm sieve. The physico-chemical parameters (pH, redox potential (ORP), electrical conductivity (EC), total dissolved solids (TDS), and salinity) were measured in the aqueous extract of soil/water (1:5 ratio) by mixing 50 g of soil in 200 mL ultrapure water for 2 h, followed by 15 min decantation, according to SR 7184-13/2001 protocol [18]. The aqueous extract was then filtered through 8 μm qualitative paper filter, and the physico-chemical parameters were further determined from the resulting extract using a portable multiparameter (WTW Multi 3320, Xylem Analytics Germany Sales GmbH & Co. KG, Weilheim, Germany).

The heavy metals investigated were Ni, Cd, Cr, Pb, Zn, Cu, Fe, and Mn. To determine the total metal content, 3 g of soil from the dried samples (24 h at 105 °C) were subjected to mineralization with aqua regia (21 mL of 12 mol l⁻¹ of HCl and 7 mL of 15.8 mol l⁻¹ HNO₃), according to ISO 11466:1995 protocol [19]. Therefore, the resulting solution was left to mineralize at room temperature overnight, followed by 2 h boiling on sand bath. Then, the extracts were filtered through 8 μm qualitative paper filter and diluted with HNO₃ (0.5 M) to a final volume of 100 mL. The analyses were performed using an atomic absorption spectrometer (AAS, Analytik Jena ZEEnit 700, Analytik Jena AG, Jena, Germany) equipped with an air-acetylene flame and a graphite furnace [20].

The analyses were performed using a method that has been validated for previous studies. The method was optimized to reach the performance parameters specified by the producer and to ensure the reliability of analytical data QA/QC criteria. The linearity of the instrumental response was studied by analyzing a set of five standard solutions with concentrations between 0.25 and 3.00 mg/L (for Cu, Fe, and Mn), and between 0.25 and 1.00 mg/L (for Cd, Cr, Ni, Pb and Zn). Each metal showed a good linearity, with regression coefficient (R²) greater than 0.999. The method reproducibility was evaluated based on the relative standard deviation for eight replicated spiked solutions. The method proved to have good reproducibility, as RSD was less than 15% for all metals. The limit of quantification (LOQ) was obtained by multiplying the standard deviation (for 8 replicated blank samples) by a factor of 10. The LOQ values ranged between 0.023 mg/L (Cd) and 0.087 mg/L (for Pb). The efficiency of the aqua regia extraction was evaluated by analyzing a certified reference material (ERM-CC141 LOAM SOIL). The results indicated a good recovery, between 71 and 115%.

3. Results

3.1. General Physico-Chemical Parameters

The analyzed soils had a pH between 7.5 and 8.3, being classified as weakly alkaline soils (Figure 2). No major differences were observed compared to the reference sampling point (P_m), where the pH ranged from 7.8 to 8 from the top to bottom. Generally, the pH was slightly higher in the soils sampled at 30 cm depth than those from 5 cm depth. It is generally known that the soil pH usually increases along with the depth [21]. All the samples had negative redox potential. The pH conditions and the redox potential do not enhance the dissolution and leaching of metals, since their dissolution increases under acidic pH conditions.

The redox potential (ORP) indicated a soil with reduction properties. The results ranged between −13 mV and −60 mV (Figure 2), with no important differences at the two depths. As in the case of pH, a slight tendency of increased values can be observed once the distance from the potentially contaminated site increases, along with the depth. The reference point had the same value (−42 mV) at both depths, but it can be considered in accordance with the average (−40 mV) of the rest of the results.

The soils had the EC between 74.6 and 368 µS/cm (Figure 2), being within the values generally registered in soils (up to 1000 µS/cm). The highest electrical conductivity (368 µS/cm, respectively, 231 µS/cm) had the soil sampled from P₁, which was very close to the potentially contaminated site. Considering that the rest of the samples are two or even three times lower in electrical conductivity (including the reference sample), it can be assumed that a chemical contamination might have occurred in P₁ area. The TDS values for P₁ confirm the supposition, as the highest values were observed in P₁-5 (239 mg/L), followed by P₁-30 (150 mg/L) (Figure 2). The increased value in P₁ can be associated with salt forms accumulated into the soil. The other soil samples had similar values for TDS, ranging between 48 and 117 mg/L, without major differences to the reference samples. The salinity was 0.0 ‰ except for soil P₁-5, where the salinity was 0.1‰, generally.

3.2. Heavy Metal Content

The metal concentrations were confronted with the Romanian thresholds set for soil pollution [22] (Figure 3), and several contaminants have exceeded the alert threshold, some of them exceeding even the intervention threshold. To ease the interpretation, each metal will be discussed individually. Considering the two different types of usage, the soil samples were compared with the corresponding level of concentration from the environmental legislation (less sensitive use—P₁, P₂, and P_3, and sensitive use—P₄, P₅, and P_m).

All the analyzed soil samples had copper content higher than 20 mg/kg, which is considered the normal value according to Ministry Order 756/1997 (Figure 3). Samples from P₁, P_2, and P₃, which correspond to less sensitive usage, had the copper level below the alert threshold (250 mg/kg). Among the sensitive usage sites, P₅ samples exceeded the alert threshold (100 mg/kg) and even the intervention threshold (200 mg/kg) at both depths (Figure 3). The other soil sample’s average (63 mg/kg) was slightly higher than the reference sample (43 mg/kg) at 5 cm, while at 30 cm, the average (62 mg/kg) was similar with P_m (66 mg/kg). These results emphasis the anthropogenic impact on copper content in P₅ samples. P₁, P₂, and P₅ had higher metal concentration at 5 cm depth, while the metal content from P₃, P₄, and P_m was increased at 30 cm depth. Since metal contamination is assumed to be propagated through air pathway, these variations might indicate anthropogenic works at soil level in that area.

Chromium presented a relatively low content, around the normal value (30 mg/kg), with the exception of P₄-5 (42 mg/kg) and P₄-30 (64 mg/kg), which registered an important overpass of this threshold (Figure 3). Additionally, the Cr concentration in P₅-5 exceeded the alert threshold. These exceedances are important even if they would have been registered just in one sampling point, because Cr is toxic for the human health even at low concentrations [23], and P₄ and P₅ correspond to the soil sensitive use, where inhabitant access is allowed. Chromium can be present in substances used for colorants, pigments, or tannery, activities that took place on the potentially contaminated site. Since the other soil samples, found closer to the site, had Cr concentrations three to four times lower than the one from P₅-5, it might indicate an air pathway of soil contamination. Considering that P₅-30 has a lower concentration than the surface (Figure 3), the Cr contamination might be a recent one, where no water infiltration into the depth has occurred yet. Due to Cr threats to human health, further analyses should be conducted to establish if Cr is also present in the bedrock, or its’ presence can be completely correlated to anthropogenic activity within the potentially contaminated site.

In the case of Pb, all the samples had higher concentrations than the normal values, including the reference samples (Figure 3). More important is the fact that all the sampling points from the residential area (P₄, P₅, P_m) revealed increased concentration, exceeding the alert threshold for sensitive soil usage (50 mg/kg). P₅ and P_m exceeded the limit at 5 cm depth, while P₄ exceeded it at both depths. P₄ was sampled from an area with more vegetation, especially trees, and near an apartment building. Gardening or other related activities for vegetation maintenance might have been performed by inhabitants, leading to an overlapping of soil layers. This might be a reason for the higher Pb content at 30 cm depth compared to the content from 5 cm depth since only P₄ had this important metal increase in the deeper soil layers. The elevated Pb concentration in the reference point (P_m-5) can be associated with a more spread contamination than initially assumed; therefore, wider areas (in the range of 1 km) should be subjected to a similar soil investigation. It is important to mention that the samples located further from the contamination source had higher Pb concentrations than those found in close proximity, thus indicating a possible air pathway for contaminant transport, as discussed in the previous metal results. Due to the increased levels of Pb in the study area, additional soil samples should be collected, including from the potentially contaminated site, if allowed, and air measurements from the activity emissions should be conducted. Thus, it would be possible to determine if the activity from the potentially contaminated site is the only source for Pb contamination, or if other anthropogenic activities should also be considered.

The highest metal contamination was in the case of Zn, where all samples considerably exceeded the normal value (100 mg/kg) (Figure 3). All the soil samples from the residential areas (P₄, P₅, P_m) exceeded both the alert threshold (300 mg/kg) and intervention threshold (600 mg/kg) for sensitive soil usage at both depths, while P₁-5 and P₂-5 samples exceeded the alert threshold (700 mg/kg) for less sensitive soil usage. Increased metal content was observed at both evaluation depths (except P₁-30), including the reference samples (P_m). Since P₃, P_4, and especially P_m had higher concentrations at 30 cm, chemical speciation should be performed in order to establish if Zn concentration is found in the residual form. If this would be the case, then the high Zn content could be linked to parent material and not to anthropogenic contamination even though the local authorities indicated such metal pollution nearby (Appendix A).

Ni concentration exceeded the normal threshold (20 mg/kg) in almost all the samples (Figure 3), except P₃-5 (18 mg/kg), but did not reach the alert threshold for both soil uses (50 mg/kg, respectively, 250 mg/kg). Except P₄ and P₅, all the samples from 30 cm depth had higher concentrations of Ni than the ones from 5 cm depth. Considering that even the reference point, P_m, had the same dispersion into the soil, it might be possible to have Ni concentration into the bedrock since Ni is an essential element found in the soil [24]. Therefore, it is possible that the Ni accumulation in the study area is a natural occurrence due to the geological substructure. The exceptions in P₄ and P₅, with increased Ni concentrations at 5 cm depth, could be associated with air contamination from another contaminated site found approximately 1 km distance from the study area, in the NE direction. The local authorities declared the area contaminated with Ni, as proven by chemical analyses (Appendix A, No. 3). P₄ and P₅ have a more exposed position to NE to SW wind directions than the other sampling points, which are partially blocked by high buildings.

The content of Cd and Mn was relatively low, their concentrations being close to the normal values (Figure 3). Cd concentrations slightly exceeded the normal threshold (1 mg/kg) in the majority of samples, while Mn did not reach the normal values (900 mg/kg) in any of the samples. Cd concentration in P₅-5 (1.9 mg/kg) was almost double that of P₅-30 (1.1 mg/kg), a situation which might indicate slight contamination from other anthropogenic sources, since P₅ registered increased metal contents of different metals. Based on these results, Cd and Mn do not pose any important risk to the environment or human health. Still, the increased concentrations of the previously discussed heavy metals indicate an extended contamination in the proximity of the potentially contaminated site.

Iron was investigated to gain a broader understanding of the soil quality. The Romanian legislation does not specify any limitations, which is why iron was compared with the reference results. Close to the surface, at 5 cm depth, all the samples (P₁–P₅) had higher Fe concentration than P_m-5 (Figure 3), while at 30 cm depth, the Fe levels were more similar with P_m-30. A metal speciation study, in order to establish the chemical form of Fe present in soil, would ease the interpretation if Fe has a natural occurrence especially close to the surface, or this result is the consequence of anthropogenic contamination [25].

The Pearson correlation coefficient (

Table 1. Pearson correlation between the analyzed parameters.

pH	pH	ORP	EC	TDS	salinity	Cu	Mn	Fe	Cr	Ni	Pb	Zn	Cd
ORP	−0.96515
EC	−0.68364	0.76416
TDS	−0.68166	0.7624	0.99999
salinity	−0.57937	0.67192	0.8473	0.84663
Cu	0.57325	−0.47585	−0.03274	−0.02962	−0.12007
Mn	−0.16707	0.19403	0.24546	0.24533	0.13877	−0.00543
Fe	0.079856	0.053152	0.38841	0.39004	0.11277	0.59391	0.59224
Cr	0.20725	−0.15614	0.029791	0.03195	−0.04462	0.77889	0.36208	0.61701
Ni	0.18099	−0.11052	0.31381	0.31668	0.082415	0.70755	0.5443	0.83355	0.78404
Pb	0.1203	−0.21329	−0.27099	−0.27062	−0.18274	0.38114	0.31785	0.16153	0.7048	0.32685
Zn	0.098228	−0.05055	−0.03619	−0.03477	0.15249	0.51031	−0.07822	0.031698	0.54598	0.19105	0.59839
Cd	0.50694	−0.51771	−0.35217	−0.34932	−0.18182	0.40436	−0.42321	−0.18796	0.2821	0.19824	0.099577	0.23924

) showed a significant direct correlation (coefficient > 0.7) between EC and ORP, TDS, and salinity: between Cu content versus Cr; Ni content; between Ni versus Fe soil level; between Cr and Pb content; and a reverse correlation between pH and OPR.

4. Discussion

According to the Romanian regulations (Law 74/2019), an area can be framed as a contaminated site after confirmation of environmental pollution through laboratory analyzes, and the assessment of the surroundings is mandatory in the range of 1 km. The metal assessment revealed increased metal concentrations in the study area. In some cases, higher amounts of heavy metals were found at 5 cm depth, while in other situations the increased metal content was determined at 30 cm depth. This situation was also observed at the reference point; therefore, it is hard to predict the contaminant pathway to soil. Where the contamination is higher near the surface, it would be expected to have an air pathway, a situation which is often observed [26]. When the contamination increased along with the depth, direct contact of contaminant with soil might have occurred, followed by rapid infiltration and accumulation into the depth. For this case, along with soil structure and texture, organic matter content and water permeability should be evaluated to determine possible pathways of metal transport into soil due to the geological background [27].

The area where the present study took place represents the former industrial platform of Cluj-Napoca. According to the local authorities (Appendix A), other potentially contaminated or contaminated sites are found in this region. Similar studies for heavy metal contamination in this particular area are rare. One of the reasons is the late national regulation related to management of contaminated sites, which was released in 2019, follow by two methodologies in 2020 and 2021, respectively. Another reason might be the private properties of the fields where the contaminations are present. Based on our knowledge, there is only another research [28] which evaluated heavy metals close to a contaminated site from the former industrial platform of Cluj-Napoca. The study indicated elevated contamination with Cd and Pb, while Cu, Zn, and Ni exceeded the normal thresholds at soil surface.

Near the investigation area are neighborhoods, and the whole area is planned for general urban redevelopment, including new apartment buildings with inhabitant facilities (shops, schools, parks, etc.). Since most of the exceedances of the analyzed heavy metals occurred in the sampling points located in the residential area (P₄ and P₅) and sometimes even in the reference point (P_m), it is important to associate the increased metal contents with possible human environmental threats.

The Cu exceeding of the intervention threshold for sensitive soil usage implies that restriction measurements for human access in the P₅ area should be applied [16,29]. Copper is known as an essential element for metabolism at low concentrations, but increased concentrations are toxic for the human organism and can lead to health problems, such as gastrointestinal diseases, or liver and kidney diseases [30,31]. Furthermore, P₅ also registered the exceeding of chromium for alert threshold. In terms of toxicity, Cr (VI) can be absorbed by the lung and gastrointestinal tract, being responsible for many health problems, including lung or respiratory cancer [32]. Therefore, further investigations should be conducted to identify the source of contamination, followed by speciation analyses to determine the chemical form of the Cr in soil.

Since increased Pb concentrations exceeding the alert threshold were found in the upper layers of soil (close to surface) in the residential areas (P₄, P_5, and P_m), it is important to prevent inhabitant soil exposure in those areas. Soil particles can be easily carried by the wind, so lead can be present in the breathing air of the citizens living there. Lead is a very dangerous heavy metal, its’ toxicity being correlated to neurological effects (especially for children), followed by hematological and renal disorders, and even decreased fertility [33]. The most extensive contamination was generated by Zn, especially in the residential area. The exceeding of the intervention threshold implies, as in the case of Cu, that human access in those areas (P₄, P_5, and P_m) should be restricted based on Romanian regulations [16,29]. Zn toxicity is mostly associated with growth and reproduction dysfunctions, even though this element is non-toxic and is an essential element at low concentrations [34].

All these implications for inhabitants should draw the attention of the local authorities and the plans for urban redevelopment should be carefully analyzed. Since the current study offers just a preliminary insight of the contamination level, the decision makers can demand a more complex evaluation of soil pollution based on the legislation for management of contaminated sites, which clearly claims the need to perform detailed investigation when preliminary assessments reveal exceedance of national soil thresholds. If the area presents the prerogatives to be framed as a contaminated site, the redevelopment should be postponed until remediation measurements are implemented.

5. Conclusions

The study focused on the soil assessment of heavy metals occurrence in the proximity of a field framed as a potentially contaminated site. The results indicated various areas of metal contamination. One of the sampling points located very close to the potentially contaminated site, P₅, revealed the highest concentrations for Cu, Cr, Pb, and Zn. Zn was the metal with the highest concentrations, and among the 12 soil samples collected for the chemical assessment, 8 of them exceeded the alert or intervention threshold. Thus, the identified contamination can pose considerable threats for the population living in the area. Normally, the inhabitants should be informed and their access to these areas should be limited or even restricted.

This preliminary study provides evidence for an extended contamination around the potentially contaminated site listed by the local authorities from Cluj-Napoca. Based on these results, the investigated area corresponding to sensitive usage can be framed as a contaminated site with Cu, Cr, Pb, and Zn. It can be assumed that the shoe manufacturing activity represents the source of contamination. To provide a more complete description of soil pollution, further analysis of the heavy metals and other possible contamination sources present in the area should be considered. Detailed chemical assessments should be undertaken for metal speciation to establish the mobility of the pollutants in the soil. These results would offer the primary background for remediation plans. Afterward, soil decontamination is mandatory, so that the local authorities to begin an adequate redevelopment of this industrial area into a residential one.