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Open AccessArticle

Radon as a Natural Tracer for Monitoring NAPL Groundwater Contamination

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Water 2020, 12(12), 3327; https://doi.org/10.3390/w12123327 - 26 Nov 2020

Abstract

In this research, the radioactive noble gas radon was used as a tracer for Non-Aqueous Phase Liquids (NAPLs) contamination, since it is much more soluble in these substances than in air or water. Soil radon remains trapped within the NAPLs, resulting in a [...] Read more.

In this research, the radioactive noble gas radon was used as a tracer for Non-Aqueous Phase Liquids (NAPLs) contamination, since it is much more soluble in these substances than in air or water. Soil radon remains trapped within the NAPLs, resulting in a local reduction in the radon concentration within close proximity to the contaminated area. This technique was applied to a contaminated site in Roma (Italy). The main residual NAPLs are total hydrocarbons and methyl-tertiary-butyl ether (MTBE), a water-soluble additive. The monitoring activities included two sampling campaigns of groundwater from 18 wells in February and May 2020. Concentration maps were produced using radon data. The results show that the radon deficit traces the location of NAPLs in the fuelling station very well, with a residual source zone extending in a NNW-SSE direction. A good correspondence between a low amount of radon and a higher concentration of NAPLs was found. A reduction in the average amount of radon in the May 2020 survey indicated a stronger remobilization of NAPLs compared to that of the February 2020 monitoring campaign. The peaks of Volatile Organic Compounds (VOCs) detected between 8–9 and 11–12 m depths indicate the presence of residual blobs of NAPLs in the vadose zone of the aquifer. Full article

(This article belongs to the Special Issue Using Natural Radionuclides as Aquatic Tracers in the Terrestrial and the Coastal/Marine Environment)

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Open AccessArticle

Submarine Groundwater Discharge (SGD) to Coastal Waters of Saipan (Commonwealth of the Northern Mariana Islands, USA): Implications for Nitrogen Sources, Transport and Ecological Effects

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Water 2020, 12(11), 3029; https://doi.org/10.3390/w12113029 - 28 Oct 2020

Abstract

Seagrass meadows and coral reefs along the coast of Saipan, a US commonwealth in the Northern Pacific, have been declining since the 1940s, possibly due to nutrient loading. This study investigated whether submarine groundwater discharge (SGD) contributes to nutrient loading and supports primary [...] Read more.

Seagrass meadows and coral reefs along the coast of Saipan, a US commonwealth in the Northern Pacific, have been declining since the 1940s, possibly due to nutrient loading. This study investigated whether submarine groundwater discharge (SGD) contributes to nutrient loading and supports primary production on Saipan’s coast. SGD can be an important source of freshwater, nutrients, and other pollutants to coastal waters, especially in oceanic islands without well-developed stream systems. Ra and Rn isotopes were used as natural tracers of SGD. Nitrate, phosphate, and ammonium concentrations, ancillary water quality parameters, δ¹⁵N and δ¹⁸O of dissolved nitrate, and δ¹⁵N of primary producer tissue were measured. Our results pointed to discharge of low-salinity groundwater containing elevated concentrations of sewage-derived N at specific locations along Saipan’s coast. High SGD areas had lower salinity and pH, higher dissolved inorganic nitrogen concentrations, and elevated primary producer δ¹⁵N, indicative of sewage nitrogen inputs. We estimated that SGD could support 730–6400 and 3000–15,000 mol C d⁻¹ of primary production in Tanapag and Garapan Lagoons, respectively, or up to approximately 60% of primary production in Garapan Lagoon. Efforts to improve water quality, reduce nutrient loading, and preserve coastal ecosystems must account for groundwater, since our results demonstrate that it is an important pathway of nitrogen delivery. Full article

(This article belongs to the Special Issue Using Natural Radionuclides as Aquatic Tracers in the Terrestrial and the Coastal/Marine Environment)

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Open AccessArticle

Investigating River Water/Groundwater Interaction along a Rivulet Section by ²²²Rn Mass Balancing

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Water 2020, 12(11), 3027; https://doi.org/10.3390/w12113027 - 28 Oct 2020

Abstract

Investigation of river water/groundwater interaction aims generally at: (i) localizing water migration pathways; and (ii) quantifying water and associated matter exchange between the two natural water resources. Related numerical models generally rely on model-specific parameters that represent the physical conditions of the catchment [...] Read more.

Investigation of river water/groundwater interaction aims generally at: (i) localizing water migration pathways; and (ii) quantifying water and associated matter exchange between the two natural water resources. Related numerical models generally rely on model-specific parameters that represent the physical conditions of the catchment and suitable aqueous tracer data. A generally applicable approach for this purpose is based on the finite element model FINIFLUX that is using the radioactive noble gas radon-222 as naturally occurring tracer. During the study discussed in this paper, radon and physical stream data were used with the aim to localize and quantify groundwater discharge into a well-defined section of a small headwater stream. Besides site-specific results of two sampling campaigns, the outcomes of the study reveal: (i) the general difficulties of conducting river water/groundwater interaction studies in small and heterogeneous headwater catchments; and (ii) the particular challenge of defining well constrained site- and campaign-specific values for both the groundwater radon endmember and the radon degassing coefficient. It was revealed that determination of both parameters should be based on as many data sources as possible and include a critical assessment of the reasonability of the gathered and used datasets. The results of our study exposed potential limitations of the approach if executed in small and turbulent headwater streams. Hence, we want to emphasize that the project was not only executed as a case study at a distinct site but rather aimed at evaluating the applicability of the chosen approach for conducting river water/groundwater interaction studies in heterogeneous headwater catchments. Full article

(This article belongs to the Special Issue Using Natural Radionuclides as Aquatic Tracers in the Terrestrial and the Coastal/Marine Environment)

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Open AccessArticle

Variability of Cosmogenic ³⁵S in Rain—Resulting Implications for the Use of Radiosulfur as Natural Groundwater Residence Time Tracer

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Water 2020, 12(10), 2953; https://doi.org/10.3390/w12102953 - 21 Oct 2020

Abstract

Information about groundwater residence times is essential for sustainable groundwater management. Naturally occurring radionuclides are suitable tools for related investigations. While the applicability of several long-lived radionuclides has been demonstrated for the investigation of long residence times (i.e., years, decades, centuries and more), [...] Read more.

Information about groundwater residence times is essential for sustainable groundwater management. Naturally occurring radionuclides are suitable tools for related investigations. While the applicability of several long-lived radionuclides has been demonstrated for the investigation of long residence times (i.e., years, decades, centuries and more), studies that focus on sub-yearly residence times are only scarcely discussed in the literature. This shortage is mainly due to the rather small number of radionuclides that are generally suitable for the purpose and show at the same time adequately short half-lives. A promising innovative approach in this regard applies cosmogenic radiosulfur (³⁵S). ³⁵S is continuously produced in the stratosphere from where it is conveyed to the troposphere or lower atmosphere and finally transferred with the rain to the groundwater. As soon as the meteoric water enters the subsurface, its ³⁵S activity decreases with an 87.4 day half-life, making ³⁵S a suitable time tracer for investigating sub-yearly groundwater ages. However, since precipitation shows a varying ³⁵S activity during the year, setting up a reliable ³⁵S input function is required for sound data evaluation. That calls for (i) an investigation of the long-term variation of the ³⁵S activity in the rain, (ii) the identification of the associated drivers and (iii) an approach for setting up a ³⁵S input function based on easily attainable proxies. The paper discusses ³⁵S activities in the rain recorded over a 12-month period, identifies natural and anthropogenic influences, and suggests an approach for setting up a ³⁵S input function applying ⁷Be as a proxy. Full article

(This article belongs to the Special Issue Using Natural Radionuclides as Aquatic Tracers in the Terrestrial and the Coastal/Marine Environment)

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Open AccessArticle

Investigating Groundwater Discharge into a Major River under Low Flow Conditions Based on a Radon Mass Balance Supported by Tritium Data

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Water 2020, 12(10), 2838; https://doi.org/10.3390/w12102838 - 13 Oct 2020

Cited by 1

Abstract

The potentially detrimental impact of groundwater discharge into rivers on the ecosystem services provided by the river makes the localization of groundwater discharge areas as well as the quantification of the associated mass fluxes an issue of major interest. However, localizing groundwater discharge [...] Read more.

The potentially detrimental impact of groundwater discharge into rivers on the ecosystem services provided by the river makes the localization of groundwater discharge areas as well as the quantification of the associated mass fluxes an issue of major interest. However, localizing groundwater discharge zones and evaluating their impact are challenging tasks because of (i) the limited number of suitable tracers and (ii) the high spatio-temporal variability of groundwater/river water interaction in general. In this study, we applied the ubiquitous naturally occurring radioactive noble gas radon (²²²Rn) as an aqueous tracer to localize and quantify groundwater discharge along a 60 km reach of the upper German part of the major river Elbe under drought conditions. All radon data processing was executed with the numerical implicit finite element model FINIFLUX, a radon mass balance-based approach, which has been developed specifically to quantify the groundwater flux into rivers. The model results were compared to the tritium (³H) distribution pattern in the studied river reach. The results of the study proved the applicability of both (i) the methodical approach (i.e., radon as tracer) and (ii) the application of FINIFLUX to drought conditions (with river discharge rates as low as 82 m³/s vs. a long time mean of 300 m³/s). Applying the model, the recorded dataset allowed differentiating between groundwater baseflow, on the one hand, and interflow and surface water runoff distributions to the river, on the other. Furthermore, the model results allowed assessing the location and the intensity of groundwater discharge into the river under low flow conditions. It was also shown that analysing discrete river water samples taken from distinct points in a major stream might lead to slightly incorrect results because of an incomplete mixing of river water and locally discharging groundwater. An integrating sampling approach (as applied for radon) is preferable here. Full article

(This article belongs to the Special Issue Using Natural Radionuclides as Aquatic Tracers in the Terrestrial and the Coastal/Marine Environment)

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Open AccessArticle

Submarine Groundwater Discharge in a Coastal Bay: Evidence from Radon Investigations

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Water 2020, 12(9), 2552; https://doi.org/10.3390/w12092552 - 12 Sep 2020

Abstract

Jiaozhou Bay, an urbanized coastal bay located in the southern part of Shandong Peninsula, China, has been deeply affected by anthropogenic activities. Here, the naturally occurring ²²²Rn isotope was used as a tracer to assess the submarine groundwater discharge (SGD) in this [...] Read more.

Jiaozhou Bay, an urbanized coastal bay located in the southern part of Shandong Peninsula, China, has been deeply affected by anthropogenic activities. Here, the naturally occurring ²²²Rn isotope was used as a tracer to assess the submarine groundwater discharge (SGD) in this bay. The time series of ²²²Rn concentrations in nearshore seawater were monitored continuously over several tidal cycles at two fixed sites (Tuandao (TD) and Hongdao (HD)) during the dry season in spring and the wet season in autumn of 2016. ²²²Rn concentrations in seawater were negatively related to the water depth, indicating the influence of tidal pumping. A ²²²Rn mass balance model revealed that the mean SGD rates were 21.9 cm/d at TD and 17.8 cm/d at HD in the dry season, and were 19.5 cm/d at TD and 26.9 cm/d at HD in the wet season. These rates were about 8–14 times the discharge rates of the local rivers. Enhanced groundwater inputs occurred at HD in the wet season, likely due to the large tidal amplitudes and the rapid response to local precipitation. Large inputs of SGD may have important influences on nutrients levels and structure, as well as the water eutrophication occurring in coastal waters. Full article

(This article belongs to the Special Issue Using Natural Radionuclides as Aquatic Tracers in the Terrestrial and the Coastal/Marine Environment)

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Open AccessArticle

Temporal Variations of Submarine Groundwater Discharge into a Tide-Dominated Coastal Wetland (Gaomei Wetland, Western Taiwan) Indicated by Radon and Radium Isotopes

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Water 2020, 12(6), 1806; https://doi.org/10.3390/w12061806 - 24 Jun 2020

Cited by 1

Abstract

Submarine groundwater discharge (SGD) is evidenced around Taiwan, but the seasonal/temporal changes of SGD have not been fully examined. Here, we report a time-series investigation of SGD into a tide-dominated coastal wetland, the Gaomei Wetland, located to the south of the Da-Chia River’s [...] Read more.

Submarine groundwater discharge (SGD) is evidenced around Taiwan, but the seasonal/temporal changes of SGD have not been fully examined. Here, we report a time-series investigation of SGD into a tide-dominated coastal wetland, the Gaomei Wetland, located to the south of the Da-Chia River’s mouth, western Taiwan, by using environmental tracers (²²²Rn, ²²⁴Ra_ex, ²²⁸Ra, δD, and δ¹⁸O). Our results showed that regardless of dry and wet seasons, the ²²²Rn activities in coastal waters were high at low tide but low at high tide. It represents the continuous input of ²²²Rn-enriched groundwater. However, the ²²⁴Ra_ex and ²²⁸Ra activities showed seasonal changes with tide conditions. In the dry season, the ²²⁴Ra_ex and ²²⁸Ra activities in coastal waters were low at low tide but high at high tide; whereas in the wet season, an opposite relation was observed with quite high ²²⁴Ra_ex and ²²⁸Ra activities in the low-tide waters. Coupled with the lower δD and δ¹⁸O values of coastal and pore waters in the dry season, in comparison to those in the wet season, it is suggested that these phenomena probably reflected a seasonal difference in the main SGD component with fresh SGD in the dry season, but saline ones in the wet season. Based on a ²²²Rn mass balance model, the estimated SGD fluxes into the Gaomei Wetland varied with tidal fluctuations and ranged from 0.2 to 25 cm d⁻¹ and from 0.1 to 47 cm d⁻¹ for the dry and wet seasons, respectively. A slightly high SGD flux occurring during the wet season at spring tide, implied a stronger tidal pumping coupled with a larger hydraulic gradient between land and sea. In this study, we demonstrated that the variation of SGD into the Gaomei Wetland is not only controlled by the seasonal changes of groundwater recharge, but also by the tidal pumping process. Full article

(This article belongs to the Special Issue Using Natural Radionuclides as Aquatic Tracers in the Terrestrial and the Coastal/Marine Environment)

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Review

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Open AccessReview

Absolute Measurement of Thoron in Surface Waters

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Derek Lane-Smith

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Michael Schubert

Water 2020, 12(11), 3083; https://doi.org/10.3390/w12113083 - 03 Nov 2020

Abstract

Thoron (radon-220, ²²⁰Rn, half-life 55.6 s) is a useful aqueous tracer suitable for applications such as locating and measuring groundwater discharge in surface waters (including the coastal ocean) and detecting radium-224 (²²⁴Ra) bearing scale inside water pipes. Generally, such applications [...] Read more.

Thoron (radon-220, ²²⁰Rn, half-life 55.6 s) is a useful aqueous tracer suitable for applications such as locating and measuring groundwater discharge in surface waters (including the coastal ocean) and detecting radium-224 (²²⁴Ra) bearing scale inside water pipes. Generally, such applications require only relative thoron measurements in the water. However, if a thoron-in-water quantification in absolute numbers is desired the knowledge of the thoron sensitivity of the measurement system is obligatory. Absolute readings would, e.g., give a measure of the ²²⁴Ra activity in the sediment, supporting the thoron, or of the quantity of ²²⁴Ra in the pipe scale. Since there is no standardised source of thoron-in-water (such as a NIST standard), there is no way to calibrate a thoron-in-water measurement system, in the usual sense. Up until now, therefore, it has not been possible to make absolute measurements of thoron in water. This paper presents a novel method of assessing the sensitivity of a mobile thoron-in-water measurement system. The paper analyses such systems and describes a straightforward experimental approach to obtain all setup-specific values of critical parameters that will allow a reasonably precise determination of the system thoron sensitivity. The method is simple enough that a thoron sensitivity calibration can be performed on site with no additional equipment. Full article

(This article belongs to the Special Issue Using Natural Radionuclides as Aquatic Tracers in the Terrestrial and the Coastal/Marine Environment)

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