Mean Residence Time Of Groundwater Research Articles

Groundwater flow system and microbial dynamics of groundwater in a headwater catchment

Besides surface water systems, groundwater also provides a habitat for prokaryotes. The prokaryotes inhabiting groundwater are affected by the environment that they grow in. Owing to the environmental specificity, some prokaryotic community constituents might thus provide information on local subsurface growth conditions as well as advective transport processes in case of rapid dislocation. In this study, we investigated the spatiotemporal distribution of prokaryotic communities in groundwater and spring water to examine the relationship between the groundwater flow system and prokaryotic dynamics. We sampled and monitored the water in a mountainous forested catchment in Kawamata Town, Fukushima Prefecture, Japan. Seven piezometers and a spring located across a ridge, hillslope, and valley were monitored. Prokaryotic information, such as density estimated by total direct counting, community constituents and diversity using the next-generation sequencing methods, physicochemical parameters, general water quality, and groundwater mean residence time using sulfur hexafluoride (SF6) were analyzed. Our results showed that the mean residence time of groundwater and spring water ranged from less than 1 year to 30 years. The prokaryotic density ranged from 5.37 × 103cells/mL to 2.01 × 106cells/mL (n = 60). As the mean residence time increased, the density of prokaryotes in the shallow subsurface also increased, whereas that of the prokaryotes in the deep subsurface decreased. The contribution of deep groundwater to the examined groundwater is inferred using the groundwater residence time and prokaryotic information, including density and specific constituents. The stable water isotopes did not suggest the deep groundwater contribution to the groundwater at the ridge in the high-rainfall period; however, the SiO2 concentration indicated that. Elucidating the prokaryotic information along with existing hydrological tracers may enable a better understanding of groundwater flow processes.

Constraining Bedrock Groundwater Residence Times in a Mountain System With Environmental Tracer Observations and Bayesian Uncertainty Quantification

AbstractGroundwater residence time distributions provide fundamental insights on the hydrological processes within watersheds. Yet, observations that can constrain groundwater residence times over broad timescales remain scarce in mountain catchment studies. We use environmental tracers (CFC‐12, SF6, 3H, and 4He) to investigate groundwater residence times along a hillslope in the East River Watershed, Colorado, USA. We develop a Bayesian inference framework that applies a Markov‐chain Monte Carlo (MCMC) approach to estimate noble gas recharge temperature, elevation, and excess‐air parameters and the resulting environmental tracer concentrations. MCMC is then used to propagate the environmental tracer uncertainties to estimates of groundwater mean residence times inferred with lumped parameter models. All samples contain 3H, CFC‐12, and SF6 in addition to terrigenic 4He, suggesting a mixture of water characterized by modern and premodern residence times. 4He exponential mean residence times range from hundreds of years at the upslope well to thousands of years at the toe‐slope well assuming average crustal production rates. We find that binary mixing residence time distributions with separate young and old mixing fractions are needed to predict the 4He, CFC‐12, SF6, and 3H observations, supporting the importance of flow path mixing in this bedrock system. Our findings that the fractured bedrock hosts groundwater with a mixture of residence times ranging from decades to millennia suggest variable recharge dynamics and flow path mixing along the hillslope and highlight the importance of characterizing groundwater systems with observations that are sensitive to transport over a broad range of residence times.

Characterizing the Groundwater Flow Regime in a Landslide Recharge Area Using Stable Isotopes: A Case Study of the Urbas Landslide Area in NW Slovenia

Slope stability strongly depends on the prevailing hydrological and hydrogeological conditions. The amount and intensity of precipitation and changing groundwater levels are important landslide triggering factors. Environmental tracers, including the chemical and stable isotope compositions of precipitation and groundwater, were used to gain insight into the groundwater dynamics of the Urbas landslide. The landslide is situated in a mountainous area with steep slopes and high precipitation amount and poses a high risk for the safety of the Koroška Bela settlement that lies downstream. The stable isotope analyses of oxygen-18 (18O) and deuterium (2H) in the precipitation and groundwater were used to estimate the groundwater mean residence time and the average altitude of the landslide recharge area. This information will help to plan and prioritize remedial landslide measures aiming to reduce the recharge of the landslide body and, thus, lower the risk of transformation of the sliding material into debris flow. The results of the chemical analysis of samples taken from springs and a piezometer show a Ca–HCO3 water type. This indicates low water–rock interaction in a landslide area composed of Upper Carboniferous and Permian clastic rocks and points to upper laying carbonate rocks and scree deposits as the main recharge area. Water samples for stable isotope analyses of δ18O and δ2H were collected from a rain gauge, springs, and a piezometer over a two-year period (2018–2020). The estimated mean recharge altitude of the groundwater at sampling points was from approximately 1700 to 1800 m a.s.l. with a mean residence time of 2–5 months.

Integrating Stable Isotopes with Mean Residence Time Estimation to Characterize Groundwater Circulation in a Metamorphic Geothermal Field in Yilan, Taiwan

This paper presents a water circulation model by combing oxygen and hydrogen stable isotopes and mean residence time (MRT) estimation in a high-temperature metamorphic geothermal field, Tuchen, in Yilan, Taiwan. A total of 18 months of oxygen and hydrogen stable isotopes of surface water and thermal water show the same variation pattern, heavier values in summer and lighter values in the rest of the year. A shift of δ18O with a relative constant δD indicates the slow fluid–rock interaction process in the study area. Two adjacent watersheds, the Tianguer River and Duowang River, exhibit different isotopic values and imply different recharge altitudes. The seasonal variation enabled us to use stable isotope to estimate mean residence time of groundwater in the study area. Two wells, 160 m and 2200 m deep, were used to estimate mean residence time of the groundwater. Deep circulation recharges from higher elevations, with lighter isotopic values, 5.9‰ and 64‰ of δ18O and δD, and a longer mean residence time, 1148 days, while the shallow circulation comes from another source with heavier values, 5.7‰ and 54.4‰ of δ18O and δD, and a shorter mean residence time, 150 days. A two-circulation model was established based on temporal and spatial distribution characteristics of stable isotopes and the assistance of MRT. This study demonstrates the usefulness of the combined usage for further understanding water circulation of other various temperatures of metamorphic geothermalfields.

Impact of Indian summer monsoon in westerly dominated water resources of western Himalayas

ABSTRACT We used stable water isotopes of oxygen and hydrogen to identify and estimate the seasonal contribution of precipitation to the regional hydrology of Sindh and Rambiara catchments of western Himalayas. The different source waters exhibit significant spatio-temporal variations that correspond to the change in seasonal meteorology, precipitation form and moisture sources. The two-component hydrograph separation based on d-excess suggests that the western disturbances (WD) contribute dominantly (76 ± 4 %) to the regional hydrology, compared to Indian summer monsoon (ISM) rainfall (24 ± 4 %). A comparison of d-excess values of WD and ISM indicates the groundwater consists of 90 ± 3 % WD sources and 10 ± 2 % ISM sources, signifying distinct seasonal variations in groundwater recharge sources. The sine wave model results showed that the annual mean residence time (MRT) of groundwater for the Sindh catchment (5.8 ± 0.6 months) is greater than the Rambiara groundwater (3.6 ± 0.5 months). The lower isotope values observed in the river water than in the precipitation suggest its origin from the snowmelt. This study provides valuable insights into the hydrological processes operating in the high altitude Himalayan catchments to facilitate the improved understanding of runoff generation mechanisms and water resource management in future climate change scenarios.

Depth to water table correction for initial carbon-14 activities in groundwater mean residence time estimation

Abstract. Carbon-14 (14C) is routinely used to determine mean residence times (MRTs) of groundwater. 14C-based MRT calculations typically assume that the unsaturated zone is in equilibrium with the atmosphere, controlling the input 14C activity. However, multiple studies have shown that unsaturated zone 14C activities are lower than atmospheric values. Despite the availability of unsaturated zone 14C data, no attempt has been made to generalise initial 14C activities with depth to the water table. We utilise measurements of unsaturated zone 14C activities from 13 studies to produce a 14C–depth relationship to estimate initial 14C activities. The technique only requires the depth to the water table at the time of sampling or an estimate of depth to water in the recharge zone to determine the input 14C activity, making it straightforward to apply. Applying this new relationship to two Australian datasets (113 14C measurements in groundwater) shows that MRT estimates were up to 9250 years younger when the 14C–depth correction was applied relative to conventional MRTs. These findings may have important implications for groundwater samples that suggest the mixing of young and old waters and the determination of the relative proportions of young and waters, whereby the estimated fraction of older water may be much younger than previously assumed. Owing to the simplicity of the application of the technique, this approach can be easily incorporated into existing correction schemes to assess the sensitivity of unsaturated zone 14C to MRTs derived from 14C data.

Tracer-aided assessment of catchment groundwater dynamics and residence time

Sustainable groundwater management is becoming increasingly important due to intensifying water deficits around the world, which highlights the necessities and challenges to explore the dynamics of groundwater at the catchment-scale. In this paper, a tracer-aided approach has been developed and applied to quantify the relationships between groundwater storage and discharge, upon which the dynamics of groundwater storage and its residence time are assessed. Robust nonlinear storage-discharge relationships are found for the eight studied catchments located in a headwater region of the Murray-Darling Basin, Australia. The dynamics of groundwater storage for all the studied catchments largely show synchronization with regional climate. The estimated highest and active groundwater storages in a year are more responsive to precipitation than the lowest storages. For all the studied catchments, during dry seasons without recharge, the depletion rate of groundwater is found to decrease with the decline in groundwater storage, which can be ascribed to the vertical heterogeneity of their unconfined aquifers. The mean residence time of groundwater represented by MRT75 (i.e. time taken for groundwater storage to diminish from its mean level to the 75th percentile) varies between 57 and 111 days across the studied catchments, suggesting considerable hydrogeological differences among the studied catchments. The groundwater dynamics observed in this research could contribute to improving hydrogeological model performance in simulating the connection between groundwater and surface water at the catchment-scale. Moreover, the findings could serve to develop guidelines for groundwater extraction when considering the constraints of environmental flow requirements.

Multi-method geochemical characterization of groundwater from a hypogene karst system

An approach, combining several geochemical methods, was used to determine the groundwater properties and components of a hypogene karst system, where sampling is restricted only to the spring sites, and with a limited number of available sampling locations. Radiogenic isotopes (3H, 14C) were used to constrain the groundwater mean residence time and separate different groundwater components. Noble gases, stable isotopes of water (δ18O, δ2H), dissolved inorganic carbon (δ13C) and dissolved sulfate (δ34S, δ18O), and major ion and trace element composition were used to identify the source of water, its chemical evolution and water–rock interactions, as well as to identify the contribution and composition of endogenic gases. This approach was applied to three low-temperature thermal springs located in Mariovo (North Macedonia) associated with fossil hypogene caves, previously identified by morphological and geochemical studies of caves and cave deposits. Based on the obtained results, the main studied springs represent an output part of a regional hypogene karst groundwater system with a deep-circulating (~1 km), old (~15 ka), thermal (≥60 °C) water, which mixes with young (<50 years), cold (<14 °C) and shallow epigene karst groundwater. The output parts are structurally controlled, at the interception of low topography and deep faults, along which the groundwater interacts with deep-seated gases, dominantly CO2 of metamorphic origin (δ13C of +4.5‰ VPDB), with some contribution of mantle helium. The thermal karst groundwater interacts at depth with volcanic rocks from the nearby Neogene-Quaternary volcanic complex, as well as with metamorphic basement rocks and granitoids.

Differentiation of natural and anthropogenic contaminant sources using isotopic and microbial signatures in a heavily cultivated coastal area.

Hydrogeochemical and multiple isotope data for groundwater samples were obtained and interpreted to discriminate anthropogenic and natural contaminant sources in a coastal aquifer underlying a heavily cultivated watershed in Hwaseong, South Korea. The local aquifers are vulnerable to contamination, due to high anthropogenic N inputs and the location close to the ocean facilitating seawater intrusion. Thus, to effectively control the groundwater quality in the study area, it is necessary to differentiate between anthropogenic and natural contaminant sources. The concentrations of NO3-N in the groundwater ranged between 0.14 and 45.6mg/L in August 2015 and 0.2-39.6mg/L in March 2016. High concentrations of Cl- (388-1107mg/L) and a high electrical conductivity (1027-2715μS/cm) were observed in the study area, suggesting that the groundwater was affected by seawater intrusion. Furthermore, δ15N-NO3-, δ34S-SO42- values and 87Sr/86Sr of groundwater were determined to reveal the origins of the natural and anthropogenic contaminants and the groundwater mean residence times (MRT) and 87Sr/86Sr ratios were used to assess the hydrogeochemical processes along the flow path in the study area. Young groundwater was affected by an anthropogenic contamination source with contributions of 26-46% adding nitrate to the aquifer, whereas old groundwater was impacted by mixing with seawater with contributions of 10-20% with low concentrations of NO3-N, but elevated concentrations of chloride and sulfate. Recently recharged uncontaminated groundwater showed oxic conditions with a diverse microbial community structure, whereas young groundwater contaminated by anthropogenic sources showed a less diverse microbial community structure. The results of this study suggest that multiple isotopes combined with groundwater MRT and microbial data can be applied to distinguish natural and anthropogenic contaminant sources in a groundwater system.

Groundwater age dating using single and time-series data of environmental tritium in the Moeda Synclyne, Quadrilátero Ferrífero, Minas Gerais, Brazil

The use of tritium in the identification of the “age” of groundwater is being used since the discovery of its potential for this purpose in 1954. The present work sought to apply and discuss the use of the different types of lumped parameter models (LPM) in the Cauê and Gandarela aquifers, in order to obtain the groundwater mean residence time based on tritium data. Water samples from several discharges related to the Cauê and Gandarela aquifers were collected during the month of September of 2017. The reconstruction of the historical series of tritium contents in the precipitation at the GNIP station CDTN-Belo Horizonte was carried out based on data from the Kaitoke (New Zealand) and Adelaide (Australia) stations in addition to the Brazilian stations Brasília, Rio de Janeiro and Salvador. The sample results and the data of the reconstructed historical series of the tritium concentration in the CDTN station during the period between 1953 and 2017, served as the basis to use the lumped parameter models with the aid of the software TracerLPM, provided by the United States Geological Survey. From the overlap of some sample points with tritium samples collected in different years by different authors, an adjustment of some models of residence time distribution was possible. The models used were exponential (EM), exponential-piston (EPM), and dispersive (DM). The model that best fit to data was the EPM model. The mean transit time for the EPM model in the analyzed points varies from 63 to 95 years, with a median value of 73 years. Differences between values of the EPM ratio parameter were verified. However, all values indicate an exponential flow component greater than the piston flow component. The EPM model suggests a long retention time of the tracer in the aquifer system. It can be used as an approximation of the retention time for environmental contamination. The exponential model (EM) and piston flow model (PFM) were used together. Based on the data of a single tritium sample collected in time, and the average value of residence time provided by each model, a first approximation of the mean residence time value can be obtained. The results suggest that this is a better approximation than assuming a pure exponential model or piston flow model to describe the data of a single tritium sample in time as were made by previous local works. In general, a short residence time was estimated for the Cauê aquifer as a whole with some discharges with long residence time that drains regional flows from the deepest part of the Cauê aquifer, reflecting the complex structural geology fabric.

Groundwater mean residence times of a subtropical barrier sand island

Abstract. Fresh groundwater on barrier islands is affected by changing sea levels and precipitation variability due to climate change and is also vulnerable to anthropogenic processes, such as contamination and groundwater over-abstraction. Constraining groundwater mean residence times (MRTs) and flow paths is essential for understanding and managing these resources. This study uses tritium (3H) and carbon-14 (14C) to determine the MRTs of groundwater along a transect across subtropical North Stradbroke Island, south-east Queensland, Australia. Hydraulic properties, major ion geochemistry and stable isotopes are used to validate residence times and to identify the processes responsible for their variability. 3H activities range from less than 0.01 to 1 TU (tritium units), which are values lower than those of local average rainfall (1.6–2.0 TU). 14C concentrations range from 62.5 to 111 pMC (percent modern carbon). Estimated MRTs determined using lumped parameter models and 3H activities range from 37 to more than 50 years. Recharge occurs over the entire island, and groundwater MRTs generally increase vertically and laterally towards the coastal discharge areas, although no systematic pattern is observed. MRTs estimated from 14C concentrations display similar spatial relationships but have a much greater range (from modern to approximately 5000 years). Water diversion and retention by lower-permeability units in the unsaturated parts of the dune systems are the most likely course for relatively long MRTs to date. The results indicate that the internal structures within the dune systems increase MRTs in the groundwater system and potentially divert flow paths. The structures produce perched aquifer systems that are wide-spread and have a significant influence on regional recharge. The geochemical composition of groundwater remains relatively consistent throughout the island, with the only irregularities attributed to old groundwater stored within coastal peat. The outcomes of this study enhance the understanding of groundwater flow, recharge diversion and inhibition for large coastal sand masses in general, especially for older sand masses that have developed structures from pedogenesis and dune movement. With respect to south-east Queensland, it allows the existing regional groundwater flow model to be refined by incorporating independent MRTs to test models' validity. The location of this large fresh groundwater reservoir, in dry and populous south-east Queensland, means that its potential to be used as a water source is always high. Background information on aquifer distribution and groundwater MRTs is crucial to better validate impact assessment for water abstraction.

Hydrochemical and isotopic approach to dynamic recharge of a dolomite aquifer in South Africa

The dolomite aquifer is the largest water source in northern South Africa. The flow dynamics of the dolomite aquifer is investigated by using analyses of the hydro-chemical parameters and isotopes (3H, δ2H, δ18O, δ13C-DIC and 14C-DIC) of spring samples. Recharge areas of the dolomite aquifer are confirmed through interpretation of the hydro-geochemical types of the spring samples. The important role of rainfall in groundwater recharge is suggested by low Na+ and Cl− concentrations and by the δ2H and δ18O values of the spring samples. Groundwater mean residence time (MRT) and its temporal and spatial distributions within the young dolomite spring system can be analyzed using an improved lumped-parameter model based on the time series of 14C-DIC, initial 14C activities and δ13C-DIC values of the spring samples collected during the 1970s and 2000s. The results show that the spring samples have about 50–80% of the initial 14C activities and the MRTs of the dolomite spring system range from ≤10–51 years. At five spring sites, the temporal distributions of groundwater MRTs are identified to be significantly influenced by the variability of the local rainfall. At the Kuruman sites only, an increasing trend of the groundwater MRTs and the evolution of the [Ca2+]/[Mg2+] ratio along the flow direction indicate an important role of deep groundwater inflow to the spring flow. The results provide basic scientific information required for sustainable management of the dolomite aquifer.

Application of environmental tracers for investigation of groundwater mean residence time and aquifer recharge in fault-influenced hydraulic drop alluvium aquifers

Abstract. Investigating groundwater residence time and recharge sources is crucial for water resource management in the alluvium aquifers of arid basins. Environmental tracers (chlorofluorocarbons, 3H, 14C, δ2H, δ18O) and groundwater hydrochemical components are used for assessing groundwater mean residence times (MRTs) and aquifer recharge in fault-influenced hydraulic drop alluvium aquifers in the Manas River basin (China). Aquifers under the Manas River upstream (south of the fault) contains very high 3H activity (41.1–60 TU), implying water recharge affected by the nuclear bomb tests of the 1960s. Carbon-14 groundwater age correlates positively with distance from mountain area (3000–5000 years in the midstream to &gt; 7000 years in the downstream) and groundwater depth, but correlates negatively to a decrease in 3H activity (1.1 TU) and more negative δ18O values. This phenomenon reveals that the source of the deeper groundwater in the semi-confined aquifer is paleo-meteoric recharge. Special attention has been paid to the estimation of MRTs using CFCs and 3H by an exponential piston flow model. The results show that MRTs vary from 19 to 101 years by CFCs and from 19 to 158 years by 3H. MRTs estimated from 3H are much longer than those from CFCs, probably due to the different time lag of liquid (3H) and gas-phase CFCs through the unsaturated zone. The MRTs estimated by CFCs show good correlations with pH and the concentrations of SiO2 and SO42-, which can provide a possible approach to estimate first-order proxies of MRTs for groundwater age. The young water fractions are investigated by the CFC binary mixing method in the south and north of the fault. Relatively modern recharge is found in the south of the fault with young (post-1940) water fractions of 87 %–100 %, whereas in the north of the fault the young water fractions vary from 12 % to 91 %. This study shows that the combination of CFCs and 3H residence time tracers can help in analysing the groundwater MRTs and the recharge sources for the different mixing end-members.

Groundwater age dating and recharge mechanism of Arusha aquifer, northern Tanzania: application of radioisotope and stable isotope techniques

The continuous abstraction of groundwater from Arusha aquifers in northern Tanzania has resulted in a decline in water levels and subsequent yield reduction in most production wells. The situation is threatening sustainability of the aquifers and concise knowledge on the existing groundwater challenge is of utmost importance. To gain such knowledge, stable isotopes of hydrogen and oxygen, and radiocarbon dating on dissolved inorganic carbon (DIC), were employed to establish groundwater mean residence time and recharge mechanism.14C activity of DIC was measured in groundwater samples and corrected using a δ13C mixing method prior to groundwater age dating. The results indicated that groundwater ranging from 1,400 years BP to modern is being abstracted from deeper aquifers that are under intensive development. This implies that the groundwater system is continuously depleted due to over-pumping, as most of the sampled wells and springs revealed recently recharged groundwater. High 14C activities observed in spring water (98.1 ± 7.9 pMC) correspond with modern groundwater in the study area. The presence of modern groundwater suggests that shallow aquifers are actively recharged and respond positively to seasonal variations.

Deep urban groundwater vulnerability in India revealed through the use of emerging organic contaminants and residence time tracers

Demand for groundwater in urban centres across Asia continues to rise with ever deeper wells being drilled to avoid shallow contamination. The vulnerability of deep alluvial aquifers to contaminant migration is assessed in the ancient city of Varanasi, India, using a novel combination of emerging organic contaminants (EOCs) and groundwater residence time tracers (CFC and SF6). Both shallow and intermediate depth private sources (<100 m) and deep (>100 m) municipal groundwater supplies were found to be contaminated with a range of EOCs including pharmaceuticals (e.g. sulfamethoxazole, 77% detection frequency, range <0.0001–0.034 μg L−1), perfluoroalkyl substances (e.g. PFOS, range <0.0001–0.033 μg L−1) as well as a number of pesticides (e.g. phenoxyacetic acid, range <0.02–0.21 μg L−1). The profile of EOCs found in groundwater mirror those found in surface waters, albeit at lower concentrations, and reflect common waste water sources with attenuation in the subsurface. Mean groundwater residence times were found to be comparable between some deep groundwater and shallow groundwater sources with residence times ranging from >70 to 30 years. Local variations in aquifer geology influence the extent of modern recharge at depth. Both tracers provide compelling evidence of significant inputs of younger groundwater to depth >100 m within the aquifer system.

The study of the interactions between groundwater and Sava River water in the Ljubljansko polje aquifer system (Slovenia)

River basin aquifers are common sites for drinking water wells as bank filtration can be a cost effective pretreatment technology. A groundwater vulnerability to pollution depends on a groundwater mean residence time and on a relative contribution of river water versus local precipitation to groundwater. Environmental isotopes of oxygen and hydrogen (δ18O and δ2H), tritium (3H) and concentrations of nitrate (NO3−) were used to investigate hydrological pathways, mean residence time and interactions between surface water and groundwater in the Ljubljansko polje aquifer system in Slovenia. δ18O and δ2H values indicate a spatial variability of the influence of individual groundwater sources inside the aquifer – local precipitation and the Sava River water. Fractions of river water in groundwater depend on the depth of perforated screens in the pumping wells and their distance from the Sava River. It was estimated that groundwater at wells Kleče 11, Hrastje 3, and Hrastje 8 is mostly composed of recently infiltrated local precipitation, while the Sava River is the dominant source of groundwater at the well Jarški prod 1. Groundwater at wells Kleče 8, Kleče 12, and Jarški prod 3 contains on average between 41% and 48% of the Sava River water. The 3H and 3H/3He methods indicate short mean residence time of groundwater present at Jarški prod (2–7 years) and Hrastje (7–8 years). A small fraction (<10%) of old groundwater is present at Kleče. Furthermore, infiltration of local precipitation influenced the levels of NO3− at Hrastje. These data extend our understanding of groundwater flow in the Ljubljansko polje aquifer system, interactions between the Sava River water/local precipitation and groundwater, and the utility of isotope tracers in evaluating the spatial distribution of groundwater vulnerability to pollution.

Naturally Occurring versus Anthropogenic Sources of Elevated Molybdenum in Groundwater: Evidence for Geogenic Contamination from Southeast Wisconsin, United States

Molybdenum (Mo) is an essential trace nutrient but has negative health effects at high concentrations. Groundwater typically has low Mo (<2 μg/L), and elevated levels are associated with anthropogenic contamination, although geogenic sources have also been reported. Coal combustion residues (CCRs) are enriched in Mo, and thus present a potential anthropogenic contamination source. Here, we use diagnostic geochemical tracers combined with groundwater residence time indicators to investigate the sources of Mo in drinking-water wells from shallow aquifers in a region of widespread CCR disposal in southeastern Wisconsin. Samples from drinking-water wells were collected in areas near and away from known CCR disposal sites, and analyzed for Mo and inorganic geochemistry indicators, including boron and strontium isotope ratios, along with groundwater tritium-helium and radiogenic 4He in-growth age-dating techniques. Mo concentrations ranged from <1 to 149 μg/L. Concentrations exceeding the U.S. Environmental Protection Agency health advisory of 40 μg/L were found in deeper, older groundwater (mean residence time >300 y). The B (δ11B = 22.9 ± 3.5‰) and Sr (87Sr/86Sr = 0.70923 ± 0.00024) isotope ratios were not consistent with the expected isotope fingerprints of CCRs, but rather mimic the compositions of local lithologies. The isotope signatures combined with mean groundwater residence times of more than 300 years for groundwater with high Mo concentrations support a geogenic source of Mo to the groundwater, rather than CCR-induced contamination. This study demonstrates the utility of a multi-isotope approach to distinguish between fossil fuel-related and natural sources of groundwater contamination.

Groundwater flow mechanism in the well-developed karst aquifer system in the western Croatia: Insights from spring discharge and water isotopes

Karst aquifers have a fundamental importance to water supply in many countries. Due to their specific hydrogeologic properties, these aquifers are sometimes difficult to use because of the high discharge variations of the karst springs and are almost always sensitive to pollution. With an aim to better understand karst aquifers, different research methods are used to study the karst groundwater system in Croatia.The spring hydrograph and the stable isotope (δ18O, δ2H) compositions in the water samples collected from the Rječina and Zvir springs and precipitation were analysed and used to characterize the karst aquifer. The recession coefficient obtained from the hydrograph analysis indicates only a fast-flow spring component at the Rječina spring. The lack of the base-flow spring component is the primary reason for the spring drying out during the dry periods. The low recession coefficient of the Zvir spring indicates a base-flow and discharge from well-drained fissures and fractures in the spring catchment area during the low water stage. A mean residence time (MRT) of groundwater was calculated for stable isotope δ18O using lumped parameter approach by applying the exponential model, combined exponential-piston and dispersion models to isotopic input (rainfall) and output (spring) data sets during 2011–2013. The MRT of 3.24 and 3.6months for the Rječina spring and 7.2months for the Zvir spring suggests recent groundwater recharge from precipitation.

Estimating mean residence time of karst groundwater in mountainous catchments of Western Himalaya, India

ABSTRACTThe mean residence time (MRT) of karst groundwater in three mountainous catchments of the Western Himalaya was estimated using multiple approaches: the tritium method, the sine wave model and tracer tests. Water samples were collected from precipitation, glacier melt, streams and karst springs for δ2H and tritium analysis during 2012 and 2013. High tritium values were observed in winter precipitation and low values in summer precipitation. The variation of tritium in karst springs was similar to that of the streams, whereas glacier melt showed lower tritium values. The MRT of cold karst springs was shorter than that of warm karst springs. The tracer breakthrough curves (TBC) retrieved for different springs suggested a short travel time for groundwater and possibly conduit flow. Deterioration of water quality and variation in flux magnitude are the two main practical consequences of the short travel time of karst groundwater in the region.EDITOR D. KoutsoyiannisASSOCIATE EDITOR K. Heal

Residence times of groundwater and nitrate transport in coastal aquifer systems: Daweijia area, northeastern China

Groundwater within the coastal aquifer systems of the Daweijia area in northeastern China is characterized by a large of variations (33–521mg/L) in NO3− concentrations. Elevated nitrate concentrations, in addition to seawater intrusion in the Daweijia well field, both attributable to anthropogenic activities, may impact future water-management practices. Chemical and stable isotopic (δ18O, δ2H) analysis, 3H and CFCs methods were applied to provide a better understanding of the relationship between the distribution of groundwater mean residence time (MRT) and nitrate transport, and to identify sources of nitrate concentrations in the complex coastal aquifer systems. There is a relatively narrow range of isotopic composition (ranging from −8.5 to −7.0‰) in most groundwater. Generally higher tritium contents observed in the wet season relative to the dry season may result from rapid groundwater circulation in response to the rainfall through the preferential flow paths. In the well field, the relatively increased nitrate concentrations of groundwater, accompanied by the higher tritium contents in the wet season, indicate the nitrate pollution can be attributed to domestic wastes. The binary exponential and piston-flow mixing model (BEP) yielded feasible age distributions based on the conceptual model. The good inverse relationship between groundwater MRTs (92–467years) and the NO3− concentrations in the shallow Quaternary aquifers indicates that elevated nitrate concentrations are attributable to more recent recharge for shallow groundwater. However, there is no significant relationship between the MRTs (8–411years) and the NO3− concentrations existing in the carbonate aquifer system, due to the complex hydrogeological conditions, groundwater age distributions and the range of contaminant source areas. Nitrate in the groundwater system without denitrification effects could accumulate and be transported for tens of years, through the complex carbonate aquifer matrix and the successive inputs of nitrogen from various sources.