Groundwater Residence Time Research Articles

Hydrological controls of a riparian wetland based on stable isotope data and model simulations.

Isotopic evidence of groundwater and stream water is frequently used to investigate water exchanges with groundwater. Monthly sampling of rain, stream water, and groundwater was conducted at Tims Branch watershed in South Carolina for the oxygen and hydrogen stable isotope (δ2H and δ18O) measurement, as well as pH and oxidation-reduction potential (ORP). Together with a mass balance perspective, it was determined that it takes a few weeks to one month for groundwater in the hyporheic zone to fully exchange with stream water. From hydrodynamic modelling, we show that substantial (up to 70 %) groundwater exchange occurs at gaining and losing sites. Groundwater exfiltration, i.e. inflow into stream water, contributes up to 4 % to stream water, with the remainder from upstream exfiltration. A 2-4 % per day renewal rate of adjacent groundwater would indirectly indicate a groundwater residence time in the order of half a month to a full month (assuming either a well-mixed case or large dispersion rate in pulse flow case), in agreement with a greatly reduced variability of δ2H and δ18O of groundwater compared to stream water and rain. This reduced variability of stable isotope signal from groundwater confirms our hypothesis that riparian groundwater mixing at Tims Branch is more of a mixed type rather than a pulse flow type. A monthly time scale is sufficient for groundwater to become anoxic at exit points into stream water resulting in the episodic production of natural organic matter- and iron-rich flocs upon oxidation.

Mobilization mechanisms and spatial distribution of arsenic in groundwater of western Bangladesh: Evaluating water quality and health risk using EWQI and Monte Carlo simulation

Arsenic (As) contamination in groundwater is emerging as a significant global concern, posing serious risks to the safety of drinking water and public health. To understand the release mechanisms, mobilization processes, spatial distribution, and probabilistic health risks of As in western Bangladesh, forty-seven samples were collected and analyzed using an atomic absorption spectrometer (AAS). The As concentrations in groundwater ranged from 1.97 to 697.4 μg L⁻1 (mean: 229.9), significantly exceeding recommended levels. The dominant hydrochemistry of As-enriched groundwater was Ca–Mg–HCO₃, with the primary sources of arsenic in groundwater being the dissolution of arsenic-bearing minerals in sediment and the recharge of aquifers from the Ganges River Basin. The assessment using the Entropy Water Quality Index revealed that the groundwater is unsuitable for drinking, with 89.36% (n = 42) of the samples surpassing the WHO's limit for arsenic. Rock-water interactions, including calcite dissolution and silicate weathering within the confined aquifer, predominantly influenced hydrochemical properties. The significant relationships among Fe, Mn, and As indicate that the reductive dissolution of FeOOH and/or MnOOH considerably contributes to the release of As from sediment into groundwater. Geochemical modeling analysis revealed that siderite and rhodochrosite precipitate into aquifer solids, suggesting a weak to moderate relationship among As, Fe, and Mn. The long residence time of groundwater, combined with the presence of a clayey aquitard, likely controls the mobilization of arsenic in the aquifer. For the first time, Monte Carlo simulations have been used in arsenic-prone areas to assess the severity of arsenic contamination in western Bangladesh. The analysis indicates that out of 100,000 people, 10 may develop cancer as a result of drinking arsenic-contaminated water, with children being more susceptible than adults.

Hydraulic recharge and element dynamics during salinization in an overexploited coastal aquifer of the world's driest zone: Atacama Desert

The management of water resources in hyper-arid coastal regions is a challenging task because proper information regarding groundwater recharge and water budget is needed for maintaining the hydraulic balance in optimal conditions, avoiding salinization and seawater intrusion. Thus, this article deals with the estimation of the hydraulic recharge and the study of the effects of salinization on the dynamics of major and trace elements in an alluvial aquifer located in the world's driest zone, the northern Atacama Desert. The result of stable water isotopes (δD and δ18O) and tritium (3H) indicated that groundwater in the area is not recent, whereas 14C results estimated a groundwater residence time ranging between 11,628 and 16,067 yBP. The estimation of the artificial recharge coming from the urban water-supply-system leaks and wastewater/river-water/groundwater infiltration during irrigation was about 19.84 hm3/year, which represents an annual negative water balance of 177 hm3/year for the aquifer. The groundwater salinization triggered by seawater intrusion (up to 32.6 %) has caused the enrichment of Li, Rb, Ca, Ba, and Sr in groundwater by cationic exchange, where the excess of aqueous Na is exchanged by these elements in the aquifer sediments. Other elements such as B, Se, Si, and Sb are enriched in groundwater by ionic strength and/or anionic exchange during salinization. The heightened B concentrations derived from the B-rich alluvial sediments were higher than the limit suggested by international guidelines, representing a risk to consumers. Vanadium seems to be unaffected by salinization, whereas Pb, Mo, As, U, and Zr did not show a clear behavior during saline intrusion. Finally, this article highlights the consequences of conducting improper water management in coastal hyper-arid regions with exacerbated agriculture.

Microbial cycling contributes to the release of dissolved inorganic phosphate into the groundwater of floodplain aquifers

Little is known about biological processes controlling inorganic phosphate (PO4) in groundwater ecosystems. Here we present analyses of groundwater samples from the Hetao Basin, China that show an increasing contribution of microbial cycling to groundwater PO4 from oxic to anoxic conditions along a flow path with phosphate-bound oxygen isotopes (δ18OPO4). Under oxic conditions, although 25–47% of the dissolved PO4 inherited the initial source signal of igneous apatite, groundwater δ18OPO4 reflected a pronounced impact of intracellular enzymatic cycling. Under anoxic conditions, dissolved PO4 carried a nearly exclusive equilibrium isotope signal, which was probably due to (i) release of PO4 with an equilibrium δ18OPO4 from Fe(III) oxides, as a result of Fe(III) reduction in the presence of Fe(III)- and sulfate-reducing bacteria; and/or (ii) cumulative microbial cycling of dissolved PO4 with increasing groundwater residence time. Our study highlights that PO4 in groundwater is tightly microbially cycled under a wide range of redox conditions and microbial cycling contributes to the release of PO4 in groundwater.

Ra isotope perspective on the hydrology and continuity of permafrost in the high Arctic

The continuous permafrost in the valleys of Svalbard is dotted by pingos, which are small hills formed by the near surface freezing of ascending groundwater. In this study, we used 3H and Ra isotopes to inquire into the sub-surface residence time of groundwater discharging at these pingos. While its low 3H suggests that the pingo-associated groundwater is basically not modern (i.e. older than 60 years), Ra isotopes imply that most water has an underground residence time of several hundred years. This is deduced from the lower than equilibrium ratios (activity ratios<21.7) of the long-lived to short-lived 226Ra/223Ra. Since the freezing age of the main body of permafrost in this area is >4000 years, the presence of younger water at depth suggests that the aquifer has been recharged after permafrost formation, which could take place via faults or through the non-frozen base of wet glaciers. This active hydrology suggests that permafrost in the valleys of Svalbard was at least locally discontinuous during the Late Holocene, with likely further implications to the release of greenhouse gases during the pre-industrial period.

An integrated approach for characterization of a fractured-rock carbonate aquifer in the Zagros Region of Iran

Karst aquifers typically exhibit a wide range of dissolution effects that include end-members of matrix, fractured-rock, and conduit-dominated types. This study employs an integrated approach involving geological studies, hydrogeological assessments, hydrochemical and isotopic analysis, and dye tracer tests to investigate the Sarvak limestone aquifer (SLA) in the Zagros Region, southwest Iran. Key characteristics of the SLA include numerous stratification boundaries, thin limestone layers with intercalated siliceous and marl impurities, extensive joint and fracture networks, predominant autogenic recharge, and absence of notable point recharge features (e.g., sinkholes, dolines), and the exchange flow with the Bakhtiari River (B-R) has made it a unique aquifer. Numerous pieces of evidence, such as low spatial and temporal changes in groundwater level, insignificant seasonal variations in the hydrochemical and isotopic composition of water samples, and the supersaturation state of groundwater with calcite and dolomite minerals, suggested a slow flow regime in many parts of the SLA. This regime is characterized by low velocity and long residence time of groundwater. The results reveal that despite the high solubility of carbonate rocks, extensive joint networks can limit significant karst development, leading to fluid flow behavior similar to that of fractured-rock aquifers. Therefore, SLA can be considered a fractured-rock and karst aquifer. The identified paths with fast flow in the dam area are unremarkable and cannot provide a large portion of the Pelle Khan Spring discharge.

Groundwater seeps are hot spots of denitrification and N2O emissions in a restored wetland

Restorations of former cranberry farms (“bogs”) aim to re-establish native wetland vegetation, promote cold water habitat, and attenuate nitrogen (N) delivery to coastal waters. It is unclear, though, how elements of restoration design such as microtopography, groundwater interception, and plant communities affect N removal via denitrification. In a recently restored riparian cranberry bog with created microtopography, we compared denitrification potential, nitrous oxide (N2O) yield of denitrification (ratio of N2O:N2O + N2 gases), in situ N2O fluxes, soil chemistry, and plant communities at the highest and lowest elevations within 20 plots and at four side-channel groundwater seeps. Denitrification potential was > 2 × greater at low elevations, which had plant communities distinct from high elevations, and was positively correlated with plant species richness (Spearman’s rho = 0.43). Despite detecting high N2O yield (0.86 ± 0.16) from low elevation soils, we observed small N2O emissions in situ, suggesting minimal incomplete denitrification even in saturated depressions. Groundwater seeps had an order of magnitude higher denitrification potentials and 100–300 × greater soil NO3− concentrations than the typically saturated low elevation soils. Groundwater seeps also had high N2O yield (1.05 ± 0.15) and higher, but spatially variable, in situ N2O emissions. Our results indicate that N removal is concentrated where soils interact with NO3–rich groundwater, but other factors such as low soil carbon (C) also limit denitrification. Designing restoration features to increase groundwater residence time, particularly in low lying, species rich areas, may promote more N attenuation in restored cranberry bogs and other herbaceous riparian wetlands.

Complex hydrology and variability of nitrogen sources in a karst watershed.

Streams draining karst areas with rapid groundwater transit times may respond relatively quickly to nitrogen reduction strategies, but the complex hydrologic network of interconnected sinkholes and springs is challenging for determining the placement and effectiveness of management practices. This study aims to inform nitrogen reduction strategies in a representative agricultural karst setting of the Chesapeake Bay watershed (Fishing Creek watershed, Pennsylvania) with known elevated nitrate contamination and a previous documented groundwater residence time of less than a decade. During baseflow conditions, streamflow did not increase with drainage area. Headwaters and the main stem lost substantial flow to sinkholes until eventually discharging along large springs downstream. Seasonal hydrologic conditions shift the flow and nitrogen load spatially among losing and gaining stream sections. A compilation of nitrogen source inputs with the geochemistry and the pattern of enrichment of δ15N and δ18O suggest that the nitrogen in streams and springs during baseflow represents a mixture of manure, fertilizer, and wastewater sources with low potential for denitrification. The pH and calcite saturation index increased along generalized flow paths from headwaters to springs and indicate shorter groundwater residence times in baseflow during the spring versus summer. Given the substantial investment in management practices, fixed monitoring sites could incorporate synoptic water sampling to properly monitor long-term progress and help inform management actions in karst watersheds. Although karst watersheds have the potential to respond to nitrogen reduction strategies due to shorter groundwater residence times, high nitrogen inputs, effectiveness of conservation practices, and release of legacy nutrients within the karst cavities could confound progress of water quality goals.

The contraction of freshwater lenses in barrier island: A combined geophysical and numerical analysis

Coastal aquifers, vital for supplying freshwater to over one billion people worldwide, often face saltwater intrusion, with barrier island aquifers playing a crucial role in this coastal system. Despite the proliferation of studies exploring the impacts of sea-level rise, storm surges, and over-pumping, the effect of droughts on barrier island aquifers salinization remains largely under-investigated. This lack of attention is particularly concerning given the heightened vulnerability of barrier island aquifers; most ultimately rely solely on aerial recharge compared to continental coastal aquifers. An in-depth understanding of recharge and salinization processes is important for sustainably managing these islands' marginal freshwater resources, especially considering climate change impacts. This study presents an evaluation approach for the response of a freshwater lens (FWL) to drought conditions, incorporating in-situ observations, geophysical measurements, and numerical modeling. The study examines the response of a shallow unconfined aquifer on an Atlantic coast barrier island to the 2020 Northeast extreme hydrological drought. It encompasses a density-driven flow model informed by time-lapse electrical resistivity imaging and in-situ measurements, including groundwater head and salinity collected from monitoring wells. Results from our approach indicate that FWL volume is reduced by 11% during the drought, but it returns to its previous volume over the following spring season, indicating that the net volume of the FWL remains unchanged on an annual scale. The mean groundwater residence time on the island is approximately a year, indicating that the barrier island aquifer is a hydrodynamically active coastal zone. These findings highlight the vulnerability and resilience of shallow unconfined barrier island aquifers to droughts and climate change.

Stable and Radio-Isotopic tools to understand the groundwater recharge and dynamics of CHASHMA-Mianwali area, Pakistan

The study conducted on “Chashma-Mianwali” area to investigate the impact of canals, lake and rivers flowing through the area on groundwater recharge and dynamics of the area. Isotopic and noble gases data suggested localize impacts of canals, lake and river on groundwater recharge. Groundwater residence time using Tracer Lump Parameter Model (LPM), and Chloroflourocarbons (CFCs) data suggest a very young water in the range of 14–59 years. The real and filtration velocities of groundwater were found to be 22.2 cm/day and 10.94 cm/day, respectively, in south-west direction and in-situ effective porosity of the aquifer was 49.3 %. The hydraulic conductivity of the aquifer was estimated with average value of 143 m/day with Transmissivity (T) of 43,127 m2/day.

Multi-proxy (CFCs, Cl, δ18O and δD) assessment of the origin, residence time and recharge of groundwater in the Bilate River Basin (BRB) of Southern Main Ethiopian Rift (SMER)

Multi-proxy (CFCs, Cl, δ18O and δD) assessment of the origin, residence time and recharge of groundwater in the Bilate River Basin (BRB) of Southern Main Ethiopian Rift (SMER)

Estimation of Groundwater Residence Time Using Radiocarbon and Stable Isotope Ratio in Dissolved Inorganic Carbon and Soil CO2

AbstractEstimation of residence time of groundwater, particularly in regions with inadequate surface waters are very important for formulating sustainable groundwater management policies. We developed a technique for extracting dissolved inorganic carbon (DIC) quantitatively from water for measuring its 14C contents and presented the analytical details here. We also measured stable carbon isotope ratio (δ13C) in soil CO2 and groundwater DIC to correct the groundwater 14C ages. In addition, 14C in soil CO2 were measured for making necessary correction in the initial activity of the recharging water. The corrected 14C contents in the groundwater samples were used to estimate their residence times employing Lumped Parameter Models (LPM), a set of mathematical models to account for the processes that take place during transport from the recharge to the sampling spots. We present a case study focused on the calculation of radiocarbon ages and residence times for a groundwater sample collected from the campus of Physical Research Laboratory in Ahmedabad, Gujarat, India. The study also includes estimations of groundwater residence times using previously measured 14C ages of groundwater samples from Gujarat, India. Various factors controlling the groundwater ages in the LPM and their applicability are discussed.

Assessments of groundwater recharge process and residence time using hydrochemical and isotopic tracers under arid climate: Insights from Errachidia basin (Central-East Morocco)

The aquifer systems in the Errachidia basin, located to the south of the High Atlas limestone, are constituted by three superimposed and exploitable aquifers (Senonian, Turonian, and Infracenomanian). These aquifers hold economic significance for the central-east region of Morocco. This area experiences an arid to Saharan climate characterized by low precipitation, high temperatures, and very high evaporation. It is one of the most exposed and vulnerable regions to climate change in Morocco. The low local rainfall cannot explain the recharge of the deep aquifers. However, the significant spring discharges from these aquifers indicate the existence of other recharge contributions.The aim of this study is to elucidate the recharge processes of the Cretaceous aquifers in the investigated basin, clarify hydraulic relationships between them and the Jurassic aquifers of the High Atlas Mountains and estimate the residence time of groundwaters. Water samples from aquifers are collected and analyzed for chemical and isotopic elements. Several statistical methods (SOM, discriminant factorial analysis, CPA) are employed. The results reveal that many samples from different aquifers exhibit clear similarities in chemical characteristics, providing evidence of water transfer between aquifers. Isotopic findings are consistent with chemical characterization and mineralization anomalies.Recharge of the investigated aquifers originates from the Jurassic limestone of the High Atlas Mountains, facilitated by the karstic nature of the Jurassic in contact with the permeable formations of the Cretaceous basin. Age dating results, using 3H and 14C values, confirm that most of the investigated groundwater results from a mixing of old and recent recharges. This process of groundwater renewability is relevant. All the findings are valuable for decision-makers to enhance their understanding of the aquifer system's functioning and inform sustainable management strategies for future sustainability.

Influence of Pleistocene glacial deposits on the transport of agricultural nitrate in the river Wensum catchment, UK

Mitigating NO3− pollution requires an understanding of the hydrological processes controlling contaminant mobilisation and transport, particularly in agricultural catchments underlain by Pleistocene glacial deposits. Focusing on the Wensum catchment in East Anglia, UK, precipitation (n = 20), stream water (n = 50), field drainage (n = 22) and groundwater (n = 84) samples collected between February–March 2011 and April–September 2012 were variously analysed for water stable isotopes (δ2HH2O and δ18OH2O), the dual-isotopes of NO3− (δ15NNO3 and δ18ONO3), groundwater residence time indicators (CFCs and SF6) and hydrochemical parameters. The residence time indicators suggested a component of modern (post-1960) groundwater throughout the sequence of glacial deposits that corresponds with the penetration of agricultural NO3−. Denitrification and lower NO3− concentrations (<8 mg L−1) are observed in the glacial tills, compared with higher NO3− concentrations (<90 mg L−1) observed under more oxidising conditions in the glacial sands and gravels. Storm hydrograph separation for two storms in April and September 2012 using two- and three-component mixing models showed a faster response with field drainage (36–38 %) and baseflow (5–37 %) contributing to the total stream discharge in areas of clay loam soils over glacial tills. In these areas, the dual stable isotopes of NO3− (δ15NNO3 = +11.8 ‰ and δ18ONO3 = +7.1 ‰) indicated a denitrified source of nitrogen from field drainage and groundwater. In comparison, a dampened response and a higher percentage of baseflow (29–80 %) was observed in areas of sandy clay loam soils over glacial sands and gravels. In these areas, mean NO3− isotopic signatures (δ15NNO3 = +7.8 ‰ and δ18ONO3 = +5.0 ‰) indicated a source of nitrified NH4+. In conclusion, understanding hydrological processes in catchments underlain by variable glacial deposits can inform nutrient management plans and cultivation practices to reduce the risk of agricultural NO3− contamination.

Multi-tracer approach to constrain groundwater flow and geochemical baseline assessments for CO2 sequestration in deep sedimentary basins

Geological storage of gases will be necessary in the push to net zero and the energy transition to reduce carbon emissions to atmosphere. These include CO2 geological storage in suitable sandstone reservoirs. Understanding groundwater flow, connectivity and hydrogeochemical processes in aquifer and storage systems is vital to prevent risk and protect important water resources, such as the Great Artesian Basin. Here, we provide a ‘tool-box’ of geochemical assessment methods to provide information on flow patterns through the basin's aquifers (changes in chemistry along flow path), stagnant versus flowing conditions (cosmogenic isotopes and noble gases), inter-aquifer connectivity and seal properties (major ions, Sr and stable isotopes), water quality (major ions and metals) and general assessments on residence times of groundwater (cosmogenic isotopes and noble gases). This information can be used with reservoir and groundwater models to inform on possible changes in the above-mentioned processes and serve as input parameters for CO2 injection impact modelling. We demonstrate the use and interpretation on an example of a potential CO2 storage geological sequestration site in the Surat Basin, part of the Great Artesian Basin, and the aquifers that overly the reservoir. The stable water isotopes are depleted compared to average rainfall and most likely indicate greater contributions from monsoonal rain events from the northern monsoonal troughs, where amount and rainout effects lead to the depletion rather than colder recharge climates. This is supported by the modern recharge temperatures from noble gases. Inter-aquifer mixing between the Precipice Sandstone reservoir and the Hutton Sandstone aquifer seems unlikely as the Sr isotope ratios are distinctly different suggesting that the Evergreen Formation is a seal in the locations sampled. Mixing, however, occurs on the edges of the basin, especially in the south-east and east where the Surat Basin transitions into the Clarence-Moreton Basin. Groundwater flow appears to be to the south in the Precipice Sandstone, with a component of flow east to the Clarence-Morton Basin. The cosmogenic isotopes and noble gases strongly indicate very long residence times of groundwater in the central south Precipice Sandstone around a proposed storage site. 14C values below analytical uncertainty, R36Cl ratios at secular equilibrium as well as high He concentrations and high 40Ar/36Ar ratios support the argument that groundwater flow in this area is extremely slow or groundwater is stagnant. The results of this study reflect the geological and hydrogeological complexities of sedimentary basins and that baseline studies, such as this one, are paramount for management strategies.

Hydrologic windows into the crystalline basement and their controls on groundwater flow patterns across the Paradox Basin, western USA

Conceptual models of sedimentary basin groundwater flow systems typically assume that the crystalline basement acts as an impermeable boundary and can be neglected. In this study, we use hydrologic models constrained by isotopic and geochemical datasets to argue that the La Sal Mountains, Utah, USA, act as a hydrologic window into the Paradox Basin’s lower aquifer system and underlying crystalline basement. We conducted a sensitivity study in which we varied crystalline basement/laccolith permeability as well as fault zone connectivity along a cross-sectional transect from the La Sal Mountains to Lisbon Valley. When the crystalline basement/laccolith units are set at relatively permeable levels (10−14 m2), simulated tracers that include total dissolved solids, oxygen isotopic composition of pore fluids (δ18O), and groundwater residence times are in closest agreement with field measurements. Model results indicate that pore fluids in the basal aquifer system underlying the Paradox Formation confining unit are a mixture of relatively young meteoric fluids and older Paradox Formation brines. The presence of faults did not significantly modify fluid exchange between the upper and lower aquifer systems. This was due, in part, to underpressuring within the Paradox Formation. Our study concludes that the Paradox Basin represents a regional recharge area for the Colorado Plateau, with groundwater discharge occurring along the Colorado River within the Grand Canyon some 375 km away to the southwest. This is only possible with a permeable crystalline basement. Our findings help explain the genesis of Mississippi Valley-type ore deposits of the US Midcontinent, where the presence of a permeable basement may be useful in addressing issues related to solute mass and energy balance.

Mechanism of denitrification in subsurface-dammed Ryukyu limestone aquifer, southern Okinawa Island, Japan

Denitrification crucially regulates the attenuation of groundwater nitrate and is unlikely to occur in a fast-flowing aquifer such as the Ryukyu limestone aquifer in southern Okinawa Island, Japan. However, evidences of denitrification have been observed in several wells within this region. This study analyzed environmental isotopes (δ15NNO3 and ẟ18ONO3) to derive the rationale for denitrification at this site. Additionally, the presence of two subsurface dams in the study area may influence the processes involved in nitrate attenuation. Herein, we analyzed 150 groundwater samples collected spatially and seasonally to characterize the variations in the groundwater chemistry and stable isotopes during denitrification. The values of δ15NNO3 and δ18ONO3 displayed a progressive trend up to +59.7 ‰ and + 21 ‰, respectively, whereas the concentrations of NO3−–N decreased to 0.1 mg L−1. In several wells, the enrichment factors of δ15NNO3 ranged from −6.6 to −2.1, indicating rapid denitrification, and the δ15NNO3 to δ18ONO3 ratios varied from 1.3:1 to 2:1, confirming the occurrence of denitrification. Denitrification intensively proceeds under conditions of depleted dissolved oxygen concentrations (<2 mg L−1), sluggish groundwater flow with longer residence times, high concentrations of dissolved organic carbon (>1.2 mg L−1), and low groundwater levels during the dry season with precipitation rates of <100 mm per month (Jun–Sep). SF6 analysis indicated the exclusive occurrence of denitrification in specific wells with groundwater residence times exceeding 30 years. These wells are located in close proximity to the major NE–SW fault system in the Komesu area, where the hydraulic gradient was below 0.005. Detailed geological and lithological investigations based on borehole data revealed that subsurface dams did not cause denitrification while the major NE–SW fault system uplifted the impermeable basement rock of the Shimajiri Group, creating a lithological gap at an equivalent depth that ultimately formed a sluggish groundwater area, promoting denitrification.

Ferric Oxyhydroxylsulfate Precipitation Improves Water Quality in an Acid Mining Lake: A Hydrogeochemical Investigation

Hydrogeochemistry of a lignite pit lake in Lusatia, Germany, was investigated. Anoxic groundwater from the dump aquifer rich in FeII (average ~5911 µmol/L) and SO4 (average ~14,479 µmol/L) contents enter the lake as subsurface inflow; oxidation and subsequent precipitation of poorly crystallized Fe-oxyhydroxysulfate (schwertmannite) occurs and causes acidification (pH~2.8). However, the removal of dissolved loads as solid phases significantly improves the groundwater quality of the downgradient as an outflow. The rainwater isotopic values (δD ~−8.88‰ and δ18O ~−65.86‰) closely matched with the groundwater showing very little isotopic modification, which points to a short residence time of groundwater. The displacement of δD and δ18O values (slope = 5.3) from the meteoric water line reflected the evaporative enrichment of the lake water. The isotopic signature also revealed longer residence times of epilimnion than the hypolimnion waters which are dominated by groundwater. The lake is dimictic and showed abrupt changes in physicochemical parameters along the interface (~0.30 m thick) when separating the epilimnion (upper 4 m) from the hypolimnion (bottom 1.5 m). Lake sediments were found to be dominated by clay size fraction occurring as laminations (thickness: 1~0.5 mm) that reflect seasonal sedimentation. Higher schwertmannite formation in the south as compared to the north (recharge side) also serves as a scavenger of potentially toxic elements which is probably a natural solution to man-made problems. Schwertmannite transformation to goethite releases sulfate which is reduced and fixed as secondary sulfide minerals over time. Overall, waters are of a Ca–SO4 to Ca–Mg–SO4 type with distinct inflow (FeII/FeIII &gt; 2.5) and outflow (FeII/FeIII &lt; 0.5) of groundwater.

Spatial distribution and enrichment mechanisms of high fluoride groundwater in geotherm-affected Pliocene aquifers of the Guide basin, China

Native high fluoride (F−) groundwater is one of the most serious environmental problems facing China and even the international society, which seriously threatens human health. The confined aquifer in the Guide basin contains high concentrations of F− and is subject to multifaceted influences, stemming not only from the geothermal activity present at the basin's bedrock but also from distinctive attributes such as elevated alkalinity, significant burial depths, and limited water circulation. Consequently, the precise mechanisms governing the enrichment of F− within the groundwater in this confined aquifer remain inadequately understood. This study aims to elucidate the migration and enrichment mechanism of F− in aquifers and assess the relative contributions of different control mechanisms responsible for high F− groundwater through systematic sampling, analysis and statistical calculations. Chemical types of confined groundwater and unconfined groundwater are Na-HCO3-Cl and Ca-HCO3, respectively. High-F− groundwater mainly exists in confined aquifers, and the content of F− gradually increases along the flow path. High content of HCO3− in confined groundwater promotes the precipitation of Ca2+, then enhances the dissolution of F−-containing minerals, resulting in F− being continuously released into groundwater. High pH, high Na/Ca molar ratios, geothermal and long residence time of groundwater are the additional factors to accelerate the release and migration of F− in groundwater. Results of redundancy analysis showed that competitive adsorption is the primary mechanism for F− enrichment in confined groundwater, followed by the geothermal effect, precipitation of carbonate minerals and residence time of groundwater time. The results of potential health risk assessment indicate that the potential health risks associated with confined groundwater are notably higher across all age groups compared to unconfined groundwater. The research results are not only helpful for the local government in reducing the threat of drinking-water fluorosis, but also provide a reference for other high-F− groundwater research with similar geological conditions in China and around the world.

Geological Trap Controlling the Residence Time of Groundwater in Assessment of Exploitation Zone for Its Sustainable Resources Case Study: The Slope of Karang Mount, Banten Province, Indonesia

The southern slope of Mount Karang is covered by complex volcanic deposits with complicated texture and structure. The study on zone or location of water resources which would be exploited required a comprehensive hydrogeological approach. Through detailed geological mapping, spring sampling, and well drilling were carried out. Representative spring water samples were taken to be analyzed in the laboratory, and to obtain the data of physical groundwater, chemical groundwater, stable isotopes 18O (oxygen-18), and deuterium contents, as well as the age of the groundwater. In general, the groundwater facies of the studied area showed Ca, Na, KHCO3 with several sites indicating changes to CaHCO3 during the dry and rainy seasons. The synthesis results of the stable isotope 18O (oxygen-18) and deuterium contents, verified by the physical and chemical groundwater controlled by geological setting in the groundwater subbasins, show the anomaly of residence time as trapped by the normal fault in the middle of the studied area. The existing normal fault might control this anomaly of residence time of groundwater surrounding site JH1, JH9, and JH20. However, the distribution of three different water source zones occurred. All group of groundwater indicated a complex flowing with geological setting controlling the physical, chemical content, and the age of the groundwater. At last, sites JH4, JH5, and JH9 show that the zones are proper to be developed as sustainability groundwater resources. Keywords: groundwater facies, stable isotope, groundwater flow, sustainable water