A chlorine-36 study of regional groundwater flow and vertical transport in southern Nevada

Chlorine-36 data for groundwater from the Death Valley regional flow system is interpreted in the context of existing conceptual models for regional groundwater flow in southern Nevada. Chlorine-36 end member compositions are defined for both recharge and chemically evolved groundwater components. The geochemical evolution of ³⁶Cl is strongly controlled by water-rock interaction with Paleozoic carbonate rocks that comprise the regional aquifer system, resulting in chemically evolved groundwater that is characteristically low in ³⁶Cl/Cl and high in Cl. Groundwater from alluvial and volcanic aquifers that overlie the regional carbonate aquifer are generally characterized by high ³⁶Cl/Cl and low Cl signatures, and are chemically distinct from water in the regional carbonate aquifer. This difference provides a means of examining vertical transport and groundwater mixing processes. In combination with other geochemical and hydrogeologic data, the end members defined here provide constraints on aquifer residence times and mixing ratios.     

Understanding long-term groundwater flow at Pahute Mesa and vicinity,Nevada National Security Site,USA, from naturally occurring geochemical and isotopic tracers

Recently collected naturally occurring geochemical and isotopic groundwater tracers were combined with historic data from the Pahute Mesa area of the Nevada National Security Site (NNSS), Nevada, USA, to provide insights into long-term regional groundwater flow patterns, mixing and recharge. Pahute Mesa was the site of 85 nuclear detonations between 1965 and 1992, many of them deeply buried devices that introduced radionuclides directly into groundwater. The dataset examined included major ions and field measurements, stable isotopes of hydrogen (δ²H), oxygen (δ¹⁸O), carbon (δ¹³C) and sulfur (δ³⁴S), and radioisotopes of carbon (¹⁴C) and chloride (³⁶Cl). Analysis of the patterns of groundwater ¹⁴C data and the δ²H and δ¹⁸O signatures indicates that groundwater recharge is predominantly of Pleistocene age, except for a few localized areas near major ephemeral drainages. Steep gradients in sulfate (SO₄) and chloride (Cl) define a region near the western edge of the NNSS where high-concentration groundwater flowing south from north of the NNSS merges with dilute groundwater flowing west from eastern Pahute Mesa in a mixing zone that coincides with a groundwater trough associated with major faults. The ³⁶Cl/Cl and δ³⁴S data suggest that the source of the high Cl and SO₄ in the groundwater was a now-dry, pluvial-age playa lake north of the NNSS. Patterns of groundwater flow indicated by the combined data sets show that groundwater is flowing around the northwest margin of the now extinct Timber Mountain Caldera Complex toward regional discharge areas in Oasis Valley.

     

Radiocarbon dating and the <Superscript>36</Superscript>Cl/Cl evolution of three Great Artesian Basin wells at Dalhousie,South Australia

The use of ¹⁴C (half-life?=?5,730 years) in modeling the evolution of the ³⁶Cl/Cl ratios in groundwater is reported for the first time. The complexity of the Cl–³⁶Cl system due to the occurrence of different Cl and ³⁶Cl sources and the difficulty of the determination of the initial groundwater ³⁶Cl/Cl ratios have raised concerns about the reliability of using ³⁶Cl (half-life?=?301 thousand years, a) as a groundwater-dating tool. This work uses groundwater ¹⁴C age as a calibrating parameter of the Cl–³⁶Cl/Cl decay-mixing models of three wells from the southwestern Great Artesian Basin (GAB), Australia. It aims to allow for the different sources of Cl and ³⁶Cl in the southwestern GAB aquifer. The results show that the initial Cl concentrations range from 245 to 320 mg/l and stable Cl is added to groundwater along flowpaths at rates ranging from 1.4 to 3.5 mg/l/ka. The ³⁶Cl content of the groundwater is assumed to be completely of atmospheric origin. The samples have different Cl–³⁶Cl/Cl mixing-decay models reflecting recharge under different conditions as well as the heterogeneity of the aquifer.     

The hydrogeology and hydrogeochemistry of the Barwon Downs Graben aquifer, southwestern Victoria, Australia

The Barwon Downs Graben lies on the northern flanks of the Otway Ranges and is situated approximately 70 km southwest of Geelong, Victoria, Australia. The major lower Tertiary Barwon Downs Graben aquifer comprises highly permeable sands and gravels interbedded with clays and silts of the hydraulically interconnected Pebble Point, Dilwyn and Mepunga Formations. Groundwater flows east into the Barwon Downs Graben from the Barongarook High, and yields ¹⁴C ages up to ～20 ka implying that recharge rates are low and, consequently, that the resource could be impacted by overabstraction. The presence of three different lithological units has led to the development of localized flow systems that has resulted in a lack of regular spatial variations in groundwater chemistry. Stable isotopic data suggests that groundwater was recharged under similar climatic conditions as of today. The major ion chemistry of the freshest groundwater is dominated by Na and HCO₃ while higher TDS groundwater, from the confining Narrawaturk Marl, is dominated by Na and Cl. Cl/Br ratios are close to rainfall suggesting that halite dissolution is not the principle source of salts. An excess of Na relative to Cl in fresher groundwater suggests that feldspar dissolution has occurred, however, water–rock interaction is limited. The concentrations of Ca, Mg, and SO₄ are controlled by silicate dissolution and ion-exchange reactions with clays.     

Assessment of hydrogeochemistry and contamination of Varamin deep aquifer,Tehran Province,Iran

The present study investigates the hydrogeochemistry and contamination of Varamin deep aquifer located in the southeast of Tehran province, Iran. The study also evaluates groundwater suitability for irrigation uses. The hydrogeochemical study was conducted by collecting and analyzing 154 groundwater samples seasonally during 2014. Based on evolutionary sequence of Chebotarev, the aquifer is in the stage of SO₄ + HCO₃ in the north half of the plain and it has evolved into SO₄ + Cl in the south half. The unusual increase in TDS and Cl^? toward the western boundaries of the aquifer indicates some anomalies. These anomalies have originated from discharge of untreated wastewater of Tehran city in these areas. The studied aquifer contains four dominant groundwater types including Na–Ca–SO₄ (55%), Na–Ca–HCO₃ (22%), Na–Cl (13%) and Ca–Cl (10%). The spatial distributions of Na–Cl and Ca–Cl water types coincide with observed anomalies. Ionic relationships of SO₄ ^2? versus Cl^? and Na⁺ versus Cl^? confirm that water–rock interaction and anthropogenic contribution are main sources of these ions in the groundwater. The main processes governing the chemistry of the groundwater are the dissolution of calcite, dolomite and gypsum along the flow path, and direct ion exchange. Reverse ion exchange controls the groundwater chemistry in the areas contaminated with untreated wastewater. Based on Na% and SAR, 10.3 and 27% of water samples are unsuitable for irrigation purposes, respectively. Regarding residual sodium carbonate, there is no treat for crop yields. Only 6% of water samples represent magnesium adsorption ratios more than 50% which are harmful and unsuitable for irrigation.     

Groundwater age,mixing and flow rates in the vicinity of large open pit mines,Pilbara region,northwestern Australia

Determining groundwater ages from environmental tracer concentrations measured on samples obtained from open bores or long-screened intervals is fraught with difficulty because the sampled water represents a variety of ages. A multi-tracer technique (Cl, ¹⁴C, ³H, CFC-11, CFC-12, CFC-113 and SF₆) was used to decipher the groundwater ages sampled from long-screened production bores in a regional aquifer around an open pit mine in the Pilbara region of northwest Australia. The changes in tracer concentrations due to continuous dewatering over 7 years (2008–2014) were examined, and the tracer methods were compared. Tracer concentrations suggest that groundwater samples are a mixture of young and old water; the former is inferred to represent localised recharge from an adjacent creek, and the latter to be diffuse recharge. An increase in ¹⁴C activity with time in wells closest to the creek suggests that dewatering of the open pit to achieve dry mining conditions has resulted in change in flow direction, so that localised recharge from the creek now forms a larger proportion of the pumped groundwater. The recharge rate prior to development, calculated from a steady-state Cl mass balance, is 6 mm/y, and is consistent with calculations based on the ¹⁴C activity. Changes in CFC-12 concentrations with time may be related to the change in water-table position relative to the depth of the well screen.     

Constraining flow paths of saline groundwater at basin margins using hydrochemistry and environmental isotopes: Lake Cooper,Murray Basin,Australia

Solutes in saline groundwater (total dissolved solids up to 37 000 mg/L) in the Lake Cooper region in the southern margin of the Riverine Province of the Murray Basin are derived by evapotranspiration of rainfall with minor silicate, carbonate and halite dissolution. The distribution of hydraulic heads, salinity, percentage modern carbon (pmc) contents, and Cl/Br ratios imply that the groundwater system is complex with vertical flow superimposed on lateral flow away from the basin margins. Similarities in major ion composition, stable (O, H, and C) isotope, and ⁸⁷Sr/⁸⁶Sr ratios between groundwater from the shallower Shepparton Formation and the deeper Calivil – Renmark aquifer also imply that these aquifers are hydraulically interconnected. Groundwater in the deeper Calivil – Renmark aquifer in the Lake Cooper region has residence times of up to 25 000 years, implying that pre-land-clearing recharge rates were <1 mm/y. As in other regions of the Murray Basin, the low recharge rates account for the occurrence of high-salinity groundwater. Shallow (<20 m) groundwater yields exclusively modern ¹⁴C ages and shows a greater influence of evaporation over transpiration. Both these observations reflect the rise of the regional water-table following land clearing over the last 200 years and a subsequent increase in recharge to 10 – 20 mm/y. The rise of the regional water-table also has increased vertical and horizontal hydraulic gradients that may ultimately lead to the export of salt from the Lake Cooper embayment into the adjacent fresher groundwater resources.     

Reliable Late-Pleistocene Stratigraphic Ages and Shorter Groundwater Travel Times fromC in Fossil Snails from the Southern Great Basin

Both aquatic and land snails are common in the geologic record, but their utility in dating is greatly restricted by their well-documented tendency to yield¹⁴C dates inconsistent with true¹⁴C ages. In this study, we examine the use of¹⁴C ages from (1) small, previously unstudied, terrestrial snails to date hosting spring deposits and from (2) cooccuring aquatic snails to constrain groundwater travel times during the last glacial period. Our study area in the southern Great Basin encompasses Yucca Mountain, site of the proposed high-level nuclear waste repository, where information on the age and extent of past high water tables and on groundwater flow times is crucial to several licensing issues. Our results show that shells of small terrestrial snails belonging toValloniasp. yield¹⁴C dates consistent with¹⁴C ages of associated carbonized wood. These results imply that these taxa can provide reliable¹⁴C age control on the broadly distributed deposits in which they have been described. In contrast, cooccurring aquatic snails from fossil spring deposits yield¹⁴C ages generally greater than the control age. This is because the aquatic shells often formed in spring waters that had an initial¹⁴C deficiency. However, the magnitude of the deficiency is much less than that observed in nearby modern springs, arguing for much higher average¹⁴C contents in late Pleistocene groundwaters in these basins. If representative, this implies shorter groundwater travel times through aquifers in southern Nevada during late-glacial time.     

Effects of sulphate reduction and geogenic CO2 incorporation on the determination of 14C groundwater ages – a case study of the Palaeogene groundwater system in north-eastern Syria

Groundwater from the Palaeogene aquifer system in north-eastern Syria has been studied using chemical and isotopic methods to determine the effects of carbonate dissolution, sulphate reduction and geogenic CO₂ incorporation in the dilution of ¹⁴C activity, and later to correct the conventional ¹⁴C water ages. The reason for this non-classical approach of correction is because the groundwater in this karstified and Nummulitic carbonate aquifer occurs under confined and partly confined conditions, and is located in an area very close to deep faults and fractures. Furthermore, the interconnection with the Upper Cretaceous formations, which commonly contain gypsum and hydrocarbons, can facilitate the processes of sulphate reduction and geogenic CO₂ incorporation, which should not be excluded. The dilution factor related to carbonate dissolution was estimated to be about 0.60–0.75. The dilution factor associated with sulphate reduction, which only depends on H₂S content, was rather low (about 0.95). However, as a result of the local tectonic setting in this area, the influence of geogenic CO₂ incorporation was clearly high. The dilution factor associated with this effect ranges between 0.24–0.64. Consequently, the corrected ¹⁴C ages are considerably reduced compared with those determined by classical models. Accordingly, the groundwater in the study area can be divided into three main groups: (1) fresh, shallow and cold water of less than 1 ka age; (2) brackish, deep and thermal water of rather old age (10.9–12.3 ka B.P.); and (3) an admixed groundwater of intermediate quality and age (1.9–6.7 ka B.P.). Electronic Publication     

Geochemical and C, O, Sr, and U-series isotopic evidence for the meteoric origin of calcrete at Solitario Wash, Crater Flat, Nevada, USA

Calcite-rich soils (calcrete) in alluvium and colluvium at Solitario Wash, Crater Flat, Nevada, USA, contain pedogenic calcite and opaline silica similar to soils present elsewhere in the semi-arid southwestern United States. Nevertheless, a ground-water discharge origin for the Solitario Wash soil deposits was proposed in a series of publications proposing elevation-dependent variations of carbon and oxygen isotopes in calcrete samples. Discharge of ground water in the past would raise the possibility of future flooding in the unsaturated zone at Yucca Mountain, Nevada, site of a proposed high-level nuclear waste repository. New geochemical and carbon, oxygen, strontium, and uranium-series isotopic data disprove the presence of systematic elevation-isotopic composition relations, which are the main justification given for a proposed ground-water discharge origin of the calcrete deposits at Solitario Wash. Values of δ¹³C (−4.1 to −7.8 per mil [‰]), δ¹⁸O (23.8–17.2‰), ⁸⁷Sr/⁸⁶Sr (0.71270–0.71146), and initial ²³⁴U/²³⁸U activity ratios of about 1.6 in the new calcrete samples are within ranges previously observed in pedogenic carbonate deposits at Yucca Mountain and are incompatible with a ground-water origin for the calcrete. Variations in carbon and oxygen isotopes in Solitario Wash calcrete likely are caused by pedogenic deposition from meteoric water under varying Quaternary climatic conditions over hundreds of thousands of years.     

Geochemistry and14C dating of groundwaters from Jurassic aquifers of North Aquitaine Basin (France)

Twenty-seven samples from a confined Lower-Middle Jurassic aquifer and an unconfined Oxfordian aquifer of the North Aquitaine Basin (France) have been analysed for their major elements, Br⁻,¹⁸O,²H,¹³C and¹⁴C contents. Hydrochemistry indicates (1) a dissolution of carbonate and anhydrite near the recharge zone and (2) a dilution of a saline water derived from a seawater/halite mixing in the deeper part of the aquifer. The mixing is also visible in a δ¹⁸O vs Cl⁻ diagram in which two different groups appear: recent waters and old waters indicating a mixing process between fresh and saline groundwaters. The composition of the saline water is likely to be 34,100±11,200 ppm in Cl, 70±20 ppm in Br and more than −3.5±07‰ vs SMOW in¹⁸O.¹³C contents indicate (1) a C exchange with CaCO₃ matrix for groundwaters near the recharge zone and (2) a participation of organic matter in the deep part of the aquifer.Residence times for waters near the area of the aquifer outcrop correspond to Holocene and Late Pleistocene periods. The depletion in stable isotopes of 10 to 15,000 y B.P. waters show a late glacial period infiltration to the aquifer. After a distance of about 10 km in the aquifer, the¹⁴C activities are 0 pmc showing the presence of ‘old’ groundwaters.     

Using environmental isotopes to evaluate the renewable capacity of a typical karst groundwater system in northern China

The karst groundwater in northern China is an important source of water supply. Its capacity for self-renewal is a key factor affecting its sustainable use. The Pingyi–Feixian karst aquifer in central and southern Shandong Province is a typical karst water source, contributing 54% to the total groundwater taken from the region. In this study, 25 groups of water samples were collected from the Pingyi–Feixian karst aquifer in November 2013. The compositions of isotopes of tritium (³H), carbon-13 (¹³C), and carbon-14 (¹⁴C) were measured. As indicated by the tritium values between 7.1 and 12.2 TU, the Pingyi–Feixian karst groundwater is primarily originated from both historical atmospheric precipitation and modern precipitation. The ¹⁴C ages corrected by δ¹³C were between 146 and 5403 years. Specifically, the shallow groundwater is younger than deep groundwater. Groundwater age tends to increase along the flow path. The ages of the groundwater in recharge area were between 146 and 1348 years, while the ages of deep groundwater in flowing area were generally between 2000 and 4000 years. The ages of the groundwater in discharge area with little anthropic exploitation were larger than 4500 years, whereas these with large amounts of exploitation were less than 1500 years. The shallower the groundwater, the stronger its capacity for renewal. The renewable capacity of karst groundwater in discharge area was significantly affected by anthropic exploitation. The karst groundwater in the areas with less exploitation showed the weakest capacity, whereas that in the area with intensive exploitation was much older and had a stronger renewable capacity.     

Using geochemical indicators to investigate groundwater mixing and residence time in the aquifer system of Djeffara of Medenine (southeastern Tunisia)

Stable isotopes (??²H, ??¹⁸O and ??¹³C) and radiocarbon (¹⁴C) have been used in conjunction with chemical data to evaluate recharge mechanisms and groundwater residence time, and to identify inter-aquifer mixing in the Djeffara multi-aquifer in semi-arid southeastern Tunisia. The southern part of this basin, the Djeffara of Medenine aquifer system, is comprised of two main aquifers of Triassic and Miocene sandstone. The Triassic aquifer presents two compartments; the first one (west of the Medenine fault system) is unconfined with a well-defined isotope fingerprint; the second compartment is deeper and confined. Multi-tracer results show groundwater of different origins, ages and salinities, and that tectonic features control groundwater flows. Fresh and brackish groundwater from the unconfined part of the Triassic aquifer was mostly recharged during the Holocene. The recharge rates of this aquifer, inferred by ¹⁴C ages, are variable and could reach 3.5?mm/year. Brackish water of the deep confined part of the Triassic aquifer has stable isotope composition and ¹⁴C content that indicates earlier recharge during late Pleistocene cold periods. Brackish to saline water of the Miocene aquifer presents variable isotope composition. Groundwater flowing through the Medenine fault system is mainly feeding the Miocene aquifer rather than the deep confined part of the Triassic aquifer.     

Testing the suitability of geologic frameworks for extrapolating hydraulic properties across regional scales

The suitability of geologic frameworks for extrapolating hydraulic conductivity (K) to length scales commensurate with hydraulic data is difficult to assess. A novel method is presented for evaluating assumed relations between K and geologic interpretations for regional-scale groundwater modeling. The approach relies on simultaneous interpretation of multiple aquifer tests using alternative geologic frameworks of variable complexity, where each framework is incorporated as prior information that assumes homogeneous K within each model unit. This approach is tested at Pahute Mesa within the Nevada National Security Site (USA), where observed drawdowns from eight aquifer tests in complex, highly faulted volcanic rocks provide the necessary hydraulic constraints. The investigated volume encompasses 40 mi³ (167 km³) where drawdowns traversed major fault structures and were detected more than 2 mi (3.2 km) from pumping wells. Complexity of the five frameworks assessed ranges from an undifferentiated mass of rock with a single unit to 14 distinct geologic units. Results show that only four geologic units can be justified as hydraulically unique for this location. The approach qualitatively evaluates the consistency of hydraulic property estimates within extents of investigation and effects of geologic frameworks on extrapolation. Distributions of transmissivity are similar within the investigated extents irrespective of the geologic framework. In contrast, the extrapolation of hydraulic properties beyond the volume investigated with interfering aquifer tests is strongly affected by the complexity of a given framework. Testing at Pahute Mesa illustrates how this method can be employed to determine the appropriate level of geologic complexity for large-scale groundwater modeling.     

Measurements and interpretations of36Cl in groundwater,Milk River aquifer,Alberta, Canada

The³⁶Cl/Cl ratios of 12 groundwater samples from the Milk River aquifer were determined by accelerator mass spectrometry. Using known Cl concentrations,³⁶Cl concentrations were deduced. Approximately linear relations were observed between the logarithm of the³⁶Cl concentration, the Cl concentration, and the distance from the recharge area along two flow paths. The results are discussed in two approaches:

• (1)in an interpretation of the linear relation between logarithm of the³⁶Cl concentration and Cl concentration excluding and includingin situ production of³⁶Cl;
• (2)in a diffusion model. The increase of the Cl concentration with the distance from the recharge area is considered to be due to diffusion of Cl from the underlying confining Colorado shale to the aquifer.

Flow velocities ranging between 0.04 and 0.14 m/a, and ages of the groundwater between 0.6 and 2 Ma are obtained at a distance of 80 km from the recharge area.     

Potential contaminant transport in the regional Carbonate Aquifer beneath Yucca Mountain, Nevada, USA

Yucca Mountain, Nevada is the site of the proposed US geologic repository for spent nuclear fuel and high-level radioactive waste. The repository is to be a mine, sited approximately 300 m below the crest of the mountain, in a sequence of variably welded and fractured mid-Miocene rhylolite tuffs, in the unsaturated zone, approximately 300 m above the water table. Beneath the proposed repository, at a depth of 2 km, is a thick sequence of Paleozoic carbonate rocks that contain the highly transmissive Lower Carbonate Aquifer. In the area of Yucca Mountain the Carbonate Aquifer integrates groundwater flow from north of the mountain, through the Amargosa Valley, through the Funeral Mountains to Furnace Creek in Death Valley, California where the groundwater discharges in a set of large springs. Data that describe the Carbonate Aquifer suggest a concept for flow through the aquifer, and based upon the conceptual model, a one-layer numerical model was constructed to simulate groundwater flow in the Carbonate Aquifer. Advective transport analyses suggest that the predicted travel time of a particle from Yucca Mountain to Death Valley through the Carbonate Aquifer might be as short as 100 years to as long 2,000 years, depending upon the porosity.     

Hydrogeochemistry and groundwater origin of the Basses-Laurentides sedimentary rock aquifer system, St. Lawrence Lowlands, Québec, Canada

A comprehensive hydrogeochemical study was carried out in the Paleozoic Basses-Laurentides sedimentary rock aquifer system in Québec over a 1500 km² study area. Groundwater samples were collected at 153 sites, characterizing all geological and hydrogeological units to a maximum depth of 140 m. Groundwater was analyzed for major, minor and trace inorganic constituents, stable isotopes δ ²H, δ ¹⁸O, and δ ¹³C of dissolved inorganic carbon (DIC), and some samples were analyzed for ³H, and ¹⁴C of DIC. The regional distribution of groundwater types shows that the hydrogeological conditions exert a dominant control on the major ions chemistry of groundwater. Preferential recharge areas are characterized by tritiated Ca-Mg-HCO₃ groundwater, and confined conditions by submodern Na-HCO₃ and Na-Cl groundwater types. Two groundwater end-members are identified in the aquifer system, modern meteoric water and Pleistocene Champlain Sea water. The region displays significant variations of groundwater geochemistry and quality controlled by glaciation, Champlain Sea invasion, lithological rock diversity, and flow system scales. This situation leads to varied groundwater types and origins within a restricted area.     

Development of climatic and vegetation conditions and the geochemical and isotopic composition in the Franconian Albvorland aquifer system

The impact of climatic and vegetation conditions over the past 15 Ka on the chemical composition and ¹⁴C dating of groundwater from the Franconian Albvorland aquifer system is discussed. Seven groundwaters over a flow distance of 25.5 km are investigated. Groundwater dating is made by ¹⁴C of dissolved inorganic carbon (DIC) and aqueous fulvic acid as well as ¹⁸O. ¹⁴C dating via fulvic acid gives groundwater ages consistent with climatic and vegetation records and variations in the groundwater composition. No correction for geochemical processes is required, since under these geochemical conditions fulvic acid remains stable over this time period and flow-distance. On the other hand, ¹⁴C dating via DIC requires correction of the ¹⁴C value due to perturbation by different geochemical processes. Up to a groundwater flow distance of approximately 17 km and an age of about 10 Ka, the ¹⁴C dating by DIC shows considerable dependence on the ¹⁴C-correction model applied. Beyond this groundwater age, ¹⁴C-DIC dating results in an overestimation by two to three ¹⁴C half-lives (T_1/2=5730 a). This deviation may result from different groundwater recharge conditions at the end of the past glaciation and geochemical processes acting on DIC that cannot be adequately characterized. The present study has implications for humic substance mediated transport of pollutants in natural aquatic systems over long time periods.     

Carbon-14 age and chemical evolution of Ca(HCO3)2-type groundwater of age less than 8,000 years in a confined sandy and muddy Pleistocene aquifer, Japan

The Pleistocene Kimitsu aquifer was selected for examination of the relationship between groundwater age and chemical evolution of Ca(HCO₃)₂-type groundwater. For the most part, the aquifer is confined and composed mainly of quartz and feldspar with a small amount of calcite. The groundwater ages calculated by ¹⁴C were adjusted by using a carbon mass-balance method and corrected for effects of ¹⁴C diffusion. Groundwater ages in the Kimitsu aquifer vary from modern (upgradient) to approximately 2,400 years at 4.4 km from the edge of the recharge area. The ¹⁴C age was verified by groundwater velocity calculated from the hydraulic gradient and hydraulic conductivity. The confined groundwater evolved to Ca(HCO₃)₂-type around 50 years after recharge and this has been maintained for more than 8,300 years due to low chemical reactivity, derived from equilibrium with calcite, kaolinite and Ca-montmorillonite. In addition, high pH prevents the dissolution of Fe and Mn. Consequently, the rate of increase in electrical conductivity ranges from 10 to 30 μS/cm per 1,000 years. On the other hand, leakage from the deep region, which is recognized from high Cl^– levels, causes remarkable increases in CH₄ and HCO₃ ^– concentrations, resulting in an apparent sulfidic zone at 500-m depth in most downgradient regions.     

Groundwater flow and residence time in a karst aquifer using ion and isotope characterization

In order to identify the origin of the main processes that affect the composition of groundwater in a karstic aquifer, a hydrogeochemical and isotopic study was carried out of water from numerous observation wells located in Sierra de Gador, a semiarid region in SE Spain. Several natural and anthropogenic tracers were used to calculate groundwater residence time within this complex aquifer system. Analysis of major ions enabled the principal geochemical processes occurring in the aquifer to be established, and the samples were classified into four distinctive solute groups according to this criterion. Dissolution of carbonate rocks determines the chemical composition of less mineralized water. In another group, the concurrent dissolution of dolomite and precipitation of calcite in gypsum-bearing carbonate aquifer, where the dissolution of relatively soluble gypsum controls the reaction, are the dominant processes. Marine intrusion results in highly mineralized waters and leads to base exchange reactions. The groundwater enrichment of minor and trace elements allowed classification of the samples into two classes that are linked to different flow patterns. One of these classes is influenced by a slow and/or deep regional flow, where the temperature is generally elevated. The influence of sulphate reduces by up to 40 % the barium concentration due to the barite precipitation. Isotope data (T, ¹⁴C) confirm the existence of recent local flows, and regional flow system, and ages of ground water may reach 8000 years. The importance of gypsum dissolution in this aquifer is proved by the δ³⁴S content.