Stable chlorine isotopic signatures and fractionation mechanism of groundwater in Anyang,China

The present work provides an online Bench II-IRMS technique for the measurement of stable chlorine isotope ratio, which is used to measure the δ³⁷Cl of 38 groundwater samples from the Karst and Quaternary aquifers in Anyang area. The regional distribution and signature of δ³⁷Cl value are characterized on the base of isotopic data. The results suggest that the δ³⁷Cl value of Quaternary groundwater decreases with increasing Cl^? concentration, and has no correlation with δ¹⁸O and δD values, but closely correlates with the depth to water table. The fractionation mechanism of the chlorine isotope is expounded according to the type of groundwater. The δ³⁷Cl value of karst water is generally positive, which is relevant to the dissolution of evaporite (gypsum mine), and may be caused by the mixing of groundwater and precipitation. The groundwater of Quaternary unconfined aquifer is mainly recharged by precipitation, and the δ³⁷Cl value of groundwater is generally negative. The δ³⁷Cl value of groundwater in Quaternary confined aquifer is more negative with increasing the depth to water level and elevated Cl^? concentration, which is possible to result from the isotope fractionation of ion filtration. The groundwater with inorganic pollutants in Quaternary unconfined aquifer has generally a positive δ³⁷Cl value.     

Identifying the recharge sources and age of groundwater in the Songnen Plain (Northeast China) using environmental isotopes

The recharge sources and groundwater age in the Songnen Plain, Northeast China, were confirmed using environmental isotopes. The isotopic signatures of the unconfined aquifers in the southeast elevated plain and the north and west piedmont, cluster along local meteoric water lines (LMWLs) with a slope of about 5. The signature of source water was obtained by the intersection of these LMWLs with the regional meteoric water line (RMWL). This finding provides evidence that the recharge water for these areas originate from the Changbai Mountains and the Low and High Hingan Mountains, respectively. Groundwater in the unconfined aquifer in the low plain yields a LMWL with a slope of 4.4; its nitrate concentration indicates the admixture of irrigation return flow. The δ-values of the unconfined aquifer in the east elevated plain plot along the RMWL, reflecting recharge by local precipitation. The mean residence time of groundwater in these aquifers is less than 50?years. However, the ¹⁴C age of the groundwater in the confined Quaternary aquifer ranges from modern to 19,500?years, and in the Tertiary confined aquifer from 3,100 to 24,900?years. Modern groundwater is mainly recharged to the Quaternary confined aquifer on the piedmont by local precipitation and lateral subsurface flow.     

Study on the groundwater sustainable problem by numerical simulation in a multi-layered coastal aquifer system of Zhanjiang,China

Assessing sustainability of coastal groundwater is significant for groundwater management as coastal groundwater is vulnerable to over-exploitation and contamination. To address the issues of serious groundwater level drawdown and potential seawater intrusion risk of a multi-layered coastal aquifer system in Zhanjiang, China, this paper presents a numerical modelling study to research groundwater sustainability of this aquifer system. The transient modelling results show that the groundwater budget was negative (\(-3826\times 10^{4}\) to \(-4502\times 10^{4 }\,\hbox {m}^{3}/\hbox {a}\)) during the years 2008–2011, revealing that this aquifer system was over-exploited. Meanwhile, the groundwater sustainability was assessed by evaluating the negative hydraulic pressure area (NHPA) of the unconfined aquifer and the groundwater level dynamic and flow velocity of the offshore boundaries of the confined aquifers. The results demonstrate that the Nansan Island is most influenced by NHPA and that the local groundwater should not be exploited. The results also suggest that, with the current groundwater exploitation scheme, the sustainable yield should be \(1.784\times 10^{8}\, \hbox {m}^{3}/\hbox {a}\) (i.e., decreased by 20% from the current exploitation amount). To satisfy public water demands, the 20% decrease of the exploitation amount can be offset by the groundwater sourced from the Taiping groundwater resource field. These results provide valuable guidance for groundwater management of Zhanjiang.     

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.     

Sources of groundwater pumpage in a layered aquifer system in the Upper Gulf Coastal Plain, USA

Understanding groundwater-pumpage sources is essential for assessing impacts on water resources and sustainability. The objective of this study was to quantify pumping impacts and sources in dipping, unconfined/confined aquifers in the Gulf Coast (USA) using the Texas Carrizo-Wilcox aquifer. Potentiometric-surface and streamflow data and groundwater modeling were used to evaluate sources and impacts of pumpage. Estimated groundwater storage is much greater in the confined aquifer (2,200?km³) than in the unconfined aquifer (170?km³); however, feasibility of abstraction depends on pumpage impacts on the flow system. Simulated pre-development recharge (0.96?km³/yr) discharged through evapotranspiration (ET, ～37%), baseflow to streams (～57%), and to the confined aquifer (～6%). Transient simulations (1980–1999) show that pumpage changed three out of ten streams from gaining to losing in the semiarid south and reversed regional vertical flow gradients in ～40% of the entire aquifer area. Simulations of predictive pumpage to 2050 indicate continued storage depletion (41% from storage, 32% from local discharge, and 25% from regional discharge capture). It takes ～100?yrs to recover 40% of storage after pumpage ceases in the south. This study underscores the importance of considering capture mechanism and long-term system response in developing water-management strategies.     

Groundwater flow patterns in the San Luis Valley,Colorado, USA revisited: an evaluation of solute and isotopic data

Groundwater systems in the San Luis Valley, Colorado, USA have been re-evaluated by an analysis of solute and isotopic data. Existing stream, spring, and groundwater samples have been augmented with 154 solute and isotopic samples. Based on geochemical stratification, three groundwater regimes have been identified within 1,200 m of the surface: unconfined, upper active confined, and lower active confined with maximum TDS concentrations of 35,000, 3,500 and 600 mg/L, respectively. The elevated TDS of northern valley unconfined and upper active confined systems result from mineral dissolution, ion exchange and methanogenesis of organic and evaporate lake sediments deposited in an ancient lake, herein designated as Lake Sipapu. Chemical evolutions along flow paths were modeled with NETPATH. Groundwater ages, and δ¹³C, δ²H and δ¹⁸O compositions and distributions, suggest that mountain front recharge is the principle recharge mechanism for the upper and lower confined aquifers with travel times in the northern valley of more than 20,000 and 30,000 ¹⁴C years, respectively. Southern valley confined aquifer travel times are 5,000 ¹⁴C years or less. The unconfined aquifer contains appreciable modern recharge water and the contribution of confined aquifer water to the unconfined aquifer does not exceed 20%.     

深层水漏斗区开采量组成变化特征与机制

为阐明强烈开采条件下华北平原深层水的开采水量来源及组成问题,以德州地区深层水漏斗分布区为重点研究区,通过基于大量钻孔资料对含水层组及其粘性土夹层或透镜体的分析和野外地下水采样、动态监测及地面沉降测量资料等分析,采用分层、分区计算方法对漏斗中心至外围的各分区弹性释水量、粘性土压密释水量、侧向流入和越流补给量进行均衡计算。结果表明:在强烈开采条件下,深层水开采量中弹性释水量和压密释水量所占比率不是常量,随着深层水位下降的幅度和深度增大、超采释压持续时间愈长、开采层中粘性土夹层或透镜体愈多,则区内弹性释水和压密释水的强度及累积地面沉降量愈大;愈靠近漏斗中心,弹性和压密释水强度愈大,相同降深下平均单位降深的地面沉降量愈小。在深层水开采量中粘性土压密释水占35.5%~61.9%,与地面沉降密切相关。     

Hydrogeochemical and isotopic evidence of groundwater evolution and recharge in aquifers in Beijing Plain, China

An investigation was conducted in Beijing to identify the groundwater evolution and recharge in the quaternary aquifers. Water samples were collected from precipitation, rivers, wells, and springs for hydrochemical and isotopic measurements. The recharge and the origin of groundwater and its residence time were further studied. The groundwater in the upper aquifer is characterized by Ca-Mg-HCO₃ type in the upstream area and Na-HCO₃ type in the downstream area of the groundwater flow field. The groundwater in the lower aquifer is mainly characterized by Ca-Mg-HCO₃ type in the upstream area and Ca-Na-Mg-HCO₃ and Na-Ca-Mg-HCO₃ type in the downstream area. The δD and δ¹⁸O in precipitation are linearly correlated, which is similar to WMWL. The δD and δ¹⁸O values of river, well and spring water are within the same ranges as those found in the alluvial fan zone, and lay slightly above or below LMWL. The δD and δ¹⁸O values have a decreasing trend generally following the precipitation → surface water → shallow groundwater → spring water → deep groundwater direction. There is evidence of enrichment of heavy isotopes in groundwater due to evaporation. Tritium values of unconfined groundwater give evidence for ongoing recharge in modern times with mean residence times <50 a. It shows a clear renewal evolution along the groundwater flow paths and represents modern recharge locally from precipitation and surface water to the shallow aquifers (<150 m). In contrast, according to ¹⁴C ages in the confined aquifers and residence time of groundwater flow lines, the deep groundwater is approximately or older than 10 ka, and was recharged during a period when the climate was wetter and colder mainly from the piedmont surrounding the plain. The groundwater exploitation is considered to be “mined unsustainably” because more water is withdrawn than it is replenished.     

Geochemical and isotopic characterization of groundwater from shallow and deep limestone aquifers system of Aleppo basin (north Syria)

Stable isotopes (δ¹⁸O, δ²H and ¹³C) and radioactivity (³H, ¹⁴C) have been used in conjunction with chemical data to evaluate the processes generating the chemical composition, reconstruct the origin of the water and groundwater residence time. The Aleppo basin is comprised of two main limestone aquifers: the first one is unconfined of Paleogene age and the second is confined of Upper Cretaceous age. The chemical data indicate that the dissolution of minerals and evaporation are the main processes controlling groundwater mineralization. The groundwater from the two aquifers is characterized by distinctive stable isotope signatures. This difference in water isotopes is interpreted in terms of difference origin and recharge period. Fresh and brackish shallow groundwater were mostly recharged during the Holocene period. The presence of ³H in several groundwater samples of this aquifer gives evidence that groundwater recharge is going on. Brackish water of the deep confined aquifer has depleted stable isotope composition and very low ¹⁴C activity that indicates recharge during the late Pleistocene cold period.     

Multivariate statistical analysis of chemical and stable isotopic data as indicative of groundwater evolution with reduced exploitation

Groundwater resources in the North China Plain (NCP) are undergoing tremendous changes in response to the operation of groundwater exploitation reduction (GWER) project. To identify groundwater evolution in this complex context, hierarchical cluster analysis (HCA) and principal component analysis (PCA) were combined to interpret an integrated dataset of stable isotopes and chemical data from four sampling campaigns in a pilot area of groundwater control. We proposed a novel HCA approach integrating stable isotopes and chemical signals, which successfully partitioned the groundwater samples into the unconfined and the confined water samples. Stable isotopic evidence showed that the lateral inflow and the surface water may contribute more to groundwater recharge in this region than local modern precipitation. The unconfined water’s main hydrochemical types were Na type with mixed anions, and Na–Cl–SO₄ type, while the confined water was mainly Na–Cl and Na–SO₄ types. Geochemical processes mainly involved the dissolution/precipitation of halite, gypsum, Glauber's salt, feldspar, calcite and dolomite, as well as the cation exchange. PCA results showed that water–rock interaction (i.e., salinity-based and alkalinity-based processes) predominated the hydrochemical evolution, along with local nitrate contamination resulting from fertilizers and domestic sewage. The GWER project regulated the natural evolution of unconfined water chemistry, and significantly reduced the unconfined water’s salinity (mainly Na⁺, Mg²⁺, SO₄^2?). This may be attributed to upward leakage from low-salinity confined water at some parts of the aquifer. Additionally, insignificant changes in the confined water’s salinity reflected that the impact of GWER on the confined aquifer was negligible. This study facilitates the groundwater classification effectively in the areas lack of geological data, and enhances the knowledge of groundwater chemical evolution in such a region where groundwater restoration is in progress, with important implications for groundwater sustainable management in similar basins worldwide.     

A study on groundwater recharge in the Anyanghe River alluvial fan,North China Plain,based on hydrochemistry,stable isotopes and tritium

Studies on groundwater recharge are essential for sustainable exploitation of groundwater resources, especially in areas of extensive groundwater exploitation such as the Anyanghe River alluvial fan (ARAF) in the North China Plain (NCP). However, the recharge sources and processes and the contribution of each recharge flow component remain unclear. This study used hydrochemistry, stable isotopes, and tritium to investigate sources and underlying processes of groundwater recharge, along with the steady flow Mixing Cell Model (MCMsf) to quantify the proportion of each source flow for the shallow confined groundwater system in the medial fan. The results showed that groundwater mainly originates from precipitation occurring on the eastern Taihang Mountain area with average elevation estimated at 700–1,000 m above sea level during the East Asia summer monsoon period since 1952. Recharge mechanisms are: (1) river water seepage for the unconfined aquifers of the proximal and medial fan; (2) lateral flow for the confined aquifers of the medial and distal fan; and (3) precipitation infiltration for the phreatic water system. The MCMsf simulation showed that the shallow confined groundwater system in the central zone of the medial fan mainly recharged by the lateral flow from the proximal fan, a constant and considerable recharge flow from the southwestern and southern hills, and river water seepage in the medial fan; the lateral recharge flow from the Zhanghe alluvial aquifer was insignificant by comparison. The results of this study can act as a valuable reference for sustainable groundwater management in the ARAF.

     

Environmental isotopic study on the recharge and residence time of groundwater in the Heihe River Basin, northwestern China

The recharge and origin of groundwater and its residence time were studied using environmental isotopic measurements in samples from the Heihe River Basin, China. δ¹⁸O and δD values of both river water and groundwater were within the same ranges as those found in the alluvial fan zone, and lay slightly above the local meteoric water line (δD=6.87δ¹⁸O+3.54). This finding indicated that mountain rivers substantially and rapidly contribute to the water resources in the southern and northern sub-basins. δ¹⁸O and δD values of groundwater in the unconfined aquifers of these sub-basins were close to each other. There was evidence of enrichment of heavy isotopes in groundwater due to evaporation. The most pronounced increase in the δ¹⁸O value occurred in agricultural areas, reflecting the admixture of irrigation return flow. Tritium results in groundwater samples from the unconfined aquifers gave evidence for ongoing recharge, with mean residence times of: less than 36 years in the alluvial fan zone; about 12–16 years in agricultural areas; and about 26 years in the Ejina oasis. In contrast, groundwater in the confined aquifers had ¹⁴C ages between 0 and 10 ka BP.     

Use of multiple age tracers to estimate groundwater residence times and long-term recharge rates in arid southern Oman

Multiple age tracers were measured to estimate groundwater residence times in the regional aquifer system underlying southwestern Oman. This area, known as the Najd, is one of the most arid areas in the world and is planned to be the main agricultural center of the Sultanate of Oman in the near future. The three isotopic age tracers ⁴He, ¹⁴C and ³⁶Cl were measured in waters collected from wells along a line that extended roughly from the Dhofar Mountains near the Arabian Sea northward 400 km into the Empty Quarter of the Arabian Peninsula. The wells sampled were mostly open to the Umm Er Radhuma confined aquifer, although, some were completed in the mostly unconfined Rus aquifer. The combined results from the three tracers indicate the age of the confined groundwater is < 40 ka in the recharge area in the Dhofar Mountains, > 100 ka in the central section north of the mountains, and up to and > one Ma in the Empty Quarter. The ¹⁴C data were used to help calibrate the ⁴He and ³⁶Cl data. Mixing models suggest that long open boreholes north of the mountains compromise ¹⁴C-only interpretations there, in contrast to ⁴He and ³⁶Cl calculations that are less sensitive to borehole mixing. Thus, only the latter two tracers from these more distant wells were considered reliable. In addition to the age tracers, δ²H and δ¹⁸O data suggest that seasonal monsoon and infrequent tropical cyclones are both substantial contributors to the recharge. The study highlights the advantages of using multiple chemical and isotopic data when estimating groundwater travel times and recharge rates, and differentiating recharge mechanisms.     

Assessing hydrochemical evolution of groundwater in limestone terrain via principal component analysis

This paper describes the use of multivariate statistical analysis to trace hydrochemical evolution in a limestone terrain at Zagros region, Iran. The study area includes a deep confined aquifer, overlaid by an unconfined aquifer. The method involves the use of principal component analysis (PCA) to assess and evaluate the hydrochemical evolution based on chemical and isotope variables of 12 piezometers drilled in both the unconfined and confined aquifers. First PCA on all variables shows that water–rock interaction under different conditions with respect to the atmospheric CO₂ is the main process responsible for chemical constituents. As a result, combinations of several ratios such as Ca/TDS, SO₄/TDS and Mg/TDS with physico-chemical and isotope variables reveal different hydrochemical evolution trend in the aquifers. Second PCA on the selective samples and variables reveals that displacement of the unconfined samples from dry to wet season follows a refreshing trend towards river samples that is characterized by reducing electrical conductivity and increasing sulphate and tritium contents. However, the refreshing trend cannot be traced in the confined aquifer samples suggesting no recharge from river to the confined aquifer. Third PCA reveals that, chemical composition of water samples in the unconfined aquifer tends to have considerable difference from each other in the end of recharge period. In contrast, the confined aquifer samples have a tendency to show similar chemical composition during recharge period in comparison to end of dry period. This difference is caused by different mechanism of recharge in the unconfined aquifer (through the whole aquifer surface) and the confined aquifer (through the limited recharge area).     

Controls on δ34S and δ18O of dissolved sulfate in aquifers of the Murray Basin,Australia and their use as indicators of flow processes

The usefulness of stable isotopes of dissolved SO₄ (δ³⁴S and δ¹⁸O) to study recharge processes and to identify areas of significant inter-aquifer mixing was evaluated in a large, semi-arid groundwater basin in south-eastern Australia (the Murray Basin). The distinct isotopic signatures in the oxidizing unconfined Murray Group Aquifer and the deeper reducing Renmark Group confined aquifer may be more sensitive than conventional chemical tracers in establishing aquifer connections. δ³⁴S values in the unconfined Murray Group Aquifer in the south and central part of the study area decrease along the hydraulic gradient from 20.8 to 0.3‰. The concomitant increasing SO₄/Cl ratios, as well as relatively low δ¹⁸O_SO4 values, suggest that vertical input of biogenically derived SO₄ via diffuse recharge is the predominant source of dissolved SO₄ to the aquifer. Further along the hydraulic gradient towards the discharge area near the River Murray, δ³⁴S values in the unconfined Murray Group Aquifer increase, and SO₄/Cl ratios decrease, due to upward leakage of waters from the confined Renmark Group Aquifer which has a distinctly low SO₄/Cl and high δ³⁴S (14.9–56.4‰). Relatively positive δ³⁴S and δ¹⁸O_SO4 values, and low SO₄/Cl in the Renmark Group Aquifer is typical of SO₄ removal by bacterial reduction. The S isotope fractionation between SO₄ and HS⁻ of ∼24‰ estimated for the confined aquifer is similar to the experimentally determined chemical fractionation factor for the reduction process but much lower than the equilibrium fractionation (∼70‰) even though the confined groundwater residence time is >300 Ka years. Mapping the spatial distribution of δ³⁴S and SO₄/Cl of the unconfined Murray Group Aquifer provides an indicative tool for identifying the approximate extent of mixing, however the poorly defined end-member isotopic signatures precludes quantitative estimates of mixing fractions.     

Groundwater recharge and evolution of water quality in China’s Jilantai Basin based on hydrogeochemical and isotopic evidence

We investigated major ions, stable isotopes, and radiocarbon dates in a Quaternary aquifer in semi-arid northwestern China to gain insights into groundwater recharge and evolution. Most deep and shallow groundwater in the Helan Mountains was fresh, with total dissolved solids <1,000 mg L^?1 and Cl^? <250 mg L^?1. The relationships of major ions with Cl^? suggest strong dissolution of evaporites. However, dissolution of carbonates, albite weathering, and ion exchange are also the major groundwater process in Jilantai basin. The shallow desert groundwater is enriched in δ¹⁸O and intercepts the local meteoric water line at δ¹⁸O = ?13.4 ‰, indicating that direct infiltration is a minor recharge source. The isotope compositions in intermediate confined aquifers resemble those of shallow unconfined groundwater, revealing that upward recharge from intermediate formations is a major source of shallow groundwater in the plains and desert. The estimated residence time of 10.0 kyr at one desert site, indicating that some replenishment of desert aquifers occurred in the late Pleistocene and early Holocene with a wetter and colder climate than at present.     

松嫩平原地下水流动模式的环境同位素标记

采用同位素水文学方法并结合传统水文地质方法,识别松嫩平原地下水流动模式。氢氧稳定同位素和地下水年龄分布表明该区地下水流动系统流动模式呈现出局部流、中间流和区域流系统。地下水中氚分布深度指示局部水流系统为现代水循环系统,以垂向运动为主要特征,循环深度一般小于50 m,山前区可达100m以下;区域流系统存在于深部承压含水层,以侧向水平径流为主要运动特征。松辽边界附近的环境同位素特征存在明显的差别,指示天然状态下可视为零通量边界。同位素示踪剂也反映出嫩江和地下水的相互作用关系,在齐齐哈尔以北,江水补给地下水;在齐齐哈尔以南,地下水向嫩江排泄。     

The integrated impacts of natural processes and human activities on groundwater salinization in the coastal aquifers of Beihai,southern China

Salinization in coastal aquifers is usually related to both seawater intrusion and water–rock interaction. The results of chemical and isotopic methods were combined to identify the origin and processes of groundwater salinization in Daguansha area of Beihai, southern China. The concentrations of the major ions that dominate in seawater (Cl^?, Na⁺, Ca²⁺, Mg²⁺ and SO ₄ ^2– ), as well as the isotopic content and ratios (²H, ¹⁸O, ⁸⁷Sr/⁸⁶Sr and ¹³C), suggest that the salinization occurring in the aquifer of the coastal plain is related to seawater and that the prevailing hydrochemical processes are evaporation, mixing, dissolution and ion exchange. For the unconfined aquifer, groundwater salinization has occurred in an area that is significantly influenced by land-based sea farming. The integrated impacts of seawater intrusion from the Beibuwan Gulf and infiltration of seawater from the culture ponds are identified in the shallowest confined aquifer (I) in the middle of the area (site BBW2). Leakage from this polluted confined aquifer causes the salinization of groundwater in the underlying confined aquifer (II). At the coastal monitoring site (BBW3), confined aquifer I and lower confined aquifer II are heavily contaminated by seawater intrusion. The weak connectivity between the upper aquifers, and the seaward movement of freshwater, prevents saltwater from encroaching the deepest confined aquifer (III). A conceptual model is presented. Above all, understanding of the origin and processes of groundwater salinization will provide essential information for the planning and sustainable management of groundwater resources in this region.     

Different isotopic evolutionary trends of δ34S and δ18O compositions of dissolved sulfate in an anaerobic deltaic aquifer system

Concentration and isotope ratios (δ³⁴S_SO4 and δ¹⁸O_SO4) of dissolved sulfate of groundwater were analyzed in a very large anaerobic aquifer system under the Lower Central Plain (LCP) (25,000 km²) in Thailand. Groundwater samples were collected in two different kinds of aquifers; type 1 with a saline water contribution and type 2 lateritic aquifers with no saline water contribution. Two different isotopic compositional trends were observed: in type 1 aquifers sulfate isotope ratios range from low values (+2.2‰ for δ³⁴S_SO4 and +8.0‰ for δ¹⁸O_SO4) to high values (+49.9‰ for δ³⁴S_SO4 and +17.9‰ for δ¹⁸O_SO4); in type 2 aquifers sulfate isotope ratios range from low values (−0.1‰ for δ³⁴S_SO4 and +12.2‰ for δ¹⁸O_SO4) to high δ¹⁸O_SO4 ratios (+18.4‰) but with low δ³⁴S_SO4 ratios (<+12.9‰). Isotopic comparison with possible source materials and theoretical geochemical models suggests that the sulfate isotope variation for type 1 aquifer groundwater can be explained by two main processes. One is the contribution of remnant seawater, which has experienced dissimilatory sulfate reduction in the marine clay, into recharge water of freshwater origin. This process accounts for the high salinity groundwater. The other process, explaining for the modest salinity groundwater, is the bacterial sulfate reduction of the mixture water between high salinity water and fresh groundwater. Isotopic variation of type 2 aquifer groundwater may also be explained by bacterial sulfate reduction, with slower reduction rate than that of the groundwater with saline water effect. The origin of groundwater sulfate with low δ³⁴S_SO4 but high δ¹⁸O_SO4 is recognized as an important topic to be examined in a future investigation.     

Paleowaters in Silurian-Devonian carbonate aquifers: Geochemical evolution of groundwater in the Great Lakes region since the Late Pleistocene

Changes in the climatic conditions during the Late Quaternary and Holocene greatly impacted the hydrology and geochemical evolution of groundwaters in the Great Lakes region. Increased hydraulic gradients from melting of kilometer-thick Pleistocene ice sheets reorganized regional-scale groundwater flow in Paleozoic aquifers in underlying intracratonic basins. Here, we present new elemental and isotopic analyses of 134 groundwaters from Silurian-Devonian carbonate and overlying glacial drift aquifers, along the margins of the Illinois and Michigan basins, to evaluate the paleohydrology, age distribution, and geochemical evolution of confined aquifer systems. This study significantly extends the spatial coverage of previously published groundwaters in carbonate and drift aquifers across the Midcontinent region, and extends into deeper portions of the Illinois and Michigan basins, focused on the freshwater-saline water mixing zones. In addition, the hydrogeochemical data from Silurian-Devonian aquifers were integrated with deeper basinal fluids, and brines in Upper Devonian black shales and underlying Cambrian-Ordovician aquifers to reveal a regionally extensive recharge system of Pleistocene-age waters in glaciated sedimentary basins. Elemental and isotope geochemistry of confined groundwaters in Silurian-Devonian carbonate and glacial drift aquifers show that they have been extensively altered by incongruent dissolution of carbonate minerals, dissolution of halite and anhydrite, cation exchange, microbial processes, and mixing with basinal brines. Carbon isotope values of dissolved inorganic carbon (DIC) range from −10 to −2‰, ⁸⁷Sr/⁸⁶Sr ratios range from 0.7080 to 0.7090, and δ³⁴S-SO₄ values range from +10 to 30‰. A few waters have elevated δ¹³C_DIC values (>15‰) from microbial methanogenesis in adjacent organic-rich Upper Devonian shales. Radiocarbon ages and δ¹⁸O and δD values of confined groundwaters indicate they originated as subglacial recharge beneath the Laurentide Ice Sheet (14-50 ka BP, −15 to −13‰ δ¹⁸O). These paleowaters are isolated from shallow flow systems in overlying glacial drift aquifers by lake-bed clays and/or shales. The presence of isotopically depleted waters in Paleozoic aquifers at relatively shallow depths illustrates the importance of continental glaciation on regional-scale groundwater flow. Modern groundwater flow in the Great Lakes region is primarily restricted to shallow unconfined glacial drift aquifers. Recharge waters in Silurian-Devonian and unconfined drift aquifers have δ¹⁸O values within the range of Holocene precipitation: −11 to −8‰ and −7 to −4.5‰ for northern Michigan and northern Indiana/Ohio, respectively. Carbon and Sr isotope systematics indicate shallow groundwaters evolved through congruent dissolution of carbonate minerals under open and closed system conditions (δ¹³C_DIC = −14.7 to−11.1‰ and ⁸⁷Sr/⁸⁶Sr = 0.7080-0.7103). The distinct elemental and isotope geochemistry of Pleistocene- versus Holocene-age waters further confirms that surficial flow systems are out of contact with the deeper basinal-scale flow systems. These results provide improved understanding of the effects of past climate change on groundwater flow and geochemical processes, which are important for determining the sustainability of present-day water resources and stability of saline fluids in sedimentary basins.