Hydrochemical and stable isotopic investigation of groundwater quality and its sustainability for irrigation in the Hammamet-Nabeul basin,northeastern Tunisia

The major ion hydrochemistry, sodium absorption ratio (SAR), sodium percentage, and isotopic signatures of Hammamet-Nabeul groundwaters were used to identify the processes that control the mineralization, irrigation suitability, and origin of different water bodies. This investigation highlights that groundwater mineralization is mainly influenced by water-rock interaction and pollution by the return flow of irrigation water. The comparison of groundwater quality with irrigation suitability standards proves that most parts of groundwater are unacceptable for irrigation and this long-term practice may result in a significant increase of the salinity and alkalinity in the soils. Based on isotopic signatures, the shallow aquifer groundwater samples were classified into (i) waters with depleted δ¹⁸O and δ²H contents, highlighting recharge by modern precipitation, and (ii) waters with enriched stable isotope contents, reflecting the significance of recharge by contaminated water derived from the return flow of evaporated irrigation waters. The deep-aquifer groundwater samples were also classified into (i) waters with relatively enriched isotope contents derived from modern recharge and mixed with shallow-aquifer groundwater and (ii) waters with depleted stable isotope contents reflecting a paleoclimatic origin. Tritium data permit to identify three origins of recharge, i.e., contemporaneous, post-nuclear, and pre-nuclear. Carbon-14 activities demonstrate the existence of old paleoclimatic recharge related to the Holocene and Late Pleistocene humid periods.     

Hydrogeological and hydrochemical investigation of groundwater using environmental isotopes (18O, 2H, 3H, 14C) and chemical tracers: a case study of the intermediate aquifer,Sfax, southeastern Tunisia

Major element concentrations and stable (δ¹⁸O and δ²H) and radiogenic (³H and ¹⁴C) isotopes in groundwater have proved useful tracers for understanding the geochemical processes that control groundwater mineralization and for identifying recharge sources in the semi-arid region of Sfax (southeastern Tunisia). Major-ion chemical data indicate that the origins of the salinity in the groundwater are the water–rock interactions, mainly the dissolution of evaporitic minerals, as well as the cation exchange with clay minerals. The δ¹⁸O and δ²H relationships suggest variations in groundwater recharge mechanisms. Strong evaporation during recharge with limited rapid water infiltration is evident in the groundwater of the intermediate aquifer. The mixing with old groundwater in some areas explains the low stable isotope values of some groundwater samples. Groundwaters from the intermediate aquifer are classified into two main water types: Ca-Na-SO₄ and Ca-Na-Cl-SO₄. The high nitrate concentrations suggest an anthropogenic source of nitrogen contamination caused by intensive agricultural activities in the area. The stable isotopic signatures reveal three water groups: non-evaporated waters that indicate recharge by recent infiltrated water; evaporated waters that are characterized by relatively enriched δ¹⁸O and δ²H contents; and mixed groundwater (old/recent) or ancient groundwater, characterized by their depleted isotopic composition. Tritium data support the existence of recent limited recharge; however, other low tritium values are indicative of pre-nuclear recharge and/or mixing between pre-nuclear and contemporaneous recharge. The carbon-14 activities indicate that the groundwaters were mostly recharged under different climatic conditions during the cooler periods of the late Pleistocene and Holocene.

     

Hydrochemical trends,palaeorecharge and groundwater ages in the fissured Chalk aquifer of the London and Berkshire Basins,UK

The hydrochemical, radiochemical, stable isotope, ¹⁴C and dissolved noble gas composition of groundwaters has been determined along two profiles across the confined, fissured Chalk aquifer of the London Basin of southern England, and for selected sites in the adjacent Berkshire Basin. During downgradient flow in the London Basin aquifer, the groundwater chemistry is modified by water–rock interactions: congruent and incongruent reaction of the carbonate lithology resulting in enhanced Mg/Ca and Sr/Ca ratios and ¹³C contents with increased residence times; redox and ion exchange reactions; and towards the centre of the Basin, mixing with a residual saline connate water stored in the Chalk matrix. There is evidence from anomalous water chemistries for a component of vertical leakage from overlying Tertiary beds into the confined aquifer as a result of historical dewatering of the aquifer. Dissolved noble gas contents indicate the climate was up to 4.5°C cooler than at present during recharge of the waters now found in the centres of both Basins; stable isotope (²H and ¹⁸O) depletions correspond to this recharge temperature change. For evolved waters having δ¹³C > −8‰ PDB a negative linear correlation is demonstrated between derived recharge temperatures and δ¹³C values, which is interpreted as mixing between relatively warm, light isotopic, fracture-borne waters and cooler stored waters of the matrix having a ¹³C signature more or less equilibrated with the Chalk. From geochemical (¹⁴C, ⁴He) age estimates, the abstracted water is interpreted as being either of wholly Holocene/post-Devensian glacial origin, or an admixture of Holocene and Late Pleistocene pre-glacial (cold stage interstadial) recharge. Devensian pleniglacial stage waters of the Last Glacial Maximum are not represented.     

A new groundwater radiocarbon correction approach accounting for palaeoclimate conditions during recharge and hydrochemical evolution: The Ledo-Paniselian Aquifer,Belgium

The particular objective of the present work is the development of a new radiocarbon correction approach accounting for palaeoclimate conditions at recharge and hydrochemical evolution. Relevant climate conditions at recharge are atmospheric pCO₂ and infiltration temperatures, influencing C isotope concentrations in recharge waters. The new method is applied to the Ledo-Paniselian Aquifer in Belgium. This is a typical freshening aquifer where recharge takes place through the semi-confining cover of the Bartonian Clay. Besides cation exchange which is the major influencing process for the evolution of groundwater chemistry (particularly in the Bartonian Clay), also mixing with the original porewater solution (fossil seawater) occurs in the aquifer. Recharge temperatures were based on noble gas measurements. Potential infiltration water compositions, for a range of possible pCO₂, temperature and calcite dissolution system conditions, were calculated by means of PHREEQC. Then the sampled groundwaters were modelled starting from these infiltration waters, using the computer code NETPATH and considering a wide range of geochemical processes. Fitting models were selected on the basis of correspondence of calculated δ¹³C with measured δ¹³C. The ¹⁴C modelling resulted in residence times ranging from Holocene to Pleistocene (few hundred years to over 40 ka) and yielded consistent results within the uncertainty estimation. Comparison was made with the δ¹³C and Fontes and Garnier correction models, that do not take climate conditions at recharge into account. To date these are considered as the most representative process-oriented existing models, yet differences in calculated residence times of mostly several thousands of years (up to 19 ka) are revealed with the newly calculated ages being mostly (though not always) younger. Not accounting for climate conditions at recharge (pCO₂ and temperature) is thus producing substantial error on deduced residence times. The derived ¹⁴C model ages are correlated with He concentrations measured in the groundwater of the aquifer. The obtained residence times show a gap between about 14 and 21 ka indicating possible permafrost conditions which inhibited any groundwater recharge.     

Investigation by multivariate analysis of groundwater composition in a multilayer aquifer system from North Africa: A multi-tracer approach

A multi-tracer approach has been carried out in the Sbeïtla multilayer aquifer system, central Tunisia, to investigate the geochemical evolution, the origin of groundwaters and their circulation patterns. It involves statistical data analysis coupled with the definition of the hydrochemical and isotopic features of the different groundwaters. Principal Components Analysis (PCA) of geochemical data used in conjunction with bivariate diagrams of major and trace elements indicate that groundwater mineralization is mainly controlled by water-rock interaction and anthropogenic processes in relation to return flow of irrigation waters. The PCA of isotopic data and bivariate conventional diagrams of stable and radiogenic isotopes i.e. δ¹⁸O vs. δ²H and δ¹⁸O vs. ¹⁴C provide valuable information about the origin and the circulation patterns of the different groundwater groups. They permit classifying groundwaters into three groups. The first group is characterized by low ³H concentrations, low ¹⁴C activities and depleted stable isotope contents. It corresponds to an old end-member in relation with palaeoclimatic recharge which occurred during the Late Pleistocene and the Early Holocene humid periods. The second group is distinguished by high to moderate ³H concentrations, high ¹⁴C activities and enriched heavy isotope signatures. It corresponds to a modern end-member originating from a mixture of post-nuclear and present-day recharge in relation to return flow of irrigation waters. The third group is characterized by an average composition of stable and radiogenic isotope signatures. It provides evidence for the mixing between the upward moving palaeoclimatic end-member and the downward moving present-day end-member.     

Sediment-water exchange of dissolved nutrients at an intertidal site in the upper reaches of the Bay of Fundy

Concentrations of dissolved nitrate, silicate, and phosphate in water flooding intertidal sediments at Pecks Cove and along the axis of Cumberland Basin, Bay of Fundy were measured throughout the year. Exchanges of dissolved nutrients between intertidal sediments and overlying water were measured by enclosing water in chambers over undisturbed sediment. Nitrate concentrations in the water usually decreased during incubations while silicate was released by sediments during summer and consumed during fall. Particles which settled in sediment traps exposed during periods of high tide were stirred in filtered seawater to measure nutrient exchange. The flux of nutrients between the intertidal sediments and settled particles and seawater was estimated from incubation experiments and the observed nitrogen content in surface sediments and suspended particulate material. There was a net import of dissolved nitrate and silicate into Cumberland Basin from Chignecto Bay during early summer, at all other times there was a net export. Despite the low primary productivity and rigorous physical environment, biological activity has a measurable impact on dissolved nutrient concentrations in the waters of Cumberland Basin.     

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.     

Diagenetic processes in Holocene carbonate sediments: Florida Bay mudbanks and islands

The sedimentology, mineralogy and pore fluid chemistry of seven cores from the Holocene sediments of Florida Bay were studied to determine the physical processes and diagenetic reactions affecting the sediments. The cores were taken in a transect from a shallow mudbank onto a small adjacent island, Jimmy Key. Steady state models of pore fluid chemistry are used to estimate the rates of various reactions. In the mudbank sediments, little carbonate mineral diagenesis is taking place. No change in sediment mineralogy is detectable and pore water profiles of Ca²⁺, Mg²⁺ and Sr²⁺ show only minor variation. Chloride concentrations indicate substantial biological mixing of seawater from the bay into the sediments in one of the cores. Pore water analyses of sulphate and alkalinity show only a low degree of sulphate depletion and a decreasing extent of sulphate reduction downcore. Models of sulphate reduction in the mudbank show that there is substantial chemical exchange between the sediment pore fluids and water from the bay probably as a result of bio-irrigation. The sulphate and alkalinity data also suggest that the underlying Pleistocene rocks contain water of near normal seawater composition. Stratigraphic analysis and δ¹³C analyses of the organic carbon in the sediments of the island cores show that the sediments were primarily deposited in a subtidal mudbank setting; only the upper 20–30 cm is supratidal in origin. Nevertheless, island formation had a significant effect on pore fluid chemistry and the types of diagenetic reactions throughout the sediment column. Chloride in the sediment pore fluids is more than twice the normal seawater concentrations over most of the depth of the cores. The constant, elevated chloride concentrations indicate that hypersaline fluids which formed in ponds on the island are advected downward through the sediments. Models of the chloride profiles yield an estimate of 2·5 cm yr^?1 as a minimum advective velocity. Changes in pore water chemistry with depth are interpreted as indicating the following sequence of reactions: (1) minor high-Mg calcite dissolution and low-Mg calcite precipitation, from 0 to 35 cm; (2) Ca- or Mg-sulphate dissolution and low-Mg calcite precipitation, from 5 to 35 cm; (3) dolomite or magnesite precipitation together with sulphate reduction, from 35 to 55 cm; and (4) little reaction below 55 cm. In addition, one or more as yet unidentified reactions must be taking place from 5 to 55 cm depth as an imbalance in possible sources and sinks of alkalinity is observed. The imbalance could be explained if chloride is not completely conservative. Despite the pore fluid chemical evidence for diagenetic reactions involving carbonate minerals, no changes in sediment mineralogy were detected in X-ray diffraction analyses, probably because of the comparatively young age of the island.     

环境同位素在北京平谷盆地山前侧向补给研究中的应用

利用地下水水化学和同位素测试分析成果,结合区域地质、水文地质条件研究了平谷北山山区侧向补给情况和中桥水源地地区第四系松散孔隙水和下伏岩溶水关系。结果表明：研究区第四系松散孔隙水和基岩岩溶地下水均来源于大气降水,地下水化学类型均为HCO-3-Ca2+?Mg2+ 型;平谷北山山前基岩岩溶水侧向补给平原区第四系松散孔隙水和下伏岩溶地下水;通过D值估算得到中桥水源地第四系浅层地下水的山区岩溶水侧向补给和垂向降水入渗补给比例为57：43;中桥水源地基岩岩溶水接受山区岩溶水侧向补给和第四系孔隙水垂向越流补给比例为87：13。研究成果为平谷地区地下水资源量评价和地下水动力场数值模型的建设提供了关键参数,为区域地下水的合理开采和有序回补涵养提供了科学依据。     

Relation between colloid composition and the environment of their formation: application to the El Berrocal site (Spain)

In the framework of nuclear waste topics, the role of colloids is more and more emphasized in relation to the transport of radioelements in natural groundwaters. We attempt here to check if the nature and physico-chemical characteristics of the colloids from the granitic site of El Berrocal are closely related to the bedrock composition.The colloids are studied in three types of groundwaters (sulfate and carbonate waters in different redox conditions) as well as colloidal suspensions extracted from clayey materials present in the fractures. The particle size distributions are performed by photon correlation spectroscopy. Their chemical composition is obtained by X-ray microanalysis after transversal ultrafiltration.The main results deal with the chemical composition of colloids. They are mainly composed of silica, aluminosilicate phases, and Fe, Ti, Al oxyhydroxides. In the case of the carbonate and oxidized hydrofacies, traces of Mg, Cu, Zn, Ba, S as well as carbonate phases (Zn, Ca) occur. In the particular case of the sulfate hydrofacies, traces of S, Mg, Cu are identified. Finally in the less oxidized and carbonate groundwater, a lot of phases consist of sulphur with Cu, Zn, Fe as well as of carbonate with Cu, Zn, Fe, Ca and Mg. For the clayey suspensions, colloids are mainly composed of illite and silica.In conclusion, the contribution of the bedrock as well as the influence of the physico-chemical conditions are discussed in order to decipher the role of neoformation and dispersion processes in the colloid formation.     

Hydrogeochemical and isotopic evolution of water in the Complexe Terminal aquifer in the Algerian Sahara

The hydrogeochemical and isotopic evolution of groundwaters in the Mio–Pliocene sands of the Complexe Terminal (CT) aquifer in central Algeria are described. The CT aquifer is located in the large sedimentary basin of the Great Oriental Erg. Down-gradient groundwater evolution is considered along the main representative aquifer cross section (south–north), from the southern recharge area (Tinrhert Plateau and Great Oriental Erg) over about 700 km. Groundwater mineralisation increases along the flow line, from 1.5 to 8 g l^?1, primarily as a result of dissolution of evaporite minerals, as shown by Br/Cl and strontium isotope ratios. Trends in both major and trace elements demonstrate a progressive evolution along the flow path. Redox reactions are important and the persistence of oxidising conditions favours the increase in some trace elements (e.g. Cr) and also NO₃ ^?, which reaches concentrations of 16.8 mg l^?1 NO₃-N. The range in ¹⁴C, 0–8.4 pmc in the deeper groundwaters, corresponds with late Pleistocene recharge, although there then follows a hiatus in the data with no results in the range 10–20 pmc, interpreted as a gap in recharge coincident with hyper-arid but cool conditions across the Sahara; groundwater in the range 24.7–38.9 pmc signifies a distinct period of Holocene recharge. All δ¹⁸O compositions are enriched relative to deuterium and are considered to be derived by evaporative enrichment from a parent rainfall around ?11‰ δ¹⁸O, signifying cooler conditions in the late Pleistocene and possibly heavy monsoon rains during the Holocene.     

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.     

Carbonate weathering and connate seawater influencing karst groundwaters in the Gevas–Gurpinar–Güzelsu basins,Turkey

There are 59 springs at the Gevas–Gurp?nar–Güzelsu basins, 38 of these springs emerge from the fractured karst aquifers (recrystallized limestone and travertine) and 21 emerge from the Yuksekova ophiolites, K?rkgeçit formation and alluvium. The groundwater samples collected from 38 out of the total of 59 springs, two streams, one lake and 12 wells were analyzed physico-chemically in the year 2002. EC and TDS values of groundwater increased from the marble (high altitude) to the ophiolites and alluvium (toward Lake Van) as a result of carbonate dissolution and connate seawater. Five chemical types of groundwater are identified: Ca–Mg–HCO₃, Mg–Ca–HCO₃, Mg–Na–HCO₃, Na–Ca–HCO₃ and Mg–Ca–Na–HCO₃. The calculations and hydrochemical interpretations show that the high concentrations of Ca²⁺, Mg²⁺ and HCO₃ ^? as predominant ions in the waters are mainly attributed to carbonate rocks and high pCO₂ in soil. Most of the karst springs are oversaturated in calcite, aragonite and dolomite and undersaturated in gypsum, halite and anhydrite. The water–rock interaction processes that singly or in combination influence the chemical composition of each water type include dissolution of carbonate (calcite and dolomite), calcite precipitation, cation exchange and freshening of connate seawater. These processes contribute considerably to the concentration of major ions in the groundwater. Stable isotope contents of the groundwater suggest mainly direct integrative recharge.     

Holocene estuarine sediments as a source of arsenic in Pleistocene groundwater in suburbs of Hanoi,Vietnam

Groundwater pollution by arsenic is a major health threat in suburban areas of Hanoi, Vietnam. The present study evaluates the effect of the sedimentary environments of the Pleistocene and Holocene deposits, and the recharge systems, on the groundwater arsenic pollution in Hanoi suburbs distant from the Red River. At two study sites (Linh Dam and Tai Mo communes), undisturbed soil cores identified a Pleistocene confined aquifer (PCA) and Holocene unconfined aquifer (HUA) as major aquifers, and Holocene estuarine and deltaic sediments as an aquitard layer between the two aquifers. The Holocene estuarine sediments (approximately 25–40 m depth, 9.6–4.8 cal ka BP) contained notably high concentrations of arsenic and organic matter, both likely to have been accumulated by mangroves during the Holocene sea-level highstand. The pore waters in these particular sediments exhibited elevated levels of arsenic and dissolved organic carbon. Arsenic in groundwater was higher in the PCA (25–94 μg/L) than in the HUA (5.2–42 μg/L), in both the monitoring wells and neighboring household tubewells. Elevated arsenic concentration in the PCA groundwater was likely due to vertical infiltration through the arsenic-rich and organic-matter-rich overlying Holocene estuarine sediments, caused by massive groundwater abstraction from the PCA. Countermeasures to prevent arsenic pollution of the PCA groundwater may include seeking alternative water resources, reducing water consumption, and/or appropriate choice of aquifers for groundwater supply.     

Hydrogeochemical investigation of surface and groundwater composition in an irrigated land in Central Tunisia

Major element concentrations, stable (δ¹⁸O and δ²H) and radiogenic (³H, ¹⁴C) isotopes determined in groundwater provided useful initial tracers for understanding the processes that control groundwater mineralization and identifying recharge sources in semi-arid Cherichira basin (central Tunisia).Chemical data based on the chemistry of several major ions has revealed that the main sources of salinity in the groundwaters are related to the water–rock interaction such as the dissolution of evaporitic and carbonate minerals and some reactions with silicate and feldspar minerals.The stable isotope compositions provide evidence that groundwaters are derived from recent recharge. The δ¹⁸O and δ²H relationships implied rapid infiltration during recharge to both the Oligocene and Quaternary aquifers, with only limited evaporation occurring in the Quaternary aquifer.Chemical and isotopic signatures of the reservoir waters show large seasonal evolution and differ clearly from those of groundwaters.Tritium data support the existence of recent recharge in Quaternary groundwaters. But, the low tritium values in Oligocene groundwaters are justified by the existence of clay lenses which limit the infiltration of meteoric water in the unsaturated zone and prolong the groundwater residence time.Carbon-14 activities confirm that groundwaters are recharged from the surface runoff coming from precipitation.     

Geochemical evolution of groundwater in carbonate aquifers in Taiyuan, northern China

Thirty-nine samples of both cold and thermal karst groundwater from Taiyuan, northern China were collected and analyzed with the aim of developing a better understanding of the geochemical processes that control the groundwater quality evolution in the region’s carbonate aquifers. The region’s karst groundwater system was divided into three geologically distinct sub-systems, namely, the Xishan Mountain karst groundwater subsystem (XMK), the Dongshan Mountain karst groundwater subsystem (DMK) and the Beishan Mountain karst groundwater subsystem (BMK). Hydrochemical properties of the karst groundwaters evolve from the recharge zones towards the cold water discharge zones and further towards the thermal water discharge zones. In the XMK and the DMK, the hydrochemical type of the groundwater evolves from HCO₃-Ca·Mg in the recharge - flow-through zone, to HCO₃·SO₄-Ca·Mg/SO₄·HCO₃-Ca·Mg in the cold water discharge zone, and further to SO₄-Ca·Mg in the thermal water discharge zone. By contrast, the water type changes from HCO₃-Ca·Mg to HCO₃·SO₄-Ca·Mg in the BMK, with almost invariable TDS and temperatures all along from the recharge to the discharge zone. The concentrations of Sr, Si, Fe, F⁻ and of some trace elements (Al, B, Li, Mn, Mo, Co, Ni) increase as groundwater temperature increases. Different hydrogeochemical processes occur in the three karst groundwater sub-systems. In the XMK and the DMK, the geochemical evolution of the groundwater is jointly controlled by carbonate dissolution/precipitation, gypsum dissolution and dedolomitization, while only calcite and dolomite dissolution/precipitation occurs in the BMK without dedolomitization. The hydrogeochemical data of the karst groundwaters were used to construct individual geochemical reaction models for each of the three different karst groundwater sub-systems. The modeling results confirm that dedolomization is the major process controlling hydrochemical changes in the XMK and the DMK. In the thermal groundwaters, the dissolution rates of fluorite, siderite and strontianite were found to exceed those of the cold karst groundwater systems, which can explain the higher concentrations of F⁻, Fe and Sr²⁺ that are found in these waters.     

The importance of unsaturated zone biogeochemical processes in determining groundwater composition, southeastern Australia

Analysis of soil, soil water and groundwater in the Mount William Creek catchment, southeastern Australia, shows that Mg²⁺ and Ca²⁺ within infiltrating rainfall are rapidly depleted by plant uptake and adsorption on clay minerals. Na⁺ and K⁺ may exhibit minor enrichment at shallow depths but are quickly readsorbed, so that cation/Cl^– ratios typical of groundwater are observed in soil water within the upper 200 cm of the soil profile for all species. The concentrations of K⁺ and Ca²⁺ in soil and groundwater are more depleted than Na⁺ and Mg²⁺ due to preferential uptake by vegetation. Removal of organic matter results in a continuing, long-term export of all major cations from the soil profiles. The processes of biogeochemical fractionation within the unsaturated zone rapidly modify the cation/Cl^– ratios of infiltrating rainfall to values characteristic of seawater. These mechanisms may have reached steady state, because groundwaters with seawater ion/Cl^– ratios are thousands of years old; the exchange sites on the soil clays are probably saturated, so cations supplied in rainfall are exported in organic matter and incorporated into recharge infiltrating into the groundwater. Much of the chemical evolution of groundwater traditionally attributed to processes within the aquifer is complete by the time recharge occurs; this evolutionary model may have broad application.     

An assessment of the origin and variation of groundwater salinity in southeastern Ghana

Groundwater from the major aquifers in southeastern part of Ghana was sampled to determine the main controls on groundwater salinity in the area. This paper uses multivariate statistical methods, conventional graphical methods and stable isotope data to determine spatial relationships among groundwaters from the different hydrogeologic units in the area on the basis of salinity. Q-mode hierarchical cluster analysis (HCA) was used to spatially classify the samples, whilst R-mode factor analysis was used to reduce the dataset into two major principal components representing the sources of variation in the hydrochemistry. Analysis of the major chemical parameters suggests that the principal component responsible for salinity increment in the area is the weathering of minerals in the aquifers. This factor is especially more significant in the upland areas away from the coast. The second factor responsible for salinity in the area is the combined effects of seawater intrusion, and anthropogenic activities. This study finds that four major spatial groundwater groups exist in the area: low salinity, acidic groundwaters which are mainly derived from the Birimian and Togo Series aquifers; low salinity, moderate to neutral pH groundwaters which are mainly from the Voltaian, Buem and Cape Coast granitoids; very high salinity waters which are not suitable for most domestic and irrigation purposes and are mainly from the Keta aquifers; and intermediate salinity groundwaters comprising groundwater from the Keta basin aquifers with minor contributions from the other major terrains. The major water type identified in this study is the Ca–Mg–HCO₃ type, which degrades into predominantly Na–Cl–SO₄ more saline groundwaters toward the coast. Stable isotope data analyses suggest that groundwater in the Voltaian aquifers is largely of recent meteoric origin. The Birimian and Togo aquifers receive a component of recharge from the tributaries of the Densu and Volta Rivers, after the waters have undergone evaporative enrichment of the heavier isotopes. In the Keta basin, recharge is mainly from precipitation but an observed enrichment of ²H and ¹⁸O isotopes is probably due to seawater and evaporative effects since the water table there is very shallow. An analysis of the irrigation quality of groundwater from the six aquifers in the study area using sodium adsorption ratio and electrical conductivity suggests that most of the aquifers supply groundwater of acceptable quality for irrigation. The only exception is the Keta Basin area, where extremely high salinities and SAR values render groundwater from this basin unsuitable for irrigation purposes.     

Do Archean chemical sediments record ancient seawater rare earth element patterns?

Rare earth elements (REE) concentrations of Archean and Proterozoic chemical sediments are commonly used as proxies to study secular trends in the geochemistry of Precambrian seawater. In addition, similarities in the REE signatures of Archean chemical sediments and modern seawater have led researchers to argue that some Archean rocks originated as biochemical precipitates (i.e., microbial carbonates) in shallow marine (e.g., peritidal) environments. However, terrestrial waters, including river water and groundwater, also commonly exhibit REE fractionation patterns that resemble modern seawater. Here, we present the seawater-like REE data for groundwaters from central México as additional evidence that these patterns are not unique to the marine environment. The shale-normalized REE patterns of the groundwaters are compared to those of modern seawater (open ocean and nearshore), Holocene reefal microbial carbonates and corals, and Archean chemical sediments using statistical means (i.e., ANOVA and Wilcoxon analyses) in order to quantify the similarities and/or differences in the REE patterns. Shale-normalized (SN) Ce anomalies and measures of REE fractionation [i.e., (La/Yb)_SN, (Pr/Yb)_SN, (Nd/Yb)_SN, and (Gd/Yb)_SN] of the central México groundwater samples are statistically indistinguishable from those of modern seawater. Moreover, except for differences in the Ce anomalies, which are lacking in Archean chemical sediments, the REE patterns of the central México groundwaters are also statistically similar to REE patterns of Archean chemical sediments, especially those of the 3.45 Ga Strelley Pool Chert. Consequently, we suggest that without additional information, it may be premature to unequivocally conclude that Archean chemical sediments record REE signatures of an Archean ocean.     

Residence time of Chalk groundwaters in the Paris Basin and the North German Basin: a geochemical approach

The comparative geochemical and isotopic study of confined and unconfined Chalk groundwaters of the Paris Basin and the N German Basin proves a significant chemical evolution during groundwater flow from the recharge zones to the deep confined aquifer. Different time dependent geochemical parameters have been tested as dating tools: Cation ratios (Sr²⁺/Ca²⁺, Mg²⁺/Ca²⁺), N–NO⁻₃, noble gas contents as paleotemperature indicators (Ne, Ar, Kr, Xe), radiogenic He, ¹³C, ¹⁴C, ¹⁸O, ²H, ³H. Cation ratios and ¹³C show the importance of incongruent dissolution processes in the Chalk aquifer. Water–rock interactions were taken into account in a multi-step dissolution model to determine radiocarbon groundwater ages. The oldest waters in the confined part of the Paris basin Chalk with maximum ¹⁴C ages of 14,000 a B.P. contain pleistocene recharge components as can be shown by a stable isotope depletion and noble gas temperatures significantly lower than in recent groundwaters. Chalk waters at the Lägerdorf site in Northern Germany show a distinct stratification with respect to residence times and hydrochemistry.