Application of carbon isotopes to detect seepage out of coalbed natural gas produced water impoundments

Coalbed natural gas (CBNG) production from coal bed aquifers requires large volumes of produced water to be pumped from the subsurface. The produced water ranges from high quality that meets state and federal drinking water standards to low quality due to increased salinity and/or sodicity. The Powder River Basin of northeastern Wyoming is a major coalbed natural gas producing region, where water quality generally decreases moving from the southeastern portion of the basin towards the center. Most produced water in Wyoming is disposed into impoundments and other surface drainages, where it may infiltrate into shallow groundwater. Groundwater degradation caused by infiltration of CBNG produced water holding impoundments into arid, soluble salt-rich soils is an issue of immense importance because groundwater is a major source for stock water, irrigation, and drinking water for many small communities in these areas. This study examines the potential of using stable C isotope signatures of dissolved inorganic C (δ¹³C_DIC) to track the fate of CBNG produced water after it is discharged into the impoundments. Other geochemical proxies like the major cations and major anions were used in conjunction with field water quality measurements to understand the geochemical differences between CBNG produced waters and ambient waters in the study area. Samples were collected from the CBNG discharge outfalls, produced water holding impoundments, and monitoring wells from different parts of the Powder River Basin and analyzed for δ¹³C_DIC. The CBNG produced waters from outfalls and impoundments have positive δ¹³C_DIC values that fall within the range of +12‰ to +22‰, distinct from the ambient regional surface and groundwaters with δ¹³C_DIC values ranging from −10‰ to −14‰. The results from the study demonstrate that these contrasting δ¹³C_DIC signatures can be used to trace seepage out of CBNG produced water impoundments into shallow groundwaters.     

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.     

Basin hydrogeological characterization using remote sensing,hydrogeochemical and isotope methods (the case of Baro-Akobo,Eastern Nile,Ethiopia)

Hydrogeochemical and isotopic signatures of the waters of the Baro-Akobo River Basin show deviation from signatures in other Ethiopian river basins. In this study, hydrogeochemical and isotope methods were employed to determine regional and local hydrogeology and characteristics of the basin. Optical, thermal and radar remote sensing products were used to update geological and structural maps of the basin and determine sampling points using the judgment sampling method. A total of 363 samples from wells, springs, rivers, lakes, swamps and rain were collected for this study, and an additional 270 water quality data sets were added from previous studies. These data were analyzed for their hydrogeochemical characteristics and isotope signatures. Analysis of the oxygen, deuterium and tritium isotopes shows the groundwater of the basin is modern water. Among all basins in Ethiopia, the Baro-Akobo Basin shows the highest enrichment. This indicates the proximity of the rainfall sources, which presumably are the Sudd and other wetlands in South Sudan. The hydrochemical properties of the waters show evapotranspiration is the dominant hydrologic process in the basin and explains the large amount of water that is lost in the lowland plain. Analysis of radon-222 shows no significant groundwater flux over the wetlands, which are part of Machar Marshes. This shows evaporation to be dominant hydrologic process in this zone. Results from all analyses help explain the limited holding capacity of the aquifers in the recharge zone and their vulnerability to anthropogenic impacts and climate variability. There is a trend of decreasing surface flow and rainfall and increasing water soil erosion.     

Hydrogeochemical parameters for assessment of groundwater quality in the upper Gunjanaeru River basin,Cuddapah District,Andhra Pradesh,South India

In the management of water resources, quality of water is just as important as its quantity. In order to know the quality and/or suitability of groundwater for domestic and irrigation in upper Gunjanaeru River basin, 51 water samples in post-monsoon and 46 in pre-monsoon seasons were collected and analyzed for various parameters. Geological units are alluvium, shale and quartzite. Based on the analytical results, chemical indices like percent sodium, sodium adsorption ratio, residual sodium carbonate, permeability index (PI) and chloroalkaline indices were calculated. The pre-monsoon waters have low sodium hazard as compared to post-monsoon season. Residual sodium carbonate values revealed that one sample is not suitable in both the seasons for irrigation purposes due the occurrence of alkaline white patches and low permeability of the soil. PI values of both seasons revealed that the ground waters are generally suitable for irrigation. The positive values of Chloroalkaline indices in post-monsoon (80%) and in pre-monsoon (59%) water samples indicate absence of base-exchange reaction (chloroalkaline disequilibrium), and remaining samples of negative values of the ratios indicate base-exchange reaction (chloroalkaline equilibrium). Chadha rectangular diagram for geochemical classification and hydrochemical processes of groundwater for both seasons indicates that most of waters are Ca–Mg–HCO₃ type. Assessment of water samples from various methods indicated that majority of the water samples in both seasons are suitable for different purposes except at Yanadipalle (sample no. 8) that requires precautionary measures. The overall quality of groundwater in post-monsoon season in all chemical constituents is on the higher side due to dissolution of surface pollutants during the infiltration and percolation of rainwater and at few places due to agricultural and domestic activities.     

Hydrogeologic controls on groundwater recharge and salinization: a geochemical analysis of the northern Hueco Bolson aquifer, Texas, USA

Identification of hydrogeologic controls on groundwater flowpaths, recharge, and salinization is often critical to the management of limited arid groundwater resources. One approach to identifying these mechanisms is a combined analysis of hydrogeologic and hydrochemical data to develop a comprehensive conceptual model of a groundwater basin. To demonstrate this technique, water samples were collected from 33 discrete vertical zone test holes in the Hueco Bolson aquifer, located within the Trans-Pecos Texas region and the primary water resource for El Paso, Texas, USA and Juárez, Mexico. These samples were analyzed for a suite of geochemical tracers and the data evaluated in light of basin hydrogeology. On the basis of δ²H and δ¹⁸O data, two regional recharge sources were recognized, one originating from western mountain-fronts and one from through-flow of the adjacent Tularosa aquifer. Chloride concentrations were strongly correlated with lithologic formations and both Cl/Br and ³⁶Cl ratios suggested the primary chloride source is halite dissolution within a specific lithologic unit. In contrast, sulfur isotopes indicated that most sulfate originates from Tularosa basin Permian gypsum sources. These results yielded a more comprehensive conceptual model of the basin, which suggested that chloride salinization of wells is the result of upconing of waters from the Fort Hancock formation.     

A Geochemical study of the groundwater in the Misli basin and environmental implications

The aim of this study was to determine geochemical properties of groundwater and thermal water in the Misli Basin and to assess thermal water intrusion into shallow groundwater due to over-extraction. According to isotope and hydrochemical analyses results, sampled waters can be divided into three groups: cold, thermal, and mixed waters. Only a few waters reach water–rock chemical equilibrium. Thermal waters in the area are characterized by Na⁺–Cl⁻–HCO₃⁻, while the cold waters by CaHCO₃ facies. On the basis of isotope results, thermal waters in the Misli basin are meteoric origin. In particular, δ¹⁸O and δ²H values of shallow groundwater vary from −10.2 to −12.2‰ and −71.2 to −82‰, while those of thermal waters range from −7.8 to −10.1‰ and from −67 to −74‰, respectively. The tritium values of shallow groundwater having short circulation as young waters coming from wells that range from 30 to 70 m in depth vary from 10 to 14 TU. The average tritium activity of groundwater in depths more than 100 m is 1.59 ± 1.16, which indicates long circulation. The rapid infiltration of the precipitation, the recycling of the evaporated irrigation water, the influence of thermal fluids and the heterogeneity of the aquifer make it difficult to determine groundwater quality changes in the Misli Basin. Obtained results show that further lowering of the groundwater table by over-consumption will cause further intrusion of thermal water which resulted in high mineral content into the fresh groundwater aquifer. Because of this phenomenon, the concentrations of some chemical components which impairs water quality in terms of irrigation purposes in shallow groundwaters, such as Na⁺, B, and Cl⁻, are highy probably expected to increase in time.     

Regional groundwater flow and geochemical evolution in the Amacuzac River Basin,Mexico

An approach is presented to investigate the regional evolution of groundwater in the basin of the Amacuzac River in Central Mexico. The approach is based on groundwater flow cross-sectional modeling in combination with major ion chemistry and geochemical modeling, complemented with principal component and cluster analyses. The hydrogeologic units composing the basin, which combine aquifers and aquitards both in granular, fractured and karstic rocks, were represented in sections parallel to the regional groundwater flow. Steady-state cross-section numerical simulations aided in the conceptualization of the groundwater flow system through the basin and permitted estimation of bulk hydraulic conductivity values, recharge rates and residence times. Forty-five water locations (springs, groundwater wells and rivers) were sampled throughout the basin for chemical analysis of major ions. The modeled gravity-driven groundwater flow system satisfactorily reproduced field observations, whereas the main geochemical processes of groundwater in the basin are associated to the order and reactions in which the igneous and sedimentary rocks are encountered along the groundwater flow. Recharge water in the volcanic and volcano-sedimentary aquifers increases the concentration of HCO₃ ^–, Mg²⁺ and Ca²⁺ from dissolution of plagioclase and olivine. Deeper groundwater flow encounters carbonate rocks, under closed CO₂ conditions, and dissolves calcite and dolomite. When groundwater encounters gypsum lenses in the shallow Balsas Group or the deeper Huitzuco anhydrite, gypsum dissolution produces proportional increased concentration of Ca²⁺ and SO₄ ^2–; two samples reflected the influence of hydrothermal fluids and probably halite dissolution. These geochemical trends are consistent with the principal component and cluster analyses.     

Age and origin of groundwater resources in the Ararat Valley,Armenia: a baseline study applying hydrogeochemistry and environmental tracers

Within the Ararat Valley (Armenia), a continuously growing water demand (for irrigation and fish farming) and a simultaneous decline in groundwater recharge (due to climate change) result in increasing stress on the local groundwater resources. This detrimental development is reflected by groundwater-level drops and an associated reduction of the area with artesian conditions in the valley centre. This situation calls for increasing efforts aimed at more sustainable water resources management. The aim of this baseline study was the collection of data that allows for study on the origin and age distribution of the Ararat Valley groundwater based on environmental tracers, namely stable (δ²H, δ¹⁸O) and radioactive (³⁵S, ³H) isotopes, as well as physical-chemical indicators. The results show that the Ararat Valley receives modern recharge, despite its (semi-)arid climate. While subannual groundwater residence times could be disproved (³⁵S), the detected ³H pattern suggests groundwater ages of several decades, with the oldest waters being recharged around 60 years ago. The differing groundwater ages are reflected by varying scatter of stable isotope and hydrochemical signatures. The presence of young groundwater (i.e., younger that the 1970s), some containing nitrate, indicates groundwater vulnerability and underscores the importance of increased efforts to achieve sustainable management of this natural resource. Since stable isotope signatures indicate the recharge areas to be located in the mountains surrounding the valley, these efforts must not be limited to the central part of the valley where most of the abstraction wells are located.

     

Groundwater resources of the middle and upper Odra River basin, southwestern Poland

 The total amount of groundwater resources in the middle and upper Odra River basin is 5200×10³ m³/d, or about 7.7% of the disposable groundwater resources of Poland. The average modulus of groundwater resources is about 1.4 L/s/km². Of the 180 'Major Groundwater Basins' (MGWB) in Poland, 43 are partly or totally located within the study area. The MGWB in southwestern Poland have an average modulus of groundwater resources about 2.28 L/s/km² and thus have abundant water resources in comparison to MGWB from other parts of the country. Several types of mineral waters occur in the middle and upper Odra River basin. These waters are concentrated especially in the Sudety Mountains. Carbon-dioxide waters, with yields of 414 m³/h, are the most widespread of Sudetic mineral waters. The fresh waters of the crystalline basement have a low mineralization, commonly less than 100 mg/L; they are a HCO₃–Ca–Mg or SO₄–Ca–Mg type of water. Various hydrochemical compositions characterize the groundwater in sedimentary rocks. The shallow aquifers are under risk of atmospheric pollution and anthropogenic effects. To prevent the degradation of groundwater resources in the middle and upper Odra River basin, Critical Protection Areas have been designated within the MGWB. Received, January 1995 Revised, May 1996, August 1997 Accepted, August 1997     

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.

     

Characterising groundwater and surface-water interconnections using hydrogeology,hydrochemistry and stable isotopes in the Ouémé Delta,southern Benin

The Mio-Pliocene aquifer of the coastal sedimentary basin of Benin is the most exploited aquifer for water supply to the urbanised region in the southern part of the country. The population explosion is putting increasing pressure on quantitative and qualitative aspects of the groundwater resources. Preventing groundwater contamination caused by surface waters requires a thorough understanding of surface-water/groundwater interactions, especially the interactions between the Mio-Pliocene aquifer and surface waters. This study aimed to investigate the interactions between groundwater and surface waters along the major rivers (Sô River and Ouémé Stream) and brooks in the Ouémé Delta. Field campaigns identified 75 springs located in the valleys which feed the rivers, and thus maintain their base flow. The piezometric results indicated, through flow direction assessment, that the Mio-Pliocene aquifer feeds Ouémé Stream and Sô River. Chemical analyses of groundwater and surface waters show similar chemical facies, and changes in the chemical composition in groundwater are also observed in the surface waters. Moreover, the isotopic signatures of surface waters are similar to those of the groundwater and springs, which led to the identification of potential groundwater discharge areas. As a result of groundwater discharge into surface waters, the fraction of groundwater in the surface water is more than 66% in the brooks, regardless of the season. In the Ouémé Stream and Sô River, the fraction of groundwater is 0–21% between June and September, while from October to March it is 47–100%.

     

Groundwater recharge,circulation and geochemical evolution in the source region of the Blue Nile River,Ethiopia

Geochemical and environmental isotope data were used to gain the first regional picture of groundwater recharge, circulation and its hydrochemical evolution in the upper Blue Nile River basin of Ethiopia. Q-mode statistical cluster analysis (HCA) was used to classify water into objective groups and to conduct inverse geochemical modeling among the groups. Two major structurally deformed regions with distinct groundwater circulation and evolution history were identified. These are the Lake Tana Graben (LTG) and the Yerer Tullu Wellel Volcanic Lineament Zone (YTVL). Silicate hydrolysis accompanied by CO₂ influx from deeper sources plays a major role in groundwater chemical evolution of the high TDS Na–HCO₃ type thermal groundwaters of these two regions. In the basaltic plateau outside these two zones, groundwater recharge takes place rapidly through fractured basalts, groundwater flow paths are short and they are characterized by low TDS and are Ca–Mg–HCO₃ type waters. Despite the high altitude (mean altitude ∼2500 masl) and the relatively low mean annual air temperature (18 °C) of the region compared to Sahelian Africa, there is no commensurate depletion in δ¹⁸O compositions of groundwaters of the Ethiopian Plateau. Generally the highland areas north and east of the basin are characterized by relatively depleted δ¹⁸O groundwaters. Altitudinal depletion of δ¹⁸O is 0.1‰/100 m. The meteoric waters of the Blue Nile River basin have higher d-excess compared to the meteoric waters of the Ethiopian Rift and that of its White Nile sister basin which emerges from the equatorial lakes region. The geochemically evolved groundwaters of the YTVL and LTG are relatively isotopically depleted when compared to the present day meteoric waters reflecting recharge under colder climate and their high altitude.     

Geochemical techniques to identify fluid sources in highly pressured casing-casing annuli (CCA)

The buildup of high pressure in the casing-casing annulus (CCA) threatens well integrity and can cause serious incidents in case of left untreated. Either, trapped water in the cement column or a dynamic water inflow represent potential fluid sources for the elevated CCA pressure. This study presents a sequential methodology to determine the provenance of CCA effluent as trigger for high pressure in newly drilled wells. Single fluid types, multicomponent mixing, and secondary fluid alteration processes were identified through inorganic geochemical techniques; in detail by monitoring the hydrochemical (major, minor, and trace elements) and stable isotopic (δ²H, δ¹⁸O) relationship between fluid candidates. As a proof-of-concept, geochemical signatures of CCA effluent from three wells were linked with potential source candidates, i.e., utilized drilling fluids (mud filtrate, supply water) from the prospect well site, groundwater from Lower - Upper Cretaceous aquifers, and Upper Jurassic - Upper Triassic formation waters from adjacent wells and fields. The detection of geochemical affinities of CCA water with groundwater from a Lower Cretaceous aquifer postulates one single lithological unit as source for active groundwater inflow. Nonreactive elements (Na⁺, Cl⁻) and environmental isotopes (δ²H, δ¹⁸O) were found to be most suited tools for primary fluid identification. The ²H/¹H and ¹⁸O/¹⁶O ratios of supply water and mud filtrate are generally close to global meteoric water and Tertiary groundwater composition, while formation water from Mesozoic units (Cretaceous, Jurassic, and Triassic) can individually be distinguished through increasing ratios in δ¹⁸O and δ²H. Compositional anomalies in SO₄²⁻ and K⁺, and extreme alkaline conditions for CCA water indicate the occurrence of secondary fluid alteration processes, likely caused by the contact of inflowing groundwater with alkaline minerals in the cement column or by fluid mixing with residuals of potassium chloride (KCl) additives from the drilling process. The geochemical techniques from this study facilitate the detection of high CCA pressure and fluid leakages sources. As a practical benefit, workover engineers are enabled to plan for potential remedial actions prior to moving the rig to affected well sites; thereby significantly reducing operational costs. Appropriate remedial solutions can be induced for safe well abandonment, plus to resume operation at the earliest time.     

Hydrochemical signatures of deep groundwater circulation in a part of the Himalayan foreland basin

Groundwater samples were analyzed from 71 springs and wells as part of a larger study in a region of compressional tectonic regime. The study site covers the Peshawar basin and surroundings in the Himalayan foreland of Pakistan. The northern portion is mountainous and the water table is discontinuous in different intermontane valleys, with abundant springs (with normal and anomalous temperatures and composition). The southern part is divided into isolated basins with a number of drilled (“deep”) and dug (“shallow”) wells. Hydrochemical signatures of elevated strontium (Sr), SiO₂, boron (B)—and the geothermometric signatures—all indicate a deep circulation of the emerging groundwater. Moreover, for several of the sample sites, water chemical compositions, measured spring and water well temperatures, and reservoir temperatures calculated for spring waters, all point to origin from deep horizons within the basin. Remarkable proximity of all the thermal and hydrochemical anomalies to major faults suggests that the waters ascended along these faults from greater depths. The area is a natural western extension of the Himalayan Geothermal Belt described in earlier literature for the eastern and central Himalayas.     

Composition and origin of organic matter in surface sediments of Lake Sarbsko: A highly eutrophic and shallow coastal lake (northern Poland)

We present an organic geochemical study of surface sediments of Lake Sarbsko, a shallow coastal lake on the middle Polish Baltic coast. The aim was to provide evidence concerning the origin of the organic matter (OM) and its compositional diversity in surface deposits of this very productive, highly dynamic water body. The content and composition of the OM in the bottom sediments were investigated at 11 sampling stations throughout the lake basin. OM sources were assigned on the basis of bulk indicators [total organic carbon (TOC), total nitrogen (TN), δ¹³C_TOC and δ¹⁵N and extractable OM yield], biomarker composition of extractable OM and compound-specific C isotope signatures. The source characterization of autochthonous compounds was verified via phytoplankton analysis. The distribution of gaseous hydrocarbons in the sediments, as well as temporal changes in lake water pH, the concentration of DIC (dissolved inorganic carbon) and δ¹³C_DIC were used to trace OM decomposition.The sedimentary OM is composed mainly of well preserved phytoplankton compounds and shows minor spatial variability in composition. However, the presence of CH₄ and CO₂ in the bottom deposits provides evidence for microbial degradation of sedimentary OM. The transformation of organic compounds in surface, bottom and pore waters via oxidative processes influences carbonate equilibrium in the lake and seasonally favours precipitation or dissolution of CaCO₃.The data enhance our understanding of the relationships between the composition of sedimentary OM and environmental conditions within coastal ecosystems and shed light on the reliability of OM proxies for environmental reconstruction of coastal lakes.     

Hydrochemistry and hydrogen sulfide generating processes in the Malm aquifer,Bavarian Molasse Basin,Germany

Knowledge about the hydrochemical conditions of deep groundwater is crucial for the design and operation of geothermal facilities. In this study, the hydrochemical heterogeneity of the groundwaters in the Malm aquifer, Germany, is assessed, and reasons for the extraordinarily high H₂S concentrations in the central part of the Bavarian Molasse Basin are proposed. Samples were taken at 16 sites, for a total of 37 individual wells, to analyze cations, anions, gas loading and composition. The hydrochemical characteristics of the Malm groundwater in the center of the Molasse Basin are rather heterogeneous. Although the groundwater in the central basin is dominated by meteoric waters, there is a significant infiltration of saline water from higher strata. Care has to be taken in the interpretation of data from geothermal sites, as effects of chemical stimulation of the boreholes may not be fully removed before the final analyses. The distribution of H₂S in the gas phase is correlated to the gas loading of the water which increases in the central basin. Temperatures, isotopic data and the sulfur mass balance indicate that H₂S in the central basin is related to thermochemical sulfate reduction (south of Munich) and bacterial sulfate reduction (north of Munich).     

A geochemical and stable isotope investigation of groundwater/surface-water interactions in the Velenje Basin,Slovenia

The geochemical and isotopic composition of surface waters and groundwater in the Velenje Basin, Slovenia, was investigated seasonally to determine the relationship between major aquifers and surface waters, water–rock reactions, relative ages of groundwater, and biogeochemical processes. Groundwater in the Triassic aquifer is dominated by HCO₃ ^–, Ca²⁺, Mg²⁺ and δ¹³C_DIC indicating degradation of soil organic matter and dissolution of carbonate minerals, similar to surface waters. In addition, groundwater in the Triassic aquifer has δ¹⁸O and δD values that plot near surface waters on the local and global meteoric water lines, and detectable tritium, likely reflecting recent (<50 years) recharge. In contrast, groundwater in the Pliocene aquifers is enriched in Mg²⁺, Na⁺, Ca²⁺, K⁺, and Si, and has high alkalinity and δ¹³C_DIC values, with low SO₄ ^2– and NO₃ ^– concentrations. These waters have likely been influenced by sulfate reduction and microbial methanogenesis associated with coal seams and dissolution of feldspars and Mg-rich clay minerals. Pliocene aquifer waters are also depleted in ¹⁸O and ²H, and have ³H concentrations near the detection limit, suggesting these waters are older, had a different recharge source, and have not mixed extensively with groundwater in the Triassic aquifer.     

Indication of hydrogen and oxygen stable isotopes on the characteristics and circulation patterns of medium-low temperature geothermal resources in the Guanzhong Basin,China

Guanzhong Basin is a typical medium-low temperature geothermal field mainly controlled by geo-pressure in the west of China.The characteristics of hydrogen and oxygen isotopes were used to analyze the flow and storage modes of geothermal resources in the basin.In this paper,the basin was divided into six geotectonic units,where a total of 121 samples were collected from geothermal wells and surface water bodies for the analysis of hydrogen-oxygen isotopes.Analytical results show that the isotopic signatures of hydrogen and oxygen throughout Guanzhong Basin reveal a trend of gradual increase from the basin edge areas to the basin center.In terms of recharge systems,the area in the south edge belongs to the geothermal system of Qinling Mountain piedmont,while to the north of Weihe fault is the geothermal system of North mountain piedmont,where the atmospheric temperature is about 0.2℃-1.8℃in the recharge areas.The main factors that affect the geothermal waterδ18O drifting include the depth of geothermal reservoir and temperature of geothermal reservoir,lithological characteristics,water-rock interaction,geothermal reservoir environment and residence time.Theδ18O-δD relation shows that the main source is the meteoric water,together with some sedimentary water,but there are no deep magmatic water and mantle water which recharge the geothermal water in the basin.Through examining the distribution pattern of hydrogen-oxygen isotopic signatures,the groundwater circulation model of this basin can be divided into open circulation type,semi-open type,closed type and sedimentary type.This provides some important information for rational exploitation of the geothermal resources.     

Chemical evolution of groundwater in the Wilcox aquifer of the northern Gulf Coastal Plain,USA

The Wilcox aquifer is a major groundwater resource in the northern Gulf Coastal Plain (lower Mississippi Valley) of the USA, yet the processes controlling water chemistry in this clastic aquifer have received relatively little attention. The current study combines analyses of solutes and stable isotopes in groundwater, petrography of core samples, and geochemical modeling to identify plausible reactions along a regional flow path ～300 km long. The hydrochemical facies evolves from Ca-HCO₃ upgradient to Na-HCO₃ downgradient, with a sequential zonation of terminal electron-accepting processes from Fe(III) reduction through SO₄ ^2? reduction to methanogenesis. In particular, decreasing SO₄ ^2? and increasing δ³⁴S of SO₄ ^2? along the flow path, as well as observations of authigenic pyrite in core samples, provide evidence of SO₄ ^2? reduction. Values of δ¹³C in groundwater suggest that dissolved inorganic carbon is contributed both by oxidation of sedimentary organic matter and calcite dissolution. Inverse modeling identified multiple plausible sets of reactions between sampled wells, which typically involved cation exchange, pyrite precipitation, CH₂O oxidation, and dissolution of amorphous Fe(OH)₃, calcite, or siderite. These reactions are consistent with processes identified in previous studies of Atlantic Coastal Plain aquifers. Contrasts in groundwater chemistry between the Wilcox and the underlying McNairy and overlying Claiborne aquifers indicate that confining units are relatively effective in limiting cross-formational flow, but localized cross-formational mixing could occur via fault zones. Consequently, increased pumping in the vicinity of fault zones could facilitate upward movement of saline water into the Wilcox.     

Ion chemistry of groundwater and the possible controls within Lhasa River Basin,SW Tibetan Plateau

In order to study the major ion chemistry and controls of groundwater, 65 groundwater samples were collected and their major ions measured from wells within Lhasa River Basin. Groundwater has the characteristics of slightly alkaline and moderate total dissolved solid (TDS). TDS concentration ranged from 122.0 to 489.9 mg/L with a median value of 271.2 mg/L. Almost all the groundwater samples suited for drinking and irrigation. The major cations of groundwater are Ca²⁺ and Mg²⁺, accounting for 59.6 and 31.3% of the cations, respectively. Meanwhile, HCO₃^? and SO₄^2? constituted about 56.7 and 36.9% of the anions, respectively, in Lhasa River Basin. The hydrochemical type of groundwater is HCO₃-SO₄-Ca-Mg. The chemical composition of groundwater samples located in the middle of Gibbs model, which indicates that the major chemical process of groundwater is controlled by rock weathering. Carbonate weathering was the dominant hydro-geochemical process controlling the concentration of major ions in groundwater within Lhasa River Basin, but silicate weathering also plays an important role.