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1.
The Regional Deep Cretaceous Aquifer (RDCA) is the principal groundwater resource in Syria. Isotope and hydrochemical data have been used to evaluate the geographic zones in terms of renewable and non-renewable groundwater and the inter-relation between current and past recharge. The chemical and isotopic character of groundwater together with radiometric 14C data reflect the existence of three different groundwater groups: (1) renewable groundwater, in RDCA outcropping areas, in western Syria along the Coastal and Anti-Lebanon mountains. The mean δ 18O value (?7.2 ‰) is similar to modern precipitation with higher 14C values (up to 60–80 pmc), implying younger groundwater (recent recharge); (2) semi-renewable groundwater, which is located in the unconfined section of the RDCA and parallel to the first zone. The mean δ 18O value (?7.0 ‰) is also similar to modern precipitation with a 14C range of 15–45 pmc; (3) non-renewable groundwater found in most of the Syrian interior, where the RDCA becomes confined. A considerable depletion in δ 18O (?8.0 ‰) relative to the modern rainfall and low values of 14C (<15 pmc) suggest that the large masses of deep groundwater are non-renewable and related to an older recharge period. The wide scatter of all data points around the two meteoric lines in the δ 18O-δ 2H diagram indicates considerable variation in recharge conditions. There is limited renewable groundwater in the mountain area, and most of the stored deep groundwater in the RDCA is non-renewable, with corrected 14C ages varying between 10 and 35 Kyr BP. 相似文献
2.
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 δ 18O and intercepts the local meteoric water line at δ 18O = ?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. 相似文献
3.
Hundreds of precipitation samples collected from meteorological stations in the Ordos Basin from January 1988 to December 2005 were used to set up a local meteoric water line and to calculate weighted average isotopic compositions of modern precipitation. Oxygen and hydrogen isotopes, with averages of ?7.8‰ and ?53.0‰ for δ18O and δD, respectively, are depleted in winter and rich in spring, and gradually decrease in summer and fall, illustrating that the seasonal effect is considerable. They also show that the isotopic difference between south portion and north portion of the Ordos Basin are not obvious, and the isotope in the middle portion is normally depleted. The isotope compositions of 32 samples collected from shallow groundwater (less than a depth of 150 m) in desert plateau range from ?10.6‰ to ?6.0‰ with an average of ?8.4‰ for δ18O and from ?85‰ to ?46‰ with an average of ?63‰ for δD. Most of them are identical with modern precipitation. The isotope compositions of 22 middle and deep groundwaters (greater than a depth of 275 m) fall in ranges from ?11.6‰ to ?8.8‰ with an average of ?10.2‰ for δ18O and from ?89‰ to ?63‰ with an average of ?76‰ for δD. The average values are significantly less than those of modern precipitation, illustrating that the middle and deep groundwaters were recharged at comparatively lower air temperatures. Primary analysis of 14C shows that the recharge of the middle and deep groundwaters started at late Pleistocene. The isotopes of 13 lake water samples collected from eight lakes define a local evaporation trend, with a relatively flat slope of 3.77, and show that the lake waters were mainly fed by modern precipitation and shallow groundwater. 相似文献
4.
The Quaternary coastal plain aquifer down gradient of the Wadi Watir catchment is the main source of potable groundwater in the arid region of south Sinai, Egypt. The scarcity of rainfall over the last decade, combined with high groundwater pumping rates, have resulted in water-quality degradation in the main well field and in wells along the coast. Understanding the sources of groundwater salinization and amount of average annual recharge is critical for developing sustainable groundwater management strategies for the long-term prevention of groundwater quality deterioration. A combination of geochemistry, conservative ions (Cl and Br), and isotopic tracers ( 87/86Sr, δ 81Br, δ 37Cl), in conjunction with groundwater modeling, is an effective method to assess and manage groundwater resources in the Wadi Watir delta aquifers. High groundwater salinity, including high Cl and Br concentrations, is recorded inland in the deep drilled wells located in the main well field and in wells along the coast. The range of Cl/Br ratios for shallow and deep groundwaters in the delta (∼50–97) fall between the end member values of the recharge water that comes from the up gradient watershed, and evaporated seawater of marine origin, which is significantly different than the ratio in modern seawater (228). The 87/86Sr and δ 81Br isotopic values were higher in the recharge water (0.70,723 < 87/86Sr < 0.70,894, +0.94 < δ 81Br < +1.28‰), and lower in the deep groundwater (0.70,698 < 87/86Sr < 0.70,705, +0.22‰ < δ 81Br < +0.41‰). The δ 37Cl isotopic values were lower in the recharge water (−0.48 < δ 37Cl < −0.06‰) and higher in the deep groundwater (−0.01 < δ 37Cl < +0.22‰). The isotopic values of strontium, chloride, and bromide in groundwater from the Wadi Watir delta aquifers indicate that the main groundwater recharge source comes from the up gradient catchment along the main stream channel entering the delta. The solute-weighted mass balance mixing models show that groundwater in the main well field contains 4–10% deep saline groundwater, and groundwater in some wells along the coast contain 2–6% seawater and 18–29% deep saline groundwater.A three-dimensional, variable-density, flow-and-transport SEAWAT model was developed using groundwater isotopes ( 87Sr/ 86Sr, δ 37Cl and δ 81Br) and calibrated using historical records of groundwater level and salinity. δ 18O was used to normalize the evaporative effect on shallow groundwater salinity for model calibration. The model shows how groundwater salinity and hydrologic data can be used in SEAWAT to understand recharge mechanisms, estimate groundwater recharge rates, and simulate the upwelling of deep saline groundwater and seawater intrusion. The model indicates that most of the groundwater recharge occurs near the outlet of the main channel. Average annual recharge to delta alluvial aquifers for 1982 to 2009 is estimated to be 2.16 × 10 6 m 3/yr. The main factors that control groundwater salinity are overpumping and recharge availability. 相似文献
5.
A study was conducted at the Fresh Kills landfill, Staten Island, New York to investigate the use of B and Li isotopes as tracers of mixing and flow in the groundwater environment. Four end-member waters are present at the Fresh Kills: freshwater, seawater, a geochemically distinct transitional groundwater (that occurs in the zone of mixing between seawater and freshwater) and landfill leachate. The δ 11B and δ 6Li values of end-member waters are distinct and have isotopic compositions that reflect the solute sources: freshwater δ 11B∼+30‰, δ 6Li∼−22‰; transition zone groundwaters δ 11B∼+20‰, δ 6Li∼−27‰; seawater δ 11B+40 to +75‰, δ 6Li−37 to−44‰; leachate δ 11B∼+10‰ (δ 6Li not determined). Those wells influenced by seawater exhibited a clear chemical mixing trend, with seawater contributions ranging from 3 to 85%. Well waters with a high percentage of seawater (>30%) had δ 11B values that were within 1‰ of the seawater value (+40‰), whereas a trend of increasing δ 11B values (+55 to +75‰) was observed for wells with a lower percentage of seawater (<30%). δ 6Li values for well waters impacted by mixing with seawater ranged from−37 to−44‰, significantly more negative than pure seawater (−31‰). This deviation from the isotopic composition of seawater, for both δ 11B and δ 6Li values, represents non-conservative behavior and is likely the result of isotopic fractionation during ion exchange reactions. The wide range of δ 11B and δ 6Li values and the distinct isotopic compositions of end-member waters makes B and Li isotopes useful for recognizing solute sources, however isotopic fractionation may limit their use as simple tracers of groundwater flow and mixing. 相似文献
6.
The recharge and origin of groundwater and its residence time were studied using environmental isotopic measurements in samples from the Heihe River Basin, China. δ 18O 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=6.87δ 18O+3.54). This finding indicated that mountain rivers substantially and rapidly contribute to the water resources in the southern and northern sub-basins. δ 18O 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 δ 18O 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. 相似文献
7.
The usefulness of stable isotopes of dissolved SO 4 (δ 34S and δ 18O) 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. δ 34S 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 SO 4/Cl ratios, as well as relatively low δ 18O SO4 values, suggest that vertical input of biogenically derived SO 4 via diffuse recharge is the predominant source of dissolved SO 4 to the aquifer. Further along the hydraulic gradient towards the discharge area near the River Murray, δ 34S values in the unconfined Murray Group Aquifer increase, and SO 4/Cl ratios decrease, due to upward leakage of waters from the confined Renmark Group Aquifer which has a distinctly low SO 4/Cl and high δ 34S (14.9–56.4‰). Relatively positive δ 34S and δ 18O SO4 values, and low SO 4/Cl in the Renmark Group Aquifer is typical of SO 4 removal by bacterial reduction. The S isotope fractionation between SO 4 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 δ 34S and SO 4/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. 相似文献
8.
An investigation using environmental isotopes (δ 18O and δD) was conducted to gain insight into the hydrological processes of the Ganga Alluvial Plain, northern India. River-water, shallow-groundwater and lake-water samples from the Gomati River Basin were analyzed. During the winter season, the δ 18O and δD compositions of the Gomati River water ranged from ?1.67 to ?7.62 ‰ and ?25.08 to ?61.50 ‰, respectively. Deuterium excess values in the river water (+0.3 to ?13 ‰) and the lake water (?20 ‰) indicate the significance of evaporation processes. Monthly variation of δ 18O and δD values of the Gomati River water and the shallow groundwater follows a similar trend, with isotope-depleted peaks for δ 18O and δD synchronized during the monsoon season. The isotopically depleted peak values of the river water (δ 18O?=??8.30 ‰ and δD?=??57.10 ‰) can be used as a proxy record for the isotopic signature of the monsoon precipitation in the Ganga Alluvial Plain. 相似文献
9.
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. 相似文献
10.
Analysis of stable isotopes and major ions in groundwater and surface waters in Belize, Central America was carried out to identify processes that may affect drinking water quality. Belize has a subtropical rainforest/savannah climate with a varied landscape composed predominantly of carbonate rocks and clastic sediments. Stable oxygen (δ 18O) and hydrogen (δD) isotope ratios for surface and groundwater have a similar range and show high d-excess (10–40.8‰). The high d-excess in water samples suggest secondary continental vapor flux mixing with incoming vapor from the Caribbean Sea. Model calculations indicate that moisture derived from continental evaporation contributes 13% to overhead vapor load. In surface and groundwater, concentrations of dissolved inorganic carbon (DIC) ranged from 5.4 to 112.9 mg C/l and δ 13C DIC ranged from −7.4 to −17.4‰. SO 42, Ca 2+ and Mg 2+ in the water samples ranged from 2–163, 2–6593 and 2–90 mg/l, respectively. The DIC and δ 13C DIC indicate both open and closed system carbonate evolution. Combined δ 13C DIC and Ca 2+, Mg 2+, and SO 42− suggest additional groundwater evolution by gypsum dissolution and calcite precipitation. The high SO 42−content of some water samples indicates regional geologic control on water quality. Similarity in the range of δ 18O, δD and δ 13C DIC for surface waters and groundwater used for drinking water supply is probably due to high hydraulic conductivities of the karstic aquifers. The results of this study indicate rapid recharge of groundwater aquifers, groundwater influence on surface water chemistry and the potential of surface water to impact groundwater quality and vise versa. 相似文献
11.
Conventional hydrogeochemical data and environmental stable isotopes are used to identify the recharge sources and the water–rock interactions in the groundwater-flowing direction within the multilayer groundwater system of the Sulin coal-mining district in the north Anhui province in China. δD and δ 18O of groundwater in the mining district decrease along the groundwater-flowing direction in the recharge areas, yet in the runoff or discharge areas, they rise and fall along average δ values ( δ 18O = ?8.68 ‰, δD = ?67.4 ‰), which are lower than average δ values of local atmospheric precipitation ( δ 18O = ?7.80 ‰, δD = ?52.4 ‰). Principal component analysis is used to analyze the conventional hydrogeochemical data (K + + Na +, Mg 2+, Ca 2+, Cl ?, SO 4 2?, HCO 3 ?, CO 3 2?) in the groundwater. The first and second principal components have large variance contributions, and represent “pyrite oxidation or groundwater hardening” and “desulfurization or cation exchange and adsorption,” respectively. From conventional hydrogeochemical data and environmental stable isotopes, it is demonstrated that groundwater of the Sulin coal-mining district is characterized by a mixing type, which is confirmed by three recharge end-members: fresh groundwater, leaching groundwater, and retained groundwater. By means of a sample dot-encompassed triangle in the scatter diagram of load scores for Component 1–Component 2, whose vertexes stand for the three end-members, a model for calculating groundwater mixing ratio is established and applied successfully to the evaluation and management of groundwater hazards in the coal-mining districts. 相似文献
12.
Stable chlorine isotopes are useful geochemical tracers in processes involving the formation and evolution of evaporitic halite. Halite and dissolved chloride in groundwater that has interacted with halite in arid non-marine basins has a δ 37Cl range of 0 ± 3‰, far greater than the range for marine evaporites. Basins characterized by high positive (+1 to +3‰), near-0‰, and negative (−0.3 to −2.6‰) are documented. Halite in weathered crusts of sedimentary rocks has δ 37Cl values as high as +5.6‰. Salt-excluding halophyte plants excrete salt with a δ 37Cl range of −2.1 to −0.8‰. Differentiated rock chloride sources exist, e.g. in granitoid micas, but cannot provide sufficient chloride to account for the observed data. Single-pass application of known fractionating mechanisms, equilibrium salt-crystal interaction and disequilibrium diffusive transport, cannot account for the large ranges of δ 37Cl. Cumulative fractionation as a result of multiple wetting-drying cycles in vadose playas that produce halite crusts can produce observed positive δ 37Cl values in hundreds to thousands of cycles. Diffusive isotope fractionation as a result of multiple wetting-drying cycles operating at a spatial scale of 1–10 cm can produce high δ 37Cl values in residual halite. Chloride in rainwater is subject to complex fractionation, but develops negative δ 37Cl values in certain situations; such may explain halite deposits with bulk negative δ 37Cl values. Future field studies will benefit from a better understanding of hydrology and rainwater chemistry, and systematic collection of data for both Cl and Br. 相似文献
13.
This paper investigated the sources and behaviors of sulfate in groundwater of the western North China Plain using sulfur and oxygen isotopic ratios. The groundwaters can be categorized into karst groundwater (KGW), coal mine drainage (CMD) and pore water (subsurface saturated water in interstices of unconsolidated sediment). Pore water in alluvial plain sediments could be further classified into unconfined groundwater (UGW) with depth of less than 30 m and confined groundwater (CGW) with depth of more than 60 m. The isotopic compositions of KGW varied from 9.3‰ to 11.3‰ for δ 34S SO4 with the median value of 10.3‰ ( n = 4) and 7.9‰ to 15.6‰ for δ 18O SO4 with the median value of 14.3‰ ( n = 4) respectively, indicating gypsum dissolution in karst aquifers. δ 34S SO4 and δ 18O SO4 values of sulfate in CMD ranged from 10.8‰ to 12.4‰ and 4.8‰ to 8.7‰ respectively. On the basis of groundwater flow path and geomorphological setting, the pore water samples were divided as three groups: (1) alluvial–proluvial fan (II 1) group with high sulfate concentration (median values of 2.37 mM and 1.95 mM for UGW and CGW, respectively) and positive δ 34S SO4 and δ 18O SO4 values (median values of 8.8‰ and 6.9‰ for UGW, 12.0‰ and 8.0‰ for CGW); (2) proluvial slope (II 2) group with low sulfate concentration (median values of 1.56 mM and 0.84 mM for UGW and CGW, respectively) and similar δ 34S SO4 and δ 18O SO4 values (median values of 9.0‰ and 7.4‰ for UGW, 10.2‰ and 7.7‰ for CGW); and (3) low-lying zone (II 3) group with moderate sulfate concentration (median values of 2.13 mM and 1.17 mM for UGW and CGW, respectively) and more positive δ 34S SO4 and δ 18O SO4 values (median values of 10.7‰ and 7.7‰ for UGW, 20.1‰ and 8.8‰ for CGW). In the present study, three major sources of sulfate could be differentiated as following: sulfate dissolved from Ordovician to Permian rocks (δ 34S SO4 = 10–35‰ and δ 18O SO4 = 7–20‰), soil sulfate (δ 34S SO4 = 5.9‰ and δ 18O SO4 = 5.8‰) and sewage water (δ 34S SO4 = 10.0‰ and δ 18O SO4 = 7.6‰). Kinetic fractionations of sulfur and oxygen isotopes as a result of bacterial sulfate reduction (BSR) were found to be evident in the confined aquifer in stagnant zone (II 3), and enrichment factors of sulfate–sulfur and sulfate–oxygen isotopes calculated by Rayleigh equation were −12.1‰ and −4.7‰ respectively along the flow direction of groundwater at depths of 60–100 m. The results obtained in this study confirm that detailed hydrogeological settings and identification of anthropogenic sources are critical for elucidating evolution of δ 34S SO4 and δ 18O SO4 values along with groundwater flow path, and this work also provides a useful framework for understanding sulfur cycling in alluvial plain aquifers. 相似文献
14.
The environmental isotopes such as deuterium and oxygen-18 and the deuterium excess values have been used to assess groundwater recharge sources and their dynamics in Khan Younis City in the Gaza Strip in Palestine. Three isotopic lines for the relationship between δ 2H and δ 18O were used in the assessment. These lines are the global meteoric water line, the local meteoric water line and the groundwater evaporation line. The δ 2H, δ 18O and D-excess values indicate that deuterium and oxygen-18 isotopes originated in the groundwater from groundwater mixing with rainfall and other water sources; the groundwater in the area recharged from rainfall from a distant source that came from the Mediterranean Sea and from other sources such as wastewater, irrigation return flow and saline water. 相似文献
15.
Groundwater and surface water samples were collected to improve understanding of the Senegal River Lower Valley and Delta
system, which is prone to salinization. Inorganic ion concentrations and environmental isotopes ( 18O, 2H and 3H) in groundwater, river, lake and precipitation were investigated to gain insight into the functioning of the system with
regard to recharge sources and process, groundwater renewability, hydraulic interconnection and geochemical evolution. The
geochemical characteristics of the system display mainly cation (Ca 2+ and/or Na +) bicarbonated waters, which evolve to chloride water type; this occurs during groundwater flow in the less mineralized part
of the aquifer. In contrast, saline intrusion and secondary brines together with halite dissolution are likely to contaminate
the groundwater to Na–Cl type. Halite, gypsum and calcite dissolution determine the major ion (Na +, Cl −, Ca 2+, Mg 2+, SO 4
2− and HCO 3
−) chemistry, but other processes such as evaporation, salt deposition, ion exchange and reverse exchange reactions also control
the groundwater chemistry. Both surface water and groundwater in the system show an evaporation effect, but high evaporated
signatures in the groundwater may be due to direct evaporation from the ground, infiltration of evaporated water or enriched
rainwater in this region. The stable isotopes also reveal two types of groundwater in this system, which geomorphologically
are distributed in the sand dunes (depleted isotopes) and in the flood plain (enriched isotopes). Consideration of the 3H content reinforces this grouping and suggests two mechanisms of recharge: contribution of enriched surface water in recharging
the flood plain groundwater and, in the sand dunes area where water table is at depth between 8 and 13 m, slow recharge process
characterized the submodern to mixed water. 相似文献
16.
In the cool temperate region of South Korea, oxygen and hydrogen isotopes of groundwater, lake water, and precipitation were studied to determine the season of groundwater recharge. All the groundwater samples, irrespective of season, on δ 18O–δ 2H scale plotted along the summer precipitation, suggesting summer precipitation largely modulates recharge. The deuterium excess values of groundwater ( d-excess) show clear seasonal difference, higher in winter (> 18‰) and lower in summer (< 10‰). And its resemblance to the summer precipitation d-excess value further suggests dominant role of summer precipitation in groundwater recharge. Based on the mass balance equation, with end-member d-excess values of seasonal precipitation and groundwater as input variables, groundwater is composed of 66% summer and 34% winter precipitation. Despite the study area being heavily forested, summer rainfall contribution higher than winter suggests that evapotranspiration effect is minimal in the region; may be due to thin sand–gravel-based porous soil overlying highly weathered granitic rock system. 相似文献
17.
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-HCO 3 type in the upstream area and Na-HCO 3 type in the downstream area of the groundwater flow field. The groundwater in the lower aquifer is mainly characterized by Ca-Mg-HCO 3 type in the upstream area and Ca-Na-Mg-HCO 3 and Na-Ca-Mg-HCO 3 type in the downstream area. The δ D and δ 18O in precipitation are linearly correlated, which is similar to WMWL. The δ D and δ 18O 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 δ D and δ 18O 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. 相似文献
18.
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 HCO 3 –, Ca 2+, Mg 2+ and δ 13C DIC indicating degradation of soil organic matter and dissolution of carbonate minerals, similar to surface waters. In addition, groundwater in the Triassic aquifer has δ 18O 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 Mg 2+, Na +, Ca 2+, K +, and Si, and has high alkalinity and δ 13C DIC values, with low SO 4 2– and NO 3 – 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. 相似文献
19.
The southwestern Chad basin is a semi-arid region with annual rainfall that is generally less than 500 mm and over 2,000 mm of evapotranspiration. Surface water in rivers is seasonal, and therefore groundwater is the perennial source of water supply for domestic and other purposes. Stable isotope has been measured for rainwater, surface water and groundwater samples in this region. The stable isotope data have been used to understand the inter-relationships between the rainwater, surface water, shallow and deep groundwater of this region. This is being used in a qualitative sense to demonstrate present day recharge to the groundwater. Stable isotope in rainwater for the region has an average value of –4‰ δ 18O and –20‰ δ 2H. Surface water samples from rivers and Lake Chad fall on the evaporation line of this average value. The Upper Zone aquifer water samples show stable isotope signal with a wide range of values indicating the complex character of the aquifer Zone with three distinguishable units. The wide range of values is attributable to waters from individual unit and/or mixture of waters of different units. The Middle and Lower aquifers Zones’ waters show similar stable isotopes values, probably indicating similarity in timing and/or mechanism of recharge. These are palaeowaters probably recharged under a climate that is different from today. The Upper Zone aquifer is presently being recharged as some of its waters show stable isotope compositions similar to those of average rainfall waters of the region. 相似文献
20.
Hydraulic fracturing of shale deposits has greatly increased the productivity of the natural gas industry by allowing it to exploit previously inaccessible reservoirs. Previous research has demonstrated that this practice has the potential to contaminate shallow aquifers with methane (CH 4) from deeper formations. This study compares concentrations and isotopic compositions of CH 4 sampled from domestic groundwater wells in Letcher County, Eastern Kentucky in order to characterize its occurrence and origins in relation to both neighboring hydraulically fractured natural gas wells and surface coal mines. The studied groundwater showed concentrations of CH 4 ranging from 0.05 mg/L to 10 mg/L, thus, no immediate remediation is required. The δ 13C values of CH 4 ranged from −66‰ to −16‰, and δ 2H values ranged from −286‰ to −86‰, suggesting an immature thermogenic and mixed biogenic/thermogenic origin. The occurrence of CH 4 was not correlated with proximity to hydraulically fractured natural gas wells. Generally, CH 4 occurrence corresponded with groundwater abundant in Na +, Cl −, and HCO 3−, and with low concentrations of SO 42−. The CH 4 and SO 42−concentrations were best predicted by the oxidation/reduction potential of the studied groundwater. CH 4 was abundant in more reducing waters, and SO 42− was abundant in more oxidizing waters. Additionally, groundwater in greater proximity to surface mining was more likely to be oxidized. This, in turn, might have increased the likelihood of CH 4 oxidation in shallow groundwater. 相似文献
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