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1.
High water demand for domestic use in Douala with over 3 million inhabitants is met mainly by shallow groundwater. Field measurements and water sampling in January 2015 were carried out to examine the major controls on the groundwater composition and spatial view of ions in the water, timing of recharge and link between the recharge process and quality of the water. Fifty-two water samples were analysed for major ions and stable hydrogen and oxygen isotopes. Low pH values (3.61–6.92) in the groundwater indicated an acidic aquifer; thus, prone to acidification. The dominant water type was Na–Cl. Nitrate, which exceeded the WHO guide value of 50 mg/l in 22% of the groundwater, poses a health problem. Mass ratios of Cl?/Br? in the water ranged from 54 to 3249 and scattered mostly along the mixing lines between dilute waters, septic-tank effluent and domestic sewage. A majority of the samples, especially the high NO3 ? shallow wells, clustered around the septic-tank effluent-end-member indicating high contamination by seepage from pit latrines; hence, vulnerable to pollution. Stable isotopes in the groundwater indicated its meteoric origin and rapid infiltration after rainfall. The δ18O values showed narrow ranges and overlaps in rivers, springs, open wells and boreholes. These observations depict hydraulic connectivity, good water mixing and a homogeneous aquifer system mainly receiving local direct uniform areal recharge from rainfall. The rapid and diffused recharge favours the leaching of effluent from the pit toilets into the aquifer; hence, the high NO3 ? and Cl? in shallow wells. Silicate weathering, ion exchange and leaching of waste from pit toilets are the dominant controls on the groundwater chemistry. Drilling of deep boreholes is highly recommended for good-quality water supply. However, due the hydraulic connection to the shallow aquifer, geochemical modelling of future effects of such an exploitation of the deeper aquifer should support groundwater management and be ahead of the field actions.  相似文献   

2.
Continual expansion of population density, urbanization, agriculture, and industry in most parts of the world has increased the generation of pollution, which contributes to the deterioration of surface water quality. This causes the dependence on groundwater sources for their daily needs to accumulate day by day, which raises concerns about their quality and hydrogeochemistry. This study was carried out to increase understanding of the geological setup and assess the groundwater hydrogeochemical characteristics of the multilayered aquifers in Lower Kelantan Basin. Based on lithological data correlation of exploration wells, the study area can be divided into three main aquifers: shallow, intermediate and deep aquifers. From these three aquifers, 101 groundwater samples were collected and analyzed for various parameters. The results showed that pH values in the shallow, intermediate and deep aquifers were generally acidic to slightly alkaline. The sequences of major cations and anions were Na+ > Ca2+ > Mg2+ > K+ and HCO3? > Cl? > SO42? > CO32?, respectively. In the intermediate aquifer, the influence of ancient seawater was the primary factor that contributed to the elevated values of electrical conductivity (EC), Cl? and total dissolved solids (TDS). The main facies in the shallow aquifer were Ca–HCO3 and Na–HCO3 water types. The water types were dominated by Na–Cl and Na–HCO3 in the intermediate aquifer and by Na–HCO3 in the deep aquifer. The Gibbs diagram reveals that the majority of groundwater samples belonged to the deep aquifer and fell in the rock dominance zone. Shallow aquifer samples mostly fell in the rainfall zone, suggesting that this aquifer is affected by anthropogenic activities. In contrast, the results suggest that the deep aquifer is heavily influenced by natural processes.  相似文献   

3.
The gravel aquifer within the Szigetköz Plain in northern Hungary is mainly fed by the infiltration of the Danube River. This infiltration process can be identified using the tritium/helium method to a distance of about 30 km away from the infiltration area of the Danube. In this study, chlorine-36 analyses are used as additional method. This natural radioisotope was also produced by nuclear bomb tests in the atmosphere. It is an ideal, stable constituent for this particular study due to its very long half-life (300,000 years), and consequently acts as an independent check of the established model of aquifer recharge. The chlorine-36 data of the analysed selected groundwater samples of this area clearly show the effect of the atmospheric nuclear bomb tests, with enhanced 36Cl/Cl ratios of up to a factor 10 higher than the unaffected groundwater of the pre-bomb period within the study area. Finally, the observed 36Cl values were introduced into a transport model with dispersion/advection-type flow of groundwater to deduce the transport parameters.  相似文献   

4.
Groundwater is the most important source of water supply in the Yeniceoba Plain in Central Anatolia,Turkey.An understanding of the geochemical evolution of groundwater is important for the sustainable development of water resources in this region.A hydrogeochemical investigation was conducted in the Plio-Quaternary aquifer system using stable isotopes(δ~(18)O andδD),tritium(~3H),major and minor elements(Ca,Na,K,Mg,Cl,SO_4,NO_3,HCO_3 and Br)in order to identify groundwater chemistry patterns and the processes affecting groundwater mineralization in this system.The chemical data reveal that the chemical composition of groundwater in this aquifer system is mainly controlled by rock/water interactions including dissolution of evaporitic minerals,weathering of silicates,precipitation/dissolution of carbonates,ion exchange,and evaporation.Based on the values of Cl/Br ratio(300 mg/l)in the Plio-Quaternary groundwater,dissolution of evaporitic minerals in aquifer contributes significantly to the high mineralization.The stable isotope analyses indicate that the groundwater in the system was influenced by evaporation of rainfall during infiltration.Low tritium values(generally1 tritium units)of groundwater reflect a minor contribution of recent recharge and groundwater residence times of more than three or four decades.  相似文献   

5.
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6.
《Applied Geochemistry》2003,18(7):1043-1063
The Memphis aquifer in southwestern Tennessee is confined to a semi-confined unconsolidated sand aquifer and is the primary municipal water source in the Memphis metropolitan area. Past studies have identified regions in the metropolitan area in which the overlying upper Claiborne confining unit lacks significant clay and provides a hydraulic connection between the shallow aquifer and the Memphis aquifer. In this study, major solute chemistry, 3H, and 3H/3He groundwater dating are used to investigate the extent and chemical effects of leakage through the confining unit to the Memphis aquifer in the vicinity of a municipal well field. The 3H/3He dates and geochemical modeling of the chemical data are used to constrain mixing fractions and the timing of modern recharge. Tritium activities of as much as 2.8 TU are observed in shallow production wells, but deeper production wells have 3H activities that approach the detection limit. Trends in water chemistry indicate vertical mixing in the aquifer of shallow Na–SO4–Cl-rich water and deeper Ca–Mg–HCO3-rich water. Water chemistry does not vary consistently with seasonal pumping, but 3H activity generally decreases during low use periods. Stable O and H isotopes show little variation and are not useful groundwater tracers for this study. The 3H-bearing, Na–SO4–Cl-rich water is interpreted to reflect recharge of modern water through the upper Claiborne confining unit. The 3H/3He dates from 5 production wells indicate modern recharge, that infiltrated 15–20 a ago, is present in the shallow production wells. Geologic data and hydrologic boundary conditions suggest that the most likely source for continued leakage is a nearby stream, Nonconnah Creek. Geochemical reaction modeling using the NETPATH computer code suggests that proportions of shallow aquifer water leaking into the Memphis aquifer range from 6 to 32%. The 3H/3He dating and NETPATH modeling results correlate well, suggesting that these complementary analytical tools provide an effective means to evaluate proportions of modern water leaking into semi-confined aquifers. These results also indicate a need to carefully consider connections between surface water and semi-confined groundwater resources in wellhead protection programs.  相似文献   

7.
 Chlorine-36 is a radionuclide with a half-life of 3.01×105a. Most 36Cl in the hydrosphere originates from cosmic radiation interacting with atmospheric gases. Large amounts were also produced by testing thermonuclear devices during 1952–58. Because the monovalent anion, chloride, is the most common form of chlorine found in the hydrosphere and because it is extremely mobile in aqueous systems, analyses of both total Cl– as well as 36Cl have been important in numerous hydrologic studies. In almost all applications of 36Cl, a knowledge of the initial, or pre-anthropogenic, levels of 36Cl is useful, as well as essential in some cases. Standard approaches to the determination of initial values have been to: (a) calculate the theoretical cosmogenic production and fallout, which varies according to latitude; (b) measure 36Cl in present-day precipitation and assume that anthropogenic components can be neglected; (c) assume that shallow groundwater retains a record of the initial concentration; (d) extract 36Cl from vertical depth profiles in desert soils; (e) recover 36Cl from cores of glacial ice; and (f) calculate subsurface production of 36Cl for water that has been isolated from the atmosphere for more than one million years. The initial value from soil profiles and ice cores is taken as the value that occurs directly below the depth of the easily defined bomb peak. All six methods have serious weaknesses. Complicating factors include 36Cl concentrations not related to cosmogenic sources, changes in cosmogenic production with time, mixed sources of chloride in groundwater, melting and refreezing of water in glaciers, and seasonal groundwater recharge that does not contain average year-long concentrations of 36Cl. Received, December 1996 · Revised, August 1997 · Accepted, August 1997  相似文献   

8.
The main factors and mechanisms controlling the groundwater chemistry and mineralization are recognized through hydrochemical data. However, water quality prediction remains a key parameter for groundwater resources management and planning. The geochemical study of groundwater of a multilayered aquifer system in Tunisia is recognized by measurements of the pH, EC, total dissolved solids (TDS), major ion concentration and nitrates of 36 samples from pumping wells covering the aquifer extension and analyzed using standard laboratory and field methods. The calcite precipitation, gypsum, anhydrite and halite dissolution, and direct and reverse ion exchange are the principal process of chemical evolution in the Nadhour-Saouaf aquifer system. Using stepwise regression, the concentration groups of (Ca, Cl, and NO3), (Cl, SO4, and Mg), and (Ca and Na) exhibit significant prediction of TDS in Plio-Quaternary, Miocene, and Oligocene aquifer levels, respectively. The highest values of R 2 and adjusted R 2 close to 1 revealed the accuracy of the developed models which is confirmed by the weak difference between the measured and estimated values varying between ?12 and 8%. The important uncertainty parameters that affected the estimated TDS are assessed by the sensitivity analysis method. The concentration of (Cl), (Ca and Cl), and (Na) are the major parameters affecting the TDS sensitivity of the Plio-Quaternary, Miocene, and Oligocene aquifer levels, respectively. Hence, the developed TDS models provide a more simple and easy alternative to other methods used for groundwater quality estimation and prediction as proven from external validation on groundwater samples unconsidered in the model construction.  相似文献   

9.
The radioactive isotope 36Cl, with a half-life of 301 ka, is a valuable chronometer for estimation of groundwater residence time up to 2 millions of years. Aerial thermonuclear fission bomb tests, performed during the late 1950s, injected a massive amount of this isotope into the atmosphere, which exceeded the natural fallout signal. Since this bomb pulse, atmospheric 36Cl deposition tends to return to natural fallout rate. The monitoring of this attenuation can provide a good opportunity to extend the use of this chronometer to shorter time spans. Venice’s lagoon alimentation zone shows groundwaters with residence times distributed over last fifty years. This permits the estimation of a continuous 36Cl deposition curve, free from latitudinal and seasonal variations of the signal. Three old groundwater samples, with residence times comprised in the range −900 to −8000 BP, allow the estimation of a mean natural deposition of 49 at m−2 s−1 and are in good agreement with 36Cl fallout observed for the last 40,000 years by (Plummer et al., 1997). For the bomb pulse period, a fallout of 5300 at m−2 s−1 was calculated. This was followed by a strong attenuation period, taking place until the 1980s, during which the fallout reached values ranging between 167 and 354 at m−2 s−1. The attenuation reached then a plateau: it experienced a slower lowering until the actual deposition, with fallout values calculated between 124 and 252 at m−2 s−1. This persistence of high deposition rate was classically attributed to biological and atmospherical recycling processes or underestimation of the natural atmospheric production of the 36Cl. Additional source of 36Cl production has been envisaged through the activation of chlorine radicals from stratospherical CFCs, leading to a 36Cl production rate comparable with that of Ar spallation from the first approximation. Lastly, the latitudinal factor of the attenuation of the fallout rate is discussed and the impact of the jet streams is proposed as an explanation for the discrepancies in the attenuation rate.  相似文献   

10.
In this study a typical coastal karst aquifer, developed in lower Cretaceous limestones, on the western Mediterranean seashore (La Clape massif, southern France) was investigated. A combination of geochemical and isotopic approaches was used to investigate the origin of salinity in the aquifer. Water samples were collected between 2009 and 2011. Three groundwater groups (A, B and C) were identified based on the hydrogeological setting and on the Cl− concentrations. Average and maximum Cl− concentrations in the recharge waters were calculated (ClRef. and ClRef.Max) to be 0.51 and 2.85 mmol/L, respectively). Group A includes spring waters with Cl− concentrations that are within the same order of magnitude as the ClRef concentration. Group B includes groundwater with Cl− concentrations that range between the ClRef and ClRef.Max concentrations. Group C includes brackish groundwater with Cl− concentrations that are significantly greater than the ClRef.Max concentration. Overall, the chemistry of the La Clape groundwater evolves from dominantly Ca–HCO3 to NaCl type. On binary diagrams of the major ions vs. Cl, most of the La Clape waters plot along mixing lines. The mixing end-members include spring waters and a saline component (current seawater or fossil saline water). Based on the Br/Clmolar ratio, the hypothesis of halite dissolution from Triassic evaporites is rejected to explain the origin of salinity in the brackish groundwater.Groundwaters display 87Sr/86Sr ratios intermediate between those of the limestone aquifer matrix and current Mediterranean seawater. On a Sr mixing diagram, most of the La Clape waters plot on a mixing line. The end-members include the La Clape spring waters and saline waters, which are similar to the deep geothermal waters that were identified at the nearby Balaruc site. The 36Cl/Cl ratios of a few groundwater samples from group C are in agreement with the mixing hypothesis of local recharge water with deep saline water at secular equilibrium within a carbonate matrix. Finally, PHREEQC modelling was run based on calcite dissolution in an open system prior to mixing with the Balaruc type saline waters. Modelled data are consistent with the observed data that were obtained from the group C groundwater. Based on several tracers (i.e. concentrations and isotopic compositions of Cl and Sr), calculated ratios of deep saline water in the mixture are coherent and range from 3% to 16% and 0% to 3% for groundwater of groups C and B, respectively.With regard to the La Clape karst aquifer, the extension of a lithospheric fault in the study area may favour the rise of deep saline water. Such rises occur at the nearby geothermal Balaruc site along another lithospheric fault. At the regional scale, several coastal karst aquifers are located along the Gulf of Lion and occur in Mezosoic limestones of similar ages. The 87Sr/86Sr ratios of these aquifers tend toward values of 0.708557, which suggests a general mixing process of shallow karst waters with deep saline fossil waters. The occurrence of these fossil saline waters may be related to the introduction of seawater during and after the Flandrian transgression, when the highly karstified massifs invaded by seawater, formed islands and peninsulas along the Mediterranean coast.  相似文献   

11.
Aquifer-based groundwater quality assessment offers critical insight into the major hydrochemical processes, and aids in making groundwater resources management decisions. The Texas Rolling Plains (TRP), spanning over 22 counties, is a major agro-ecological region in Texas from where highest groundwater nitrate (NO3 ?) levels in the state have been reported. In this study, we present a comparative assessment of major hydrochemical facies pertaining to NO3 ? contamination and a host of species such as sulfate (SO4 2?), chloride (Cl?), and total dissolved solids (TDS) in different water use classes in the Seymour and Blaine aquifers, underlying the TRP. Aquifer-stratified groundwater quality information from 1990 to 2010 was obtained from the Texas Water Development Board and aggregated over decadal scale. High groundwater salinization was found in the municipal water use class in the Blaine aquifer with about 100, 87 and 50 % of observations exceeding the secondary maximum contaminant level for TDS, SO4 2?, and Cl?, respectively in the 2000s (2000–2010). The NO3-contamination was more alarming in the Seymour aquifer with 82 and 61 % of observations, respectively, exceeding the maximum contaminant level (MCL) in the irrigation and municipal water use classes in the 2000s. Salinization was more influenced by SO4 2? and Cl? in the Blaine aquifer and by NO3 ? in the Seymour aquifer. High NO3 ? (>MCL) observations in the Seymour aquifer occurred in the Ca–HCO3 and Ca–Mg–HCO3 facies, the domains of fresh water recharge and anthropogenic influences (e.g., agricultural activities, waste disposal). High SO4 2?, Cl? and TDS observations in the Blaine aquifer dominated the Ca–Cl, Na–Cl, and mixed Ca(Mg)–SO4(Cl) facies indicating evaporite dissolution, mixing and solute exchange, and lack of fresh recharge.  相似文献   

12.
The present study investigates the hydrogeochemistry and contamination of Varamin deep aquifer located in the southeast of Tehran province, Iran. The study also evaluates groundwater suitability for irrigation uses. The hydrogeochemical study was conducted by collecting and analyzing 154 groundwater samples seasonally during 2014. Based on evolutionary sequence of Chebotarev, the aquifer is in the stage of SO4 + HCO3 in the north half of the plain and it has evolved into SO4 + Cl in the south half. The unusual increase in TDS and Cl? toward the western boundaries of the aquifer indicates some anomalies. These anomalies have originated from discharge of untreated wastewater of Tehran city in these areas. The studied aquifer contains four dominant groundwater types including Na–Ca–SO4 (55%), Na–Ca–HCO3 (22%), Na–Cl (13%) and Ca–Cl (10%). The spatial distributions of Na–Cl and Ca–Cl water types coincide with observed anomalies. Ionic relationships of SO4 2? versus Cl? and Na+ versus Cl? confirm that water–rock interaction and anthropogenic contribution are main sources of these ions in the groundwater. The main processes governing the chemistry of the groundwater are the dissolution of calcite, dolomite and gypsum along the flow path, and direct ion exchange. Reverse ion exchange controls the groundwater chemistry in the areas contaminated with untreated wastewater. Based on Na% and SAR, 10.3 and 27% of water samples are unsuitable for irrigation purposes, respectively. Regarding residual sodium carbonate, there is no treat for crop yields. Only 6% of water samples represent magnesium adsorption ratios more than 50% which are harmful and unsuitable for irrigation.  相似文献   

13.
Groundwater residence time in the Kulnura–Mangrove Mountain aquifer was assessed during a multi-year sampling programme using general hydrogeochemistry and isotopic tracers (H2O stable isotopes, δ13CDIC, 3H, 14C and 87Sr/86Sr). The study included whole-rock analysis from samples recovered during well construction at four sites to better characterise water–rock interactions. Based on hydrogeochemistry, isotopic tracers and mineral phase distribution from whole-rock XRD analysis, two main groundwater zones were differentiated (shallow and deep). The shallow zone contains oxidising Na–Cl-type waters, low pH, low SC and containing 3H and 14C activities consistent with modern groundwater and bomb pulse signatures (up to 116.9 pMC). In this shallow zone, the original Hawkesbury Sandstone has been deeply weathered, enhancing its storage capacity down to ～50 m below ground surface in most areas and ～90 m in the Peats Ridge area. The deeper groundwater zone was also relatively oxidised with a tendency towards Ca–HCO3-type waters, although with higher pH and SC, and no 3H and low 14C activities consistent with corrected residence times ranging from 11.8 to 0.9 ka BP. The original sandstone was found to be less weathered with depth, favouring the dissolution of dispersed carbonates and the transition from a semi-porous groundwater media flow in the shallow zone to fracture flow at depth, with both chemical and physical processes impacting on groundwater mean residence times.

Detailed temporal and spatial sampling of groundwater revealed important inter-annual variations driven by groundwater extraction showing a progressive influx of modern groundwater found at >100 m in the Peats Ridge area. The progressive modernisation has exposed deeper parts of the aquifer to increased NO3? concentrations and evaporated irrigation waters. The change in chemistry of the groundwater, particularly the lowering of groundwater pH, has accelerated the dissolution of mineral phases that would generally be inactive within this sandstone aquifer triggering the mobilisation of elements such as aluminium in the aqueous phase.  相似文献   

14.
The need for more agricultural or residential land has encouraged reclamation at the coastal areas of Korea since 1200 ad (approximately). The groundwaters of these reclaimed areas could be expected to reveal hydrogeochemical properties different from those of areas directly affected by seawater intrusion. The purpose of this study, therefore, was to examine the salinization of shallow groundwater in a coastal reclaimed area and to identify the effect of land reclamation on groundwater quality. Major cations and anions, iodide, total organic carbon, δD, δ 18O and δ 13C were measured to assist the hydrogeochemical analysis. Chloride, δD and δ 18O data clearly show that the Na–Cl type water results from mixing of groundwater with seawater. In particular, the δD and δ 18O of Ca+Mg–Cl+NO3 type groundwaters are close to the meteoric water line, but Na–Cl type waters enriched in chloride are 18O-enriched with respect to the meteoric water line. Meanwhile, carbon isotopic data and I/Cl ratios strongly suggest that there are various sources of salinity. The δ 13C values of Na–Cl type groundwaters are generally similar to those of Ca+Mg–Cl+NO3 type waters, which are depleted in 13C with respect to seawater. I/Cl ratios of Na–Cl type groundwater are 10–100 times higher than that of seawater. Because the reclamation has incorporated a large amount of organic matter, it provides optimum conditions for the occurrence of redox processes in the groundwater system. Therefore, the salinization of groundwater in the study area seems to be controlled not only by saltwater intrusion but also by other effects, such as those caused by residual salts and organic matter in the reclaimed sediments.  相似文献   

15.
 The sustainability of community water supplies drawn from shallow aquifers in the arid southwest of the Northern Territory has been evaluated using the radioactive isotope chlorine-36 (36Cl). These aquifers include fractured sandstones of the Ngalia Basin, fractured metamorphic rocks and Cainozoic sands and gravels. 36Cl/Cl ratios for these shallow, regional groundwaters exhibit a bimodal distribution with peaks at 205 (±7) and 170 (±7)×10–15. The higher ratio probably represents modern (Holocene) recharge, diluted with windblown salts from local playa lakes, and occurs mostly around the margin of the basin. The lower ratio corresponds to a 36Cl "age", or mean residence time, of 80–100 ka, implying that the last major recharge occurred during the last interglacial interval (Oxygen Isotope Stage 5). These values are mainly observed in the interior of the Ngalia Basin. Lower values of the 36Cl/Cl ratio measured near playa lakes are affected by addition of chloride from remobilised salts. Finite carbon-14 (14C) data for the groundwaters are at variance with the 36Cl results, but a depth profile suggests low recharge, allowing diffusion of recent atmospheric carbon to the water table. The 36Cl results have important implications for groundwater management in this region, with substantial recharge only occurring during favourable, wet, interglacial climatic regimes; most community water supplies are dependent on these "old" waters. Received, September 1997 · Revised, August 1998, March 1999 · Accepted, March 1999  相似文献   

16.
Deep Quaternary groundwater is the main source for industrial, domestic, and agricultural water supply in the North China Plain (NCP). There is currently a regional decline of groundwater levels, deterioration of water quality and environmental geological problems induced by increasing exploitation of the NCP Quaternary aquifer system. To trace sources and transport processes of dissolved Cl− in a regional aquifer system and to reveal hydrogeological characteristics of Quaternary complexes, δ37Cl, δ18O and δD, and chemical compositions (including F−, Cl−, Br−) of the deep groundwater sampled from the northern flow system of the NCP were measured along the west–east groundwater flow paths. The measured δ37Cl values decreased from 0.39‰ to −2.22‰ (SMOC) along the groundwater flow direction, with increasing Cl− concentrations. Marine aerosol input via rainfall is the main source of Cl− in the deep groundwater near the recharge areas, and subsequent evaporation/evapotranspiration appears to be responsible for Cl− accumulation. Mixing of recharge water with water of high-Cl− and low-δ37Cl accounts for the pattern of δ37Cl and Cl− concentration observed in Aquifer-3 along the west–east transect. The water with high-Cl and low-δ37Cl is likely from pore water released from compacted clays induced by over-exploitation of deep groundwater, suggesting that clay is a dominant subsurface source of Cl− for groundwater where a regional depression cone is present in the Quaternary aquifers. The groundwater of Aquifer-4 in the Huang-Hua depression is potentially mixed with an upward flux of Cl− from the Neogene aquifer through subvertical faults. Diffusion and ion filtration are two mechanisms invoked to explain the highly negative δ37Cl data for groundwater of Aquifer-4 in the Yanshan–Haixing areas, which provides new insight into solute migration and the hydraulic relationship in the strongly exploited groundwater system. This study using the conservative solute Cl− provides additional important information for further investigations of the geochemistry of a wide range of reactive solutes in the Quaternary aquifer system, so guiding water resource management.  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-HCO3 type in the upstream area and Na-HCO3 type in the downstream area of the groundwater flow field. The groundwater in the lower aquifer is mainly characterized by Ca-Mg-HCO3 type in the upstream area and Ca-Na-Mg-HCO3 and Na-Ca-Mg-HCO3 type in the downstream area. The δD and δ18O in precipitation are linearly correlated, which is similar to WMWL. The δD and δ18O values of river, well and spring water are within the same ranges as those found in the alluvial fan zone, and lay slightly above or below LMWL. The δD and δ18O values have a decreasing trend generally following the precipitation → surface water → shallow groundwater → spring water → deep groundwater direction. There is evidence of enrichment of heavy isotopes in groundwater due to evaporation. Tritium values of unconfined groundwater give evidence for ongoing recharge in modern times with mean residence times <50 a. It shows a clear renewal evolution along the groundwater flow paths and represents modern recharge locally from precipitation and surface water to the shallow aquifers (<150 m). In contrast, according to 14C ages in the confined aquifers and residence time of groundwater flow lines, the deep groundwater is approximately or older than 10 ka, and was recharged during a period when the climate was wetter and colder mainly from the piedmont surrounding the plain. The groundwater exploitation is considered to be “mined unsustainably” because more water is withdrawn than it is replenished.  相似文献   

18.
El Shalal-Kema area is located east of Aswan town and Nile River. The Quaternary sediments (unconsolidated material of sands, gravels, and clays intercalation) represent the main aquifer in the studied area. Its water is under unconfined condition, and the water table is shallow (vary from 7.5 to 16.3 m). The concerned aquifer is recharged mainly from Aswan Dam Lake, from the excess irrigation water and from septic tanks, where the area is not served by sewage system. The direction of the groundwater movement is generally from south to north. The transmissivity values of the Quaternary aquifer (from three pumping tests) are relatively high (vary from 1,996 to 3,029 m2/day). The exploitation of groundwater is carried out where there is continuous withdrawal for industrial and domestic uses with a total average quantity of groundwater of 71,304 m3 per day (25.67 million m3 per year). The hydrochemical characteristics of the Quaternary aquifer is studied based on the chemical analysis of 29 groundwater and four surface water samples collected from different sites. The chemical composition of the groundwater is dominated by calcium Ca2+ from the cations and bicarbonate (HCO 3 ? ) from the anions, and the order of cation abundance is Ca2+ > Na+ > Mg2+ > K+ and HCO 3 ? > SO 4 2? > Cl? among the anions. The groundwater types are normal chloride water, normal sulfate water, and normal carbonate water. The hypothetical salt combination revealed the presence of different salts arranged in terms of their predominant as Ca(HCO3)2, Mg(HCO3)2, NaCl, Na2SO4, MgSO4, KCL, NaHCO3, MgCl2, CaSO4, and K2SO4. The analytical measurements to the NO2 and NH3 reveal that their values decrease in summer and increase in winter due to the stoppage of pumping which leads to the increase of the wastewater quantities that reach the groundwater. The chemical and microbiological analyses show that the aquifer in this area is contaminated with fecal and disease-causing bacteria. The main cause of this contamination is the outflow from the septic tanks; therefore, the construction of sewage network is a vital solution. Chlorination is important to disinfect the groundwater at the tanks before its distribution to the houses.  相似文献   

19.
Corrected groundwater 14C ages from the carbonate aquifer in Yucca Flat at the former Nevada Test Site (now the Nevada National Security Site), USA, were evaluated by comparing temporal variations of groundwater 36Cl/Cl estimated with these 14C ages with published records of meteoric 36Cl/Cl variations preserved in packrat middens (piles of plant fragments, fecal matter and urine). Good agreement between these records indicates that the groundwater 14C ages are reasonable and that 14C is moving with chloride without sorbing to the carbonate rock matrix or fracture coatings, despite opposing evidence from laboratory experiments. The groundwater 14C ages are consistent with other hydrologic evidence that indicates significant basin infiltration ceased 8,000 to 10,000 years ago, and that recharge to the carbonate aquifer is from paleowater draining through overlying tuff confining units along major faults. This interpretation is supported by the relative age differences as well as hydraulic head differences between the alluvial and volcanic aquifers and the carbonate aquifer. The carbonate aquifer 14C ages suggest that groundwater velocities throughout much of Yucca Flat are about 2 m/yr, consistent with the long-held conceptual model that blocking ridges of low-permeability rock hydrologically isolate the carbonate aquifer in Yucca Flat from the outlying regional carbonate flow system.  相似文献   

20.
The present work provides an online Bench II-IRMS technique for the measurement of stable chlorine isotope ratio, which is used to measure the δ37Cl of 38 groundwater samples from the Karst and Quaternary aquifers in Anyang area. The regional distribution and signature of δ37Cl value are characterized on the base of isotopic data. The results suggest that the δ37Cl value of Quaternary groundwater decreases with increasing Cl? concentration, and has no correlation with δ18O and δD values, but closely correlates with the depth to water table. The fractionation mechanism of the chlorine isotope is expounded according to the type of groundwater. The δ37Cl value of karst water is generally positive, which is relevant to the dissolution of evaporite (gypsum mine), and may be caused by the mixing of groundwater and precipitation. The groundwater of Quaternary unconfined aquifer is mainly recharged by precipitation, and the δ37Cl value of groundwater is generally negative. The δ37Cl value of groundwater in Quaternary confined aquifer is more negative with increasing the depth to water level and elevated Cl? concentration, which is possible to result from the isotope fractionation of ion filtration. The groundwater with inorganic pollutants in Quaternary unconfined aquifer has generally a positive δ37Cl value.  相似文献   

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