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1.
Horizontal drilling and hydraulic fracturing have enhanced unconventional hydrocarbon recovery but raised environmental concerns related to water quality. Because most basins targeted for shale‐gas development in the USA have histories of both active and legacy petroleum extraction, confusion about the hydrogeological context of naturally occurring methane in shallow aquifers overlying shales remains. The Karoo Basin, located in South Africa, provides a near‐pristine setting to evaluate these processes, without a history of conventional or unconventional energy extraction. We conducted a comprehensive pre‐industrial evaluation of water quality and gas geochemistry in 22 groundwater samples across the Karoo Basin, including dissolved ions, water isotopes, hydrocarbon molecular and isotopic composition, and noble gases. Methane‐rich samples were associated with high‐salinity, NaCl‐type groundwater and elevated levels of ethane, 4He, and other noble gases produced by radioactive decay. This endmember displayed less negative δ13C‐CH4 and evidence of mixing between thermogenic natural gases and hydrogenotrophic methane. Atmospheric noble gases in the methane‐rich samples record a history of fractionation during gas‐phase migration from source rocks to shallow aquifers. Conversely, methane‐poor samples have a paucity of ethane and 4He, near saturation levels of atmospheric noble gases, and more negative δ13C‐CH4; methane in these samples is biogenic and produced by a mixture of hydrogenotrophic and acetoclastic sources. These geochemical observations are consistent with other basins targeted for unconventional energy extraction in the USA and contribute to a growing data base of naturally occurring methane in shallow aquifers globally, which provide a framework for evaluating environmental concerns related to unconventional energy development (e.g., stray gas).  相似文献   

2.
Assessing natural vs. anthropogenic sources of methane in drinking water aquifers is a critical issue in areas of shale oil and gas production. The objective of this study was to determine controls on methane occurrences in aquifers in the Eagle Ford Shale play footprint. A total of 110 water wells were tested for dissolved light alkanes, isotopes of methane, and major ions, mostly in the eastern section of the play. Multiple aquifers were sampled with approximately 47 samples from the Carrizo‐Wilcox Aquifer (250‐1200 m depth range) and Queen City‐Sparta Aquifer (150‐900 m depth range) and 63 samples from other shallow aquifers but mostly from the Catahoula Formation (depth <150 m). Besides three shallow wells with unambiguously microbial methane, only deeper wells show significant dissolved methane (22 samples >1 mg/L, 10 samples >10 mg/L). No dissolved methane samples exhibit thermogenic characteristics that would link them unequivocally to oil and gas sourced from the Eagle Ford Shale. In particular, the well water samples contain very little or no ethane and propane (C1/C2+C3 molar ratio >453), unlike what would be expected in an oil province, but they also display relatively heavier δ13Cmethane (>?55‰) and δDmethane (>?180‰). Samples from the deeper Carrizo and Queen City aquifers are consistent with microbial methane sourced from syndepositional organic matter mixed with thermogenic methane input, most likely originating from deeper oil reservoirs and migrating through fault zones. Active oxidation of methane pushes δ13Cmethane and δDmethane toward heavier values, whereas the thermogenic gas component is enriched with methane owing to a long migration path resulting in a higher C1/C2+C3 ratio than in the local reservoirs.  相似文献   

3.
Clusters of elevated methane concentrations in aquifers overlying the Barnett Shale play have been the focus of recent national attention as they relate to impacts of hydraulic fracturing. The objective of this study was to assess the spatial extent of high dissolved methane previously observed on the western edge of the play (Parker County) and to evaluate its most likely source. A total of 509 well water samples from 12 counties (14,500 km2) were analyzed for methane, major ions, and carbon isotopes. Most samples were collected from the regional Trinity Aquifer and show only low levels of dissolved methane (85% of 457 unique locations <0.1 mg/L). Methane, when present is primarily thermogenic (δ13C 10th and 90th percentiles of ?57.54 and ?39.00‰ and C1/C2+C3 ratio 10th, 50th, and 90th percentiles of 5, 15, and 42). High methane concentrations (>20 mg/L) are limited to a few spatial clusters. The Parker County cluster area includes historical vertical oil and gas wells producing from relatively shallow formations and recent horizontal wells producing from the Barnett Shale (depth of ～1500 m). Lack of correlation with distance to Barnett Shale horizontal wells, with distance to conventional wells, and with well density suggests a natural origin of the dissolved methane. Known commercial very shallow gas accumulations (<200 m in places) and historical instances of water wells reaching gas pockets point to the underlying Strawn Group of Paleozoic age as the main natural source of the dissolved gas.  相似文献   

4.
Water and gas samples were collected from (1) nine shallow groundwater aquifers overlying Marcellus Shale in north‐central West Virginia before active shale gas drilling, (2) wells producing gas from Upper Devonian sands and Middle Devonian Marcellus Shale in southwestern Pennsylvania, (3) coal‐mine water discharges in southwestern Pennsylvania, and (4) streams in southwestern Pennsylvania and north‐central West Virginia. Our preliminary results demonstrate that the oxygen and hydrogen isotope composition of water, carbon isotope composition of dissolved inorganic carbon, and carbon and hydrogen isotope compositions of methane in Upper Devonian sands and Marcellus Shale are very different compared with shallow groundwater aquifers, coal‐mine waters, and stream waters of the region. Therefore, spatiotemporal stable isotope monitoring of the different sources of water before, during, and after hydraulic fracturing can be used to identify migrations of fluids and gas from deep formations that are coincident with shale gas drilling.  相似文献   

5.
Geochemical evaluation of the sources and movement of saline groundwater in coastal aquifers can aid in the initial mapping of the subsurface when geological information is unavailable. Chloride concentrations of groundwater in a coastal aquifer near San Diego, California, range from about 57 to 39,400 mg/L. On the basis of relative proportions of major‐ions, the chemical composition is classified as Na‐Ca‐Cl‐SO4, Na‐Cl, or Na‐Ca‐Cl type water. δ2H and δ18O values range from ?47.7‰ to ?12.8‰ and from ?7.0‰ to ?1.2‰, respectively. The isotopically depleted groundwater occurs in the deeper part of the coastal aquifer, and the isotopically enriched groundwater occurs in zones of sea water intrusion. 87Sr/86Sr ratios range from about 0.7050 to 0.7090, and differ between shallower and deeper flow paths in the coastal aquifer. 3H and 14C analyses indicate that most of the groundwater was recharged many thousands of years ago. The analysis of multiple chemical and isotopic tracers indicates that the sources and movement of saline groundwater in the San Diego coastal aquifer are dominated by: (1) recharge of local precipitation in relatively shallow parts of the flow system; (2) regional flow of recharge of higher‐elevation precipitation along deep flow paths that freshen a previously saline aquifer; and (3) intrusion of sea water that entered the aquifer primarily during premodern times. Two northwest‐to‐southeast trending sections show the spatial distribution of the different geochemical groups and suggest the subsurface in the coastal aquifer can be separated into two predominant hydrostratigraphic layers.  相似文献   

6.
To investigate the origin and behaviour of nitrate in alluvial aquifers adjacent to Nakdong River, Korea, we chose two representative sites (Wolha and Yongdang) having similar land‐use characteristics but different geology. A total of 96 shallow groundwater samples were collected from irrigation and domestic wells tapping alluvial aquifers. About 63% of the samples analysed had nitrate concentrations that exceeded the Korean drinking water limit (44·3 mg l?1 NO3?), and about 35% of the samples had nitrate concentrations that exceeded the Korean groundwater quality standard for agricultural use (88·6 mg l?1 NO3?). Based on nitrogen isotope analysis, two major nitrate sources were identified: synthetic fertilizer (about 4‰ δ15N) applied to farmland, and animal manure and sewage (15–20‰ δ15N) originating from upstream residential areas. Shallow groundwater in the farmland generally had higher nitrate concentrations than those in residential areas, due to the influence of synthetic fertilizer. Nitrate concentrations at both study sites were highest near the water table and then progressively decreased with depth. Nitrate concentrations are also closely related to the geologic characteristics of the aquifer. In Yongdang, denitrification is important in regulating nitrate chemistry because of the availability of organic carbon from a silt layer (about 20 m thick) below a thin, sandy surface aquifer. In Wolha, however, conservative mixing between farmland‐recharged water and water coming from a village is suggested as the dominant process. Mixing ratios estimated based on the nitrate concentrations and the δ15N values indicate that water originating from the village affects the nitrate chemistry of the shallow groundwater underneath the farmland to a large extent. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

7.
Traditional aquifer vulnerability techniques primarily rely on spatial property data for a region and are limited by their ability to directly or indirectly assess flow and transport processes occurring from the surface to depth within an aquifer system. The main objective of this study was to investigate groundwater vulnerability in terms of aquifer interconnectivity and flow dynamics. A combination of stable isotopes, groundwater age‐dating (radiocarbon), and geomorphic/geogenic spatial analyses was applied to a regional, highly developed coastal aquifer to explain the presence of nitrate at depth. The average δ13C value (?17.3 ± 2‰ VPDB, n = 27) is characteristic of groundwater originating from locally infiltrated precipitation through extensively cultivated soils. The average δ18O and δD values (?4.0 ± 0.1‰ VSMOW, n = 27; δD: ?19.3 ± 1‰ VSMOW, n = 27, respectively) are similar to precipitation water derived from maritime sources feeding the region's surface water and groundwater. Stable and radioactive isotopes reveal significant mixing between shallow and deep aquifers due to high velocities, hydraulic connection, and input of local recharge water to depths. Groundwater overdevelopment enhances deeper and faster modern water downward flux, amplifying aquifer vulnerability. Therefore, aquifer vulnerability is a variable, dependent on the type and degree of stress conditions experienced by a groundwater system as well as the geospatial properties at the near surface.  相似文献   

8.
Environmental concerns regarding the potential for drinking water contamination in shallow aquifers have accompanied unconventional energy development in the northern Appalachian Basin. These activities have also raised several critical questions about the hydrogeological parameters that control the naturally occurring presence and migration of hydrocarbon gases in shallow aquifers within petroliferous basins. To interrogate these factors, we analyzed the noble gas, dissolved ion, and hydrocarbon gas (molecular and isotopic composition) geochemistry of 98 groundwater samples from south‐central New York. All samples were collected ?1km from unconventional drilling activities and sample locations were intentionally targeted based on their proximity to various types of documented fault systems. In agreement with studies from other petroliferous basins, our results show significant correlations between elevated levels of radiogenic [4He], thermogenic [CH4], and dissolved ions (e.g., Cl, Br, Sr, Ba). In combination, our data suggest that faults have facilitated the transport of exogenous hydrocarbon‐rich brines from Devonian source rocks into overlying Upper Devonian aquifer lithologies over geologic time. These data conflict with previous reports, which conclude that hydrodynamic focusing regulates the occurrence of methane and salt in shallow aquifers and leads to elevated levels of these species in restricted flow zones within valley bottoms. Instead, our data suggest that faults in Paleozoic rocks play a fundamental role in gas and brine transport from depth, regulate the distribution of their occurrence in shallow aquifers, and influence the geochemistry of shallow groundwater in this petroliferous basin.  相似文献   

9.
Unconventional natural gas extraction from tight sandstones, shales, and some coal‐beds is typically accomplished by horizontal drilling and hydraulic fracturing that is necessary for economic development of these new hydrocarbon resources. Concerns have been raised regarding the potential for contamination of shallow groundwater by stray gases, formation waters, and fracturing chemicals associated with unconventional gas exploration. A lack of sound scientific hydrogeological field observations and a scarcity of published peer‐reviewed articles on the effects of both conventional and unconventional oil and gas activities on shallow groundwater make it difficult to address these issues. Here, we discuss several case studies related to both conventional and unconventional oil and gas activities illustrating how under some circumstances stray or fugitive gas from deep gas‐rich formations has migrated from the subsurface into shallow aquifers and how it has affected groundwater quality. Examples include impacts of uncemented well annuli in areas of historic drilling operations, effects related to poor cement bonding in both new and old hydrocarbon wells, and ineffective cementing practices. We also summarize studies describing how structural features influence the role of natural and induced fractures as contaminant fluid migration pathways. On the basis of these studies, we identify two areas where field‐focused research is urgently needed to fill current science gaps related to unconventional gas extraction: (1) baseline geochemical mapping (with time series sampling from a sufficient network of groundwater monitoring wells) and (2) field testing of potential mechanisms and pathways by which hydrocarbon gases, reservoir fluids, and fracturing chemicals might potentially invade and contaminate useable groundwater.  相似文献   

10.
Karst aquifers are well known for their intricate stratigraphy and geologic structures, which make groundwater characterization challenging because flowpaths and recharge sources are complex and difficult to evaluate. Geochemical data, collected from ten closely spaced production wells constructed in two karst aquifers (Bangor Limestone (Mb) and Tuscumbia Limestone/Fort Payne Chert (Mftp)) in Trussville, north‐central Alabama, illustrate two distinctive groundwater end‐members: (1) higher major ion, dissolved inorganic carbon, conductivity, alkalinity concentrations, heavier δ13C ratios (max: −10.2 ± 0.2‰ Vienna Pee Dee Belemnite (PDB)) and lower residence times (mean: 19.5 ± 2 years, n = 2) of groundwater in the Mb aquifer and (2) lower constituent concentrations, lighter δ13C ratios (min: −13.4 ± 0.2‰ PDB) and longer residence times of groundwater (mean: 23.6 ± 2 years, n = 4) in the Mftp aquifer. Summer and fall data and the binary mixing model show aquifer inter‐flow mixing along solution fractures and confirms the distinctive groundwater geochemistry of the two aquifers. Lowering of static water levels over the summer (drawdown from 2 to 5.2 m) leads to more reducing groundwater conditions (lower Eh values) and slightly enriched δ18O and δD ratios during the fall [δ18O: −4.8 ± 0.1 to −5.4 ± 0.1‰ Vienna Standard Mean Oceanic Water (VSMOW), n = 9; δD: −25.4 ± 1 to −27.4 ± 1‰ VSMOW, n = 9] when compared with summer season samples (δ18O: −5.1 ± 0.1 to −5.7 ± 0.1‰ VSMOW, n = 11; δD: −25.0 ± 1 to −30.6 ± 1‰ VSMOW, n = 11). GIS analyses confirm the localized origin of recharge to the investigated aquifers. The combination of GIS, field parameters and geochemistry analyses can be successfully used to identify recharge sources, evaluate groundwater flow and transport pathways and to improve understanding of how groundwater withdrawals impact the sustainability and susceptibility to contamination of karst aquifers. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

11.
The coastal confined aquifer in the Gulf of Urabá (Colombia) is an important water source for the banana agro‐industry as well as for urban and rural communities. However, the main processes controlling recharge and mixing in the aquifer are still poorly understood. Hydrochemical analyses and stable isotope monitoring were conducted to (a) determine groundwater recharge origin, mean groundwater age, and the main processes governing groundwater chemistry and the potential mixing of marine water and the influence of diffusive processes from the two surrounding aquitard layers. Hydrochemical data indicate that the main processes affecting the dissolved chemical composition include cation exchange, dissolution of carbonated and CO2, and silicate weathering. δ18O and δ2H compositions combined with 14C data highlight the differences in climatic conditions between the recharge zone and the confined section of the aquifer, which is close to the Atlantic Ocean. Groundwater samples with 14C ages from recent to 28,300 years BP show a depleted isotopic trend ranging from ?6.43‰ to ?9.14‰ in δ18O and from ?43.2‰ to ?65.7‰ in δ2H. The most depleted δ18O and δ2H compositions suggest a cooler recharge climate than the current conditions (corresponding to the last glacial period of the late Pleistocene). Depleted δ13C values in the total dissolved inorganic carbon indicate the existence of organic material oxidation processes within the geologic formation. These results can be used or transferred to enhance groundwater modelling efforts in other confined coastal aquifers of South America where scarcity of long‐term monitoring data limits water resources planification under a changing climate.  相似文献   

12.
We characterize the precipitation and groundwater in a mountainous (peaks slightly above 3000 m a.s.l.), semi‐arid river basin in SE Spain in terms of the isotopes 18O and 2H. This basin, with an extension of about 7000 km2, is an ideal site for such a study because fronts from the Atlantic and the Mediterranean converge here. Much of the land is farmed and irrigated both by groundwater and runoff water collected in reservoirs. A total of approximately 100 water samples from precipitation and 300 from groundwater have been analysed. To sample precipitation we set up a network of 39 stations at different altitudes (800–1700 m a.s.l.), with which we were able to collect the rain and snowfall from 29 separate events between July 2005 and April 2007 and take monthly samples during the periods of maximum recharge of the aquifers. To characterize the groundwater we set up a control network of 43 points (23 springs and 20 wells) to sample every 3 months the main aquifers and both the thermal and non‐thermal groundwater. We also sampled two shallow‐water sites (a reservoir and a river). The isotope composition of the precipitation forms a local meteoric water line (LMWL) characterized by the equation δD = 7·72δ18O + 9·90, with mean values for δ18O and δD of − 10·28‰ and − 69·33‰, respectively, and 12·9‰ for the d‐excess value. To correlate the isotope composition of the rainfall water with groundwater we calculated the weighted local meteoric water line (WLMWL), characterized by the equation δD = 7·40δ18O + 7·24, which takes into account the quantity of water precipitated during each event. These values of (dδD/dδ18O)< 8 and d‐excess (δD–8δ18O)< 10 in each curve bear witness to the ‘amount effect’, an effect which is more manifest between May and September, when the ground temperature is higher. Other effects noted in the basin were those of altitude and the continental influence. The isotopic compositions of the groundwater are represented by the equation δD = 4·79δ18O − 18·64. The groundwater is richer in heavy isotopes than the rainfall, with mean values of − 8·48‰ for δ18O and − 59·27‰ for δD. The isotope enrichment processes detected include a higher rate of evaporation from detrital aquifers than from carbonate ones, the effects of recharging aquifers from irrigation return flow and/or from reservoirs' leakage and enrichment in δ18O from thermal water. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

13.
The vertical portion of a shale gas well, known as the “tophole” is often drilled using an air‐hammer bit that may introduce pressures as high as 2400 kPa (350 psi) into groundwater while penetrating shallow aquifers. A 3‐D TOUGH2 model was used to simulate the flow of groundwater under the high hydraulic heads that may be imposed by such trapped compressed air, based on an observed case in West Virginia (USA) in 2012. The model realizations show that high‐pressure air trapped in aquifers may cause groundwater to surge away from the drill site at observable velocities. If dissolved methane is present within the aquifer, the methane can be entrained and transported to a maximum distance of 10.6 m per day. Results from this study suggest that one cause of the reported increase in methane concentrations in groundwater near shale gas production wells may be the transport of pre‐existing methane via groundwater surges induced by air drilling, not necessarily direct natural gas leakage from the unconventional gas reservoir. The primary transport mechanisms are advective transport of dissolved methane with water flow, and diffusive transport of dissolved methane.  相似文献   

14.
Understanding the source of dissolved methane in drinking‐water aquifers is critical for assessing potential contributions from hydraulic fracturing in shale plays. Shallow groundwater in the Texas portion of the Haynesville Shale area (13,000 km2) was sampled (70 samples) for methane and other dissolved light alkanes. Most samples were derived from the fresh water bearing Wilcox formations and show little methane except in a localized cluster of 12 water wells (17% of total) in a approximately 30 × 30 km2 area in Southern Panola County with dissolved methane concentrations less than 10 mg/L. This zone of elevated methane is spatially associated with the termination of an active fault system affecting the entire sedimentary section, including the Haynesville Shale at a depth more than 3.5 km, and with shallow lignite seams of Lower Wilcox age at a depth of 100 to 230 m. The lignite spatial extension overlaps with the cluster. Gas wetness and methane isotope compositions suggest a mixed microbial and thermogenic origin with contribution from lignite beds and from deep thermogenic reservoirs that produce condensate in most of the cluster area. The pathway for methane from the lignite and deeper reservoirs is then provided by the fault system.  相似文献   

15.
The hydrology of oxygen‐18 (18O) isotopes was monitored between 1995 and 1998 in the Allt a' Mharcaidh catchment in the Cairngorm Mountains, Scotland. Precipitation (mean δ18O=−7·69‰) exhibited strong seasonal variation in δ18O values over the study period, ranging from −2·47‰ in the summer to −20·93‰ in the winter months. As expected, such variation was substantially damped in stream waters, which had a mean and range of δ18O of −9·56‰ and −8·45 to −10·44‰, respectively. Despite this, oxygen‐18 proved a useful tracer and streamwater δ18O variations could be explained in terms of a two‐component mixing model, involving a seasonally variable δ18O signature in storm runoff, mixing with groundwater characterized by relatively stable δ18O levels. Variations in soil water δ18O implied the routing of depleted spring snowmelt and enriched summer rainfall into streamwaters, probably by near‐surface hydrological pathways in peaty soils. The relatively stable isotope composition of baseflows is consistent with effective mixing processes in shallow aquifers at the catchment scale. Examination of the seasonal variation in δ18O levels in various catchment waters provided a first approximation of mean residence times in the major hydrological stores. Preliminary estimates are 0·2–0·8 years for near‐surface soil water that contributes to storm runoff and 2 and >5 years for shallow and deeper groundwater, respectively. These 18O data sets provide further evidence that the influence of groundwater on the hydrology and hydrochemistry of upland catchments has been underestimated. Copyright © 2000 John Wiley & Sons, Ltd.  相似文献   

16.
The stable isotope ratios of groundwater sulfate (34S/32S, 18O/16O) are often used as tracers to help determine the origin of groundwater or groundwater contaminants. In agricultural watersheds, little is known about how the increased use of sulfur as a soil amendment to optimize crop production is affecting the isotopic composition of groundwater sulfate, especially in shallow aquifers. We investigated the isotopic composition of synthetic agricultural fertilizers and groundwater sulfate in an area of intensive agricultural activity, in Ontario, Canada. Groundwater samples from an unconfined surficial sand aquifer (Lake Algonquin Sand Aquifer) were analyzed from multi-level monitoring wells, riverbank seeps, and private domestic wells. Fertilizers used in the area were analyzed for sulfur/sulfate content and stable isotopic composition (δ18O and/or δ34S). Fertilizers were isotopically distinct from geological sources of groundwater sulfate in the watershed and groundwater sulfate exhibited a wide range of δ34S (−6.9 to +20.0‰) and δ18O (−5.0 to +13.7‰) values. Quantitative apportionment of sulfate sources based on stable isotope data alone was not possible, largely because two of the potential fertilizer sulfate sources had an isotopic composition on the mixing line between two natural geological sources of sulfate in the aquifer. This study demonstrates that, when sulfate isotope analysis is being used as a tracer or co-tracer of the origin of groundwater or of contaminants in groundwater, sulfate derived from synthetic fertilizer needs to be considered as a potential source, especially when other parameters such as nitrate independently indicate fertilizer impacts to groundwater quality.  相似文献   

17.
Horizontal drilling and hydraulic fracturing have enabled hydrocarbon recovery from unconventional reservoirs, but led to natural gas contamination of shallow groundwaters. We describe and apply numerical models of gas‐phase migration associated with leaking natural gas wells. Three leakage scenarios are simulated: (1) high‐pressure natural gas pulse released into a fractured aquifer; (2) continuous slow leakage into a tilted fractured formation; and (3) continuous slow leakage into an unfractured aquifer with fluvial channels, to facilitate a generalized evaluation of natural gas transport from faulty natural gas wells. High‐pressure pulses of gas leakage into sparsely fractured media are needed to produce the extensive and rapid lateral spreading of free gas previously observed in field studies. Transport in fractures explains how methane can travel vastly different distances and directions laterally away from a leaking well, which leads to variable levels of methane contamination in nearby groundwater wells. Lower rates of methane leakage (≤1 Mcf/day) produce shorter length scales of gas transport than determined by the high‐pressure scenario or field studies, unless aquifers have low vertical permeabilities (≤1 millidarcy) and fractures and bedding planes have sufficient tilt (～10°) to allow a lateral buoyancy component. Similarly, in fractured rock aquifers or where permeability is controlled by channelized fluvial deposits, lateral flow is not sufficiently developed to explain fast‐developing gas contamination (0‐3 months) or large length scales (～1 km) documented in field studies. Thus, current efforts to evaluate the frequency, mechanism, and impacts of natural gas leakage from faulty natural gas wells likely underestimate contributions from small‐volume, low‐pressure leakage events.  相似文献   

18.
Recharge areas of the Guarani Aquifer System (GAS) are particularly sensitive and vulnerable to climate variability; therefore, the understanding of infiltration mechanisms for aquifer recharge and surface run‐off generation represent a relevant issue for water resources management in the southeastern portion of the Brazilian territory, particularly in the Jacaré‐Pepira River watershed. The main purpose of this study is to understand the interactions between precipitation, surface water, and groundwater using stable isotopes during the strong 2014–2016 El Niño Southern Oscillation event. The large variation in the isotopic composition of precipitation (from ?9.26‰ to +0.02‰ for δ18O and from ?63.3‰ to +17.6‰ for δ2H), mainly associated with regional climatic features, was not reflected in the isotopic composition of surface water (from ?7.84‰ to ?5.83‰ for δ18O and from ?49.7‰ to +33.6‰ for δ2H), mainly due to the monthly sampling frequency, and groundwater (from ?7.04‰ to ?7.76‰ for δ18O and from ?49.5‰ to ?44.7‰ for δ2H), which exhibited less variation throughout the year. However, variations in deuterium excess (d‐excess) in groundwater and surface water suggest the occurrence of strong secondary evaporation during the infiltration process, corresponding with groundwater level recovery. Similar isotopic composition in groundwater and surface water, as well as the same temporal variations in d‐excess and line‐conditioned excess denote the strong connectivity between these two reservoirs during baseflow recession periods. Isotopic mass balance modelling and hydrograph separation estimate that the groundwater contribution varied between 70% and 80%, however, during peak flows, the isotopic mass balance tends to overestimate the groundwater contribution when compared with the other hydrograph separation methods. Our findings indicate that the application of isotopic mass balance methods for ungauged rivers draining large groundwater reservoirs, such as the GAS outcrop, could provide a powerful tool for hydrological studies in the future, helping in the identification of flow contributions to river discharge draining these areas.  相似文献   

19.
The impact of landfill contaminated groundwater along a reach of a small stream adjacent to a municipal landfill was investigated using stable carbon isotopes as a tracer. Groundwater below the stream channel, groundwater seeping into the stream, groundwater from the stream banks and stream water were sampled and analysed for dissolved inorganic carbon (DIC) and the isotope ratio of DIC (δ13CDIC). Representative samples of groundwater seeping into the stream were collected using a device (a ‘seepage well’) specifically designed for collecting samples of groundwater seeping into shallow streams with soft sediments. The DIC and δ13CDIC of water samples ranged from 52 to 205 mg C/L and ?16·9 to +5·7‰ relative to VPDB standard, respectively. Groundwater from the stream bank adjacent to the landfill and some samples of groundwater below the stream channel and seepage into the stream showed evidence of δ13C enriched DIC (δ13CDIC = ?2·3 to +5·7‰), which we attribute to landfill impact. Stream water and groundwater from the stream bank opposite the landfill did not show evidence of landfill carbon (δ13CDIC = ?10·0 to ?16·9‰). A simple mixing model using DIC and δ13CDIC showed that groundwater below the stream and groundwater seeping into the stream could be described as a mixture of groundwater with a landfill carbon signature and uncontaminated groundwater. This study suggests that the hyporheic zone at the stream–groundwater interface probably was impacted by landfill contaminated groundwater and may have significant ecological implications for this ecotone. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

20.
Groundwater in the Bengal Basin is badly polluted by arsenic (As) which adversely affects human health. To provide low‐As groundwater for As mitigation, it was sought across 235 km2 of central West Bengal, in the western part of the basin. By drilling 76 boreholes and chemical analysis of 535 water wells, groundwater with <10 µg/L As in shallow aquifers was found under one‐third of a study area. The groundwater is in late Pleistocene palaeo‐interfluvial aquifers of weathered brown sand that are capped by a palaeosol of red clay. The aquifers form two N‐S trending lineaments that are bounded on the east by an As‐polluted deep palaeo‐channel aquifer and separated by a shallower palaeo‐channel aquifer. The depth to the top of the palaeo‐interfluvial aquifers is mostly between 35 and 38 m below ground level (mbgl). The palaeo‐interfluvial aquifers are overlain by shallow palaeo‐channel aquifers of gray sand in which groundwater is usually As‐polluted. The palaeosol now protects the palaeo‐interfluvial aquifers from downward migration of As‐polluted groundwater in overlying shallow palaeo‐channel aquifers. The depth to the palaeo‐interfluvial aquifers of 35 to 38 mbgl makes the cost of their exploitation affordable to most of the rural poor of West Bengal, who can install a well cheaply to depths up to 60 mbgl. The protection against pollution afforded by the palaeosol means that the palaeo‐interfluvial aquifers will provide a long‐term source of low‐As groundwater to mitigate As pollution of groundwater in the shallower, heavily used, palaeo‐channel aquifers. This option for mitigation is cheap to employ and instantly available.  相似文献   

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