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1.
The U.S. EPA 2000 Radionuclide Rule established a maximum contaminant level (MCL) for uranium of 30 µg/L. Many small community water supplies are struggling to comply with this new regulation. At one such community, direct push (DP) methods were applied to obtain hydraulic profiling tool (HPT) logs and install small diameter wells in a section of alluvial deposits located along the Platte River. This work was conducted to evaluate potential sources of elevated uranium in the Clarks, Nebraska drinking water supply. HPT logs were used to understand the hydrostratigraphy of a portion of the aquifer and guide placement of small diameter wells at selected depth intervals. Low-flow sampling of the wells provided water quality parameters and samples for analysis to study the distribution of uranium and variations in aquifer chemistry. Contrary to expectations, the aquifer chemistry revealed that uranium was being mobilized under anoxic and reducing conditions. Review of the test well and new public water supply well construction details revealed that filter packs extended significantly above the screened intervals of the wells. These filter packs were providing a conduit for the movement of groundwater with elevated concentrations of uranium into the supply wells and the community drinking water supply. The methods applied and lessons learned here may be useful for the assessment of unconsolidated aquifers for uranium, arsenic, and many other drinking water supply contaminants.  相似文献   

2.
Arsenic in groundwater is a serious problem in New England, particularly for domestic well owners drawing water from bedrock aquifers. The overlying glacial aquifer generally has waters with low arsenic concentrations but is less used because of frequent loss of well water during dry periods and the vulnerability to surface‐sourced bacterial contamination. An alternative, novel design for shallow wells in glacial aquifers is intended to draw water primarily from unconsolidated glacial deposits, while being resistant to drought conditions and surface contamination. Its use could greatly reduce exposure to arsenic through drinking water for domestic use. Hypothetical numerical models were used to investigate the potential hydraulic performance of the new well design in reducing arsenic exposure. The aquifer system was divided into two parts, an upper section representing the glacial sediments and a lower section representing the bedrock. The location of the well, recharge conditions, and hydraulic properties were systematically varied in a series of simulations and the potential for arsenic contamination was quantified by analyzing groundwater flow paths to the well. The greatest risk of arsenic contamination occurred when the hydraulic conductivity of the bedrock aquifer was high, or where there was upward flow from the bedrock aquifer because of the position of the well in the flow system.  相似文献   

3.
Arsenic concentrations can be managed with a relatively simple strategy of grouting instead of completely destroying a selected interval of well. The strategy of selective grouting was investigated in Antelope Valley, California, where groundwater supplies most of the water demand. Naturally occurring arsenic typically exceeds concentrations of 10 µg/L in the water produced from these long-screened wells. The vertical distributions of arsenic concentrations in intervals of the aquifer contributing water to selected supply wells were characterized with depth-dependent water-quality sampling and flow logs. Arsenic primarily entered the lower half of the wells where lacustrine clay deposits and a deeper aquifer occurred. Five wells were modified by grouting from below the top of the lacustrine clay deposits to the bottom of the well, which reduced produced arsenic concentrations to less than 2 µg/L in four of the five wells. Long-term viability of well modification and reduction of specific capacity was assessed for well 4-54 with AnalyzeHOLE, which creates and uses axisymmetric, radial MODFLOW models. Two radial models were calibrated to observed borehole flows, drawdowns, and transmissivity by estimating hydraulic-conductivity values in the aquifer system and gravel packs of the original and modified wells. Lithology also constrained hydraulic-conductivity estimates as regularization observations. Well encrustations caused as much as 2 µg/L increase in simulated arsenic concentration by reducing the contribution of flow from the aquifer system above the lacustrine clay deposits. Simulated arsenic concentrations in the modified well remained less than 3 µg/L over a 20-year period.  相似文献   

4.
Deposition from at least three episodes of glaciation left a complex glacial-drift aquifer system in central Illinois. The deepest and largest of these aquifers, the Sankoty-Mahomet Aquifer, occupies the lower part of a buried bedrock valley and supplies water to communities throughout central Illinois. Thin, discontinuous aquifers are present within glacial drift overlying the Sankoty-Mahomet Aquifer. This study was commissioned by local governments to identify possible areas where a regional water supply could be obtained from the aquifer with minimal adverse impacts on existing users. Geologic information from more than 2,200 existing water well logs was supplemented with new data from 28 test borings, water level measurements in 430 wells, and 35 km of surface geophysical profiles. A three-dimensional (3-D) hydrostratigraphic model was developed using a contouring software package, a geographic information system (GIS), and the 3-D geologic modeling package, EarthVision. The hydrostratigraphy of the glacial-drift sequence was depicted as seven uneven and discontinuous layers, which could be viewed from an infinite number of horizontal and vertical slices and as solid models of any layer. Several iterations were required before the 3-D model presented a reasonable depiction of the aquifer system. Layers from the resultant hydrostratigraphic model were imported into MODFLOW, where they were modified into continuous layers. This approach of developing a 3-D hydrostratigraphic model can be applied to other areas where complex aquifer systems are to be modeled and is also useful in helping lay audiences visualize aquifer systems.  相似文献   

5.
Water from the San Joaquin Delta, having chloride concentrations up to 3590 mg/L, has intruded fresh water aquifers underlying Stockton, California. Changes in chloride concentrations at depth within these aquifers were evaluated using sequential electromagnetic (EM) induction logs collected during 2004 through 2007 at seven multiple‐well sites as deep as 268 m. Sequential EM logging is useful for identifying changes in groundwater quality through polyvinyl chloride‐cased wells in intervals not screened by wells. These unscreened intervals represent more than 90% of the aquifer at the sites studied. Sequential EM logging suggested degrading groundwater quality in numerous thin intervals, typically between 1 and 7 m in thickness, especially in the northern part of the study area. Some of these intervals were unscreened by wells, and would not have been identified by traditional groundwater sample collection. Sequential logging also identified intervals with improving water quality—possibly due to groundwater management practices that have limited pumping and promoted artificial recharge. EM resistivity was correlated with chloride concentrations in sampled wells and in water from core material. Natural gamma log data were used to account for the effect of aquifer lithology on EM resistivity. Results of this study show that a sequential EM logging is useful for identifying and monitoring the movement of high‐chloride water, having lower salinities and chloride concentrations than sea water, in aquifer intervals not screened by wells, and that increases in chloride in water from wells in the area are consistent with high‐chloride water originating from the San Joaquin Delta rather than from the underlying saline aquifer.  相似文献   

6.
In the upper Midwest, USA, elevated arsenic concentrations in public drinking water systems are associated with the lateral extent of northwest provenance late Wisconsin-aged drift. Twelve percent of public water systems located within the footprint of this drift (212 of 1764) exceed 10 microg/L arsenic, which is the U.S. EPA's drinking water standard. Outside of the footprint, only 2.4% of public water systems (52 of 2182) exceed 10 microg/L arsenic. Both glacial drift aquifers and shallow bedrock aquifers overlain by northwest provenance late Wisconsin-aged sediment are affected by arsenic contamination. Evidence suggests that the distinct physical characteristics of northwest provenance late Wisconsin-aged drift--its fine-grained matrix and entrained organic carbon that fosters biological activity--cause the geochemical conditions necessary to mobilize arsenic via reductive mechanisms such as reductive desorption and reductive dissolution of metal oxides. This study highlights an important and often unrecognized phenomenon: high-arsenic sediment is not necessary to cause arsenic-impacted ground water--when "impacted" is now defined as >10 microg/L. This analysis also demonstrates the scientific and economic value of using existing large but imperfect statewide data sets to observe and characterize regional-scale environmental problems.  相似文献   

7.
Warner KL 《Ground water》2001,39(3):433-442
The lower Illinois River Basin (LIRB) covers 47,000 km2 of central and western Illinois. In the LIRB, 90% of the ground water supplies are from the deep and shallow glacial drift aquifers. The deep glacial drift aquifer (DGDA) is below 152 m altitude, a sand and gravel deposit that fills the Mahomet Buried Bedrock Valley, and overlain by more than 30.5 m of clayey till. The LIRB is part of the USGS National Water Quality Assessment program, which has an objective to describe the status and trends of surface and ground water quality. In the DGDA, 55% of the wells used for public drinking-water supply and 43% of the wells used for domestic drinking water supply have arsenic concentrations above 10 micrograms/L (a new U.S. EPA drinking water standard). Arsenic concentrations greater than 25 micrograms/L in ground water are mostly in the form of arsenite (AsIII). The proportion of arsenate (AsV) to arsenite does not change along the flowpath of the DGDA. Because of the limited number of arsenic species analyses, no clear relations between species and other trace elements, major ions, or physical parameters could be established. Arsenic and barium concentrations increase from east to west in the DGDA and are positively correlated. Chloride and arsenic are positively correlated and provide evidence that arsenic may be derived locally from underlying bedrock. Solid phase geochemical analysis of the till, sand and gravel, and bedrock show the highest presence of arsenic in the underlying organic-rich carbonate bedrock. The black shale or coal within the organic-rich carbonate bedrock is a potential source of arsenic. Most high arsenic concentrations found in the DGDA are west and downgradient of the bedrock structural features. Geologic structures in the bedrock are potential pathways for recharge to the DGDA from surrounding bedrock.  相似文献   

8.
Patterns and Rates of Ground-Water Flow on Long Island, New York   总被引：3，自引：0，他引：3  
Increased ground-water contamination from human activities on Long Island has prompted studies to define the pattern and rate of ground-water movement. A two-dimensional, fine-mesh, finite-element model consisting of 11,969 nodes and 22,880 elements was constructed to represent ground-water flow along a north-south section through central Long Island. The model represents average hydrologic conditions within a corridor approximately 15 miles wide. The model solves discrete approximations of both the potential and stream functions. The resulting flownet depicts flow paths and defines the vertical distribution of flow within the section. Ground-water flow rates decrease with depth. Sixty-two percent of the water flows no deeper than the upper glacial (water-table) aquifer, 38 percent enters the underlying Magothy aquifer, and only 3.1 percent enters the Lloyd aquifer. The limiting streamlines for flow to the Magothy and Lloyd aquifers indicate that aquifer recharge areas are narrow east-west bands through the center of the island. The recharge area of the Magothy aquifer is only 5.4 miles wide; that of the Lloyd aquifer is less than 0.5 miles. The distribution of ground-water traveltime and a flownet are calculated from model results; both are useful in the investigation of contaminant transport or the chemical evolution of ground water within the flow system. A major discontinuity in traveltime occurs across the streamline which separates the flow subsystems of the two confined aquifers. Water that reaches the Lloyd aquifer attains traveltimes as high as 10,000 years, whereas water that has not penetrated deeper than the Magothy aquifer attains traveltimes of only 2,000 years. The finite-element approach used in this study is particularly suited to ground-water systems that have complex hydrostratigraphy and cross-sectional symmetry.  相似文献   

9.
David F. Boutt 《水文研究》2017,31(7):1479-1497
This study analyzes a long‐term regional compilation of water table response to climate variability based on 124 long‐term groundwater wells distributed across New England, USA, screened in a variety of geologic materials. The New England region of the USA is located in a humid‐temperature climate underlain by low‐storage‐fractured metamorphic and crystalline bedrock dissected by north–south trending valleys filled with glacial and post‐glacial valley fill sediments. Uplands are covered by thin glacial till that comprises more than 60% of the total area. Annual and multi‐annual responses of the water table to climate variability are assessed to understand how local hydraulic properties and hydrogeologic setting (located in recharge/discharge region) of the aquifer influence the hydrologic sensitivity of the aquifer system to climate variability. This study documents that upland aquifer systems dominated by thin deposits of surface till comprise ~70% of the active and dynamic storage of the region. Total aquifer storage changes of +5 to ?7 km3 occur over the region during the study interval. The storage response is dominated by thin and low permeability surficial till aquifer that fills and drains on a multi‐annual basis and serves as the main mechanism to deliver water to valley fill aquifers and underlying bedrock aquifers. Whereas the till aquifer system is traditionally neglected as an important storage reservoir, this study highlights the importance of a process‐based understanding of how different landscape hydrogeologic units contribute to the overall hydrologic response of a region.  相似文献   

10.
A study was conducted to evaluate monitored natural attenuation (MNA) as a remedy for arsenic in groundwater at a former phosphate mining and manufacturing facility. The mineralogy, speciation, and lability of arsenic in phosphatic wastes present in soils were characterized using sequential extraction procedures, leaching experiments, batch adsorption tests, and microchemical speciation analysis. A PHREEQC-based reactive transport model was also parameterized using these laboratory results, and it was used to evaluate the importance of identified attenuation mechanisms on arsenic concentrations along a vertical flow path from a shallow, alluvial aquifer to the underlying Floridan aquifer. Arsenic was found to occur in several chemical forms in phosphatic wastes, including unstable sulfide minerals, adsorbed surface complexes, and relatively insoluble phosphate and oxide minerals. Most arsenic was associated with stable minerals. The reactive transport model predicted that historical leaching of solid-phase waste materials in soils would not have generated enough arsenic to explain the concentrations observed in downgradient groundwater; instead, the source of arsenic to groundwater was likely acidic and saline process water that infiltrated though unlined ponds and ditches during historical manufacturing operations. A key factor affecting the long-term effectiveness of natural attenuation of arsenic in groundwater is the occurrence and stability of iron oxyhydroxides in aquifer sediments. According to laboratory and reactive transport model results, sufficient levels were found to be present at the site to effectively limit arsenic migration at concentrations exceeding drinking water standards in the future in the Floridan aquifer. This study presents the geochemical evaluations that are needed to satisfy EPA guidelines on determining whether or not MNA is an acceptable remedy for a site. It specifically details the characterization and modeling that were used to demonstrate effectiveness at a site where MNA was ultimately selected as the remedy for arsenic in groundwater.  相似文献   

11.
The buried-valley aquifers that are common in the glacial deposits of the northern hemisphere are a typical case of the strip aquifers that occur in many parts of the world. Pumping from a narrow strip aquifer leads to much greater drawdown and much more distant drawdown effects then would occur in a sheet aquifer with a similar transmissivity and storage coefficient. Widely used theories for radial flow to wells, such as the Theis equation, are not appropriate for narrow strip aquifers. Previously published theory for flow to wells in semiconfined strip aquifers is reviewed and a practical format of the type curves for pumping-test analysis is described. The drawdown response of strip aquifers to pumping tests is distinctive, especially for observation wells near the pumped well. A case study is presented, based on extensive pumping test experience for the Estevan Valley Aquifer in southern Saskatchewan, Canada. Evaluation of groundwater resources in such buried-valley aquifers needs to take into account the unusually large drawdowns in response to pumping.  相似文献   

12.
This study characterized the redox conditions in arsenic‐affected groundwater aquifers of the Lanyang plain, Taiwan. Discriminant analysis was adopted to delineate three redox zones (oxidative, transitional and reductive zones) in different aquifers and yielded 92·3% correctness on groundwater quality data. Arsenic is mainly distributed in the reductive zone, and arsenic distribution in the shallow aquifer is mainly affected by surface activities. According to PHREEQC modelling results, possible mechanisms for arsenic release to groundwater in Lanyang plain are explored. Arsenic released to groundwater in the oxidative zone (zone 1) is primarily caused by the oxidations of arsenic‐bearing pyrite minerals, and arsenate is the predominant species. While the reductive dissolution of Fe‐oxides are responsible for the high arsenic concentration found in the transitional and reductive zones (zones 2 and 3), arsenite is the predominant species. The reduction potential of groundwater rises as the depths and zones increase. Some sulphates may be reduced to form sulphide ions, which then react with arsenic to form arseno‐sulphide deposits (such as realgar, orpiment) and then slightly lower groundwater arsenic concentrations. A conceptual diagram which summarized the possible release processes of arsenic in different redox zones along groundwater flow in Lanyang plain is postulated. Arsenic‐bearing pyrite and arsenopyrite (FeAsS) are oxidized as they are exposed to the infiltrated oxygenated rainwater, releasing soluble arsenate Fe(II) and SO42? into zone 1. The dissolution of arsenic‐rich Fe‐oxides due to the onset of reducing conditions in zones 2 and 3 is responsible for the mobility of arsenic and likely to be the primary mechanism of arsenic release to groundwater in the Lanyang plain Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

13.
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   

14.
Arsenic in glacial aquifers: sources and geochemical controls   总被引：1，自引：0，他引：1  
A total of 176 wells in sand-and-gravel glacial aquifers in central Illinois were sampled for arsenic (As) and other chemical parameters. The results were combined with archived and published data from several hundred well samples to determine potential sources of As and the potential geochemical controls on its solubility and mobility. There was considerable spatial variability in the As concentrations. High concentrations were confined to areas smaller than 1 km in diameter. Arsenic and well depth were uncorrelated. Arsenic solubility appeared to be controlled by oxidation-reduction (redox) conditions, especially the presence of organic matter. Geochemical conditions in the aquifers are typically reducing, but only in the most reducing water does As accumulate in solution. In wells in which total organic carbon (TOC) was below 2 mg/L and sulfate (SO4(2-)) was present, As concentrations were low or below the detection limit (0.5 microg/L). Arsenic concentrations >10 microg/L were almost always found in wells where TOC was >2 mg/L and SO4(2-) was absent or at low concentrations, indicating post-SO4 (2-)reducing conditions. Iron (Fe) is common in the aquifer sediments, and Fe oxide reduction appears to be occurring throughout the aquifers. Arsenic is likely released from the solid phase as Fe oxide is reduced.  相似文献   

15.
Ammonium occurrence in a salinized lowland coastal aquifer (Ferrara,Italy)   总被引：1，自引：0，他引：1  
To understand the spatial distribution and the possible sources of high ammonium concentration in the unconfined coastal aquifer of the Po River Delta lowland (Italy), a detailed characterization of inorganic nitrogen species was completed. A total of 59 boreholes were drilled to collect core samples and groundwater samples of the aquifer. Among them, four piezometers, located along a representative flow line, were chosen to monitor the inorganic nitrogen concentrations with depth. Ammonium is the prevalent nitrogen inorganic species in groundwater, and its concentration increases with depth and salinity. Very high ammonium concentrations are found in coincidence with peaty sediments in the salinized anoxic aquifer and in the low‐lying aquitard. In particular, the elevated ammonium concentration derives from mineralization of organic matter present in fine sediments deposited in paleo‐marsh environments. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

16.
Field-based experiments were designed to investigate the release of naturally occurring, low to moderate (< 50 microg/L) arsenic concentrations to well water in a confined sandstone aquifer in northeastern Wisconsin. Geologic, geochemical, and hydrogeologic data collected from a 115 m2 site demonstrate that arsenic concentrations in ground water are heterogeneous at the scale of the field site, and that the distribution of arsenic in ground water correlates to solid-phase arsenic in aquifer materials. Arsenic concentrations in a test well varied from 1.8 to 22 microg/L during experiments conducted under no, low, and high pumping rates. The quality of ground water consumed from wells under typical domestic water use patterns differs from that of ground water in the aquifer because of reactions that occur within the well. Redox conditions in the well can change rapidly in response to ground water withdrawals. The well borehole is an environment conducive to microbiological growth, and biogeochemical reactions also affect borehole chemistry. While oxidation of sulfide minerals appears to release arsenic to ground water in zones within the aquifer, reduction of arsenic-bearing iron (hydr)oxides is a likely mechanism of arsenic release to water having a long residence time in the well borehole.  相似文献   

17.
Methods for predicting aquifer sensitivity to contamination typically ignore geochemical factors that affect the occurrence of contaminants such as nitrate. Use of geochemical information offers a simple and accurate method for estimating aquifer sensitivity to nitrate contamination. We developed a classification method in which nitrate-sensitive aquifers have dissolved oxygen concentrations > 1.0 mg/L, Eh values >250 mV, and either reduced iron concentrations < 0.1 mg/L or total iron concentrations < 0.7 mg/L. We tested the method in four Minnesota aquifer systems having different geochemical and hydrologic conditions. A surficial sand aquifer in central Minnesota exhibited geochemical zonation, with a rapid shift from aerobic to anaerobic conditions 5 m below the water table. A fractured bedrock aquifer in east-central Minnesota remained aerobic to depths of 50 m, except in areas where anaerobic ground water discharged upward from an underlying aquifer. A bedrock aquifer in southeast Minnesota exhibited aerobic conditions when overlain by surficial deposits lacking shale, whereas anaerobic conditions occurred under deposits that contained shale. Surficial sand aquifers in northwest Minnesota contained high concentrations of sulfate and were anaerobic throughout their extent. Nitrate-nitrogen was detected at concentrations exceeding 1 mg/L in 135 of 149 samples classified as sensitive. Nitrate was not detected in any of the 109 samples classified as not sensitive. We observed differences between our estimates of sensitivity and existing sensitivity maps, which are based on methods that do not consider aquifer geochemistry. Because dissolved oxygen, reduced iron, and Eh are readily measured in the field, use of geochemistry provides a quick and accurate way of assessing aquifer sensitivity to nitrate contamination.  相似文献   

18.
Surface water is a scarce resource in Namibia with about sixty percent of Namibia's population dependent on groundwater for drinking purposes. With increasing population, the country faces water challenges and thus groundwater resources need to be managed properly. One important aspect of Integrated Water Resources Management is the protection of water resources, including protection of groundwater from contamination and over-exploitation. This study explores vulnerability mapping as a basic tool for protecting groundwater resources from pollution. It estimates groundwater vulnerability to pollution in the upper Niipele sub-basin of the Cuvelai-Etosha in Northern Namibia using the DRASTIC index. The DRASTIC index uses GIS to estimate groundwater vulnerability by overlaying different spatially referenced hydrogeological parameters that affect groundwater contamination. The study assesses the discontinuous perched aquifer (KDP) and the Ohangwena multi-layered aquifer 1 (KOH-1). For perched aquifers, point data was regionalized by a hydrotope approach whereas for KOH-1 aquifer, inverse distance weighting was used. The hydrotope approach categorized different parts of the hydrogeological system with similar properties into five hydrotopes. The result suggests that the discontinuous perched aquifers are more vulnerable than Ohangwena multi-layered aquifer 1. This implies that vulnerability increases with decreasing depth to water table because contaminants have short travel time to reach the aquifer when they are introduced on land surface. The nitrate concentration ranges between 2 and 288 mg/l in perched aquifers while in Ohangwena multi-layered aquifer 1, it ranges between 1 and 133 mg/l. It was observed that perched aquifers have high nitrate concentrations than Ohangwena 1 aquifer, which correlates well with the vulnerability results.  相似文献   

19.
The coastal aquifers and inland waters of the Long Xuyen Quadrangle and Ca Mau Peninsula of southern Vietnam have been significantly impacted by sea water intrusion (SI) as a result of recent anthropogenic activities. This study identified the evolution and spatial distribution of hydrochemical conditions in coastal aquifers at this region using Hydrochemical Facies Evolution Diagram (HFE-D) and Geographical Information System mapping. Hydraulic heads and water chemistry were measured at 31 observation wells in four layered aquifers during dry and rainy seasons in early (2005), and more recent (2016), stages of agricultural development. Hydrochemical facies associated with intrusion or freshening stages were mapped in each aquifer after assigning mixing index values to each facies. The position of groundwater freshening and SI phases differed in Holocene, Upper Pleistocene, Middle Pleistocene, and Lower Pleistocene aquifers. The geographic position of freshening and intrusion fronts differ in dry and rainy seasons, and shifted after 11 years of groundwater abstraction in all four aquifers. The spatial and temporal differences in hydrochemical facies distributions according to HFE-D reflect the relative impact of SI in the four aquifers. The study results provide a better understanding of the evolution of groundwater quality associated with SI in a peninsular coastal aquifer system, and highlight the need for improving groundwater quality and management in similar coastal regions.  相似文献   

20.
Local surface water and stormflow were infiltrated intermittently from a 40-ha basin between September 2003 and September 2007 to determine the feasibility of recharging alluvial aquifers pumped for public supply, near Stockton, California. Infiltration of water produced a pressure response that propagated through unconsolidated alluvial-fan deposits to 125 m below land surface (bls) in 5 d and through deeper, more consolidated alluvial deposits to 194 m bls in 25 d, resulting in increased water levels in nearby monitoring wells. The top of the saturated zone near the basin fluctuates seasonally from depths of about 15 to 20 m. Since the start of recharge, water infiltrated from the basin has reached depths as great as 165 m bls. On the basis of sulfur hexafluoride tracer test data, basin water moved downward through the saturated alluvial deposits until reaching more permeable zones about 110 m bls. Once reaching these permeable zones, water moved rapidly to nearby pumping wells at rates as high as 13 m/d. Flow to wells through highly permeable material was confirmed on the basis of flowmeter logging, and simulated numerically using a two-dimensional radial groundwater flow model. Arsenic concentrations increased slightly as a result of recharge from 2 to 6 μg/L immediately below the basin. Although few water-quality issues were identified during sample collection, high groundwater velocities and short travel times to nearby wells may have implications for groundwater management at this and at other sites in heterogeneous alluvial aquifers.  相似文献   

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