Domestic Water Wells Research Articles

Regional Scale Assessment of Shallow Groundwater Vulnerability to Contamination from Unconventional Hydrocarbon Extraction.

Concerns over unconventional oil and gas (UOG) development persist, especially in rural communities that rely on shallow groundwater for drinking and other domestic purposes. Given the continued expansion of the industry, regional (vs local scale) models are needed to characterize groundwater contamination risks faced by the increasing proportion of the population residing in areas that accommodate UOG extraction. In this paper, we evaluate groundwater vulnerability to contamination from surface spills and shallow subsurface leakage of UOG wells within a 104,000 km2 region in the Appalachian Basin, northeastern USA. We test a computationally efficient ensemble approach for simulating groundwater flow and contaminant transport processes to quantify vulnerability with high resolution. We also examine metamodels, or machine learning models trained to emulate physically based models, and investigate their spatial transferability. We identify predictors describing proximity to UOG, hydrology, and topography that are important for metamodels to make accurate vulnerability predictions outside their training regions. Using our approach, we estimate that 21,000-30,000 individuals in our study area are dependent on domestic water wells that are vulnerable to contamination from UOG activities. Our novel modeling framework could be used to guide groundwater monitoring, provide information for public health studies, and assess environmental justice issues.

Methods for Estimating Locations of Housing Units Served by Private Domestic Wells in the United States Applied to 2010.

In 1990, the last time the decennial census included a question on domestic drinking water source, it was estimated that private domestic water wells (PDWs) supplied household water to about 15.1 million housing units (15% of the population) in the United States (U.S.). PDWs are not regulated by the Safe Drinking Water Act, and with few exceptions, are not subject to the water quality testing required of public water suppliers. We expanded two methods in estimating housing units reliant on PDWs from an Oklahoma pilot study (Weaver et al. 2017), nationally. Both use 1990 census data on drinking water sources as a baseline. The first method uses housing unit change and private well drilling logs for 20 states. This allows for the rate of well use to change between 1990 and 2010 in these states. The second, based solely on housing unit change, assumes a constant rate of well use. Ordinary least squares regression demonstrated (R2 = 0.78) that the methods yield similar estimates for nationwide well use. Using the housing unit change method, it is estimated that in 2010, 23 million housing units were reliant on PDWs (17% of the population). We provide these estimates at the census block group and census block resolution. This dataset will assist in a better understanding of the reliance on PDWs in the U.S., and position local, tribal, state, and national groups to better protect this water resource from contaminant sources.Research Impact Statement:The work provides improved estimates of the spatial distribution of housing units reliant on private domestic wells in the United States and a foundation to protect this water supply at all levels of government.

Using isotopes to determine the natural and anthropogenic processes influencing water quality in household wells of Chronic kidney disease of unknown origin (CKDu) endemic Medawachchiya area, Sri Lanka

The groundwater in the dry climatic, agricultural regions of Sri Lanka has been a focus of research for more than two decades as a likely contributor to the epidemic chronic kidney disease that does not have an identified causality (CKDu). The intense agriculture that heavily depends on irrigation and excessive use of agrochemicals, as well as the increasing population that prompt over-extraction and sewage discharge presents increasing threats to these groundwaters. Particulate organic matter (POM), stable environmental isotopes (13C, 15N, 18O and 2H) together with hydrochemistry were used to determine the natural and anthropogenic sources of solutes, and their dynamics in these waters. Twenty nine domestic drinking water wells and spring were sampled from CKDu endemic Medawachchiya area during the dry season of 2018. POM of most waters were found to have aquatic origin resulted from biological processes that assimilate nutrients in groundwater. The isotopic compositions (18O and 15N) of nitrate provided evidence for anthropogenic nitrate inputs in some wells and denitrification of potentially anthropogenic nitrates in others. These waters were recharged locally and subjected to varying degrees of evaporation. The water–rock interactions, evaporation, and anthropogenic inputs significantly influence shallow drinking water quality in wells (<4 m below ground surface). In contrast, deeper domestic drinking wells (<4 m below ground surface) potentially connect with deep groundwater. This study emphasises the importance of the integrated use of POM, environmental isotopes, and water chemistry in identifying the dynamics of solutes in groundwater, thereby determining the overall groundwater quality, especially in investigating complex environmental health problems like CKDu.

Arsenic concentrations after drinking water well installation: Time-varying effects on arsenic mobilization

Chronic exposure to geogenic arsenic via drinking water is a worldwide health concern. However, effects of well installation and operation on arsenic concentrations and mobilization are not well understood. This knowledge gap impacts both reliable detection of arsenic in drinking water and effective public health recommendations to reduce exposure to arsenic. This study examines changes in arsenic and redox geochemistry over one year following installation of 254 new domestic water wells in three regions of the north-central USA that commonly have elevated arsenic concentrations. Our regions' geologic settings share some important characteristics with other high-arsenic aquifers: igneous bedrock aquifers; or late Pleistocene-age glacial sand and gravel aquifers interbedded with aquitards. Over the study, arsenic concentrations increased by 16% or more in 25% of wells in glacial aquifer regions, and the redox conditions changed towards more reducing. In wells in the bedrock region, there was no significant change in arsenic concentrations, and redox conditions changed towards more oxidizing. Our findings illustrate the importance of understanding short- to moderate-term impacts of well installation and operation on arsenic and aqueous chemistry, as it relates to human exposure. Our study informs water quality sampling requirements, which currently do not consider the implications sampling timing with respect to well installation. Evaluating arsenic concentrations in samples from new wells in the context of general regional pH and redox conditions can provide information regarding the degree of disequilibrium created by well drilling. Our analysis approach may be transferable and scalable to similar aquifer settings across the globe.

Detecting and explaining why aquifers occasionally become degraded near hydraulically fractured shale gas wells

Extensive development of shale gas has generated some concerns about environmental impacts such as the migration of natural gas into water resources. We studied high gas concentrations in waters at a site near Marcellus Shale gas wells to determine the geological explanations and geochemical implications. The local geology may explain why methane has discharged for 7 years into groundwater, a stream, and the atmosphere. Gas may migrate easily near the gas wells in this location where the Marcellus Shale dips significantly, is shallow (∼1 km), and is more fractured. Methane and ethane concentrations in local water wells increased after gas development compared with predrilling concentrations reported in the region. Noble gas and isotopic evidence are consistent with the upward migration of gas from the Marcellus Formation in a free-gas phase. This upflow results in microbially mediated oxidation near the surface. Iron concentrations also increased following the increase of natural gas concentrations in domestic water wells. After several months, both iron and SO42- concentrations dropped. These observations are attributed to iron and SO42- reduction associated with newly elevated concentrations of methane. These temporal trends, as well as data from other areas with reported leaks, document a way to distinguish newly migrated methane from preexisting sources of gas. This study thus documents both geologically risky areas and geochemical signatures of iron and SO42- that could distinguish newly leaked methane from older methane sources in aquifers.

Enhancing the spatial coverage of a regional high-quality hydraulic conductivity dataset with estimates made from domestic water-well specific-capacity tests

The spatial coverage of hydraulic conductivity (K) values for large-scale groundwater investigations is often poor because of the high costs associated with hydraulic testing and the large areas under investigation. Domestic water wells are ubiquitous and their well logs represent an untapped resource of information that includes mandatory specific-capacity tests, from which K can be estimated. These specific-capacity tests are routinely conducted at such low pumping rates that well losses are normally insignificant. In this study, a simple and practical approach to augmenting high-quality K values with reconnaissance-level K values from water-well specific-capacity tests is assessed. The integration of lesser quality K values from specific-capacity tests with a high-quality K data set is assessed through comparisons at two different scales: study-area-wide (a 600-km2 area in Ontario, Canada) and in a single geological formation within a portion of the broader study area (200 km2). Results of the comparisons demonstrate that reconnaissance-level K estimates from specific-capacity tests approximate the ranges and distributions of the high-quality K values. Sufficient detail about the physical basis and assumptions that are invoked in the development of the approach are presented here so that it can be applied with confidence by practitioners seeking to enhance their spatial coverage of K values with specific-capacity tests.

Identification of blind geothermal resources in Surprise Valley, CA, using publicly available groundwater well water quality data

Geothermal resource exploration is generally limited to areas with surface expressions of thermal activity (fumaroles and hot springs), or relies on expensive geophysical exploration techniques. In this study, the hidden subsurface distribution of geothermal fluids has been identified using a free and publicly available water quality dataset from agricultural and domestic water wells in Surprise Valley, northeastern California. Thermally evolved waters in Surprise Valley have element ratios that vary in response to Ca carbonate and Mg silicate mineral precipitation, and have elevated total dissolved solids (TDS). The arid climate in Surprise Valley leads to surface water evaporation in a closed basin, producing high TDS Na-Cl-CO3-SO4 brines in three ephemeral alkali lakes and in shallow groundwater under elevated soil CO2 conditions. Evaporated fluids in Surprise Valley follow a chemical divide that leads to Ca carbonate and Mg silicate mineral precipitation. Plots of dissolved element ratios can be used to distinguish groundwater affected by evaporation from water affected by thermal water-rock interaction, however it is challenging to select components for plotting that best illustrate different fluid evolution mechanisms. Here, we use a principal component analysis of centered log-ratio transformed data, coupled with geochemical models of fluid evaporation and thermal mixing pathways, to identify components to plot that distinguish between groundwater samples influenced by evaporation from those influenced by thermal processes. We find that groundwater samples with a thermal signature come from wells that define a coherent, linear geographical distribution that closely matches the location of known and inferred faults. Modification of the general approach employed here provides promise for identifying blind geothermal resources in other locations, by applying low-cost geochemical modeling and statistical techniques to areas where large groundwater quality geochemical datasets are available.

Using multi-isotopes (34S, 18O, 2H) to track local contamination of the groundwater from Hongshan-Zhaili abandoned coal mine, Zibo city, Shandong province

The Hongshan-Zhaili coal mine was extensively mined for several decades, lies in Zibo city, Shandong province. After closed in 1994, the coal strata water level raised above the Ordovician limestone water level, then led to the cross strata pollution, which greatly affected the normal use of local people. The Ordovician limestone aquifer is the important drinking water aquifer for local community which covered under the coal strata. Thirty-two domestic water wells near the former coal mine were sampled for water chemistry and stable-isotope analysis of water and dissolved SO42−. Groundwater plots close to the Global Meteoric Water Line and reveals evidence of recharging from atmospheric precipitation. The analysis of sensitive components NO3−, SO42−, 34S in groundwater shows an oxidation environment, promoting the pyrite oxidation. The result of 34S isotopes indicates that the Ordovician limestone water SO42− mainly derived from the atmospheric precipitation SO42−, and the contamination of the Ordovician limestone water from coal strata water has occurred locally in the study area, the Ordovician limestone water is enriched in SO42− and depleted 34S in sulfate.

Forensic analysis of tertiary-butyl alcohol (TBA) detections in a hydrocarbon-rich groundwater basin.

Tertiary-butyl alcohol (TBA), a high-production volume (HPV) chemical, was sporadically detected in groundwater and coalbed methane (CBM) wells in southeastern Colorado's hydrocarbon-rich Raton Basin. TBA concentrations in shallow water wells averaged 75.1μg/L, while detections in deeper CBM wells averaged 14.4μg/L. The detection of TBA prompted a forensic investigation to try to identify potential sources. Historic and recent data were reviewed to determine if there was a discernable pattern of TBA occurrence. Supplemental samples from domestic water wells, monitor wells, CBM wells, surface waters, and hydraulic fracturing (HF) fluids were analyzed for TBA in conjunction with methyl tertiary-butyl ether (MTBE) and ethyl tertiary-butyl ether (ETBE), proxies for evidence of contamination from reformulated gasoline or associated oxygenates. Exploratory microbiological sampling was conducted to determine if methanotrophic organisms co-occurred with TBA in individual wells. Meaningful comparisons of historic TBA data were limited due to widely varying reporting limits. Mapping of TBA occurrence did not reveal any spatial patterns or physical associations with CBM operations or contamination plumes. Additionally, TBA was not detected in HF fluids or surface water samples. Given the widespread use of TBA in industrial and consumer products, including water well completion materials, it is likely that multiple diffuse sources exist. Exploratory data on stable isotopes, dissolved gases, and microbial profiling provide preliminary evidence that methanotrophic activity may be producing TBA from naturally occurring isobutane. Reported TBA concentrations were significantly below a conservative risk-based drinking water screening level of 8000μg/L derived from animal toxicity data.

Septic systems as sources of organic wastewater compounds in domestic drinking water wells in a shallow sand and gravel aquifer

Domestic drinking water wells serve 44 million people in the US and are common globally. They are often located in areas served by onsite wastewater treatment systems, including septic systems, which can be sources of biological and chemical pollutants to groundwater. In this study we tested 20 domestic drinking water wells in a sand and gravel aquifer on Cape Cod, Massachusetts, USA, for 117 organic wastewater compounds (OWCs) and for inorganic markers of septic system impact. We detected 27 OWCs, including 12 pharmaceuticals, five per- and polyfluoroalkyl substances (PFASs), four organophosphate flame retardants, and an artificial sweetener (acesulfame). Maximum concentrations of several PFASs and pharmaceuticals were relatively high compared to public drinking water supplies in the US. The number of detected OWCs and total concentrations of pharmaceuticals and of PFASs were positively correlated with nitrate, boron, and acesulfame and negatively correlated with well depth. These wells were all located in areas served exclusively by onsite wastewater treatment systems, which are likely the main source of the OWCs in these wells, although landfill leachate may also be a source. Our results suggest that current regulations to protect domestic wells from pathogens in septic system discharges do not prevent OWCs from reaching domestic wells, and that nitrate, a commonly measured drinking water contaminant, is a useful screening tool for OWCs in domestic wells. Nitrate concentrations of 1mg/L NO3-N, which are tenfold higher than local background and tenfold lower than the US federal drinking water standard, were associated with wastewater impacts from OWCs in this study.

Methane occurrence is associated with sodium‐rich valley waters in domestic wells overlying the Marcellus shale in New York State

AbstractPrior work suggests spatial parameters (e.g., landscape position, distance to nearest gas well) can be used to estimate the amount of dissolved methane in domestic drinking water wells overlying the deep Marcellus Shale. New York (NY) provides an opportunity to investigate methane occurrence prior to expansion of high‐volume hydraulic fracturing because unconventional gas production is currently banned in the state. We sampled domestic groundwater wells for methane in 2013 (n = 137) across five counties of NY bordering Pennsylvania, and then resampled a subset of those wells in 2014 for methane concentrations and δ13C‐CH4 and δD‐CH4. The majority of waters from wells sampled (77%) had low concentrations of methane (&lt;0.1 mg/L), and only 5% (n = 7) had actionable levels of methane (&gt;10 mg/L). Dissolved methane concentrations did not change as a function of proximity to existing vertical gas wells, nor other parameters indicating subsurface planes of weakness (i.e., faults or lineaments). Methane levels were significantly higher in wells closer to hydrography flow lines, and most strongly correlated to Na‐HCO3 water type. The distribution of methane between Ca‐HCO3 (n = 76) and Na‐HCO3 (n = 23) water types significantly differed (p &lt; 0.01), with median methane concentrations of 0.002 and 0.78 mg/L, respectively. Combined classification of sampled waters based on the dominant water cation, well topographic position, and geologic unit of well completion effectively identified wells with a greater than 50% probability of having methane concentrations exceeding 1 mg/L. Such classification schemes may be useful as a screening tool to assess natural versus gas production‐related sources of methane in domestic wells.

Pre-drilling water-quality data of groundwater prior to shale gas drilling in the Appalachian Basin: Analysis of the Chesapeake Energy Corporation dataset

Unconventional natural gas production in the Marcellus and Utica formations of the Northeastern United States raises concerns about potential impacts to shallow groundwater. We examined and interpreted 13,040 analyses from pre-drilling groundwater samples from domestic water wells in northeastern (NE) Pennsylvania and 8004 samples from water wells in the “Western Area” which includes southwest Pennsylvania, eastern Ohio, and north-central West Virginia. These samples were acquired on behalf of Chesapeake Energy Corporation as part of its local pre-drilling water supply monitoring program. We evaluated concentrations of major ions and metals relative to federal drinking-water-quality standards upon which regulatory decisions are often based. Chesapeake’s dataset, the most comprehensive for these areas, shows that exceedance of at least one water-quality standard occurs in 63% of water well samples in NE Pennsylvania and 87% in the Western Area. In NE Pennsylvania, 10% of the samples exceeded one or more of the United States Environmental Protection Agency’s (USEPA) primary maximum contaminant levels (MCLs) for drinking-water supplies, 46.1% of the samples exceeded one or more of USEPA secondary maximum contaminant levels (SMCLs), and another 7% exceeded one or more of USEPA health advisory or regional screening levels for tap water.In the Western Area 8% of samples exceeded one or more MCLs, 65% exceeded one or more SMCLs, and 15% exceeded one or more health advisory or regional screening levels for tap water. Chesapeake’s dataset, orders of magnitude larger than any in previously published literature, shows that water-quality exceedances relate to factors such: as where the sample occurs within the groundwater flow system, the natural groundwater chemical type (hydrochemical facies), the geologic unit producing the water, and/or the topographic position (valley versus upland). Our comparison of these results to historical groundwater data from NE Pennsylvania, which pre-dates most unconventional shale gas development, shows that the recent pre-drilling geochemical data is similar to historical data. We see no broad changes in variability of chemical quality in this large dataset to suggest any unusual salinization caused by possible release of produced waters from oil and gas operations, even after thousands of gas wells have been drilled among tens of thousands of domestic wells within the two areas studied. Our evaluation also agrees with early researchers such as Piper (1933) and Lohman (1939, 1937) who found that the saline waters in both areas underlie fresher groundwater. The saline water is naturally-occurring connate brine or salt water which has not been flushed by circulating meteoric water; rather than vertical migration of salt water from deep strata such as the Marcellus shale as suggested by Warner et al. (2012). Elevated metals concentrations, particularly iron and manganese, partly relate to sample turbidity; dissolved metals would provide a more accurate measurement of metals in shallow groundwater than does the total metals analysis typically required by regulations.

Methane concentrations in water wells unrelated to proximity to existing oil and gas wells in northeastern Pennsylvania.

Recent studies in northeastern Pennsylvania report higher concentrations of dissolved methane in domestic water wells associated with proximity to nearby gas-producing wells [ Osborn et al. Proc. Natl. Acad. Sci. U. S. A. 2011 , 108 , 8172 ] and [ Jackson et al. Proc. Natl. Acad. Sci. U. S. A. , 2013 , 110 , 11250 ]. We test this possible association by using Chesapeake Energy's baseline data set of over 11,300 dissolved methane analyses from domestic water wells, densely arrayed in Bradford and nearby counties (Pennsylvania), and near 661 pre-existing oil and gas wells. The majority of these, 92%, were unconventional wells, drilled with horizontal legs and hydraulically fractured. Our data set is hundreds of times larger than data sets used in prior studies. In contrast to prior findings, we found no statistically significant relationship between dissolved methane concentrations in groundwater from domestic water wells and proximity to pre-existing oil or gas wells. Previous analyses used small sample sets compared to the population of domestic wells available, which may explain the difference in prior findings compared to ours.

Evaluating Groundwater Pollution Using Hydrochemical Data: Case Study (Al Wahat Area East of Libya)

Water is one of the most challenging current and future natural resources, which will directly affect the environment and development by the changes in its quantity, quality and regional distribution. However, Water quality is the critical factor that influences human health and irrigation proposer. This work aims to investigate hydrochemical analysis and geochemical processes influencing the groundwater of Al Wahat area (Jalou, Awjla and Jukherra), which is located in central east Libya. Thirty four water samples collected from domestic and agricultural water wells were analyzed and used for conventional classification techniques which were Piper, Durov and Stiff diagrams to evaluate geochemical processes. Cluster analysis was used to identify the water type and ions concentration and distribution. Results show significant increase of dissolved salts, especially Nitrates. Elevated nitrates concentration can be attributed to either the disposal of untreated sewage water from disposal ponds and septic tanks or the infiltration of irrigation water saturated with fertilizing chemicals. Therefore, irrigation wells revealed that suffering from nitrate contamination caused an increase of the chance of nitrate pollution. In addition, contour maps present a sudden increase in the total dissolved salts (TDS) in the northeastern part coincident with the highest of secondary ions of NO3 content, indicating the infiltration of irrigation water which is responsible partially for the groundwater degradation. Hydrogeochemical facie is NaCl type and enrichment of Na+ and Cl- can be attributed to urban untreated wastewaters and high rate of evapotranspiration. The concentrations of heavy elements such as Zn, Pb, Cu, Cd, Ni and Cr were low and within the WHO ranges.

Using stable isotopes (S, O) of sulfate to track local contamination of the Madison karst aquifer, Montana, from abandoned coal mine drainage

The Stockett–Centerville area, south of Great Falls, Montana, was extensively mined for coal in the late 1800s through the early 1950s. After closure, the underground mine portals were sealed and the workings became partially flooded. The coal seams are rich in pyrite, and many of the mines have horizontal drains that are point sources of acid mine drainage (AMD) which flow into dry streambeds and soak into the ground. The Mississippian Madison Limestone is located stratigraphically below the coal and is an important source of drinking water. Twenty-six domestic water wells completed in the Madison near the former coal mines were sampled for water chemistry and stable-isotope analysis of water and dissolved SO42-. Madison aquifer groundwater plots close to the meteoric water line and reveals no clear evidence of mixing with water from the mine drains, the latter being slightly enriched in 18O by evaporation. In contrast, dissolved SO42- in the wells has isotopic compositions that suggest mixing from three end-member sources: (1) regional Madison aquifer groundwater with moderate SO42-concentrations (110–150mg/L) that is isotopically enriched (δ34S=20‰; δ18O=14‰); (2) AMD water with very high SO42- concentrations (>1.9g/L) that is isotopically depleted (δ34S=−15‰; δ18O=−11‰); and (3) local recharge water with low SO42- concentrations (<80mg/L) and intermediate SO4-isotope characteristics (δ34S=−10‰; δ18O=−6‰). The percentage of SO42- derived from AMD is estimated to range from 0% to nearly 80%. Wells with the highest proportion of AMD SO42- have higher SO42- concentrations, but otherwise show no mining-associated water quality problems, such as low pH or elevated concentrations of trace metals, underscoring the ability of the Madison aquifer to buffer water quality. Most of the wells with evidence of AMD incursion are older wells that were not cased over their entire depth. It is possible that the open-hole method of completion has allowed some AMD-related water to infiltrate by gravity to the lower Madison aquifer. In this study, stable isotopes of SO42- proved to be a more powerful and sensitive tool to track AMD contamination compared to stable isotopes of water or conventional water-quality sampling.

Site characterization and monitoring of natural attenuation indicator parameters in a fuel contaminated coastal aquifer: Karaduvar (Mersin, SE Turkey)

This paper presents results from a site characterization and monitoring study at Karaduvar area (Mersin, SE Turkey), where high concentrations of refined petroleum products have been detected in domestic and irrigation water wells. The saturated and unsaturated zones in the deltaic aquifer are contaminated by large quantities of gasoline and diesel range fuel hydrocarbons (GRHs and DRHs) released from diverse sources that include accidental spills, storage tank fires, pipeline breaks, deliberate discharge of waste petroleum products from slop tanks and illegal tanker truck washing facilities. At the site, due to the complex nature of the pollution sources, overlapping contaminant plumes exist and cover an area of about 0.5 km2. In both polluted and unpolluted parts of the aquifer, monitoring of groundwater physicochemical parameters in a total of 55 sampling points was carried out between 2006 and 2007. The results show that the terminal electron acceptors (e.g. dissolved oxygen, nitrate, Mn(IV), Fe(III), sulfate) were reduced near the source area(s) indicating presence of actively operating biodegradation processes at the site. Close to the contaminant source area(s), conditions in the plume are highly anoxic and reducing; where high amounts of transformation products (e.g. bicarbonate, dissolved iron, and manganese) are present in solution. Additionally, at the site, excessive pumping, careless land use, and deliberate wastewater discharges significantly deteriorated the quality and quantity of groundwater. Excessive groundwater pumping for industrial and agricultural uses has resulted in substantial water level declines (2–3 m) near the coastal part where seawater intrusion threatens the groundwater resources.

An approach for predicting groundwater recharge in mountainous watersheds

Summary Predicting groundwater supply for an entire watershed in mountainous terrain required an approach that considered a wide range in data availability between valley bottom and headwater areas, large change in elevation, and steep topography. The methodology utilized the MIKE-SHE numerical code to simulate overland flow, actual evapotranspiration and recharge for data-rich areas, and a simpler, seasonal water budget for data-limited areas. Recharge estimates were combined to form spatially variable recharge boundary conditions for a larger-scale groundwater flow model of the entire mountainous watershed. Research focused on the BX Creek watershed, located in the north Okanagan Basin in British Columbia, one of Canada’s fastest growing and most water-limited regions. Groundwater recharge was found to vary from 0 to 20 mm/yr at lower elevations, and from 20 to 50 mm/yr at higher elevations. Simulation of the whole flow system illustrated that 58% of the groundwater flux from upland areas occurs through a relatively narrow alluvial fan aquifer that extends to the valley bottom, and the remaining recharge is nearly equally divided between groundwater flow through the mountain block (20%) and direct recharge (22%). Geochemical data from domestic water wells within the watershed suggest that water in the alluvial aquifer and bedrock are generally similar (i.e., common origin); however, stable isotope data indicate that groundwater in the alluvial aquifer may be derived from snowmelt recharge at a different time and elevation than snowmelt recharge to the bedrock. The combination of modelling results and complimentary geochemical and isotopic analyses of surface water and groundwater, provide an adequate first-order approximation of groundwater flow in the watershed.

Modeling Herbicide Movement to Ground Water in Irrigated Sandy Soils of the San Joaquin Valley, California

A semi-empirical probabilistic transport model was developed to simulate simazine and diuron well water concentrations in an agriculturally intensive coarse soil region of Fresno County, California. Model inputs included five random variables: the organic carbon normalized soil adsorption coefficient, root zone degradation, application rate, depth to ground water, and ground water recharge age. Transport was simulated in two phases: initial transport through the root zone using the mechanistic model LEACHM, and a second empirically-based phase that simulated transport from the bottom of the root zone to the water table, and ultimately to domestic water wells. Best-fit calibration estimates for combined deep vadose/shallow ground water degradation half-lives were 330 d and 455 d for simazine and diuron, respectively. Simulations based on these fitted half-lives yielded coefficients of determination and root mean square errors of 0.973 and 0.993, and 0.367 and 0.364 for observed vs predicted simazine and diuron concentration percentiles, respectively. The calibrated model output also described the observed relationships of decreasing detection frequency and decreasing concentration with increasing depth to ground water. A novel contribution is the calculation of regional mass budgets for the herbicides. In modeling results, 9–54% and 2–16% (10th–90th percentiles) of applied simazine and diuron, respectively, leached out of the 1.5 m root zone, while the upper 10th percentile of well water concentrations corresponded to approximately 1–2% of application for both pesticides.

English

Major proposed changes in drainage control from a large underground mine pool required a detailed assessment of the hydrology, geochemistry, and impacts on the receiving streams. Proposed changes entail relocation of the withdrawal and treatment facilities to an adjoining, but separate watershed and increasing the pumping rate by 35%. Fifteen major and numerous smaller mines in two coal seams comprise the mine pool system. Degree of interconnection between mines ranges from horizontal and vertical seepage through natural fractures, subsidence-induced fractures, and coal cleat to open pass-throughs, slopes, and shafts. Water levels of several mines rise and fall in a mirrored fashion with only a few meters of head difference. An adjacent mine pool with 59.7 meters of head and an intact barrier ranging from 9.4 to 457 meters thick contributes at least 28% to the discharge rate. Mine storage capacity (5.38 billion liters) equates to a porosity of about 11%, a significant reduction from the original extraction volume of 63%. Mean ground water yield of the complex is 28 million liters per day. Recharge rate of 0.41 liters per minute per hectare is less than expected due to the thick overburden over much of the mine complex. The mine complex responds sharply to large precipitation events and large-volume pumping due to the relatively low storage volume and large aerial extent of the mine complex. Water levels respond to large precipitation events within three days and rises exceeding one meter have been recorded. Conversely, current maximum pumping at 35.6 million liters per day will draw the pool down an average of 0.09 meters per day. The proposed pumping of 37.9 million liters per day will, over the long term, exceed ground- water recharge and will dewater large portions of the mine complex. This will adversely impact the water quality, may induce additional subsidence, and could dewater some domestic water wells. Addition of water from an adjacent mine would allow the discharge of 37.9 million liters per day.

Effects of anisotropic transmissivity on a contaminant plume at Nemo, South Dakota

Abstract In 1996 low concentrations of ethylene dibromide (EDB) were found in thirteen domestic water wells in Precambrian metamorphic rocks at Nemo, South Dakota. The source of the contaminant is believed to be the result of pesticides disposed at a U.S. Forest Service Work Station area in the 1970s. Monitoring wells were installed and ground water from nine of them contained EDB. Sixteen additional wells within three kilometers of Nemo were sampled but EDB was not detected. Water in two domestic wells one kilometer south-southeast of Nemo had high concentrations, 13 and 2.2 micrograms per liter of EDB. The impacted landowners were initially provided bottled water by the USFS. The USFS then drilled new wells outside of the contaminant plume which now provide water to the impacted landowners. An equipotential map indicates that ground water should flow easterly. However the contaminant plume extends towards the south-southeast, in the direction as the foliation in the Precambrian phyllite and quartzite at Nemo. This foliation strongly influences the direction of contaminant migration. Based on an equipotential map and the known plume orientation, a graphical solution of the transmissivity anisotropy indicates that the major transmissivity is oriented N 10 degrees W and is approximately 0.73 m 2 /d. The minor transmissivity is 0.13 m 2 /d. A computer solution utilizing anisotropic transmissivity generated a plume which agrees with the field data.