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USGS2012

Modeling groundwater flow and quality

In most areas, rocks in the subsurface are saturated with water at relatively shallow depths. The top of the saturated zone—the water table—typically occurs anywhere from just below land surface to hundreds of feet below the land surface. Groundwater generally fills all pore spaces below the water table and is part of a continuous dynamic flow system, in which the fluid is moving at velocities ranging from feet per millennia to feet per day (Fig. 33.1). While the water is in close contact with the surfaces of various minerals in the rock material, geochemical interactions between the water and the rock can affect the chemical quality of the water, including pH, dissolved solids composition, and trace-elements content. Thus, flowing groundwater is a major mechanism for the transport of chemicals from buried rocks to the accessible environment, as well as a major pathway from rocks to human exposure and consumption. Because the mineral composition of rocks is highly variable, as is the solubility of various minerals, the human-health effects of groundwater consumption will be highly variable.

Book chapter↗
USGS2012

Estimating the benefits of land imagery in environmental applications: a case study in nonpoint source pollution of groundwater

Moderate-resolution land imagery (MRLI) is crucial to a more complete assessment of the cumulative, landscape-level effect of agricultural land use and land cover on environmental quality. If this improved assessment yields a net social benefit, then that benefit reflects the value of information (VOI) from MRLI. Environmental quality and the capacity to provide ecosystem services evolve because of human actions, changing natural conditions, and their interaction with natural physical processes. The human actions, in turn, are constrained and redirected by many institutions and regulations such as agricultural, energy, and environmental policies. We present a general framework for bringing together sociologic, biologic, physical, hydrologic, and geologic processes at meaningful scales to interpret environmental implications of MRLI applications. We set out a specific application using MRLI observations to identify crop planting patterns and thus estimate surface management activities that influence groundwater resources over a regional landscape. We tailor the application to the characteristics of nonpoint source groundwater pollution hazards in Iowa to illustrate a general framework in a land use-hydrologic-economic system. In the example, MRLI VOI derives from reducing the risk of both losses to agricultural production and damage to human health and other consequences of contaminated groundwater.

Book chapter↗
USGS1990

Transformation of Monoaromatic hydrocarbons to organic acids in anoxic groundwater environment

The transformation of benzene and a series of alkylbenzenes was studied in anoxic groundwater of a shallow glacial-outwash aquifer near Bemidji, Minnesota, U.S.A. Monoaromatic hydrocarbons, the most water-soluble components of crude oil, were transported downgradient of an oil spill, forming a plume of contaminated groundwater. Organic acids that were not original components of the oil were identified in the anoxic groundwater. The highest concentrations of these oxidized organic compounds were found in the anoxic plume where a decrease in concentrations of structurally related alkylbenzenes was observed. These results suggest that biological transformation of benzene and alkylbenzenes to organic acid intermediates may be an important attenuation process in anoxic environments. The transformation of a complex mixture of hydrocarbons to a series of corresponding oxidation products in an anoxic subsurface environment provides new insight into in situ anaerobic degradation processes.

Minnesota↗
USGS1986

U.S. Geological Survey toxic Waste-Groundwater Contamination Program, fiscal year 1985

In fiscal year 1982, the U S Geological Survey began an interdisciplinary research thrust entitled Toxic Waste-Groundwater Contamination Program The objective of the thrust was to provide earth sciences information necessary to evaluate and mitigate existing groundwater contamination problems resulting from the planned or inadvertant disposal of wastes and from certain land-use practices, and to improve future waste disposal and land-use practices The program supports process-oriented and interdisciplinary field research, and regional groundwater quality studies This article provides an overview of the current (Fiscal Year 1985) activities of the Toxic Waste Program

Environmental Geology and Water Sciences↗
USGS2005

Groundwater control of mangrove surface elevation: shrink and swell varies with soil depth

We measured monthly soil surface elevation change and determined its relationship to groundwater changes at a mangrove forest site along Shark River, Everglades National Park, Florida. We combined the use of an original design, surface elevation table with new rod-surface elevation tables to separately track changes in the mid zone (0?4 m), the shallow root zone (0?0.35 m), and the full sediment profile (0?6 m) in response to site hydrology (daily river stage and groundwater piezometric pressure). We calculated expansion and contraction for each of the four constituent soil zones (surface [accretion and erosion; above 0 m], shallow zone [0?0.35 m], middle zone [0.35?4 m], and bottom zone [4?6 m]) that comprise the entire soil column. Changes in groundwater pressure correlated strongly with changes in soil elevation for the entire profile (Adjusted R2 5 0.90); this relationship was not proportional to the depth of the soil profile sampled. The change in thickness of the bottom soil zone accounted for the majority (R2 5 0.63) of the entire soil profile expansion and contraction. The influence of hydrology on specific soil zones and absolute elevation change must be considered when evaluating the effect of disturbances, sea level rise, and water management decisions on coastal wetland systems.

Estuaries↗
USGS2006

Spatial analysis of land use and shallow groundwater vulnerability in the watershed adjacent to Assateague Island National Seashore, Maryland and Virginia, USA

Spatial relations between land use and groundwater quality in the watershed adjacent to Assateague Island National Seashore, Maryland and Virginia, USA were analyzed by the use of two spatial models. One model used a logit analysis and the other was based on geostatistics. The models were developed and compared on the basis of existing concentrations of nitrate as nitrogen in samples from 529 domestic wells. The models were applied to produce spatial probability maps that show areas in the watershed where concentrations of nitrate in groundwater are likely to exceed a predetermined management threshold value. Maps of the watershed generated by logistic regression and probability kriging analysis showing where the probability of nitrate concentrations would exceed 3 mg/L (>0.50) compared favorably. Logistic regression was less dependent on the spatial distribution of sampled wells, and identified an additional high probability area within the watershed that was missed by probability kriging. The spatial probability maps could be used to determine the natural or anthropogenic factors that best explain the occurrence and distribution of elevated concentrations of nitrate (or other constituents) in shallow groundwater. This information can be used by local land-use planners, ecologists, and managers to protect water supplies and identify land-use planning solutions and monitoring programs in vulnerable areas. ?? 2006 Springer-Verlag.

Environmental Geology↗
USGS2007

Using chemical and microbiological indicators to track the impacts from the land application of treated municipal wastewater and other sources on groundwater quality in a karstic springs basin

Multiple chemical constituents (nutrients; N, O, H, C stable isotopes; 64 organic wastewater compounds, 16 pharmaceutical compounds) and microbiological indicators were used to assess the impact on groundwater quality from the land application of approximately 9.5 million liters per day of treated municipal sewage effluent to a sprayfield in the 960-km2 Ichetucknee Springs basin, northern Florida. Enriched stable isotope signatures (?? 18O and ??2H) were found in water from the effluent reservoir and a sprayfield monitoring well (MW-7) due to evaporation; however, groundwater samples downgradient from the sprayfield have ??18O and ??2H concentrations that represented recharge of meteoric water. Boron and chloride concentrations also were elevated in water from the sprayfield effluent reservoir and MW-7, but concentrations in groundwater decreased substantially with distance downgradient to background levels in the springs (about 12 km) and indicated at least a tenfold dilution factor. Nitrate-nitrogen isotope (??15N-NO3) values above 10 ??? in most water samples were indicative of organic nitrogen sources except Blue Hole Spring (??15N-NO3 = 4.6-4.9 ???), which indicated an inorganic source of nitrogen (fertilizers). The detection of low concentrations the insect repellent N,N-diethyl-metatoluamide (DEET), and other organic compounds associated with domestic wastewater in Devil's Eye Spring indicated that leakage from a nearby septic tank drainfield likely has occurred. Elevated levels of fecal coliforms and enterococci were found in Blue Hole Spring during higher flow conditions, which likely resulted from hydraulic connections to upgradient sinkholes and are consistent with previoius dye-trace studies. Enteroviruses were not detected in the sprayfield effluent reservoir, but were found in low concentrations in water samples from a downgradient well and Blue Hole Spring during high-flow conditions indicating a human wastewater source. The Upper Floridan aquifer in the Ichetucknee Springs basin is highly vulnerable to contamination from multiple anthropogenic sources throughout the springs basin. ?? 2007 Springer-Verlag.

Environmental Geology↗
USGS1998

Assessment of natural attenuation of aromatic hydrocarbons in groundwater near a former manufactured-gas plant, South Carolina, USA

Shallow, anaerobic groundwater near a former manufactured-gas plant (MGP) in Charleston, South Carolina, USA, contains mono- and polycyclic aromatic hydrocarbons (MAHs and PAHs, respectively). Between 1994 and 1997, a combination of field, laboratory, and numerical-flow and transport-model investigations were made to assess natural attenuation processes affecting MAH and PAH distributions. This assessment included determination of adsorption coefficients (K(ad)) and first-order biodegradation rate constants (K(bio)) using aquifer material from the MGP site and adjacent properties. Naphthalene adsorption (K(ad) = 1.35 x 10-7 m3/mg) to aquifer sediments was higher than toluene adsorption (K(ad) = 9.34 x 10-10 m3/mg), suggesting preferential toluene transport relative to naphthalene. However, toluene and benzene distributions measured in January 1994 were smaller than the naphthalene distribution. This scenario can be explained, in part, by the differences between biodegradation rates of the compounds. Aerobic first-order rate constants of 14C-toluene, 14C-benzene, and 14C-naphthalene degradation were similar (-0.84, -0.03, and 0.88 day-1, respectively), but anaerobic rate constants were higher for toluene and benzene (-0.002 and -0.00014 day-1, respectively) than for naphthalene (-0.000046 day-1). Both areal and cross-sectional numerical simulations were used to test the hypothesis suggested by these rate differences that MAH compounds will be contained relative to PAHs. Predictive simulations indicated that the distributions of toluene and benzene reach steady-state conditions before groundwater flow lines discharge to an adjacent surface-water body, but do discharge low concentrations of naphthalene. Numerical predictions were 'audited' by measuring concentrations of naphthalene, toluene, and benzene at the site in early 1997. Measured naphthalene and toluene concentrations were substantially reduced and the areal extent of contamination smaller than was both observed in January 1994 and predicted for 1997. Measured 1997 benzene concentrations and distribution were shown to be relatively unchanged from those measured in 1994, and similar to predictions for 1997.The natural attenuation processes affecting mono- and polycyclic aromatic hydrocarbons (MAHs and PAHs, respectively) distributions in groundwater near a former manufactured-gas plant in South Carolina, USA was evaluated. This assessment included determination of adsorption coefficients and first-order biodegradation rate constants. Detailed results obtained in the study are presented.

South Carolina↗
USGS2002

Interactions between groundwater and surface water: The state of the science

The interactions between groundwater and surface water are complex. To understand these interactions in relation to climate, landform, geology, and biotic factors, a sound hydrogeoecological framework is needed. All these aspects are synthesized and exemplified in this overview. In addition, the mechanisms of interactions between groundwater and surface water (GW-SW) as they affect recharge-discharge processes are comprehensively outlined, and the ecological significance and the human impacts of such interactions are emphasized. Surface-water and groundwater ecosystems are viewed as linked components of a hydrologic continuum leading to related sustainability issues. This overview concludes with a discussion of research needs and challenges facting this evolving field. The biogeochemical processes within the upper few centimeters of sediments beneath nearly all surface-water bodies (hyporheic zone) have a profound effect on the chemistry of the water interchange, and here is where most of the recent research has been focusing. However, to advance conceptual and other modeling of GW-SW systems, a broader perspective of such interactions across and between surface-water bodies is needed, including multidimensional analyses, interface hydraulic characterization and spatial variability, site-to-region regionalization approaches, as well as cross-disciplinary collaborations.

Hydrogeology Journal↗
USGS2002

Using groundwater levels to estimate recharge

Accurate estimation of groundwater recharge is extremely important for proper management of groundwater systems. Many different approaches exist for estimating recharge. This paper presents a review of methods that are based on groundwater-level data. The water-table fluctuation method may be the most widely used technique for estimating recharge; it requires knowledge of specific yield and changes in water levels over time. Advantages of this approach include its simplicity and an insensitivity to the mechanism by which water moves through the unsaturated zone. Uncertainty in estimates generated by this method relate to the limited accuracy with which specific yield can be determined and to the extent to which assumptions inherent in the method are valid. Other methods that use water levels (mostly based on the Darcy equation) are also described. The theory underlying the methods is explained. Examples from the literature are used to illustrate applications of the different methods.

Hydrogeology Journal↗
USGS2003

Chlorine-36 in groundwater of the United States: Empirical data

Natural production of the radionuclide chlorine - 36 ( 36 Cl) has provided a valuable tracer for groundwater studies. The nuclear industry, especially the testing of thermonuclear weapons, has also produced large amounts of 36 Cl that can be detected in many samples of groundwater . In order to be most useful in hydrologic studies, the natural production prior to 1952 should be distinguished from more recent artificial sources. The object of this study was to reconstruct the probable preanthropogenic levels of 36 Cl in groundwater in the United States . Although significant local variations exist, they are superimposed on a broad regional pattern of 36 Cl/Cl ratios in the United States . Owing to the influence of atmospherically transported ocean salt, natural ratios of 36 Cl/total Cl are lowest near the coast and increase to a maximum in the central Rocky Mountains of the United States .

Hydrogeology Journal↗
USGS2003

A thick lens of fresh groundwater in the southern Lihue Basin, Kauai, Hawaii, USA

A thick lens of fresh groundwater exists in a large region of low permeability in the southern Lihue Basin, Kauai, Hawaii, USA. The conventional conceptual model for groundwater occurrence in Hawaii and other shield-volcano islands does not account for such a thick freshwater lens. In the conventional conceptual model, the lava-flow accumulations of which most shield volcanoes are built form large regions of relatively high permeability and thin freshwater lenses. In the southern Lihue Basin, basin-filling lavas and sediments form a large region of low regional hydraulic conductivity, which, in the moist climate of the basin, is saturated nearly to the land surface and water tables are hundreds of meters above sea level within a few kilometers from the coast. Such high water levels in shield-volcano islands were previously thought to exist only under perched or dike-impounded conditions, but in the southern Lihue Basin, high water levels exist in an apparently dike-free, fully saturated aquifer. A new conceptual model of groundwater occurrence in shield-volcano islands is needed to explain conditions in the southern Lihue Basin.

Hydrogeology Journal↗
USGS2004

Hydrochemical tracers in the middle Rio Grande Basin, USA: 1. Conceptualization of groundwater flow

Chemical and isotopic data for groundwater from throughout the Middle Rio Grande Basin, central New Mexico, USA, were used to identify and map groundwater flow from 12 sources of water to the basin, evaluate radiocarbon ages, and refine the conceptual model of the Santa Fe Group aquifer system. Hydrochemical zones, representing groundwater flow over thousands to tens of thousands of years, can be traced over large distances through the primarily siliciclastic aquifer system. The locations of the hydrochemical zones mostly reflect the “modern” predevelopment hydraulic-head distribution, but are inconsistent with a trough in predevelopment water levels in the west-central part of the basin, indicating that this trough is a transient rather than a long-term feature of the aquifer system. Radiocarbon ages adjusted for geochemical reactions, mixing, and evapotranspiration/dilution processes in the aquifer system were nearly identical to the unadjusted radiocarbon ages, and ranged from modern to more than 30 ka. Age gradients from piezometer nests ranged from 0.1 to 2 year cm –1 and indicate a recharge rate of about 3 cm year –1 for recharge along the eastern mountain front and infiltration from the Rio Grande near Albuquerque. There has been appreciably less recharge along the eastern mountain front north and south of Albuquerque.

Rio Grande Basin↗
USGS2007

Effects of intraborehole flow on groundwater age distribution

Environmental tracers are used to estimate groundwater ages and travel times, but the strongly heterogeneous nature of many subsurface environments can cause mixing between waters of highly disparate ages, adding additional complexity to the age-estimation process. Mixing may be exacerbated by the presence of wells because long open intervals or long screens with openings at multiple depths can transport water and solutes rapidly over a large vertical distance. The effect of intraborehole flow on groundwater age was examined numerically using direct age transport simulation coupled with the Multi-Node Well Package of MODFLOW. Ages in a homogeneous, anisotropic aquifer reached a predevelopment steady state possessing strong depth dependence. A nonpumping multi-node well was then introduced in one of three locations within the system. In all three cases, vertical transport along the well resulted in substantial changes in age distributions within the system. After a pumping well was added near the nonpumping multi-node well, ages were further perturbed by a flow reversal in the nonpumping multi-node well. Results indicated that intraborehole flow can substantially alter groundwater ages, but the effects are highly dependent on local or regional flow conditions and may change with time.

Hydrogeology Journal↗
USGS2007

Temporal trends in concentrations of DBCP and nitrate in groundwater in the eastern San Joaquin Valley, California, USA

Temporal monitoring of the pesticide 1,2-dibromo-3-chloropropane (DBCP) and nitrate and indicators of mean groundwater age were used to evaluate the transport and fate of agricultural chemicals in groundwater and to predict the long-term effects in the regional aquifer system in the eastern San Joaquin Valley, California. Twenty monitoring wells were installed on a transect along an approximate groundwater flow path. Concentrations of DBCP and nitrate in the wells were compared to concentrations in regional areal monitoring networks. DBCP persists at concentrations above the US Environmental Protection Agency's maximum contaminant level (MCL) at depths of nearly 40 m below the water table, more than 25 years after it was banned. Nitrate concentrations above the MCL reached depths of more than 20 m below the water table. Because of the intensive pumping and irrigation recharge, vertical flow paths are dominant. High concentrations (above MCLs) in the shallow part of the regional aquifer system will likely move deeper in the system, affecting both domestic and public-supply wells. The large fraction of old water (unaffected by agricultural chemicals) in deep monitoring wells suggests that it could take decades for concentrations to reach MCLs in deep, long-screened public-supply wells, however. ?? Springer-Verlag 2007.

Hydrogeology Journal↗
USGS2007

A simple daily soil-water balance model for estimating the spatial and temporal distribution of groundwater recharge in temperate humid areas

Quantifying the spatial and temporal distribution of natural groundwater recharge is usually a prerequisite for effective groundwater modeling and management. As flow models become increasingly utilized for management decisions, there is an increased need for simple, practical methods to delineate recharge zones and quantify recharge rates. Existing models for estimating recharge distributions are data intensive, require extensive parameterization, and take a significant investment of time in order to establish. The Wisconsin Geological and Natural History Survey (WGNHS) has developed a simple daily soil-water balance (SWB) model that uses readily available soil, land cover, topographic, and climatic data in conjunction with a geographic information system (GIS) to estimate the temporal and spatial distribution of groundwater recharge at the watershed scale for temperate humid areas. To demonstrate the methodology and the applicability and performance of the model, two case studies are presented: one for the forested Trout Lake watershed of north central Wisconsin, USA and the other for the urban-agricultural Pheasant Branch Creek watershed of south central Wisconsin, USA. Overall, the SWB model performs well and presents modelers and planners with a practical tool for providing recharge estimates for modeling and water resource planning purposes in humid areas. ?? Springer-Verlag 2007.

Hydrogeology Journal↗
USGS2007

Environmental tracers as indicators of karst conduits in groundwater in South Dakota, USA

Environmental tracers sampled from the carbonate Madison aquifer on the eastern flank of the Black Hills, South Dakota, USA indicated the approximate locations of four major karst conduits. Contamination issues are a major concern because these conduits are characterized by direct connections to sinking streams, high groundwater velocities, and proximity to public water supplies. Objectives of the study were to estimate approximate conduit locations and assess possible anthropogenic influences associated with conduits. Anomalies of young groundwater based on chlorofluorocarbons (CFCs), tritium, and electrical conductivity (EC) indicated fast moving, focused flow and thus the likely presence of conduits. ??18O was useful for determining sources of recharge for each conduit, and nitrate was a useful tracer for assessing flow paths for anthropogenic influences. Two of the four conduits terminate at or near a large spring complex. CFC apparent ages ranged from 15 years near conduits to >50 years in other areas. Nitrate-N concentrations >0.4 mg/L in groundwater were associated with each of the four conduits compared with concentrations ranging from <0.1 to 0.4 mg/L in other areas. These higher nitrate-N concentrations probably do not result from sinking streams but rather from other areas of infiltration. ?? Springer-Verlag 2007.

Hydrogeology Journal↗
USGS2008

Investigating local variation in groundwater recharge along a topographic gradient, Walnut Creek, Iowa, USA

Groundwater recharge is an important component to hydrologic studies but is known to vary considerably across the landscape. The purpose of this study was to examine 4 years of water-level behavior in a transect of four water-table wells installed at Walnut Creek, Iowa, USA to evaluate how groundwater recharge varied along a topographic gradient. The amount of daily water-table rise (WTR) in the wells was summed at monthly and annual scales and estimates of specific yield (Sy) were used to convert the WTR to recharge. At the floodplain site, Sy was estimated from the ratio of WTR to total rainfall and in the uplands was based on the ratio of baseflow to WTR. In the floodplain, where the water table is shallow, recharge occurred throughout the year whenever precipitation occurred. In upland areas where the water table was deeper, WTR occurred in a stepped fashion and varied by season. Results indicated that the greatest amount of water-table rise over the 4-year period was observed in the floodplain (379 mm), followed by the upland (211 mm) and sideslopes (122 mm). Incorporating spatial variability in recharge in a watershed will improve groundwater resource evaluation and flow and transport modeling. ?? Springer-Verlag 2008.

Hydrogeology Journal↗