Vulnerability Of Shallow Groundwater Research Articles

Groundwater vulnerability assessment using AHP-DRASTIC-GALDIT comprehensive model: a case study of Binhai New Area, Tianjin, China.

Binhai New Area (BHNA), as one of the most economically and industrially regions in the Haihe River Basin, China, is seriously affected by seawater intrusion and groundwater over-exploitation. Groundwater vulnerability assessment (GVA) is an effective tool to protect the groundwater resources from being polluted. In this study, vertical and horizontal groundwater conditional factors were first assessed separately by two different models. The AHP-DRASTIC model was used to evaluate the intrinsic groundwater vulnerability and the AHP-GALDIT model was used to evaluate the specific groundwater vulnerability to seawater intrusion. Then, a GIS-based overlaying approach was used to get the comprehensive shallow groundwater vulnerability. The results of the comprehensive model showed that the vulnerability areas of very low, low, medium, and high account for 1.37%, 11.36%, 60.56%, and 26.71%, respectively. Finally, to effectively manage the groundwater in the study area, two remediation areas, two control areas, and one protected area were determined based on the comprehensive groundwater vulnerability maps. This study can not only promote the development of rational exploitation of shallow groundwater and prevention of groundwater pollution in BHNA but also provide a framework for future research in the GVA on the coast.

Control of organic contaminants in groundwater by passive sampling and multivariate statistical analysis

Organic contaminants in groundwater are among the most challenging chemical compound contaminants today, particularly when it comes to understanding their occurrence, origin, and relations in groundwater, as well as the transport processes, fate, and environmental impacts involved. This paper presents the use of active carbon fibre (AFC) passive sampling and multivariate statistical processing of the results to predict the possible occurrence of organic compounds (OCs) in groundwater and to determine the origin of various anthropogenic activity. This study aims to deepen our knowledge on the control of OCs in groundwater by introducing a multi-analytical and multi-elemental holistic approach, using the Dravsko polje aquifer, the largest intergranular aquifer in Slovenia, as an example. The occurrence of OCs in groundwater was determined by means of ACFs and compared against the characteristics of the recharge area and the type of compounds detected. We combined hierarchical cluster analysis (HCA) and principal component analysis (PCA) to identify the relationship between different OCs in groundwater. The relationships between their occurrence, environmental setting and type of compound were determined using multiple linear regression (MLR). From the total of 343 organic compounds detected using passive sampling, 47 were included in further statistical analysis. MLR shows that the environmental setting is one of the most important factors affecting the different types of pollutants in groundwater. MLR models were calculated for different sources of pollution (agricultural, urban, and industrial) based on the environmental setting, land use, agglomeration, infrastructure networks, and hydrogeological characteristics of the aquifer. By means of HCA and PCA, we identified the relationships between different OCs in groundwater. As expected, the strongest correlations were found between primary compounds and their degradation products (e.g. atrazine and desethylatrazine) and compounds of similar use (e.g. atrazine and propazine, also desethylatrazine and propazine, atrazine and simazine). Some of them were also found to have a similar molecular structure (e.g. palmitic and stearic acid, 5-methoxygramine and 5-methoxytryptamine). The use of the same substances in different environments (agricultural/urban) makes them markers of both (different) origins. Therefore, it is particularly important to determine the combination of markers of different origin using multivariate statistical methods, especially in the case of mixed land use. This study identifies the main factors influencing the distribution of groundwater OCs and thus contributes to a more comprehensive understanding of the vulnerability of shallow groundwater to surface-derived contamination in similar environments.

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.

Omnipresent distribution of herbicides and their transformation products in all water body types of an agricultural landscape in the North German Lowland

The research of the environmental fate of pesticides has demonstrated that applied compounds are altered in their molecular structure over time and are distributed within the environment. To assess the risk for contamination by transformation products (TP) of the herbicides flufenacet and metazachlor, the following four water body types were sampled in a small-scale catchment of 50 km2 in 2015/2016: tile drainage water, stream water, shallow groundwater, and drinking water of private wells. The TP were omnipresent in every type of water body, more frequently and in concentrations up to 10 times higher than their parent compounds. Especially metazachlor sulfonic acid, metazachlor oxalic acid, and flufenacet oxalic acid were detected in almost every drainage and stream sample. The transformation process leads to more mobile and more persistent molecules resulting in higher detection frequencies and concentrations, which can even occur a year or more after the application of the parent compound. The vulnerability of shallow groundwater and private drinking water wells to leaching compounds is proved by numerous positives of metazachlor-TP with maximum concentrations of 0.7 μg L−1 (drinking water) and 20 μg L−1 (shallow groundwater) of metazachlor sulfonic acid. Rainfall events during the application period cause high discharge of the parent compound and lower release of TP. Later rainfall events lead to high displacement of TP. For an integrated risk assessment of water bodies, the environmental behavior of pesticide-TP has to be included into regular state-of-the-art water quality monitoring.

Application of DRASTIC Entropy Weight Model Method in Groundwater Vulnerability Evaluation in Ordos Area

The northeastern part of the Ordos Basin is the main recharge area of regional groundwater, the groundwater resources are relatively scarce. The main water supply source in the area is shallow groundwater, and there are many industrial and mining enterprises in the district. The potential groundwater pollution risk is high, and the the shallow groundwater vulnerability evaluation in the region is of great significance for groundwater resources protection.The weight of each indicator of the traditional DRASTIC model is fixed and does not change with the regional conditions, which may cause deviations in the evaluation results. This time, based on the DRASTIC model, the entropy weight coefficient method is introduced to determine the index weight, and the DRASTIC entropy weight model is established to obtain a more scientific and close to the actual conditions of the study area, and provide an important reference and basis for the protection of regional groundwater resources.

Emerging organic contaminants in groundwater under a rapidly developing city (Patna) in northern India dominated by high concentrations of lifestyle chemicals.

Aquatic pollution from emerging organic contaminants (EOCs) is of key environmental importance in India and globally, particularly due to concerns of antimicrobial resistance, ecotoxicity and drinking water supply vulnerability. Here, using a broad screening approach, we characterize the composition and distribution of EOCs in groundwater in the Gangetic Plain around Patna (Bihar), as an exemplar of a rapidly developing urban area in northern India. A total of 73 EOCs were detected in 51 samples, typically at ng.L-1 to low μg.L-1 concentrations, relating to medical and veterinary, agrochemical, industrial and lifestyle usage. Concentrations were often dominated by the lifestyle chemical and artificial sweetener sucralose. Seventeen identified EOCs are flagged as priority compounds by the European Commission, World Health Organisation and/or World Organisation for Animal Health: namely, herbicides diuron and atrazine; insecticides imidacloprid, thiamethoxam, clothianidin and acetamiprid; the surfactant perfluorooctane sulfonate (and related perfluorobutane sulfonate, perfluorohexane sulfonate, perfluorooctanoic acid and perfluoropentane sulfonate); and medical/veterinary compounds sulfamethoxazole, sulfanilamide, dapson, sulfathiazole, sulfamethazine and diclofenac. The spatial distribution of EOCs varies widely, with concentrations declining with depth, consistent with a strong dominant vertical flow control. Groundwater EOC concentrations in Patna were found to peak within ∼10km distance from the River Ganges, indicating mainly urban inputs with some local pollution hotspots. A heterogeneous relationship between EOCs and population density likely reflects confounding factors including varying input types and controls (e.g. spatial, temporal), wastewater treatment infrastructure and groundwater abstraction. Strong seasonal agreement in EOC concentrations was observed. Co-existence of limited transformation products with associated parent compounds indicate active microbial degradation processes. This study characterizes key controls on the distribution of groundwater EOCs across the urban to rural transition near Patna, as a rapidly developing Indian city, and contributes to the wider understanding of the vulnerability of shallow groundwater to surface-derived contamination in similar environments.

Regional Aquifer Vulnerability and Pollution Sensitivity Analysis of Drastic Application to Dahomey Basin of Nigeria.

Shallow groundwater vulnerability mapping of the southwestern Nigeria sedimentary basin was assessed in this study with the aim of developing a regional-based vulnerability map for the area based on assessing the intrinsic ability of the aquifer overlying beds to filter and degrade migrating pollutant. The mapping includes using the established seven parameter-based DRASTIC vulnerability methodology. Furthermore, the developed vulnerability map was subjected to sensitivity analysis as a validation approach. This approach includes single-parameter sensitivity, map removal sensitivity, and DRASTIC parameter correlation analysis. Of the Dahomey Basin, 21% was classified as high-vulnerability and at risk of pollution, 61% as moderate vulnerability, and 18% as low vulnerability. Low vulnerability areas of the basin are characterised by thick vadose zones, low precipitation, compacted soils, high slopes, and high depth to groundwater. High-vulnerability areas which are prone to pollution are regions closer to the coast with flat slopes and frequent precipitation. Sensitivity of the vulnerability map show the greatest impact with the removal of topography, soil media, and depth to groundwater and least impact with the removal of the vadose zone. Due to the subjectivity of the DRASTIC method, the most important single parameter affecting the rating system of the Dahomey Basin DRASTIC map is the impact of the vadose zone, followed by the net recharge and hydraulic conductivity. The DRASTIC vulnerability map can be useful in planning and siting activities that generate pollutants (e.g., landfill, soak away, automobile workshops, and petrochemical industries) which pollute the environment, groundwater, and eventually impact the environmental health of the Dahomey Basin’s inhabitants.

Intrinsic vulnerability assessment of shallow groundwater to pollution using the GOD Method by utilizing geoelectric data: a case study of the city of Pasuruan, East Java, Indonesia

Groundwater is one kind of clean water resource. The increased development of human activities, such as agriculture, industry, and settlements, has a cumulative impact on groundwater conditions. The main purpose of this research is to assess the intrinsic vulnerability of shallow groundwater to pollution in the City of Pasuruan. The utilized method in the intrinsic vulnerability assessment of shallow groundwater is the GOD method, by utilizing geoelectric data. The parameters used in the calculation are aquifer layer type (G), soil layer type above the aquifer layer (O), and ground water depth (D). The City of Pasuruan has a moderate to high index of intrinsic vulnerability, and shallow water at the study site is categorized as vulnerable to pollutants.

Estimation of intrinsic vulnerability of shallow groundwater in Jombang District, Jombang Regency, East Java, Indonesia based on Aquifer Vulnerability Index

The level of safety of groundwater to pollution hazards can be identified based on the level of vulnerability of the groundwater to pollution. Determination of groundwater vulnerability intrinsically can be done by using the Aquifer Vulnerability Index (AVI) method based on two physical parameters i.e. the thickness of each layer above the uppermost of saturated aquifer surface and the estimated hydraulic conductivity of each of these sedimentary layers. The objective of this study was to determine the level of groundwater vulnerability intrinsically based on the vulnerability index value of aquifers by using the AVI method. This study was conducted based on sample of 25 dug wells scattered in the Jombang District of Jombang Regency, East Java, Indonesia. Based on the results of the analysis using the AVI method, it can be stated that the hydraulic resistance logarithm in the study area were found to be in the range from -5.22 to 3.84 with the intrinsic vulnerability of groundwater to pollution hazards from extremely high to low.

Groundwater vulnerability assessment using the GALDIT model and the improved DRASTIC model: a case in Weibei Plain, China.

This study employed a modified DRASTIC model (AHP-DRASTIC model) and GALDIT model to evaluate the inherent vulnerability of shallow groundwater in Weibei Plain in Shandong Province of China and its vulnerability to seawater intrusion. The AHP-DRASTIC model uses the analytic hierarchy process (AHP) to determine the weight of each parameter and reduces the subjectivity of evaluation. The vulnerability map generated by the AHP-DRASTIC model shows four types of vulnerability: high (25.0%), higher (28.0%), moderate (29.7%), and low (17.3%), and the high-vulnerability areas are mainly distributed in the area north of Qingxiang Town and south of Changyi County. The distribution of high-vulnerability areas mainly related to the depth of groundwater table is 4-8m, and the recharge of rainfall is 100-175mm/year. The vulnerability map generated by the GALDIT model shows four types of vulnerability: high (33.5%), higher (23.4%), moderate (22.1%), and low (21.0%), and the high-vulnerability areas are mainly distributed in the coastal areas of Hanting District-Zhuli Town, the areas north of Linqu County, and the areas south of Shouguang County. The distribution of high-vulnerability areas mainly related to the distance between these areas and the coast is < 2.5km, with aquifer thickness > 15m. Total dissolved solid, NO3-, Cl-, and SO42- are used to verify the accuracy of the DRASTIC model, the AHP-DRASTIC model, and the GALDIT model. The results show that the AHP-DRASTIC model is more suitable for the assessment of inherent vulnerability of shallow groundwater in the study area than the DRASTIC model, and human activities have a major impact on the verification of vulnerability and should be considered when conducting groundwater vulnerability verification. The results of this study can provide grounds for groundwater management and protection and land use planning in the study area and provide new ideas for groundwater vulnerability assessment in coastal areas.

Assessing Groundwater Resource Vulnerability by Coupling GIS-Based DRASTIC and Solute Transport Model in Ajmer District, Rajasthan

Groundwater aquifer vulnerability has been assessed by incorporating the major geological and hydrogeological factors that affect and control the groundwater contamination using GIS-based DRASTIC model along with solute transport modeling. This work demonstrates the potential of GIS to derive a vulnerability map by overlying various spatially referenced digital data layers (i.e., depth to water, net recharge, aquifer media, soil media, topography, the impact of vadose zone and hydraulic conductivity) that portrays cumulative aquifer sensitivity ratings in Kishangarh, Rajasthan. It provides a relative indication of groundwater aquifer vulnerability to contamination. The soil moisture flow and solute transport regimes of the vadose zone associated with specific hydrogeological conditions play a crucial role in pollution risk assessment of the underlying groundwater resources. An effort has been made to map the vulnerability of shallow groundwater to surface pollutants of thestudy area, using soil moisture flow and contaminant transport modeling. The classical advection-dispersion equation coupled with Richard’s equation is numerically simulated at different point locations for assessing the intrinsic vulnerability of the valley. The role of soil type, slope, and the land-use cover is considered for estimating the transient flux at the top boundary from daily precipitation and evapotranspiration data of the study area. The time required by the solute peak to travel from the surface to the groundwater table at the bottom of the soil profile is considered as an indicator of avulnerability index. Results show a high vulnerability in the southern region, whereas low vulnerability is observed in the northeast and northern parts. The results have recognized four aquifer vulnerability zones based on DRASTIC vulnerability index (DVI), which ranged from 45 to 178. It has been deduced that approximately 18, 25, 34, and 23% of the area lies in negligible, low, medium and high vulnerability zones, respectively. The study may assist in decision making related to theplanning of industrial locations and the sustainable water resources development of the selected semi-arid area.

An integrated approach for management of agricultural non-point pollution sources in the Czech Republic

We report a new, integrated approach to the identification and localization of potential critical areas of non-point agricultural water pollution in the Czech Republic. The methodology is presented in model catchments of IV. order, namely the Hrejkovický and Bilinský brooks in the water reservoir Orlik watershed. The risk rate of non-point source pollution is evaluated with the help of GIS tools, integrating the assessment of geomorphology, land use and soil conditions within a territory in conjunction with the agricultural tile drainage systems. Besides the areas prone to erosion and ‘direct protection localities’ along water courses, spots with increased potential of nutrient leaching into groundwater and drainage water are delineated, based on a synthetic map of shallow groundwater vulnerability. These spots are classified using analysis of the Valuated Soil Ecological Units (VSEU) code, while agricultural drainage systems are identified according to the documents of the former Agricultural Water Management Authority of the Czech Republic. Results from geographical analysis show differences between intrinsic parameters of model catchments, which influence the vulnerability potential towards diverse types of non point pollution. The potential pollution thread of surface and groundwater by leaching is relatively high in both catchments, in the Bilinský brook catchment due to prevailing arable land on first and second vulnerability classes within the tile drainage subcatchments and in the Hrejkovický brook catchment due to dense occurence of first and second vulnerable clasess in the catchment area, though comprised from a third by grassland. Water erosion potential jeopardy is bigger within the Bilinský brook catchment with 10% of arable land having the average soil loss 4–10 t ha−1 y−1, contrary to 2% within the Hrejkovický brook catchment. Presented approach can help in prioritizing protective measures and management strategies in a catchment to curtail the negative impacts of non point agricultural pollution in water bodies and in the whole environment.

Assessment of shallow groundwater vulnerability in Dahei River Plain based on AHP and DRASTIC

Assessment of shallow groundwater vulnerability in Dahei River Plain based on AHP and DRASTIC

Identifying the Source of Methane in Groundwater in a ‘virgin’ Area with Regards to Shale Gas Exploitation: A Multi-isotope Approach

The Upper Ordovician Utica Shale located in the St. Lawrence Lowlands (Quebec, Canada) represents a promising reservoir of unconventional gas, which is still ‘virgin’ with respect to fracking due to a de facto moratorium. A project was initiated in order to evaluate the vulnerability of shallow groundwater with respect to potential future activities carried out at depth. The geochemical aspect of the project, relying on isotopes of various compounds from shallow groundwater and rock samples, will help establish baseline gas concentrations in the aquifer, evaluating whether gas concentrations and isotopic ratios vary over time, and identifying the source(s) of methane.

Groundwater Vulnerability Assessment to Contamination Using Soil Moisture Flow and Solute Transport Modeling

The soil moisture flow and solute transport regimes of the vadose zone associated with specific hydrogeological conditions play a crucial role in pollution risk assessment of the underlying groundwater resources. An effort has been made to map the vulnerability of shallow groundwater to surface pollutants of Doon Valley, an intermontane watershed in North India situated between the lesser Himalayas and Shiwalik ranges, using soil moisture flow and contaminant transport modeling. The classical advection dispersion equation coupled with the Richard’s equation is numerically simulated at different point locations for assessing the intrinsic vulnerability of the valley. The role of soil type, slope, and land-use cover is considered for estimating the transient flux at the top boundary from daily precipitation and evapotranspiration data of the study area. The time required by solute peak to travel from the surface to the groundwater table at the bottom of the soil profile is considered as an indicator of vulnerability index. Results show a high vulnerability in the southern region, whereas low vulnerability is observed in the northeast and northern parts of the valley. These findings are in line with the observed low water table depths, less runoff, and higher hydraulic conductivity of the vadose zone material in the southern part of the valley. The study may assist in decision making related to planning of industrial locations and the sustainable water resources development of the valley.

Influence of macroporosity on preferential solute and colloid transport in unsaturated field soils

Transport of solutes and colloids in soils, particularly those subject to preferential flow along macropores, is important for assessing the vulnerability of shallow groundwater to contamination. The objective of this study was to investigate flow and transport phenomena for dissolved and colloid tracers during large infiltration events in partially saturated, macroporous soils. Controlled tracer infiltration tests were completed at two field sites in southern Ontario. A tension infiltrometer (TI) was used to infiltrate water with dissolved Brilliant Blue FCF dye simultaneously with 3.7 µm and 0.53 µm diameter fluorescent microspheres. Infiltration was conducted under maximum infiltration pressure heads ranging from − 5.2 to − 0.4 cm. All infiltration test sites were excavated to examine and photograph dye-stained flow patterns, map soil features, and collect samples for microsphere enumeration. Results indicated that preferential transport of dye and microspheres via macropores occurred when maximum pressure heads were greater than − 3.0 cm, and the corresponding infiltration rates exceeded 2.0 cm h − 1 . Dye and microspheres were detected at depths greater than 70 cm under the highest infiltration rates from both sites. Microsphere concentrations in the top 5–10 cm of soil decreased by more than two orders of magnitude relative to input concentrations, yet remained relatively constant with depth thereafter. There was some evidence for increased retention of the 3.7 µm microspheres relative to the 0.53 µm microspheres, particularly at lower infiltration pressures where straining and attachment mechanisms are most prevalent. Microspheres were observed within dye stained soil matrix surrounding individual macropores, illustrating the significance of capillary pressures in controlling the vertical migration of both tracers in the vicinity of the macropores. Overall, microsphere distributions closely followed the dye patterns, with microsphere concentrations at all depths directly related to the intensity (or concentration) of dye staining. It is concluded that the flow system influenced transport to a much greater degree than differences between dissolved and colloidal species, and hence a dye tracer could serve as a reasonable surrogate for colloid distributions in the vadose zone following individual infiltration events.

A GIS-based DRASTIC model for assessing shallow groundwater vulnerability in the Zhangye Basin, northwestern China

Groundwater vulnerability is a cornerstone in evaluating the risk of groundwater contamination and developing management options to preserve the quality of groundwater. Based on the professional model (DRASTIC model) and geographical information system (GIS) techniques, this paper carries out the shallow groundwater vulnerability assessment in the Zhangye Basin. The DRASTIC model uses seven environmental parameters (depth to water, net recharge, aquifer media, soil media, topography, impact of vadose zone, and hydraulic conductivity) to characterize the hydrogeological setting and evaluate aquifer vulnerability. According to the results of the shallow groundwater vulnerability assessment, the Zhangye Basin can be divided into three zones: low groundwater vulnerability risk zone (risk index 140). Under the natural conditions, the middle and high groundwater vulnerability risk zones of the Zhangye Basin are mainly located in the groundwater recharge zones and the important cities. The high, middle and low groundwater vulnerability risk zones of the Zhangye Basin cover around 17, 21 and 62% of study area, respectively.

Determination of pesticides multiresidues in shallow groundwater in a cotton-growing region of Mato Grosso, Brazil

Aiming to evaluate the contamination of groundwater by pesticides in cotton growing areas, an SPE-based method (styrene-divinylbenzene copolymer - SDVB) was developed for the simultaneous determination of twelve pesticides in water by HPLC/DAD. The method was validated and average recoveries ranged from 73 to 113%, with a relative standard deviation of 2 to 16%. Detection limits ranged from 0.06 to 0.57 µg L-1. The method was applied to groundwater samples (110) from cotton fields located in Primavera do Leste, Mato Grosso state, Brazil. Eight pesticides (acetamiprid, aldicarb, carbendazin, carbofuran, diuron, imidacloprid, methomyl and teflubenzuron) were found in the analyzed groundwater samples and 18% of them contained at least one of the pesticides (with concentrations ranging from 0.78 to 68.79 µg L-1). In some cases, detected concentrations exceeded the target levels set by the European Union. These findings confirm the vulnerability of shallow groundwater in the aforementioned areas to pesticide contamination.

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.

Vulnerability of Shallow Groundwater and Drinking-Water Wells to Nitrate in the United States

Two nonlinear models were developed at the national scale to (1) predict contamination of shallow ground water (typically < 5 m deep) by nitrate from nonpoint sources and (2) to predict ambient nitrate concentration in deeper supplies used for drinking. The new models have several advantages over previous national-scale approaches. First, they predict nitrate concentration (rather than probability of occurrence), which can be directly compared with water-quality criteria. Second, the models share a mechanistic structure that segregates nitrogen (N) sources and physical factors that enhance or restrict nitrate transport and accumulation in ground water. Finally, data were spatially averaged to minimize small-scale variability so that the large-scale influences of N loading, climate, and aquifer characteristics could more readily be identified. Results indicate that areas with high N application, high water input, well-drained soils, fractured rocks or those with high effective porosity, and lack of attenuation processes have the highest predicted nitrate concentration. The shallow groundwater model (mean square error or MSE = 2.96) yielded a coefficient of determination (R(2)) of 0.801, indicating that much of the variation in nitrate concentration is explained by the model. Moderate to severe nitrate contamination is predicted to occur in the High Plains, northern Midwest, and selected other areas. The drinking-water model performed comparably (MSE = 2.00, R(2) = 0.767) and predicts that the number of users on private wells and residing in moderately contaminated areas (>5 to < or =10 mg/L nitrate) decreases by 12% when simulation depth increases from 10 to 50 m.