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Nutrients National Synthesis Project

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

By Bernard T. Nolan and Kerie J. Hitt

Abstract
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 (R²) 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, R² = 0.767) and predicts that the number of users on private wells and residing in moderately contaminated areas (>5 to less than or equal to 10 mg/L nitrate) decreases by 12% when simulation depth increases from 10 to 50 m.

Table of Contents
Introduction
Methods
Results and discussion
  Shallow GWAVA model
  Drinking-water GWAVA model
  Model application
Acknowledgments
Supporting information available
Literature cited

• Report (0.5 mb PDF)
• Supporting information
• Geospatial data sets used in the models:
  Shallow ground water (gwava-s)
  Deeper supplies used for drinking (gwava-dw)