National Water-Quality Assessment (NAWQA) Program
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By Bernard T. Nolan, Barbara C. Ruddy, Kerie J. Hitt, and Dennis R. Helsel
[Environmental Science and Technology, vol. 31, no. 8, 1997, p. 2229-2236]Abstract
Nitrate contamination of groundwater occurs in predictable
patterns, based on findings of the U.S. Geological
Survey's (USGS) National Water Quality Assessment (NAWQA)
Program. The NAWQA Program was begun in 1991 to
describe the quality of the Nation's water resources, using
nationally consistent methods. Variables affecting nitrate
concentration in groundwater were grouped as "input"
factors (population density and the amount of nitrogen
contributed by fertilizer, manure, and atmospheric sources)
and "aquifer vulnerability" factors (soil drainage characteristic
and the ratio of woodland acres to cropland acres in
agricultural areas) and compiled in a national map that shows
patterns of risk for nitrate contamination of groundwater.
Areas with high nitrogen input, well-drained soils, and low
woodland to cropland ratio have the highest potential for
contamination of shallow groundwater by nitrate. Groundwater
nitrate data collected through 1992 from wells less
than 100 ft deep generally verified the risk patterns shown on
the national map. Median nitrate concentration was 0.2
mg/L in wells representing the low-risk group, and the maximum
contaminant level (MCL) was exceeded in 3% of the wells.
In contrast, median nitrate concentration was 4.8 mg/L in
wells representing the high-risk group, and the MCL was
exceeded in 25% of the wells.
Table of Contents
Introduction
Methods
Limitations of national-scale risk analysis
Results and discussion
The risk of nitrate contamination varies across the United States
Nitrate in groundwater generally follows the risk map
Well type influences groundwater quality
Acknowledgments
Literature cited