National Water-Quality Assessment (NAWQA) Project
Nitrate concentrations in shallow wells are elevated across the Nation (shown in red), particularly in agricultural areas associated with some of the most intense applications of fertilizer and manure in the Nation. Natural features, such as related to geology, hydrology, and soils, and natural processes, such as related to geochemistry, can affect the transport of nitrate over the land and into the ground, making some aquifers more vulnerable to contamination than others. Spatially extensive geographic maps of these factors, along with monitoring data, are used in USGS models to extrapolate and forecast nitrate to unsampled and yet comparable areas (see map). The models predict that nitrate concentrations are likely to increase in aquifers used for public drinking-water supplies during the next decade as this shallow groundwater with high nitrate moves downward into deeper aquifers.
The full study and findings can be accessed at: http://water.usgs.gov/nawqa/nutrients/pubs/circ1350/