National Water-Quality Assessment (NAWQA) Program
A combination of groundwater modeling, chemical and dissolved gas analyses, and chlorofluorocarbon age dating of water was used to determine the relation between changes in agricultural practices and nitrate concentrations in ground water of a glacial outwash aquifer in west-central Minnesota (Puckett and Cowdery, 2002).
Groundwater ages showed that maximum residence times were on the order of 50-70 years. Reconstructed nitrate concentrations, estimated from measured nitrate and dissolved nitrogen gas, showed that nitrate concentrations have been increasing in the aquifer since the 1940s. Since the mid- to late-1960s, nitrate concentrations have been above the 10 milligrams per liter (mg/L) maximum contaminant level (MCL) at most sites. Puckett and others (1999) showed that most of the nitrogen reaching the water table in the study area originated as fertilizer, and this increase in nitrate concentrations over time corresponds to a similar increase in the use of fertilizer nitrogen in the study area, particularly after the late-1960s.