Trends in Nitrate Concentrations in Agricultural Areas of the United States: Implications for Aquifers and Streams
U.S. Geological Survey
Use of industrially fixed nitrogen (N) fertilizer for agricultural purposes has increased more than 20-fold in the United States (U.S.) and more than 45-fold globally since 1945. As a result, there has been growing concern about the consequences of increases in the amounts of anthropogenic N circulating in the atmosphere, hydrosphere, and biosphere. The U.S. Geological Survey?s National Water-Quality Assessment Program has collected groundwater samples along flow paths in more than 20 agricultural areas and across a variety of hydrogeologic settings to evaluate the trends and transformations of agricultural chemicals. Historical trends in N fluxes to groundwater were evaluated by relating the recharge dates of groundwater samples, estimated using tracer (e.g., chlorofluorocarbon) concentrations, with concentrations of the parent compound at the time of recharge, estimated as the sum of the molar concentrations of the parent compound and its transformation products in the age-dated sample. Results from this analysis indicate that the median concentrations of nitrate in recharging groundwater have increased markedly over the last 50 years, from 4 mg/L (as N) in samples that recharged prior to 1983 to 7.5 mg/L (as N) in samples that recharged after 1983. Nitrate concentration trends in recharging groundwater were related to increases in the amount of fertilizer applied. Estimates of the portion of applied N reaching the water table ranged from 4 to 49 percent among the sites, with a median value of 14 percent. Increasing nitrate concentrations in shallow groundwater pose a potential risk not only to these waters but also to deeper groundwater and streams. This risk will be reduced to the extent that nitrate is removed by denitrification along subsurface flow paths, a process that is directly related to the rates of reduction of dissolved oxygen. Dissolved oxygen reduction rates varied widely within and between sites, with higher rates (e.g., >0.13 yr-1) often found in riparian zones. However, in several watersheds, low dissolved oxygen reduction rates (<0.05 yr-1) were observed in both upland and riparian zones, indicating that denitrification rates may be too low at these sites to prevent increases in the concentrations of nitrate in groundwater and streams from legacy sources of nitrogen.