Can We Determine Background Nitrate Concentration in Groundwater?
U.S. Geological Survey
The background concentration of nitrate in groundwater is a threshold value above which influence by anthropogenic sources is indicated. This value provides the lowest threshold for modeling the vulnerability of wells to nitrate contamination and can be used to identify areas where the nitrate concentration may be increasing. Relative background concentration within a regional aquifer system can vary depending, in part, on land-management practices, hydrogeology, and climate. We needed to determine the background nitrate concentration for the purpose of assessing nitrate in private wells in the unconsolidated glacial aquifer system in the United States. Nitrate concentration and distribution were characterized in private wells of the glacial aquifer system based on data collected over the last 20 years by the U.S. Geological Survey National Water-Quality Assessment (NAWQA) Program. Reference monitoring wells were installed, as part of the NAWQA program, in areas minimally affected by human activities, including two to three monitoring wells in each of 15 studies, and these wells have been sampled to assess the background nitrate concentration. The mean nitrate concentration in these reference wells is 1.2 mg/L as N but the median is only 0.13 mg/L as N, indicating substantial variability across the aquifer system. Nitrate concentrations in 75 percent of the reference wells are below 0.49 mg/L as N. The background nitrate concentration in the glacial aquifer system ranged from 0.14 to 4 mg/L in previous studies. Based on this analysis, the background concentration of nitrate in the glacial aquifer system is low compared to other regional aquifer systems in the United States.