Nitrate in the Columbia Aquifer, central Delmarva Peninsula, Maryland

A study of nitrate in water from 604 wells tapping the Columbia aquifer on the Delmarva Peninsula in eastern Maryland describes the factors that affect nitrate variability. Samples were collected from 196 randomly selected wells and analyzed for nitrogen species. Many were also analyzed for major ions. In addition, results of 313 nitrate analyses were randomly selected from county health department files. About 95 analyses of water samples collected from 1945 to 1978 were also evaluated. The frequency distribution of the nitrate analyses is bimodal, with 25 percent of the sample ranging from 0 to about 0.42 milligrams per liter (mg/L) nitrate as nitrogen (N), and the median is about 0.1 mg/L; the rest ranges from 0.42 to 58 mg/L, and the median is about 5.9 mg/L. The overall median nitrate concentration is about 3.5 mg/L as N. Over half of the samples had nitrate concentrations of 3 mg/L as N or higher, indicating that the water in the aquifer has been affected by human activity. Nitrate-nitrogen concentrations exceeded the water-quality standard of 10 mg/L in 15 percent of the samples established by the U.S. Environmental Protection Agency. The major factors affecting nitrate concentration are the presence of a nitrogen source, hydrogeological conditions, and the soil drainage. Sites with poorly drained soils may have a lower nitrate concentration either because the soils block the entrance of nitrate into the aquifer or because the aquifer under a poorly drained soil is associated with a chemical environment that promotes denitrification. (USGS)