Nutrients in Shallow Ground Waters Beneath Relatively Undeveloped Areas in the Conterminous United States

Abstract

Nutrient concentrations in shallow (well depth of 30 meters or less) ground waters of relatively undeveloped areas were evaluated to determine background conditions relative to agricultural and urban land uses. Lands comprising 67 percent or greater forest or range, 10 percent or less agricultural land, and 10 percent or less urban land were used to represent relatively undeveloped areas. Data subsets from the U.S. Geological Survey’s National Water-Quality Assessment Program (81 wells) and retrospective studies (320 wells) yielded 75th percentile nitrate concentrations of 0.51 and 1.1 milligrams per liter, respectively, in shallow ground water beneath relatively undeveloped areas. The value of 1.1 milligrams per liter is a reasonable upper bound estimate of relative background concentration of nitrate in shallow ground waters in the United States and incorporates effects of nominal nitrogen load to susceptible aquifers. Relative background concentration of nitrate is variable and depends in part on land use, rock type, and climate. Median nitrate concentration was significantly greater in ground water beneath rangeland (1.20 milligrams per liter) than beneath forest land (0.06 milligram per liter). Median nitrate concentration in ground water beneath rangeland was 1.4 2.7 milligrams per liter in susceptible aquifers, which consist of coarse-textured deposits or fractured rock. Increased relative background concentration of nitrate in rangeland areas likely results from evaporative concentration of nominal nitrogen load associated with natural organic and inorganic sources in hydrogeologically susceptible settings. INTRODUCTION Nutrient concentration in shallow ground water (from wells 30 meters deep or less) beneath relatively undeveloped land was evaluated to enable meaningful assessment of anthropogenic effects on ground-water quality. Previous studies of nutrients in watersheds have emphasized anthropogenic sources such as inorganic fertilizer, animal manure, and airborne nitrogen compounds emitted by utilities and automobiles (Nolan and others, 1997; Nolan and Stoner, 2000; Puckett, 1995; Smith and others, 1997). Nitrate is of particular concern because it is common in ground water, can persist for years, and at high concentration has been linked to adverse health effects (Centers for Disease Control and Prevention, 1996; Fan and Steinberg, 1996; Ward and others, 1996). Nitrate also is derived from natural sources, including native organic matter and selected minerals. Measuring nitrate and other nutrients in ground water from reference areas unaffected by agriculture, cities, and other human influences would yield natural background levels against which elevated nitrate concentration could be compared to assess the degree of human influence. Most watersheds in the United States, however, have been affected by human activity to some degree. In this study, data collected or compiled for the U.S. Geological Survey (USGS) National WaterQuality Assessment (NAWQA) Program were analyzed to infer relative background concentrations of nutrients in shallow, recently recharged ground water. Shallow ground water was analyzed because it is most likely to show recent land-use effects when compared with deeper, older ground water. Shallow ground water is used for drinking-water purposes in many areas and has been the focus of recent studies of aquifer vulnerability (Nolan and others, 1997; Nolan, 2001). Separation of natural and anthropogenic concentrations of nitrate to determine the level of human influence would enhance the usefulness of aquifer vulnerability assessments.