Nitrate in Groundwater of the United States, 1991-2003 KAREN R. BUROW,* ,† BERNARD T. NOLAN, ‡ MICHAEL G. RUPERT, § AND NEIL M. DUBROVSKY † U.S. Geological Survey, Placer Hall, 6000 J Street, Sacramento, California 95819, U.S. Geological Survey, 413 National Center, Reston, Virginia 20192, and U.S. Geological Survey, 201 E. Ninth Street, Pueblo, Colorado 81003 Received February 17, 2010. Revised manuscript received May 7, 2010. Accepted May 25, 2010. An assessment of nitrate concentrations in groundwater in the United States indicates that concentrations are highest in shallow, oxic groundwater beneath areas with high N inputs. During 1991-2003, 5101 wells were sampled in 51 study areas throughout the U.S. as part of the U.S. Geological Survey National Water-Quality Assessment (NAWQA) program. The well networks reflect the existing used resource represented by domestic wells in major aquifers (major aquifer studies), and recently recharged groundwater beneath dominant land-surface activities (land-use studies). Nitrate concentrations were highest in shallow groundwater beneath agricultural land use in areas with well-drained soils and oxic geochemical conditions. Nitrate concentrations were lowest in deep groundwater where groundwater is reduced, or where groundwater is older and hence concentrations reflect historically low N application rates. Classification and regression tree analysis was used to identify the relative importance of N inputs, biogeochemical processes, and physical aquifer properties in explaining nitrate concentrations in groundwater. Factors ranked by reduction in sum of squares indicate that dissolved iron concentrations explained most of the variation in groundwater nitrate concentration, followed by manganese, calcium, farm N fertilizer inputs, percent well- drained soils, and dissolved oxygen. Overall, nitrate concentrations in groundwater are most significantly affected by redox conditions, followed by nonpoint-source N inputs. Other water- quality indicators and physical variables had a secondary influence on nitrate concentrations. Introduction The natural global nitrogen (N) cycle has been extensively altered by human activities, doubling the rate of N inputs into the terrestrial N cycle (1). The global increase in the use of N fertilizer over the last several decades has also led to increased leaching and runoff of N that threaten water quality, especially in agricultural areas where elevated nitrate con- centrations are common (2-9). Although this increase in the use of fertilizer has been important for increasing crop production for food in the face of increasing population (10), questions have been raised as to the sustainability of these practices without causing further impairment of water resources (11). Groundwater supplies more than 33% of the water used for public drinking-water supply in the United States (12). Contamination of groundwater by nitrate is of concern because elevated concentrations can affect human health. Additionally, many surface water bodies receive significant groundwater discharge, and excess N in groundwater can lead to ecological disturbances in receiving surface water (13-15). Elevated concentrations of nitrate in groundwater are highly variable, however, and although the overall increase in N fertilizer use may generally correspond to higher nitrate concentrations (16), it is sometimes difficult to link high nitrate concentrations in groundwater directly to overlying N inputs (17, 18). In addition to complex unsaturated zone processes that influence the N leaching to the water table (2, 19, 20), nitrate may also be affected by physical and chemical processes within the aquifer that result in some areas being more vulnerable to nitrate contamination than others (21-24). To understand where and how nitrate concentrations become elevated requires an understanding of the sources of nitrate and the factors that control how nitrate moves through the hydrologic system. This, in turn, can aid in the development of effective management practices for the most vulnerable areas. Numerous studies at regional and local scales have examined the relation between nitrate concentrations in groundwater and governing factors; however, relatively few studies have evaluated nitrate concentrations across wide regions, representing multiple aquifers and diverse hydro- geologic conditions. Since 1991, The U.S. Geological Survey (USGS) National Water-Quality Assessment (NAWQA) program has been monitoring groundwater quality in the primary aquifers of the U.S. using a common network design, sampling methods, and consistently developed explanatory variables. Results of the first 20 study areas (2130 wells sampled in 1991-1995) reported by ref 5 indicated that nitrate concentrations were highest beneath shallow agricultural land, in areas charac- terized by high N inputs, well-drained soils, fractured bedrock, and in irrigated areas. This analysis of NAWQA results builds upon the initial assessment (5). Since this time, sampling was done in an additional 2971 wells in an additional 31 study areas, for a total of 5101 wells sampled during 1991-2003 in 51 study areas throughout the U.S. The more extensive data and expanded geographic coverage available for the current study reinforces many of the previously reported findings, allows more detailed analyses of each topic, and supports new findings. This paper describes the occur- rence and distribution of nitrate in groundwater and presents statistical comparisons using a greater number of regionally available variables than reported by ref 5, including geochem- ical variables, groundwater age, and other aquifer and well construction characteristics that were not considered previ- ously. Additionally, the relative importance of these variables was further evaluated. The objective was to identify the most influential factors controlling groundwater nitrate concen- trations across the wide range of environmental settings in the U.S., based on an extensive data set that included environmental factors, water quality indicators from ground- water samples, and well construction data. * Corresponding author phone: 916-278-3087; e-mail: krburow@ usgs.gov. † U.S. Geological Survey, Sacramento, CA. ‡ U.S. Geological Survey, Reston, VA. § U.S. Geological Survey, Pueblo, CO. Environ. Sci. Technol. 2010, 44, 4988–4997 4988 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 13, 2010 10.1021/es100546y  2010 American Chemical Society Published on Web 06/11/2010