Analysis of nitrate (NO3-N) concentration trends in 25 ground-water-quality management areas, Idaho, 1961-2001

1 term trend assessments are needed. Addition of these kinds of information would allow assess­ ment of trends associated with hydrology and geology of each area and would provide a much stronger basis for trend assessment than currently possible. INTRODUCTION In Idaho, ground water currently is the principal source of about 95 percent of public water supplies and most rural-domestic and livestock supplies (E. Hagan, Idaho Department of Water Resources, IDWR, oral commun., 2001). Many drinking-water supplies are pumped from relatively shallow zones where groundwater quality has great potential for degradation by landand water-use activities. One indicator of water quality, and one of the most widespread contaminants 1 in Idaho ground water related to land and water uses, is dissolved nitrate-nitrogen (NO3–N) concentrations. 2 The maximum U.S. Environmental Protection Agency (EPA, 2000, p. 9) limit for nitrate concentra­ tions in public water supplies is 10 mg/L. Concentra­ tions of nitrate in Idaho ground water prior to land and water development probably were less than 1 mg/L (Parliman, 2000, p. 1), but elevated nitrate concentra­ tions in ground water have been reported for more than 40 years (see Selected References). The median con­ centration (50th percentile) of nitrate in Idaho ground water from 1990 through 2000, for example, was about 1.5 mg/L, but the range of concentrations was less than 0.05 to 110 mg/L. Widespread elevated nitrate concentrations histor­ ically have been detected in some areas of Idaho more frequently than in other areas. In 1998, personnel with the Idaho Department of Environmental Quality (IDEQ) conducted a preliminary assessment of statewide nitrate concentrations and delineated 33 ground-water-quality management areas (nitrate priority areas) where more than 25 percent of nitrate concentrations exceeded 5 mg/L (S. Short, IDEQ, written commun., 2000; fig. 1). In July 2000, IDEQ convened the Ground-Water Monitoring Technical Committee to discuss groundwater-quality management issues in Idaho. One goal of 1Contaminants are components of ground-water quality that can limit the water’s suitability for use or can represent degradation of water quality. 2In this report, nitrate is reported as nitrogen, NO3–N, in milligrams per liter (mg/L), equivalent to parts per million. the committee was to prioritize the 33 ground-waterquality management areas where nitrate has signifi­ cantly degraded ground-water quality; one component of the prioritization process was identification of nitrate concentration trends within each area. In December 2000, the U.S. Geological Survey (USGS), in coopera­ tion with IDEQ, began a study to compile and assess nitrate data for ground water in the 33 priority areas. The primary objective of the study was to determine whether statistically significant trends in nitrate con­ centrations were discernible. On the basis of statistical trend analysis results, nitrate trends would be defined as increasing, decreasing, or neither increasing nor decreasing (no trend). Areas would be identified where data were insufficient for statistical assessment. The purpose of this report is to present findings of these trend analyses. DATA COMPILATION The first task of the nitrate trends study was to identify sources of electronically transferable nitrate data in Idaho and combine these data into one data set. Data were compiled from data bases maintained by the following agencies: (1) Idaho Department of Agricul­ ture (IDAG)—ground-water-monitoring networks and a one-time analysis of water from wells at dairies statewide, (2) IDEQ—individual ground-water-quality investigations statewide and the Drinking Water Infor­ mation Management System (DWIMS) public watersupply data base, (3) USGS—individual groundwater-quality investigations statewide by Idaho and Utah Districts, and (4) the USGS/IDWR Statewide Ambient Ground-Water Quality Monitoring Program (Statewide GW-QW Program). 3 Data included ground-water analyses but not anal­ yses of water from springs, drains, or thermal water (greater than 84.5°F) sources. Nitrite plus nitrate as nitrogen (NO2+NO3–N) and nitrate-nitrogen (NO3–N) data were moved into one Microsoft Excel 2000 spreadsheet. Minimum required information for each analysis included sample date and agency source. Min­ imum required location information for each well 3A description of this program is available at http://www.idwr. state.id. us/planpol/techserv/gwmon/statewide.htm 4USGS laboratory analyses are reported in units of nitrite plus nitrate as nitrogen (NO2+NO3–N). Idaho State Laboratory analyses are reported in units of nitrate as nitrogen (NO3–N). Concentrations of nitrite in ground water generally are negligible. For purposes of this study, USGS and IDEQ analyses are comparable. 2 Nitrate Trends in 25 Ground-Water-Quality Management Areas, Idaho, 1961–2001 117° EXPLANATION Nitrate priority area