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

Abstract

In Idaho, drinking-water supplies are pumped from relatively shallow groundwater zones where water quality has great potential for degradation by land-and water-use activities. One indicator of water quality, and one of the most widespread con taminants in Idaho ground water related to land and water uses, is dissolved nitrate. In December 2000, the U.S. Geological Sur vey, in cooperation with the Idaho Department of Environmental Quality, began a study to compile and assess nitrate data for ground water in 25 groundwater quality management areas in Idaho. The primary objective of the study was to determine whether statistically significant trends in groundwater concentrations were discernible. Data included groundwater analyses but not analyses of water from springs, drains, or thermal water sources. A total of 8,465 nitrate analyses were compiled from 2,931 wells in the 25 priority areas; analyses dates ranged from June 1961 to February 2001. A time-period comparison was used to assess general trends in nitrate concentrations within individual priority areas. A time-series comparison was used to assess trends in nitrate concentrations from selected wells in each priority area. In time-period assessments, general trends in an area were evaluated by compiling nitrate data for selected time intervals to determine whether the population distributions were significantly dif ferent between one period and another. Data within each priority area were sorted into decades-1970s, 1980s, and 1990s-for long-term trend assessment. The 1990s data also were divided into sets of selected years corresponding to Statewide Ambient GroundWater Quality Program sampling cycles-1991 through 1994, 1995 through 1998, and a partial cycle, 1999 through 2000-for shortterm trend assessment. Data were analyzed by summary statistics, boxplots, and the Mann-Whitney statistical test. Long-term increasing trends in nitrate concen trations were evident for 6 of 25 priority areas, and long-term decreasing trends were evident for 4 of 25 areas. Short-term increasing trends were evident for 7 of 25 areas, and a short-term decreasing trend was evident for only 1 area. No long-term nitrate trends were evident for 7 of 25 areas, and no shortterm trends were evident for 15 of 25 areas. Data were insufficient for long-term trend assessment in 8 areas and for short-term assessment in 2 areas. Time-series trend analyses were conducted on data from wells with 7 or more nitrate analyses and longest periods of record, at least 10 years between oldest and most recent analyses. Because long-term records were available for so few wells and well construction data were not available for several of these wells, time-series analyses were not helpful to the nitrate trend assessment study. Trend results may be strongly affected by well construction, hydrogeologic environments, and changes in density and areal distribution of wells and analyses. The utility of nitrate trend assess ments in priority areas would be improved by more consistent and specific well location d...