NATURAL AND ANTHROPOGENIC NITRATE IN GROUND WATER: HIGH PLAINS AQUIFER

Nitrate is a common contaminant in the High Plains aquifer, which underlies an area of about 450,000 km2 in parts of eight western States. Water pumped from the aquifer for irrigation supports one of the largest agricultural economies in the United States, and agriculture is an important source of the nitrate in ground water. Large deposits of natural nitrate in the unsaturated zone in some areas also have been proposed as important sources of ground-water nitrate. Hydrologic, chemical, and isotopic data from the unsaturated and saturated zones were used to improve understanding of nitrate sources in High Plains ground water. A 10,000 year record of paleorecharge from the aquifer indicates that nitrate derived from natural sources differed in concentration and in nitrogen isotopic composition from nitrate in recent recharge (< 50 years) under irrigated fields. Chemical transit times through unsaturated zones determined on the basis of chloride mass balance and tritium profiles indicate that natural nitrate deposits were not likely to be transported to the water table under current climatic conditions, but they could reach the water table in a timeframe of several decades because of mobilization by irrigation return flow. In general, results from this study indicate that natural nitrate deposits did not contribute substantially to large ground-water nitrate concentrations in recent recharge, but nitrate loads to the aquifer could increase in the future as natural nitrate deposits mobilized by irrigation return flow eventually reach the water table. A major uncertainty with this analysis is the distribution and abundance of these deposits in the unsaturated zone. Data indicate that the spatial distribution of natural nitrate deposits in the unsaturated zone is complex and could be dependent on various factors such as climate, slope, soil type, and vegetation on both regional and local scales.