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This report describes the reconnaissance phase of a study to determine the occurrence of agricultural chemicals from nonpoint sources in ground water in six areas, which are representative of the major provinces of the High Plains aquifer in Nebraska. Nitrate and triazine-herbicide concentrations in ground water were assessed to establish preliminary relations between these constituents and selected hydrogeologic, climatic, and land-use variables. In 1984, water from 82 wells in the 6 study areas was analyzed for nitrate, and water from 57 of the 82 wells was analysed for triazine herbicides. Data for 9 of the 21 independent variables suspected of affecting concentrations of nitrate and triazine herbicides in ground water were compiled from the 82 well sites. The variables and their ranges are: hydraulic gradient (XI), 0.0006-0.0053; hydraulic conductivity (X2), 5-149 feet per day; specific discharge (X3), 0.0128-0.2998 foot per day; depth to water (X4), 3-239 feet; well depth (X5), 40-550 feet; annual precipitation (X6), 12.0-39.3 inches; soil permeability (X7), 0.76-9.0 inches; irrigation-well density (X8), 0-8 irrigation wells per square mile; and annual nitrogen fertilizer use (X9), 0-260 pounds of nitrogen per acre. Nitrate concentrations ranged from less than 0.1 to 45 milligrams per liter as nitrogen. Triazine-herbicide concentrations were detected in samples from five of the six study areas in concentrations ranging from less than 0.1 to 2.3 micrograms per liter. Statistical tests indicated that there were significant differences in nitrate concentrations among the six study areas, while no significant differences in triazine-herbicide concentrations were found. Concentrations of nitrate and triazine herbicide were determined, by use of nonparametric statistics, to be significantly larger in more intensively irrigated areas than in less intensively irrigated areas. Preliminary correlations with the independent variables and nitrate concentrations indicated significant relations at the 95-percent confidence level with variables X2, X5, and X8. Correlations with triazine-herbicide concentrations indicated significant relations with variables X2, X3, X5, X6 and X8, and with nitrate concentrations (X10). By use of a simple multiple-regression technique, variables X5, X8, and X9 explained about 51 percent of the variation in nitrate concentrations. Variables X3 and X5 explained about 60 percent of the variation in triazine-herbicide concentrations. With the addition of nitrate concentration as an independent variable, two variables, X10 and X3, explained 84 percent of the total variation in triazine-herbicide concentrations.

Nonpoint-source agricultural chemicals in ground water in Nebraska; preliminary results for six areas of the High Plains Aquifer