Variables that Affect Agricultural Chemicals in Groundwater in Nebraska

Agricultural chemicals from nonpoint sources in groundwater are present in the major provinces of the High Plains aquifer in Nebraska. Nitrate and triazine-herbicide concentrations in groundwater were assessed to establish preliminary relations between these constituents and selected hydrogeologic, climatic, and land-use variables. Also, macropore flow paths were measured in an attempt to delineate their contribution to non-point source pollution from the study areas. Water from 82 wells in six study areas was analyzed for nitrate; water from 57 of the 82 wells was analyzed for triazine herbicides. Twenty-one independent variables were identified that could potentially affect chemical concentrations in groundwater. Data for 9 of 21 independent variables suspected of affecting concentrations of nitrate and triazine herbicides in groundwater were collected from the well sites. The nine variables and their measured ranges were hydraulic gradient, 0.0006–0.0053; hydraulic conductivity, 1.5–45.4 m (5–149 ft) per day; specific discharge, 0.004–0.091 m (0.0128–0.2998 ft) per day; depth to water, 0.91–76 m (3–250 ft); well depth, 12–168 m (40–550 ft); annual precipitation, 30–100 cm (12.0–39.3 in.); soil permeability, 1.9–23 cm (0.76–9.0 in.); irrigation-well density, 0–8 irrigation wells per 2.59 km2 (1 square mile); and annual nitrogen fertilizer use, 0–118 kg (0–260 lb) of nitrogen per acre. Macropore flow is listed in percent, average per study area based on determinations from dye studies. In this instance, macropore flow is used to also entail preferential flow paths. Nitrate concentrations ranged from 0.1 to 45 mgL−1. Triazine-herbicide concentrations were detected in samples from five of the six study areas in concentrations ranging from 0.1 to 2.3 μL−1. Analysis indicated that there were significant differences in nitrate concentrations (averages—at 95 % confidence level using Kendall Test) among the six study areas; no significant differences in triazine-herbicide concentrations were found. Concentrations of nitrate and triazine herbicide were determined (using contingency-table analysis), to be significantly larger in more intensively irrigated areas compared to less intensively irrigated areas. Preliminary correlations with the independent variables and nitrate concentrations indicated significant relations at the 95 % confidence level with variables hydraulic conductivity, well depth, and irrigation well density. Correlations with triazine-herbicide concentrations indicated significant relations with hydraulic conductivity, specific discharge, well depth, annual precipitation, and irrigation well density, as well as nitrate concentrations. Simple multiple-regression technique indicated that well depth and density and fertilizer use explained about 51 % of the variation in nitrate concentrations. Specific discharge and well depth explained about 60 % of the variation in triazine-herbicide concentrations. Macropore flow paths and specific discharge explained 84 % of the total variation in triazine-herbicide concentrations. The use of trade names in this report is for identification purposes only and does not constitute endorsement by the U.S. Geological Survey.