WELL SAMPLING FOR AGRICHEMICALS IN HIGH-CAPACITY SYSTEMS

In irrigated areas of Nebraska, agrichemical contamination of shallow groundwater is characterized by much less areal variability than is co mmonly associated with point-source contaminated areas. Existing high capacity irrigation wells represent a cost-effective sampling network for describing the areal extent of nonpoint-source contamination. Vari ability in nitrate concentrations during pumping is important to both the regulator and the farmer because the nitrate in the irrigation wat er can meet some of the crop's N-requirement and the nitrate concentra tion is an integral factor in preparing an accurate N-fertilizer budge t, which may be mandated in Ground Water Quality Management and Specia l Protection Areas. In order to ascertain the variability in the conce ntrations of agrichemical contaminants during pumping, nitrate and atr azine concentrations were measured in successive samples taken during a 24-hour period from five high capacity irrigation wells located in h eavily irrigated areas of Nebraska. In four of the wells stable analyt e concentrations were attained in <0.25 hours after which fluctuations in concentration hardly exceeded the analytical method variability. I n established areas of nonpoint-source contamination with transmissive formations, Papadopulos and Cooper's estimates for equilibration time between the well and aquifer best approximate the actual time necessa ry for the stabilization of these agrichemical concentrations in pumpe d groundwater. The progressive decrease in the nitrate concentration d uring the pumping of the fifth well reflects the radial variability in analyte concentration in the aquifer around the well and correlates w ell with land use, geology, and well design. In emerging areas of nonp oint-source contamination with complex geology and short screened inte rvals, longer pumping times may be necessary to attain uniform concent rations.