SAMPLING OF NONPOINT-SOURCE CONTAMINATION IN HIGH-CAPACITY WELLS

In order to ascertain the variability in analytical concentrations tha t could occur during pumping of high-capacity irrigation wells, nitrat e and atrazine concentrations were measured during prolonged pumping. Five high-capacity irrigation wells were selected in heavily irrigated areas of Nebraska with non-point source contamination. The wells were screened in different sediment types which included sands and gravels , sandstone, and fractured siltstone. Results indicated that in four o f the wells random variations in the concentrations of the two analyte s were dampened during the first 0.25 hr of pumping after which fluctu ations in concentrations hardly exceeded the analytical method precisi on. Papadopulos and Cooper (1967) estimates for equilibration time bet ween the well and aquifer best approximate the actual time necessary f or the stabilization of these agrochemicals m pumped groundwater in es tablished areas of non-point source contamination. The progressive dec rease in nitrate and atrazine concentrations during the pumping of the fifth well, which is in an area of emerging non-point source contamin ation, reflects the radial variability in analytical concentrations in the aquifer around the well and correlates well with land use, geolog ical conditions and well design. In this case, longer pumping times pr ovided the data necessary for estimation of the spacial distribution o f contaminants. Therefore, in the more transmissive formations, sample s can be taken after 0.25 hr of pumping. Longer purge times may be nec essary in emerging areas of non-point source contamination with comple x lithology and short-screen wells.