In order to ascertain the variability in analytical concentrations that could occur during pumping of high-capacity irrigation wells, nitrate 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 of the wells random variations in the concentrations of the two analytes were dampened during the first 0.25 hr of pumping after which fluctuations in concentrations hardly exceeded the analytical method precision. Papadopulos and Cooper (1967) estimates for equilibration time between the well and aquifer best approximate the actual time necessary for the stabilization of these agrochemicals in pumped groundwater in established areas of non-point source contamination. The progressive decrease in nitrate and atrazine concentrations during the pumping of the fifth well, which is in an area of emerging non-point source contamination, reflects the radial variability in analytical concentrations in the aquifer around the well and correlates well with land use, geological conditions and well design. In this case, longer pumping times provided the data necessary for estimation of the spatial distribution of contaminants. Therefore, in the more transmissive formations, samples can be taken after 0.25 hr of pumping. Longer purge times may be necessary in emerging areas of non-point source contamination with complex lithology and short-screen wells.

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