ANALYSIS OF NITRATE-NITROGEN MOVEMENT NEAR HIGH-CAPACITY IRRIGATION WELLS

A Galerkin finite-element model coupled with a particle tracking routi ne was developed to analyze the flow and transport dynamics near a hig h-capacity irrigation well. The model was used to compute the head dis tribution around the pumping well, to determine the area of influence, and to define ground water flowlines during short-term pumping period s typical of those used to collect water quality samples from high-cap acity wells. in addition to hypothetical example results, the model wa s used to qualitatively analyze data obtained from pump-and-sample exp eriments conducted in an unconfined alluvial aquifer within the Platte River valley of south-central Nebraska where nitrate-nitrogen (NO3-N) contamination is prevalent. Simulation results of both the hypothetic al and field cases suggest that short-term pumping events impact a lim ited volume of aquifer. The area of influence and flowlines are affect ed by aquifer anisotropy, pumping rate, and well construction characte ristics. Ground water above or below the screened intervals does not e nter a partially penetrating well in anisotropic aquifers. In aquifers where NO3-N concentration varies vertically and horizontally, wafer q uality samples from an irrigation, or other high-capacity, well provid e only limited information about ground water contamination. A numeric al model is thus recommended for calculating the area of influence and determining flowlines around high-capacity wells so that information derived from water quality samples collected at the wellhead can be be tter interpreted.