Analysis of nitrate-nitrogen movement near high-capacity irrigation wells

A Galerkin finite-element model coupled with a particle tracking routine was developed to analyze the flow and transport dynamics near a high-capacity irrigation well. The model was used to compute the head distribution around the pumping well, to determine the area of influence, and to define ground water flowlines during short-term pumping periods typical of those used to collect water quality samples from high-capacity wells. In addition to hypothetical example results, the model was used to qualitatively analyze data obtained from pump-and-sample experiments 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 hypothetical and field cases suggest that short-term pumping events impact a limited volume of aquifer. The area of influence and flowlines are affected by aquifer anisotropy, pumping rate, and well construction characteristics. Ground water above or below the screened intervals does not enter a partially penetrating well in anisotropic aquifers. In aquifers where NO₃-N concentration varies vertically and horizontally, water quality samples from an irrigation; or other high-capacity, well provide only limited information about ground water contamination. A numerical 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 better interpreted.