loadings to groundwater from irrigated croplands, lawns, and on-site wastewater drain fields were simulated using the Nitrogen Loss and Environmental Assessment Package-Geographic Information System (NLEAP-GIS) 4.2 model in Ogden Valley, Utah. The study determined the influence of domestic wastewater and nitrogen fertilizers applied to lawns and fields on NO 3 -N loadings to the shallow, unconfined aquifer in the drainage area of the south fork of the Ogden River. Groundwater NO 3 -N concentrations were estimated from the NLEAP-GIS 4.2 simulated leaching losses. Annual leaching rates (kg N ha -1 y -1 ) from the drain-fields and the lawns were, respectively, more than 2.6-and 1.1-fold higher than from the croplands. Total leaching losses (kg N y -1 ) from the croplands and lawns were, respectively, 70-and 50-fold higher than total loads from drain-fields. Lawns and drain-fields had lower total leaching losses than the cropland because the total area was smaller than the cropland. The model predicted that a 50% reduction in lawn fertilizer application rate would result in a 36% decline in leaching. A 50% reduction in irrigation water application rate only reduced predicted leaching by 18%. NLEAP was able to predict NO 3 -N concentrations (1.9 ± 0.3 mg N L -1 [1.9 ± 0.3 ppm]) resulting from blending leachate into groundwater within the range of the NO 3 -N concentrations measured in two wells in the study area. Predicted residual soil NO 3 -N concentrations matched measured concentrations only where assumed initial NO 3 concentration and fertilization practices were reasonably accurate.
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