Nitrate-nitrogen (NO3-N) 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 NO3-N loadings to the shallow, unconfined aquifer in the drainage area of the south fork of the Ogden River. Groundwater NO3-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 NO3-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 NO3-N concentrations measured in two wells in the study area. Predicted residual soil NO3-N concentrations matched measured concentrations only where assumed initial NO3 concentration and fertilization practices were reasonably accurate.
