Technical Abstract: A long-term crop irrigation project with treated wastewater south of Dodge City in southwestern Kansas is the focus of this study. The use of treated wastewater in that area, which consists of mainly silty clay loam soils, has resulted in high nitrate-nitrogen concentrations (10 – 50 mg/kg) in the upper 15-m profile, and also in the underlying deep ground water, suggesting that preferential flow processes may have occurred at the study area. The goal of this two-year monitoring project is to assess how and under what circumstances nitrogen nutrients under treated wastewater-irrigated corn can reach the deep (20 – 45 m) ground water of the underlying High Plains aquifer, and what can realistically be done to minimize this problem. To achieve this goal, we collected deep cores for physical and chemical properties characterization; installed neutron moisture probe access tubes and suction lysimeters for periodic measurements; sampled area monitoring, irrigation, and domestic wells; performed dye tracer experiments to demonstrate the existence of macropores; and obtained climatic, crop, irrigation, and fertilization application rate data, and additional information for use in the comprehensive Root Zone Water Quality Model (RZWQM) to identify key parameters and processes that influence nitrogen losses in the study area. We demonstrated that preferential transport processes do indeed take place and that significant amounts of nitrogen are accumulating in the thick vadose zone. RZWQM simulations indicated that macropore flow is generated particularly during heavy rainfall events but during our 2005-2006 simulations the total macropore flow generated was relatively small (3% of precipitation). Our calibrated model indicated that reducing current levels of corn fertilization using treated wastewater by half substantially increases nitrogen use efficiency (NUE), while reducing current irrigation amounts by 25 percent (at the 50 percent fertilization levels) does not appreciably decrease NUE and corn yields.