Nonpoint source pollution of surface and groundwater resulting from agricultural management practices is a major water quality problem. This problem was assessed on a demonstration watershed in the Cape Fear River Basin of North Carolina, during a five-year study. Groundwater was monitored in a row crop field (corn/wheat/soybean) and a swine waste spray field (Coastal bermuda grass). Groundwater nitrate-N concentrations averaged 6.5 mg/L in the row crop field. Nitrate-N concentrations in groundwater at the swine waste spray field exceeded 80 mg/L. Nitrate-N concentrations were simulated in both fields with the GLEAMS model. The GLEAMS model simulated groundwater nitrate-N concentrations with mean residuals (simulated-observed) ±1.3 mg/L and ±19 mg/L, respectively, for the row crop and the swine waste spray field. Groundwater nitrate-N concentrations have been reduced in the spray field by using improved management practices and the GLEAMS model simulated this nitrate-N concentration reduction. These simulation results show that the GLEAMS model can be used to predict nitrate-N loading of groundwater of these agricultural management systems.