Modeling fertilization impacts on nitrate leaching and groundwater contamination with HYDRUS-1D and MT3DMS

Agriculture is recognized as the major source of groundwater nitrate (NO3-) contamination; yet quantifying its effects is still challenging in large, due to the difficulty to track the transformation and fates of agricultural nitrogen (N) in soils and aquifers. In this study, a HYDRUS-1D model was adopted to explore water content and NO3- distribution in the unsaturated zone above groundwater table in an agricultural area. The resulting estimates of water flux and NO3- leaching through the unsaturated zone were used as input data in the application of the groundwater flow model Visual MODFLOW and mass transport model MT3DMS via a concentration recharge boundary. Nitrate leaching occurred mainly between May and September, accounting for 64% of the annual total. Four fertilizer application scenarios were developed, and their effects were compared in predictive simulations of groundwater NO3- concentrations using MT3DMS and quantitative analysis of NO3--contaminated areas, the spatial and temporal distribution of groundwater NO3- concentration using ArcGIS. Predictions in the business as usual (scenario 1) showed that NO3- continued to accumulate in groundwater in the study area, with the maximum increased from 14 to 18 mg L-1 in 10 years. In the scenario 2 (2% increase in fertilizer application rate), peak groundwater NO3- was expected to exceed 20 mg L-1 in 2027. However, in both scenarios 3 (2% reduction in fertilizer application) and 4 (4% reduction in fertilization), the maximal NO3- concentrations were predicted to be lower than 12 mg L-1 in 2027. The integration of HYDRUS-1D, MT3DMS and GIS models offers a powerful tool for evaluating agricultural management impacts on aquifer water quality.