DEVELOPMENT OF A FIELD-SCALE MODEL TO SIMULATE NITRATE TRANSPORT IN GROUNDWATER FROM AGRICULTURAL LANDS

Groundwater is a very important source of drinking water for over 2 billion people worldwide. Nitrogen-based fertilizers are applied on agricultural lands to enhance crop production and meet the global food demand. Excess nutrients can leach into aquifers and contaminate groundwater with nitrate. Groundwater modelling can help to evaluate the impact of changing agricultural practices on nitrate dynamics in subsurface by applying different stressors or input conditions to numerical models calibrated using observed field conditions. For this purpose, groundwater contamination by nitrate was investigated in a sand plain aquifer in Norfolk County, Southern Ontario (Canada) known for intensive agricultural practices. A network of 24 monitoring wells was selected to monitor groundwater elevations and nitrate concentrations in the aquifer. Water table depths and groundwater samples were collected bi-monthly from June 2014 to May 2016. Pumping well and irrigation data for the study region were collected from Provincial and other sources and were used in the flow simulation. A regional-scale flow model was developed using HydroGeoSphere, which is state-of-the-science three dimensional groundwater modelling software. The model was calibrated for flow conditions using several available calibration targets including the monitoring well network. The results showed good agreement between modelled and measured data. Once the regional model was developed using HydroGeoSphere, a field-scale transport model was developed to simulate the impact of agricultural activities on nitrate concentrations in study area using a one-dimensional unsaturated zone model (DRAINMOD). Sandy soil conditions and water table depths beneath the root zone allowed coupling of the one-dimensional unsaturated nitrate transport model with HydroGeoSphere. The DRAINMOD nitrate output was used as the top boundary condition in the HydroGeoSphere model to simulate three-dimensional nitrate transport in the saturated zone. The field-scale model was also successfully calibrated for both groundwater flow and nitrate transport. The field-scale model is used to examine the impacts of several crop and best management practice scenarios, such as crop rotations and variable rate fertilization, on groundwater quality.