Application of a Nitrate Fate and Transport Model to the Abbotsford-Sumas Aquifer, Whatcom County, Washington

The Abbotsford-Sumas aquifer is a shallow, unconfined aquifer located in an agriculturally intensive area in northwestern Washington and southwestern British Columbia. Due to aquifer characteristics and surface land use, the Abbotsford-Sumas aquifer has had a history of nitrate contamination from non-point sources. As such, nutrient managers are interested in predictive tools to evaluate management strategies. I assessed the effectiveness of a GIS based nitrate fate and transport model developed specifically for the Abbotsford-Sumas aquifer by Almasri and Kaluarachchi (2004) as a predictive tool for nutrient management. This model couples four sub-models that collectively estimate nutrient loading, predict soil-nitrogen dynamics (NLEAP), calculate groundwater velocity (MODFLOW), and nitrate fate and transport in groundwater (MT3D). The model was used to validate measured nitrate concentrations in the aquifer, and to assess the impact of land use changes and irrigation on nitrate concentrations. Validating nitrate concentrations was difficult due to the model’s design as a single layer aquifer. For those well sites with similar modeled and measured depths, the model was fairly effective at predicting nitrate concentration. Previous work has shown that nitrate is stratified in the Abbotsford-Sumas aquifer, but this fate and transport model estimates the same nitrate concentration for an entire water column. The model was sensitive to land use changes; however, the scale of the model is too coarse to capture local changes and seasonal variation. Changes in irrigation rate and concentration showed little change in resulting nitrate leaching. This lack of response is contrary to previous work, and indicates that the model underestimates irrigation’s impact on groundwater nitrate concentrations