Assessment of the impact of nutrient management practices on nitrate contamination in the Abbotsford-Sumas aquifer.

The impact of recent changes to nutrient management practices in raspberry fields on the loading and subsequent transport of nitrate through the vadose zone of the Abbotsford-Sumas aquifer is investigated numerically. Previous studies have shown that nitrate concentrations in the aquifer have remained relatively stable despite a shift in nutrient management practices. Using an estimate of net annual available nitrogen in fields that are fertilized using synthetic fertilizer, nitrate concentrations as a function of time and depth through the vadose zone are simulated from spring to late fall. Results indicate rapid leaching of nitrate owing to the permeable nature of the aquifer and suggest that nitrate loading to the water table may occur earlier than previously thought, possibly due to spring rains. For an average fertilizer application rate of 90 kg of N/ha, the simulated nitrate concentration on Oct 1 within the top 1 m of soil is 33 mg of N/kg, while the residual soil nitrate measured in late September was 37 mg of N/kg. Taking into account the effects of dilution within the saturated zone, the simulated peak nitrate concentration is similar to average observed peak concentrations in a shallow monitoring well. A solution is offered for estimating nitrate concentration at the water table as a function of the rate of synthetic fertilizer applied to raspberry fields.