Linking Root Zone Nitrate Leaching to Groundwater Nitrate Loading In the Abbotsford-Sumas Aquifer, British Columbia, Canada.

The unconfined trans-boundary Abbotsford-Sumas aquifer supplies drinking water for more than 100,000 people. High annual precipitation, intensive agricultural (small fruit, poultry) production and its highly conductive sand and gravel composition make the aquifer very susceptible to groundwater nitrate contamination. Average groundwater nitrate concentrations have been consistently above Canadian drinking water guidelines (10 mg/L NO₃-N) since the initiation of regional groundwater monitoring in the early 1990’s. Groundwater flow in the aquifer is almost horizontal, and consequently it is difficult to directly link agricultural management practices or seasonal processes with nitrate loading to groundwater using more traditional sampling approaches. We are using an interdisciplinary approach to link the magnitude and timing of nitrate movement from the soil root zone in a grower field with nitrate concentration at the water table.  Nitrate loss from the root zone is estimated biweekly using passive capillary samplers. A detailed vertical profile of nitrate concentration at the water table is measured monthly using vertical passive diffusion sampling and the groundwater nitrate mass flux estimated using hydrogeologic principles.  Significant leaching from the soil zone occurred in early fall, while peak nitrate concentrations arrived at the (4 to 6 m deep) water table in late winter.   The nitrate flux leaving the soil zone was comparable to that estimated in the groundwater.  This approach provides a direct and quantitative basis on which better management practices can be based.