TRANSBOUNDARY TRANSPORT IN THE ABBOTSFORD-SUMAS AQUIFER, BRITISH COLUMBIA AND NORTHWEST WASHINGTON STATE

The Abbotsford-Sumas aquifer is a shallow, predominantly unconfined aquifer that spans regions in southwest British Columbia, Canada and northwest Washington, USA. The glacial outwash aquifer occupies approximately 200 sq-km within the Fraser and Nooksack lowlands and serves as a water supply for more than 100,000 people in the USA and Canada. Because of intensive agricultural practices in the lowlands and high precipitation on well-drained soils, the aquifer has a history of nitrate contamination. Groundwater in the aquifer flows in a southerly direction, which results in a transboundary contribution by non-point nitrogen sources in British Columbia to elevated groundwater nitrate concentrations in Washington State.

We are monitoring groundwater and surface water nitrate and nitrogen isotopes in a 10 sq-km study area adjacent to the international boundary in northwest Washington to assess nitrate distributions and to identify both local and British Columbia components of groundwater nitrate contamination. Monthly sampling of 25 domestic wells in the study area started in July 2002. To date, 21 of the 25 domestic wells have yielded median nitrate values above 3 mg-N/L, characteristic of non-point agricultural sources. Median nitrate concentrations above the regulatory MCL of 10 mg-N/L have been observed in 13 of the wells sampled. The highest nitrate concentrations measured exceed 40 mg-N/L. Because of low vertical hydraulic gradients in the region, nitrate concentrations (>10 mg-N/L) measured in deeper wells, and in wells near the international border, suggest agricultural sources in British Columbia. Wells with the highest median concentrations (>20 mg-N/L) are from shallow regions of the aquifer and are assumed to be related to local agricultural practices in the study area. Nitrogen isotope data indicate mainly manure and manure mixed with synthetic fertilizer as sources for the nitrate.

In addition to assessing the source and distribution of nitrate contamination, we are also studying the nature and effectiveness of natural denitrification processes operating in the aquifer. Our goal is to provide scientific information that will be useful to farmers and agencies on both sides of the border for establishing effective management plans for protecting drinking water in the region.