INVESTIGATING AQUIFER HETEROGENEITY USING GROUND-PENETRATING RADAR: IMPLICATIONS FOR NITRATE TRANSPORT IN THE ABBOTSFORD-SUMAS AQUIFER, BRITISH COLUMBIA, CANADA AND WASHINGTON, USA

Nitrate contamination of the Abbotsford-Sumas aquifer in the central Fraser Valley has become a significant problem over the last 30 years. Nitrate concentrations above the maximum allowable concentration (10 mg/L NO₃-N) have been recorded in many of the aquifer's wells since the 1970's. This trans-national aquifer, approximately 160 km² in area, is situated on the boundary between British Columbia (BC) and Washington State. The aquifer is a source of water for approximately 100,000 people. A regional groundwater flow model, developed previously for the aquifer, captures a considerable degree of spatial heterogeneity of the surficial materials. However, at a scale necessary for modeling nitrate transport, the glaciofluvial deposits comprising the aquifer are complex, with interbedded and cross-cutting sand and gravels. It is anticipated that local heterogeneity will result in complex permeable pathways that might variably influence the movement of nitrate through the aquifer. A series of ground-penetrating radar (GPR) surveys were conducted at Agriculture and Agri-Food Canada's Pacific Agriculture Research Centre (PARC) in Abbotsford, BC in an attempt to investigate the scale of heterogeneity present locally. Several boreholes at the site are used to constrain the interpretation. The GPR profiles give no indication of lateral continuity of beds, but do suggest that there is heterogeneity at a small-scale, as evidenced by the cross-cutting nature of the glaciofluvial deposits. However, the scale of these heterogeneities is too small to resolve using the GPR, and thus, cannot be adequately represented in the model. This suggests that heterogeneity may be a limiting factor in modeling nitrate transport in this aquifer.