Application of geophysics and numerical modelling in studying aquifer heterogeneity and nitrate transport, Abbotsford-Sumas aquifer, British Columbia, and Washington, USA

Heterogeneity within the sand and gravel deposits of the Abbotsford-Sumas aquifer has a significant impact upon groundwater movement and nitrate transport. Using GPR and borehole logging, the scale of heterogeneity was determined, with fining upward sequences up to 5 m thick and continuous over 10’s of metres. Smaller heterogeneities were also identified visually in a local gravel pit. Various approaches were examined to represent this heterogeneity within a local groundwater flow mode l. The use of vertical anisotropy proved to be most realistic. Ages determined from the model were 60-80% lower than measured isotopic ages due to the inability to adequately represent the tortuosity of the flow paths. The spatial distribution and temporal variation of nitrate in the aquifer provided initial and calibration nitrate concentrations for the nitrate transport model. Nitrate concentrations thought to be reaching the aquifer based on recent BMPs are not sufficient to produce the observed nitrate concentrations.