Spatial and temporal variation of nitrate contamination was assessed in a study area within the Canadian portion of the Abbotsford/Sumas Aquifer that covers an area of 100km 2 . Elevated nitrate concentrations were observed over a wide portion of the aquifer. Only minor decreases in nitrate concentrations were measured with depths up to 20m below the water table. The results are consistent with a wide-spread, non-point source of contamination. The spatial distribution of high nitrate concentrations appeared to correlate with agricultural land use patterns. A nitrogen budget calculation was performed based on nitrogen additions and losses to the root zone of the agricultural land in a study area located over the aquifer, using 1971, 1981 and 1991 census data. Nitrogen additions exceeded N removals by 134, 185, and 245kg Nha - 1 in 1971, 1981, and 1991, respectively, indicating a high potential for nitrate leaching to occur. The increase in the nitrogen surplus was primarily the result of changes in land use. In particular, the agricultural land base decreased by almost 20%, and there was a shift from animal production systems which require a local land base for crop production and grazing to animal production where the feed is imported. The N balance calculations were internally consistent with N budgets constructed for individual agricultural operations over the aquifer, and confirm that agricultural production is a primary contributor to elevated nitrate concentrations in the aquifer. The results highlight the potential for intensification of animal production, and the de-coupling of animal and crop production, to affect ground water quality.