Use of High Resolution Vertical Concentration Profiles to Associate Groundwater Nitrate with Agricultural Land Use

Abstract

Nitrate derived from manure and mineral fertilizers is adversely impacting groundwater quality in the Abbotsford-Sumas Aquifer (ASA). The main objective was to identify potential relationships between groundwater geochemistry in the ASA and upgradient land use, specifically raspberry, blueberry and non-agricultural fields. High resolution vertical diffusion sampling of four monitoring wells were utilized. Groundwater samples were analyzed for chloride, sulphate, nitrate-N, and 15N and 18O of NO3-N. Time series geochemical and isotopic profiles revealed differences in groundwater chemistry within and between wells. At wells with consistent groundwater flow direction and relatively uniform upgradient land use, it was possible to differentiate land uses. Specifically, shallow groundwater captured downgradient of blueberry fields had comparable or higher nitrate-N concentrations than raspberry fields, which were both higher than non-agricultural areas. Elsewhere and deeper in the vertical profiles, seasonal variations in groundwater flow directions made it difficult to differentiate upgradient land uses because crop types varied spatially and temporally.