Tracing Ground Water Recharge in an Agricultural Watershed with Isotopes

Abstract

Abstract Nitrate and pesticide use in the 800 km 2 Raisin River agricultural watershed in eastern Ontario, Canada, threatens the quality of ground water in the highly exploited regional carbonate aquifer overlain by sandy till. To assess local recharge contributions through the cultivated fields, monthly monitoring of ground water levels, geochemistry, and environmental isotopes (δ 2 H H2O , (δ 13 C DIC ) was carried out for 12 wells over a 14‐month period. Seasonal water level variations suggest that recharge is constrained to spring and late fall when transpiration is minimized and the ground is not frozen. However, 2 H monitoring shows that early summer precipitation also contributes to recharge. Variations in (δ 2 H values between monitoring sites suggest a local component to recharge. δ 13 C DIC was used to distinguish between dissolved inorganic carbon (DIC) originating from natural (C 3 ) vegetation and DIC from the cultivated corridors where corn is grown (C 4 vegetation). Seasonal variations in δ 13 C DIC are remarkably coherent for all wells, with uniform trends to positive values during periods of low water table elevation. During periods of high water table (spring and late fall), δ 13 C DIC values are between ‐13 and ‐ 16%c VPDB (Vienna Peedee Belemnite), reflecting DIC originating from a dominantly natural (C 3 ) vegetation. When ground water levels are low (summer and mid‐winter), δ 13 C DIC values shift to between ‐11 and ‐l%c. The seasonal enrichments in δ 13 C DIC are clear evidence for a local contribution to recharge by direct infiltration though the fields. This contribution is enhanced during periods of low water level, likely due to drainage from the phreatic aquifer. High DOC (dissolved organic carbon) concentrations (> 10 to 30 mg‐C/L) correlate with periods of high water levels indicating infiltration of labile organics to the carbonate aquifer. The work carried out for this paper shows that the conjunctive use of environmental isotope geochemistry and physical parameters are fundamental to assessing the risk of ground water contamination in agricultural watersheds.