Tracing ground water recharge in an agricultural watershed with isotopes

Nitrate and pesticide use in the 800 km(2) Raisin River agricultural waters hed in eastern Ontario, Canada, threatens the quality of ground water in th e highly exploited regional carbonate aquifer overlain by sandy till, To as sess local recharge contributions through the cultivated fields, monthly mo nitoring of ground water levels, geochemistry, and environmental isotopes ( delta(2)H(H2O), delta(13)C(DIC)) was carried out for 12 wells over a 14-mon th period, Seasonal water level variations suggest that recharge is constrained to spr ing and late fall when transpiration is minimized and the ground is not fro zen, However, H-2 monitoring shows that early summer precipitation also con tributes to recharge. Variations in delta(2)H values between monitoring sit es suggest a local component to recharge, delta(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 del ta(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), delta(13)C(DIC) values are bet ween - 13 and - 16 parts per thousand VPDB (Vienna Peedee Belemnite), refle cting DIC originating from a dominantly natural (C-3) vegetation. When grou nd water levels are low (summer and mid-winter), delta(13)C(DIC) values shi ft to between - 11 and -7 parts per thousand, The seasonal enrichments in d elta(13)C(DIC) are clear evidence for a local contribution to recharge by d irect infiltration though the fields, This contribution is enhanced during periods of low water level, likely due to drainage from the phreatic aquife r, High DOC (dissolved organic carbon) concentrations (> 10 to 30 mg-Cn) co rrelate 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 envir onmental isotope geochemistry and physical parameters are fundamental to as sessing the risk of ground water contamination in agricultural watersheds.