Isotopic determination of snow-water contribution to soil water and groundwater

Abstract

The stable isotopes oxygen-18 and deuterium were used to determine the seasonal contributions of snow and rain to the soil and groundwater systems of a prairie agricultural site during the period, 1985–1987. Using measured isotope values and a temperature-isotope relationship the isotope values of precipitation during the previous 10 years at Edmonton, Canada, were determined. Winter precipitation had average oxygen-18 and deuterium values of −25.6%. and −194%., respectively; spring through autumn rains had weighted values of −16.2%. and −125%., respectively. Soil samples taken in the top 4m indicated that mixing of infiltrating waters was generally complete by a depth of 30 cm. With algebraic manipulation of measured isotope concentrations of soil and precipitation waters with known physical processes, it was possible to estimate the seasonal composition of the waters in the top 4 m. After correction for evaporation, both soil water (0–0.9 m) and shallow groundwater (3–4 m) had lower oxygen-18 values (−18.7%.; and −20.3%., respectively) than the weighted average annual precipitation concentration (−18.1%.). Using isotopes the snow-water composition of soil water was estimated to be 27% and of groundwater to be 44%. These values are higher than the proportion of winter precipitation of total annual precipitation (21% with rains ⩽5 mm day −1 excluded). The higher snow-water content of groundwater, along with the fact that the groundwater underwent less fractionation owing to evaporation than soil waters, could be the result of lateral flow from nearby small depressions where snowmelt runoff waters accumulated during spring. These findings emphasize the importance of the contribution of winter precipitation to groundwater recharge.