ISOTOPIC DETERMINATION OF SNOW-WATER CONTRIBUTION TO SOIL-WATER AND GROUNDWATER

The stable isotopes oxygen-18 and deuterium were used to determine the seasonal contributions of snow and rain to the soil and groundwater s ystems of a prairie agricultural site during the period, 1985-1987. Us ing measured isotope values and a temperature-isotope relationship the isotope values of precipitation during the previous 10 years at Edmon ton, Canada, were determined. Winter precipitation had average oxygen- 18 and deuterium values of -25.6 parts per thousand and -194 parts per thousand, respectively; spring through autumn rains had weighted valu es of -16.2 parts per thousand and -125 parts per thousand, respective ly. Soil samples taken in the top 4 m indicated that mixing of infiltr ating waters was generally complete by a depth of 30 cm. With algebrai c manipulation of measured isotope concentrations of soil and precipit ation waters with known physical processes, it was possible to estimat e the seasonal composition of the waters in the top 4 m. After correct ion for evaporation, both soil water (0-0.9 m) and shallow groundwater (3-4 m) had lower oxygen-18 values (-18.7 parts per thousand; and -20 .3 parts per thousand, respectively) than the weighted average annual precipitation concentration (-18.1 parts per thousand). 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 less-than-or-equal-to 5 mm day-1 excluded). The higher snow-water cont ent of groundwater, along with the fact that the groundwater underwent less fractionation owing to evaporation than soil waters, could be th e 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.