Seasonal Variations of Oxygen-18 in Soil Moisture and Estimation of Recharge in Esker and Moraine Formations: Paper presented at the Nordic Hydrological Conference (Nyborg, Denmark, August - 1984)

The use of Oxygen-18 as a natural tracer of soil moisture and estimation of recharge in different types of soils is presented and discussed. The 8 values of ¹⁸O in summer and winter precipitation in southern Sweden differ by about 6-8‰ SMOW, giving the soil moisture a distinct isotopic profile. For instance, meltwater having low δ¹⁸O labels the upper soil horizon with isotopically light water and pushes down the isotopically heavy old moisture contributed by the preceding summer precipitation. The soil moisture thus exhibits a cyclic pattern having low and high ¹⁸O contents. The temporal displacements of soil moisture layers depleted or enriched in ¹⁸O have been used to calculate seasonal and annual recharge in esker and moraine formations. In Uppsala region the annual recharge varies from 230-300 mm. These estimates are reliable since ¹⁸O depleted soil moisture layers contributed by two years consecutive meltperiods have been observed in the soil profile.