Stable isotope values of the groundwater as function of the soil properties

The stable isotopes oxygen-18 (¹⁸ O) and deuterium (² H) are naturally abundant in precipitation. Due to fractionation effects, a seasonally variable distribution of water isotopes occurs in the precipitation of temperate and continental regions. Water isotopes, being conservative tracers, are ideal for investigating subsurface flow processes. They reveal information about soil water fluxes like evaporation, transpiration, downward infiltration and others that are difficult to determine by other techniques. We are using the observations from meteorological station and monthly average values of stable isotopes (δ¹⁸ O and δ² H) in precipitation in Riga, Latvia to model the isotopic composition of soil water and groundwater recharge. A simplified soil root-zone model as fully mixed reservoir is used. It is assumed that all the precipitation is infiltrated immediately, and water is lost from the soil due to evapotranspiration as calculated with Penman-Monteith equation and any water in excess of field capacity is exported to the groundwater. We have found that the range of the average isotopic composition of groundwater recharge in different soils types is similar to the uncertainty of the groundwater isoscape values in the region. The approach can be applied to refine the existing isoscape maps by considering the relationship between soil properties, evapotranspiration and groundwater recharge and infer groundwater recharge processes.