Conference Paper

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

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Abstract

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 (δ 18 O and δ 2 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.

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... Stable isotope ratios of groundwater, especially phreatic groundwater, are expected to reflect values of modern precipitation input (Clark and Fritz, 1997;Darling et al., 2003;Filippini et al., 2015;Ma et al., 2017;Rozanski, 1985) because it is the main source of water in aquifers. Groundwater recharge and variations of δ 18 O and δ 2 H are controlled by factors such as soil type, vegetation coverage and type, precipitation type and seasonality (Barbecot et al., 2018;Birkel et al., 2018;Crosbie et al., 2015;Jasechko et al., 2014;Kalvāns et al., 2018;Matiatos and Wassenaar, 2019;Raidla et al., 2016;Sánchez-Murillo and Birkel, 2016;Wassenaar et al., 2009). ...
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