Isotopic composition of atmospheric precipitation and shallow groundwater in Olkiluoto: O-18, H-2 and H-3

Isotopic composition of oxygen and hydrogen in local precipitation is a key parameter in modelling of local water circulation. This study was initiated in order to provide systematic monthly records of the isotope content of atmospheric precipitation in the Olkiluoto area and to establish the relation between local rainfall and shallow groundwater. During January 2005 - June 2008 a total of 35 cumulative monthly rainfall samples and 38 shallow groundwater samples were collected and their O-18, H-2 (= D) and H-3 values were detected. Based on the three years' monitoring, the weighted annual mean isotope values of the precipitation and the mean values for groundwater are -12.15 per mille and -11.31 per mille for oxygen, -86.8 per mille and -80.4 per mille for hydrogen and 10.2 and 9.9 TU for tritium, respectively. In general, the isotope signatures of atmospheric precipitation and groundwater at Olkiluoto are close to unity and represent the typical isotope values of this area in Finland. The coastal location of the study area is reflected in the isotopic composition of oxygen and hydrogen in the precipitation. In contrast to inland rainfall, the temporal correlation between the stable isotope ratios and surface temperatures is poor and the seasonal isotopic variations are decreased. The seasonal behaviour of tritium concentrations in Olkiluoto precipitation follows a typical H-3 pattern observed in the northern hemisphere and, hence, not any significant input of secondary tritium can be suggested during this monitoring period. The precipitation data were used to establish the local meteoric water line (LMWL) for the Olkiluoto area. The line is formulated as: δD = 7.29 δO-18 + 2.01. (orig.)