Regional and seasonal trends in the oxygen and hydrogen isotope ratios of Finnish groundwaters: a key for mean annual precipitation

The isotopic composition of oxygen and hydrogen in Finnish groundwaters was studied as part of two hydrogeological research programs during 1995–2001. A groundwater data set of 983 samples with good geographical coverage and a long-term groundwater data set from 19 study sites were included. The isotopic composition of oxygen and hydrogen in groundwaters shows a systematic shift from higher δ18O and δD values in the south to lower values in the north, the difference being approximately 4‰ for oxygen and 34‰ for hydrogen. The δ18O values are correlated with the mean annual surface temperature and the slope is close to that reported for the precipitation in European stations. In order to evaluate the relationship between the δ18O and δD values in precipitation and those in local groundwater, precipitation was collected at one site in southern Finland. No precipitation isotope records have been published in northern Finland, and those from groundwaters were therefore compared to published precipitation data from northern Sweden. No significant differences were observed between the mean annual δ18O and δD values in precipitation and those in local groundwaters. These groundwater data provide the first approximation of regional trends in the isotopic composition of precipitation in Finland.