An insight into water stable isotope signatures in temperate catchment

Stable isotopes are used to decipher hydrological processes in watershed research. A two-year monthly monitoring of hydrogen and oxygen stable isotope ratios (delta H-2 and delta O-18) in a temperate catchment in Norther Europa, Latvia was undertaken. Isotope ratios in common water types - raised bog, confined groundwater, unconfined groundwater and surface water - were measured. We found characteristic signatures of isotope ratios for each of these four water types. The average isotope ratios of different water types ranged from -80.8 to -68.3 parts per thousand for delta H-2 and -11.46 to -8.76 parts per thousand for delta O-18, with standard deviations from 18 to 25 parts per thousand and 0.10 to 1.59 parts per thousand, respectively. The isotope ratios of the stream base flow were consistent with the groundwater isoscape and seasonally enriched by evaporation. Most enriched water are found in raised bogs and large lakes. The most depleted water is found in a spring discharging phreatic groundwater in a forest site with sandy soil. The most enriched water was associated with significant short-lived precipitation events, resulting in impoundment of water on the land surface and its enrichment due to evaporation. The novelty of this study is that this enriched isotope signal is propagated throughout the hydrological system, temporarily albeit significantly shifting isotope ratios of phreatic groundwater and surface runoff. Further case studies are needed to affirm if this is a regionally significant mechanism controlling isotope ratios of surface and subsurface water. The observed difference between the average delta O-18 of phreatic groundwater at two locations was 0.9 parts per thousand. We suggest that the differences are due to different land use and soil conditions.