In-situ stream water isotope (δ18O, δ2H) measurements reveal ecohydrologic nuances in an Arctic Finland watershed

Understanding the changing Arctic hydrologic system has taken on new urgency as the state of sea ice, moisture sources, and precipitation patterns have rapidly shifted compared to the past. Measuring and monitoring these processes is entering a new phase as we utilize new technology that reveals patterns and processes previously out of reach with standard campaign sampling.

In June 2018, we installed a Picarro Continuous Water Sampler and isotopic analyzer to a second-order stream in Pallas-Yllästunturi National Park, Finland. This new integrated system allows us to continuously measure the stable isotope geochemistry (δ¹⁸O and δ²H) of stream water in-situ, and in parallel with standard measurements of stream water discharge, temperature, DOC and pH, as well as event-based precipitation isotope measurements. Initial data observations indicate that during the prolonged mid-summer 2018 drought, mean stream water δ¹⁸O and δ²H values were -14 ± 1 ‰ and -99 ± 2 ‰, respectively. These values reflect the fundamental recharge of the basin being driven by snow melt water (typical snow δ¹⁸O -17 ‰). We observed that the sub-diurnal response to minor episodic enriched rain events of δ¹⁸O -8 ‰ are not detected in stream water δ¹⁸O and δ²H, likely reflecting sustained drought and local groundwater recharge instead. Our baseline drought measurements will allow us to partition and quantify hydrologic thresholds for enrichment of stream water isotopes as precipitation is detected in the system. These exceptionally high-frequency (10/min) measurements of stream water isotopes provide insight as to how the extraordinary drought affects the mobilization of water in the catchment and an assessment of how future changes in precipitation patterns and seasonality will affect Arctic watersheds. This high-resolution quantification of stream water geochemistry will further allow more accurate hydrograph separation and provide the basis to investigate watershed scale ecohydrological pathways, including in-stream biogeochemical and biological processes such as the phenology of invertebrate hatches now and in the past.