Heini Postila

A combination of thermal infrared (TIR) images captured by uncrewed aircraft systems (UASs) and manual geophysical measurements with a GEM-2 device were used to better plan a water sampling campaign at an abandoned Au-Cu mine and an active Ni-Cu-PGE mine in the sub-arctic boreal zone in northern Finland. Anomalies from the TIR images and GEM-2 conductivity values were used to focus the water sampling. The hydrogeochemistry and isotopic compositions of oxygen (δ¹⁸O), hydrogen (δ²H), strontium (⁸⁷Sr/⁸⁶Sr), and sulfur (δ³⁴S) were analyzed to better understand the flow regime of possible effluent waters at these mine sites. The TIR images were useful in pinpointing sites where groundwater and surface water were potentially interacting. This was confirmed with the oxygen and hydrogen isotopic data. Isotopic values for the two groundwater solutes used here (Sr and S), reflected the local geology and the biogeochemical environment at the mine sites. The electrical conductivity values obtained from the GEM-2 measurements were influenced by the local geochemistry, particularly the presence of conductive sulfide-bearing paraschist rock. The anomalies from the UAS-TIR and GEM-2 data revealed sampling sites well suited for tracking potential effluent waters at the two mine sites.

The influence of seasonally frozen ground (SFG) on water, energy, and solute fluxes is important in cold climate regions. The hydrological role of permafrost is now being actively researched, but the influence of SFG has received less attention. Intuitively, SFG restricts (snowmelt) infiltration, thereby enhancing surface runoff and decreasing soil water replenishment and groundwater recharge. However, the reported hydrological effects of SFG remain contradictory and appear to be highly site- and event-specific. There is a clear knowledge gap concerning under what physiographical and climate conditions SFG is more likely to influence hydrological fluxes. We addressed this knowledge gap by systematically reviewing published work examining the role of SFG in hydrological partitioning. We collected data on environmental variables influencing the SFG regime across different climates, land covers, and measurement scales, along with the main conclusion about the SFG influence on the studied hydrological flux. The compiled dataset allowed us to draw conclusions that extended beyond individual site investigations. Our key findings were: (a) an obvious hydrological influence of SFG at small-scale, but a more variable hydrological response with increasing scale of measurement, and (b) indication that cold climate with deep snow and forest land cover may be related to reduced importance of SFG in hydrological partitioning. It is thus increasingly important to understand the hydrological repercussions of SFG in a warming climate, where permafrost is transitioning to seasonally frozen conditions.