Geochemical characterization of Lake Pyhäjärvi catchment and potential applicability of hydrogeochemical separation in understanding groundwater-surface water interaction

Lake Pyhäjärvi, the largest lake in southwestern Finland, has been under considerable external nutrient loading for the past decades. Rivers Pyhäjoki and Yläneenjoki are the only major input rivers of the lake and are the source of most of the external nutrient loading to the lake. The hydrogeochemistry of the Lake Pyhäjärvi catchment, as well as the catchments of the two input rivers, and groundwater-surface water interaction were evaluated using a wide array of geochemical tracers (major ions, dissolved silica, stable isotopes of oxygen and hydrogen, electrical conductivity and radon-222). Additionally, the feasibility of using mass balance based methods to separate river waters of the two input rivers to their respective source components was evaluated independently with each tracer analysed. In the context of this study, stable isotopes alone were deemed plausible and were only usable in the River Pyhäjoki catchment as there was not enough difference between river water and precipitation (new water) stable isotope proportions in River Yläneenjoki. Employing the stable isotopes of oxygen, mass balance based modelling was attempted to separate the hydrograph of River Pyhäjoki to its end-members (new water and old water). Based on the obtained data, the hydrogeochemical content of the surface waters of each subcatchment (Pyhäjärvi, Pyhäjoki and Yläneenjoki) differed significantly. Additionally, the groundwaters were clearly distinguished from surface waters. Differences were most apparent in stable isotope proportions as well as dissolved silica, and in the case of groundwaters in radon-222 concentrations. This categorization was further supported by a hierarchical cluster analysis. Surface waters showed varying signs of evaporation, whereas groundwaters retained the stable isotope characteristics of mean annual precipitation. Dissolved silica concentrations appeared to be mostly affected by the amount of easily soluble silica in the sediment, water residence time, as well as biological uptake in the surface waters. Lithology seemed to be the controlling factor in radon concentrations, with areas of granitic bedrock having the highest concentrations. The hydrograph separation of River Pyhäjoki gave results between 66–88% of old water in the river at the time of sampling, consisting likely mostly of groundwater. Though this result corresponds well with recent similar studies in the area, there were considerable sources of uncertainty, therefore making the result best thought of as indicative. However, there was clear evidence of groundwater-surface water interaction in all of the subcatchments of Lake Pyhäjärvi, with clear evidence of groundwater discharging into the lake, lake water infiltrating into an aquifer near the shore of the lake, as well as signs of groundwater discharging into the two input rivers.