The Khibiny alkaline massif on the Kola Peninsula (Fig. 1d) consists of igneous rocks with titanite and apatite-nepheline deposits (Kogarko, 2018). The massif is surrounded by two larger lakes: the Umbozero Lake to the east and the Imandra Lake to the west with lake areas of 320 and 880 km2, respectively (Fig 1; Dauvalter and Kashulin, 2010; Dinu et al., 2020). The Khibiny mountain valleys (Fig. 1e) are characterized by sandy till deposits and a forest-tundra ecosystem, while the higher plateaus (with mountain peaks up to 1000 meter above the Umbozero and Imandra Lakes) hold mostly weathered rocks and Arctic tundra (Pereverzev, 2010). The average annual temperature for the Khibiny massif was −0.59 °C between 1993 and 2017 according to data from the Climatic Research Unit (CRU) at the University of East Anglia described by Harris et al. (2014; see Supporting Information S1 on details). For the same time period, the annual average precipitation was 715 mm/yr and annual average actual evapotranspiration was 290 mm/yr (based on CRU data and same calculation procedure as for the Imetjoki catchment). About 210 days per year were snow covered between 1998 and 2007 (Callaghan et al., 2011). The southwestern part of the Khibiny massif is drained by the Belaya River that has a catchment area of 142 km2 and an average annual discharge of 1900 l/s (ID 22 in Fig. 1; derived from CRU data following the water balance calculation steps of Fischer et al. 2020) before reaching the Imandra Lake. Furthermore, the Belaya River (main branch) starts at the outlet of the Bolshoy Vudyavr Lake gathering tributary inflows from the Yuksporiok, Saamsky and Vudyavriok sub-catchments (Fig. 1e). The southeastern part of the massif is drained by the Vuonnemiok Stream with an annual average discharge of 1300 l/s (ID 25 in Fig. 1; based on water balance calculations) that empties in the Umbozero Lake. Kirovsk City is the main settlement in the Khibiny massif where there are five apatite-nepheline ore deposits (Fig. 1e) that have been actively mined since the 1930s. They are now owned by the company PhosAgro. The mining activity has led to elevated surface water concentrations of strontium (Sr) and aluminum (Al), as well as copper (Cu), zink (Zn), nickel (Ni) and chromium (Cr) in the Yuksporiok and Vuonnemiok basins (Efimov et al., 2019; Malinovsky et al., 2002). The Kola Peninsula also hosts several Cu-Ni ore smelters that have for decades emitted large volumes of sulfur dioxide into the air resulting in acid rain and high sulfur and metal atmospheric deposition (Moiseenko and Bazova, 2016).
Field campaigns and analyses
Surface water sampling and measurement campaigns were conducted in the Imetjoki catchment in the spring (end of May during snowmelt) and in the late summer (end of August) of 2017. Apart from element concentrations and general water quality parameters (e.g., pH, dissolved organic carbon etc.; presented in Fischer et al. 2020) sulfur isotopes (δ34SSO4-values in SO42-) were measured. In total 11 locations were sampled for sulfur isotopes and sulfur concentrations in the spring and 13 locations were sampled in the summer (the latter used here for comparison, being presented in Fischer et al., 2022). Nine sampling locations were common for both campaigns. The sampling locations were distributed over the upstream areas (undisturbed by mining; IDs 1‒2 in Fig.1), the mining site itself (directly affected by AMD; IDs 3‒9), and downstream of the mining site (IDs 10‒12, 14). Two adjacent rivers were also sampled for reference (IDs 13, 15). The same sampling and analysis procedures were followed in both the spring and summer campaigns as described in Fischer et al. (2022), making the results directly comparable. The sulfur isotopic composition was determined from collecting 2L water samples that were dripped through ion exchange resin columns to collect the sulfate in water and for easier transportation to the laboratory. Instrumental analyses were carried out with an elemental analyzer (CarloErba NC2500) coupled to a stable isotope ratio mass spectrometer (Finnigan Thermo Delta plus; analytical accuracy was ±0.2‰) at the laboratories of the Department of Geological Sciences at Stockholm University. Sulfur isotopes are reported in parts per million (‰) through the δ-notation relative to the Vienna-Canyon Diablo Troilite (V-CDT) standard: