The measurement campaigns in the Imetjoki catchment in the spring (May) and summer (August) of 2017 revealed high surface water concentrations (15-5000 µg/L) of Cu, Zn and Cd at and downstream of the mining site, where the dissolved organic carbon (DOC) ranged between 1.3 to 11.6 mg/L and pH-levels were between 3.3 and 7.1 (Fischer et al. 2020). No systematic differences in concentrations could be seen between the seasons except for DOC which was higher in all samples (on average 30%) from the summer compared to the spring. The measured sulfur isotope composition (δstream ) and total sulfur (S) concentrations (cstream ) are plotted in Figure 2a together with a so called “theoretical mixing line” which shows a hypothetic proportional mixing between the two end-members without the influence of MSR. The mixing line is defined through corresponding average regional values for each end-member (Table 1) where the grey area conveys the ±1 standard deviation range. Overall,δstream -values measured in the spring (May) in Imetjoki (hollow circles; Fig. 2a) lie closer to the theoretical mixing line than δstream measured in the summer (August). This is further illustrated in Figure 2b where the highest difference in δstream between spring and summer was seen in the Imetjärvi Lake inlet (ID 4; +2.8‰) and the Northern Lake outlet (ID 3; +2.5‰). The samples from the Northern Lake, Downstream 1, and Maria Lake sites (IDs 3, 5, 10) all show an increase incstream by 60-70% over the summer, while the values for the stream in the lower Industrial area (ID 9) only increased by 11% (i.e., from 20.5 mg/L in the spring to 22.8 mg/L in the summer; Table S2 in SI). The only decrease in cstream was detected in samples from the small brook in the upper Industrial area (ID 8) where the concentration decreased from 48 to 17 mg/L over the summer.
The surface waters within the Khibiny catchments showed clearly higher metal concentrations (3-24 µg/L for Cu, Zn and Cr) close to the mining areas compared to the concentrations in the upstream areas unaffected by mining (with average concentrations of 0.2-0.7 µg/L for Cu, Zn and Cr; see Supporting Information Table S6‒7 for full analysis results). DOC levels were generally low in upstream areas (~1 mg/L), and increased only slightly (1.7-3.2 mg/L) in the mining-impacted and downstream areas. pH ranged between 6.8 and 10.6 for all Khibiny samples and all measured (total) sulfur concentrations occurred in the form of sulfate. All of the measured δstream -values in Khibiny are falling either on or within 1 standard deviation from the theoretical mixing line (Fig. 2c), which indicates that end-member mixing alone (with no/little isotopic fractionation) explains most of the spread in the values. Compared to the wide spread in sulfur isotopic data within the Imetjoki catchment, the Khibiny values formed two clear clusters; all upstream sampling points (IDs 16‒19, 23) grouped towards the top left corner with a small range in bothδstream and cstream , displaying a clear signal from the atmospheric deposition end-member, while results from the mining impacted sampling points (IDs 20, 21, 24) grouped in the lower right corner in a similarly narrow range, likely portraying the bedrock end-member (see detailed results in Table S6‒7 in SI). The Yuksporiok 3 data (ID 21) represents a 3-day average value from which the corresponding coefficient of variation (CV) were 0.22 and 0.14 for δstream and cstream , respectively. A clear mining-impacted signal was derived from the downstream Vuonnemiok 3 sampling point (ID 25) while samples from the Umba River (ID 26) and Belaya River (ID 22) show a mix of the two end-members, however still within 1 standard deviation from the mixing line.
Calculated MSR in the Imetjoki and Khibiny catchments