where R represents the34S/32S ratio.
Surface water was sampled from rivers draining the Khibiny massif in late August in 2017 using an equivalent sampling strategy as the one performed in the Imetjoki catchment (following Fischer et al., 2020; Fischer et al., 2022). More specifically, the Belaya catchment and Vuonnemiok catchments were sampled within three zones: the upstream areas unaffected by the mining sites (IDs 16‒19, 23 in Fig. 1), the mining areas comprising the mine effluents (IDs 20‒21, 24), and the downstream areas (IDs 22, 25). An additional sample was collected from the Umba River (ID 26; draining the Umbozero Lake catchment) which corresponds to about 30 km downstream of the Vuonnemiok catchment.
Isotopic fractionation and mixing scheme
Both the Imetjoki and Khibiny catchments were assumed to have two major sulfur isotopic end-members; sulfur from atmospheric deposition (δdep ; composed of sulfur from both sea spray and fossil fuel emissions) and geogenic sulfur from weathered bedrock (δrock ). This allowed us to apply the sulfur isotopic fractionation and mixing scheme developed by Fischer et al. (2022) to quantify MSR within each site. In summary, the method compares stream water field measurements (representing potential post-MSR conditions) with theoretical predictions derived from sulfur end-member mixing (representing initial or pre-MSR conditions). Deviations between these two are assumed to be due to MSR. Although other microbial processes apart from MSR could contribute to isotopic deviations, they either represent only smaller/negligible fractionation (e.g., assimilatory sulfate reduction; Sharp, 2017) or they can be registered only when sulfur concentrations are much higher (e.g. sulfur disproportionation in marine/laboratory environments; Böttcher et al., 2005) than in our study environments. A theoretical stream water sample (δsample ) is then in the considered conservative case composed of proportional fractions of each end-member (fdep and frock , respectively), according to the principle of mass balance: