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