Microbial sulfate reduction (MSR) as nature-based solution (NBS)
to mine drainage: Contrasting spatio-temporal conditions in northern
Europe
Sandra Fischer1*, Carl-Magnus
Mörth2, Gunhild Rosqvist1, Sergey R.
Chalov3, Vasiliy Efimov3, Jerker
Jarsjö1
1 Department of Physical Geography and the Bolin
Centre for Climate Research, Stockholm University, SE-106 91 Stockholm,
Sweden
2 Department of Geological Sciences, Stockholm
University, SE-106 91 Stockholm, Sweden
3 Faculty of Geography, M.V. Lomonosov Moscow State
University Leninskie Gory 1, 119991 Moscow, Russia
* Corresponding author: Sandra Fischer
(sandra.fischer@natgeo.su.se)
Key Points:
- Despite colder conditions, Arctic catchments may host microbes that
reduce between 5-20% of the river-transported sulfate concentrations
- Favorable landscape characteristics include vegetated terrain with
lakes and wetlands enabling longer microbial contact times
- Catchment-scale microbial sulfate reduction can act as a nature-based
solution for (acid) mine drainage
Abstract
An emerging solution in mine waste remediation is the use of biological
processes, such as microbial sulfate reduction (MSR), to immobilize
metals, reducing their bioavailability and buffering the pH of acid mine
drainage. Apart from laboratory tests and local observations of natural
MSR in e.g. single wetlands, little is known about spatio-temporal
characteristics of freshwater MSR from multiple locations within entire
hydrological catchments. We here applied an isotopic fractionation
(δ34S-values in
SO42-) and Monte-Carlo based mixing
analysis scheme to detect MSR and its variation across two major mining
regions (Imetjoki, Sweden and Khibiny, Russia) in the Arctic part of
Europe under different seasonal conditions. Results indicate a range of
catchment-scale MSR-values in the Arctic of ∼ 5-20% where the low end
of the range was associated with the non-vegetated, mountainous terrain
of the Khibiny catchment, having low levels of dissolved organic carbon
(DOC). The high-end of the range was related to vegetated conditions
provided by the Imetjoki catchment that also contains wetlands, lakes
and local aquifers. These prolong hydrological residence times and
support MSR hot-spots reaching values of ∼40%. Present results
additionally show evidence of MSR-persistence over different seasons,
indicating large potential, even under relatively cold conditions, of
using MSR as part of nature-based solutions to mitigate adverse impacts
of (acid) mine drainage. The results call for more detailed
investigations regarding potential field-scale correlations between MSR
and individual landscape and hydro-climatic characteristics, which e.g.
can be supported by the here utilized isotopic fractionation and mixing
scheme.
Introduction
Microbial sulfate reduction (MSR) has increasingly been investigated for
its potential to immobilize metals and reduce their bioavailability
while also increasing the pH of acid mine drainage (AMD; e.g., Nielsen
et al., 2018). The process involves microbes (bacteria and archaea)
converting sulfate into sulfide that together with toxic dissolved
metals precipitate into less mobile forms. Laboratory bioreactor
experiments on MSR show a metal retention of 70% or more under
favorable conditions (e.g., Sinharoy et al., 2020; Zhang and Wang,
2016). The activity is depending on several factors such as carbon and
sulfate supply, oxygen level, pH, and temperature (Xu and Chen, 2020).
MSR has also been observed in the field at certain locations and time
periods, e.g., in individual wetlands or near tailing deposits at
particular points of time (Mandernack et al., 2000; Praharaj and Fortin,
2004). Recently, Fischer et al. (2022) additionally showed evidence of
on-going and considerable MSR in multiple locations (so called “hot
spots”) within an AMD-impacted catchment (Imetjoki, Northern Sweden),
which is essential if MSR is to be used as an effective mitigation
solution for spatially extensive mining sites and their downstream
regions. However, large knowledge gaps remain regarding catchment-scale
MSR in freshwater systems, where specific catchment and seasonal
conditions could differ substantially from site to site. It is therefore
not known to which extent MSR more generally could provide a basis for
viable bioremediation, for instance as part of nature-based solutions
for sites impacted by (acid) mine drainage across the world. This
includes the Arctic, which counts as one of the world’s larger mining
regions with numerous examples of large-scale mine drainage development,
and where cold conditions and pronounced seasonality may hamper the
activity of freshwater sulfur-reducing microbes (SRM).
Current evidence shows that point locations that are relatively
favorable for MSR contain soil and sediments with sufficiently high
organic matter content to support the metabolism of SRM, and that they
are associated with wetlands, lakes or groundwater systems that prolong
hydrological residence times and the general contact time for SRM
(Hampton et al., 2019; Lindström et al., 2005). Such characteristics are
found in the Imetjoki catchment where relatively high catchment-scale
MSR was detected through a summer (August) field campaign (Fischer et
al., 2022). Here we hypothesize that, in contrast, low-MSR catchments
should have relatively limited vegetation/forest cover and steep
topography that limits the number of lakes and wetlands as well as
reduces residence times and therefore also SRM contact times. A northern
European example of this is the Khibiny region in northwestern Russia
where an active apatite mining complex is located (Efimov et al., 2019).
The Khibiny catchments have been increasingly industrialized and
polluted as a result of more than 90 years of ore extraction (Malinovsky
et al., 2002).
Apart from impacts on MSR by regional topography and land cover
conditions, it is reasonable to assume that MSR is impacted by the
seasonality of hydro-climatic conditions. For instance, laboratory
experiments generally show increased microbial activity with higher
temperatures (e.g. Pellerin et al., 2020), although steady-state
batch-tests have shown that cold-tolerant bacteria successfully may
reduce metal concentrations (Nielsen et al., 2018; Virpiranta et al.,
2019). Field studies in temperate climates show indications of locally
increased MSR during warmer summer periods (e.g., Praharaj and Fortin,
2004), although there are also reports on potentially high MSR-levels
during the winter (e.g., Björkvald et al., 2009; Fortin et al., 2000).
Colder regions furthermore have a strong seasonal effect in runoff
generation (e.g. frozen conditions vs. spring flood) implying that the
mixing of water from different landscape compartments differs greatly
over the year. This, together with annual fluctuations of water
temperature, fundamentally change the ambient conditions for SRM,
supporting our working hypothesis that large-scale MSR-values should
vary over a hydrological year. A better understanding of the magnitude
of such potential large-scale seasonal variations would be desirable in
assessments of the overall effectiveness of MSR as a suitable mitigation
solution. For the Arctic, for example, the suitability would most likely
be related to whether or not the measures would be efficient for only a
few favorable months per year.
The degree of MSR detected in water samples from catchments can be
calculated using the isotopic fractionation model developed by Fischer
et al. (2022). The method is based on the fact that sulfur isotope
composition (of δ34S in
SO42-) in surface water show a
distinct signal from SRM preferentially taking up 32S
during sulfur reduction while leaving 34S in the
remaining sulfate. To quantify for the first time the large-scale MSR
sensitivity to contrasting catchment and seasonal conditions, we apply
the isotopic fractionation model by Fischer et al. (2022) on to two
major cold-climate mining-impacted regions: the Imetjoki catchment in
northern Sweden containing the abandoned Nautanen copper mines (where
ambient conditions were shown to be favorable for MSR during summer),
and the actively mined Khibiny catchments on the Kola Peninsula, Russia.
Specifically, we aim to quantify large-scale MSR under (i) contrasting
catchment conditions (i.e., spatial characteristics) of Imetjoki and
Khibiny, and (ii) contrasting seasonal conditions (i.e., temporal
characteristics) in Imetjoki, extending snap-shot observations of high
MSR in late summer (Fischer et al., 2022) with observations during less
favorable snow-melt conditions in spring.
Materials and methods
Site descriptions
The Imetjoki catchment in northern Sweden covers 6.6
km2 (Fig. 1a‒c) and is underlain by igneous bedrock
with sulfide deposits hosting iron oxide-copper-gold (IOCG)
mineralization (Martinsson et al., 2016). Copper was mined mainly in
five deposits distributed over the Imetjoki catchment. The mining
operations were carried out between 1902 and 1908, resulting in about 20
000 ton of untreated mine waste still remaining on the site polluting
the near-by environment (Fischer et al., 2020). The annual average
temperature was −1.6 °C (seasonal variation of 11 to −13 °C) and the
annual average precipitation was 560 mm/yr between the years 1993 and
2017 (Fischer et al., 2020). Annual average actual evaporation was 260
mm/yr (Fischer et al., 2020) and about 200 days per year were snow
covered during the same time period (Berglöv et al., 2015). Forest and
wetlands cover most of the Imetjoki catchment area (SGU, 2020), except
for the former so called “Industrial area” where freely exposed
tailings prevent re-vegetation (see also the detailed site
characterization in Fischer et al. 2020).