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:
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).