Microbially-mediated oxidation of trace element-bearing sulfide minerals in sandstones of Trempealeau County, WI

This is an experimental project designed to gain insight into the kinetics of biologically-mediated sulfide mineral (e.g. pyrite) oxidation in sandstone aquifer sediments from Trempealeau County in western Wisconsin. The motivation for the work is the potential for degradation of drinking water quality through both natural and frac sand mining induced groundwater flow through reduced, pyrite-bearing geological materials, where pyrite oxidation can generate acidity and lead to release of toxic trace elements into solution. Prior work by Zambito and colleagues at the Wisconsin Geological and Natural History Survey (WGNHS) has led to a conceptual model of the distribution of trace metal-bearing sulfides (mainly pyrite) and their potential vulnerability to oxidation in the Tunnel City Group (TCG) –Wonewoc Formation (WF) contact interval strata in western Wisconsin (Zambito et al., 2018a; Zambito et al., 2018b). The goal of our experiments is to obtain information on the rate of reaction of these sulfides with oxygen in both the presence and absence of natural groundwater microorganisms under (at least initially) circumneutral pH conditions. Although the central role of microorganisms in pyrite oxidation at low pH has been extensively studied in the context of acid mine/rock drainage generation (Schippers, 2004), the role of microorganisms in mediating pyrite oxidation at circumneutral pH is not well understood. Our recent work with microorganisms from other pyrite-bearing subsurface sediments (Percak-Dennett et al., 2017; Napieralski et al., 2018) has conclusively shown that chemolithotrophic bacteria (i.e. organisms that gain energy for growth from inorganic compounds) can accelerate neutral-pH aerobic oxidation of both synthetic and specimen pyrite up to 10-fold relative to abiotic controls. Additional experiments demonstrated that pyrite oxidation which starts at neutral pH can lead to dramatic pH decrease in poorly-buffered systems. These novel findings have key implications for the oxidation kinetics of natural sulfide minerals in sedimentary environments such as those in Trempealeau County sandstones. However, to our knowledge no previous studies have documented the influence of biological activity on oxidation of naturally-occurring disseminated pyrite in any such subsurface materials.

Core materials (obtained from the WGNHS core repository) from both reduced (pyrite-rich, Fe oxide poor) and oxidized (pyrite-poor, Fe oxide rich) TCG and WF strata will be suspended in aerobic groundwater from the TCG-WF contact interval. Changes in aqueous (e.g. pH, sulfate, trace elements) and solid-phase geochemical (e.g. Fe/S oxidation state; trace element partitioning) parameters will be monitored over a period of months in live versus heat-killed (autoclaved) microcosms. Microbial community composition will be analyzed periodically via small subunit ribosomal RNA (rRNA) gene sequence analysis, and the total genomic potential of microbial communities that arise in the biotic reactors will be assessed by “shotgun” metagenomic sequence analysis at the conclusion of the experiments. Based on prior findings, we hypothesize that the presence of living microorganisms will dramatically accelerate pyrite oxidation, leading to the release of trace elements to solution depending on (primarily) the evolving pH conditions in the reactors. The materials to be used in these experiments were chosen specifically to provide insight into how changes in pH may control rates of pyrite oxidation and trace element release, as the TCG materials contain abundant (ca. 8% dry weight) carbonate whereas the WF materials contain virtually no carbonate and are thus poorly-buffered. In addition, the comparative behavior of pyrite-rich vs. pyrite-poor materials from the same formation will provide direct insight into the impact of pyrite oxidation on both geochemical conditions and microbial community composition. Parallel experiments will be conducted in reactors amended with powderized specimen pyrite, which is highly enriched in various trace elements, to serve as positive controls relative to the impact of microbial activity on pyrite oxidation and trace metal release. An additional series of experiments will be conducted using meteoric (rain) water as the fluid phase to gain insight into how passage of surface recharge through materials that are exposed/excavated during the course of frac sand mining operations might be expected to impact local groundwater quality. The absence of significant carbonate alkalinity in rain water compared to typical Wisconsin groundwaters is expected to have a fundamental impact on geochemical conditions in the reactors, here again depending on the buffering capacity of the aquifer solids.

The proposed experiments will generate novel scientific findings suitable for peer-review publication in major scientific journals (e.g. Environmental Science and Technology, Ground Water, Hydrogeology Journal). In addition, our results (together with batch reaction numerical modeling) will be useful for development and parameterization of reactive transport models that assess risks to groundwater quality in both natural and frac sand mining-impacted subsurface environments. In particular, careful microscopic analysis of disseminated pyrite and calcite in the subsurface materials will provide the starting point for incorporating information on biotic/abiotic pyrite oxidation kinetics into models that utilize standard surface area-based mineral dissolution/precipitation rate laws (e.g. Steefel and MacQuarrie, 1996). Our results will also provide perspective to stakeholders in Trempealeau County who have concern about controls their drinking water quality, specifically in relation to local frac sand mining impacts versus natural hydrogeological processes. Our findings will also shed light on the potential contribution of microbial activity on oxidative dissolution of pyrite and input of arsenic to the St. Peter Sandstone groundwater aquifer in eastern Wisconsin (e.g. Schreiber et al., 2000; Root et al., 2010).