Biogeochemical Analysis of Groundwater and Lake Water at a CCR Impoundment
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Biogeochemistry, as it relates to CCR, affords a rich complement of reactions that can alter CCR chemistry in ways that may affect the environmental mobility and toxicity of metals and other inorganics derived from CCR.

The mineral characteristics of CCR lend themselves to reactions with microorganisms, similar to reactions between microbes and the inorganic constituents of rock, soil, and sediments. These reactions are known to occur in other environments through the oxidation or reduction of water-soluble consitutent species and their reaction with metabolic byproducts to form insoluble metal complexes and crystals. The processes by which microorganisms interconvert inorganics between mobile and immobile states may occur naturally or the environment may be modified to favor beneficial reactions that immobilize inorganics. Similarly, environmental conditions may exist that support microbial actions that mobilize metals; therefore, understanding CCR associated microbiota can play an important role in controlling metals migration and associated impacts to receptors. A confidential utility client retained Geosyntec to perform a biogeochemical assessment at one of their site’s groundwater surface interface (GSI) to identify environmental and biological factors that may contribute to observed elevated arsenic concentrations.

Geosyntec’s Scope of Services

To evaluate the influence of groundwater and lake water biogeochemistry within the site’s compliance monitoring network, Geosyntec began by completing a preliminary analysis of the groundwater chemistry and chemical concentration data. Geochemical data analysis focused on arsenic and iron concentrations with respect to site location, field measured parameters, and the lake water surface elevation. In addition to the geochemistry, Geosyntec utilized next generation sequencing (NGS) to characterize key microbial populations and their metabolic potential at the GSI. NGS has been shown to be successful in characterizing microbial populations in a number of habitats including leachate, sediment, groundwater/surface water. The results were evaluated with the intent to correlate site conditions groundwater geochemistry, chemical concentration data, and microbiology with the arsenic concentrations that have been observed to vary between sampling events. Results from this study indicate biogeochemical reactions alter groundwater conditions and contribute to the observed fluctuations in arsenic mobility, especially when coupled with changes in lake elevation and suspected inputs of dissolved oxygen and possibly other redox-altering agents in near shore groundwater and from the upgradient influx of water into the aquifer beneath the landfill. While it is unclear whether the microbial community is actively altering groundwater geochemistry or just responding to environmental conditions, a substantial diversity of biochemical reactivity was detected from the biomass and analytical results from this sampling event. This indicates the naturally occurring bacteria are very likely to affect the Site’s groundwater chemistry, including the solubility of arsenic.

Notable Accomplishments

The work completed by Geosyntec, which is among the first applications of molecular technologies for supporting GSI evaluations at CCR facilities, provides a holistic perspective on how arsenic concentrations may be affected by environmental conditions and microbial reactions. By looking at the combined biogeochemical effect on arsenic, the client was able to improve their understanding of the biogeochemistry of the system, thus refining their conceptual site model to include microbial metabolic processes. More importantly, the client was able to leverage the data in partial support of an approved plan to close the landfill in-place.

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Project Summary

  • Location: Midwest, USA
  • Client: Confidential Utility Client
  • Project Practice Areas: Contaminated Sites, Water & Wastewater
  • Type of Facility: Utility
  • Services Provided: Biogeochemical data assessment of the groundwater-surface interface (GSI), Evaluation of natural attenuation mechansisms for inorganic elements, Expansion of the conceptual site model to include microbiological reactions, Metagenomic community analysis
  • Type of Work: Biogeochemical assessment
  • Governing Regulation: State regulations