State Water Resources Research Institute Program


Project ID: 2012OH262B
Title: Discriminating Biotic and Abiotic Arsenic Release Processes under Highly Reduced Ground Water Conditions
Project Type: Research
Start Date: 3/01/2012
End Date: 2/28/2013
Congressional District: 15
Focus Categories: Geochemical Processes, Toxic Substances, Water Quality
Keywords: Arsenic, Methanogenesis, Biogeochemistry, Microbial Community
Principal Investigators:Lenhart, John (The Ohio State University); Mouser, Paula J (Ohio State University)
Federal Funds: $ 29,124
Non-Federal Matching Funds: $ 58,304,
Abstract: Scientific studies link chronic ingestion of inorganic arsenic (As) to an increased incidence of cancer in humans and in 2006 the United States Environmental Protection Agency lowered the Maximum Contaminant Level (MCL) of As in drinking water from 50 µg/L to 10 µg/L. Concentrations of naturally occurring inorganic arsenic that exceed this level are routinely measured in the ground water in many locations across Ohio. These sites are not limited to specific aquifer types, with elevated As concentrations measured in all three of the major aquifer types in Ohio: carbonate bedrock, sandstone bedrock, and glacial deposits. However, elevated concentrations of As in the glacial sand and gravel aquifers are of primary concern since these formations are relied on more heavily for use in domestic and public supplies. Coincident with the elevated arsenic concentrations in Ohio's glacial deposits is the presence of certain chemically reducing conditions. Both iron-reducing and methanogenic conditions are specifically associated with arsenic accumulation in the ground water. Under iron-reducing conditions, the proximate cause of elevated ground water As concentrations is the reductive dissolution of iron oxides and subsequent release of accumulated arsenic. Under methanogenic conditions, however, details of the aquifer properties or biogeochemical conditions that promote arsenic release remain poorly understood. This is problematic because in Ohio ground water from methanogenic aquifers exceed the MCL nearly 50 percent of the time.

Microbiological processes are known to be the primary drivers for controlling redox conditions in subsurface environments. These processes are tied to the availability of nutrients and energy resources that dictate microbial ecology and community structure, in particular iron, sulfate and organic matter. In Ohio, since arsenic speciation in ground water systems depends on the redox state it is by extension also dependent upon the composition and activity of the microbial community within that system. Whether these communities dictate arsenic speciation indirectly, through maintaining specific methanogenic redox conditions suited for arsenic release, or directly, via specific arsenate-reducing metabolic pathways, is currently unknown. Isolating such abiotic and biotic mechanisms requires specific and simultaneous analyses of the chemical properties of the system through detailed solution-phase and solid-phase analyses, as well the biological properties via microbial community analysis and sequencing techniques.

In this research we propose to investigate arsenic release and sequestration processes under methanogenic and transient redox conditions, with the goals to (1) characterize mechanisms and pathways responsible for arsenic release from aquifer solids under methanogenic conditions and (2) relate redox conditions and changes in arsenic release/sequestration to dominant microbial community members. Ground water and sediments isolated from specific iron-reducing, sulfate-reducing and methanogenic horizons will be used as the medium for this study. Experiments will be conducted under anaerobic conditions in order to investigate arsenic release and sequestration processes under ambient redox conditions. Using dissolved organic matter (DOM) isolated from the site, we will also investigate how changes in DOM concentrations modify redox conditions and subsequently arsenic speciation. The composition of the water and microbial community structures will be monitored through the course of the experiments. Select samples will also be analyzed using X-ray absorption spectroscopy to determine the dominant solid-phase arsenic species. The outcome of this work will provide needed information regarding dominant abiotic and biotic mechanisms responsible for controlling arsenic speciation in methanogenic and transient redox systems. Such information is critical for protecting the health of the approximately 5 million residents of Ohio that rely on ground water for their daily needs.

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