Characterization of Organic Carbon and Its Bioavailability in Recharge Waters and Aquifer Sediments: Implications for Groundwater Arsenic Contamination in Bangladesh

Abstract:

Arsenic contamination of groundwater in Bangladesh affects millions of people, as groundwater is the primary source of both drinking and irrigation water in the country. The arsenic is of geologic origin, naturally-occurring in the aquifer sediment. However, the source of organic carbon that fuels the microbial reactions responsible for mobilizing arsenic off the sediment and into the groundwater has been debated for over a decade. The outstanding question is whether this organic carbon is sedimentary carbon that was co-deposited when the aquifers were formed, or surface-derived organic carbon transported into the subsurface along with recharge water. The answer to this question has implications for managing the contamination problem.

Here we present results of recent laboratory incubations of aquifer sediment with recharge waters collected from our field site in Bangladesh. The incubations revealed a hitherto undocumented pool of biodegradable sedimentary organic carbon. Despite the carbon being old (thousands of years), it was rapidly utilized by the native microbial population. The results imply that within the aquifer this pool of sedimentary organic carbon is largely unavailable to the microbial community, but that chemical and/or physical perturbations to the subsurface, induced, for example, by large-scale groundwater pumping or microbial activity, could mobilize this bioavailable organic carbon off the sediment. Currently, we are using Fourier Transform Ion Cyclotron Resonance Mass Spectrometry and spectroscopic techniques to understand the initial character of the mobilized organic carbon in our incubation experiments, and to track how its composition changes over time as it is degraded by microbes. These efforts will help clarify the in situ processes that could destabilize the sedimentary organic carbon and identify the components that make the carbon biologically available. Collectively, our data suggest a possible role for both surface-derived and sedimentary organic carbon in fueling the microbial reactions that mobilize arsenic into groundwater: surface-derived carbon can stimulate microbial reactions that target the solid phase and destabilize bioavailable sedimentary organic carbon that can then fuel further microbial reactions (i.e., the “priming” hypothesis).