BIODEGRADABLE ORGANIC CARBON IN AQUIFER SEDIMENT FROM BANGLADESH: IMPLICATIONS FOR GROUNDWATER ARSENIC CONTAMINATION

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 from laboratory incubations of aquifer sediment with recharge waters collected from our field site in Bangladesh. The incubations revealed a pool of biodegradable sedimentary organic carbon that was rapidly utilized by native microbial populations. FT-ICR-MS analysis indicated that the sedimentary organic carbon was highly heterogeneous, containing lipids, proteins, carbohydrates, lignins, tannins and condensed aromatics. The native microbial community utilized a wide variety of these carbon compounds, including those typically considered recalcitrant (e.g., aromatics). The results imply that within the aquifer, this pool of sedimentary organic carbon was largely unavailable to the microbial community (i.e., physically, chemically and/or energetically protected). However, 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. Our results indicate 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).