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