US EPA

1519071 

Journal Article 

Advection of surface-derived organic carbon fuels microbial reduction in Bangladesh groundwater 

Mailloux, BJ; Trembath-Reichert, E; Cheung, J; Watson, M; Stute, M; Freyer, GA; Ferguson, AS; Ahmed, KM; Alam, MJ; Buchholz, BA; Thomas, J; Layton, AC; Zheng, Y; Bostick, BC; van Geen, A 

In Press 

Yes 

Proceedings of the National Academy of Sciences
ISSN: 0027-8424
EISSN: 1091-6490 

110 

14 

5331-5335 

English 

23487743 

10.1073/pnas.1213141110 

WOS:000318037800031 

Chronic exposure to arsenic (As) by drinking shallow groundwater causes widespread disease in Bangladesh and neighboring countries. The release of As naturally present in sediment to groundwater has been linked to the reductive dissolution of iron oxides coupled to the microbial respiration of organic carbon (OC). The source of OC driving this microbial reduction-carbon deposited with the sediments or exogenous carbon transported by groundwater-is still debated despite its importance in regulating aquifer redox status and groundwater As levels. Here, we used the radiocarbon ((14)C) signature of microbial DNA isolated from groundwater samples to determine the relative importance of surface and sediment-derived OC. Three DNA samples collected from the shallow, high-As aquifer and one sample from the underlying, low-As aquifer were consistently younger than the total sediment carbon, by as much as several thousand years. This difference and the dominance of heterotrophic microorganisms implies that younger, surface-derived OC is advected within the aquifer, albeit more slowly than groundwater, and represents a critical pool of OC for aquifer microbial communities. The vertical profile shows that downward transport of dissolved OC is occurring on anthropogenic timescales, but bomb (14)C-labeled dissolved OC has not yet accumulated in DNA and is not fueling reduction. These results indicate that advected OC controls aquifer redox status and confirm that As release is a natural process that predates human perturbations to groundwater flow. Anthropogenic perturbations, however, could affect groundwater redox conditions and As levels in the future.