14C Analysis of Microbial DNA from Bangladesh Groundwater: Implications for Deep Aquifer Sustainability

Over 50 million people in Bangladesh are exposed to arsenic contaminated groundwater. Arsenic is mobilized from Bangladesh sediments into groundwater via microbial heterotrophic respiration. During respiration, microbes incorporate proximal organic carbon (OC) into cellular structures such as DNA. The source of this OC is debated and could be derived from the sediment or recharge waters. Previous studies have used indirect methods to determine the OC source by collecting methane, dissolved OC, and dissolved inorganic carbon samples from aquifers. To directly determine the source of the OC used by microbes inducing arsenic release, we developed a method for ¹⁴C analysis of microbial DNA from the arsenic contaminated aquifers. Microbes were collected by filtering over 2000L of groundwater through 0.2µm filters at Site F in the Lashkardi Village, Araihazar, using sampling wells from the shallow and deep aquifers. Microbial DNA was extracted and purified, and three DNA samples were submitted for radiocarbon analysis. Radiocarbon results from the two shallow aquifer samples were inconclusive due to carbon contamination from gel electrophoresis DNA extraction materials. Gel extraction is not a viable method of DNA extraction for radiocarbon analysis, and we are currently developing purification procedures that do not introduce carbon. An alternate purification method was used for the deeper low-arsenic Pleistocene Dupa Tilla aquifer sample, which yielded a robust ¹⁴C signature. The deep aquifer ¹⁴C analysis indicates the presence of a young OC source. These results suggest the need for further study of the effects of young OC intrusion into the deep aquifer in Bangladesh.