title: Advection of surface-derived organic carbon fuels microbial reduction in Bangladesh groundwater.
abstract: 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.
authors.affiliation: Environmental Science Department, Barnard College, New York, NY 10027, USA. bmaillou@barnard.edu
authors.forename: Brian J
authors.initials: BJ
authors.lastname: Mailloux
authors.forename: Elizabeth
authors.initials: E
authors.lastname: Trembath-Reichert
authors.forename: Jennifer
authors.initials: J
authors.lastname: Cheung
authors.forename: Marlena
authors.initials: M
authors.lastname: Watson
authors.forename: Martin
authors.initials: M
authors.lastname: Stute
authors.forename: Greg A
authors.initials: GA
authors.lastname: Freyer
authors.forename: Andrew S
authors.initials: AS
authors.lastname: Ferguson
authors.forename: Kazi Matin
authors.initials: KM
authors.lastname: Ahmed
authors.forename: Md Jahangir
authors.initials: MJ
authors.lastname: Alam
authors.forename: Bruce A
authors.initials: BA
authors.lastname: Buchholz
authors.forename: James
authors.initials: J
authors.lastname: Thomas
authors.forename: Alice C
authors.initials: AC
authors.lastname: Layton
authors.forename: Yan
authors.initials: Y
authors.lastname: Zheng
authors.forename: Benjamin C
authors.initials: BC
authors.lastname: Bostick
authors.forename: Alexander
authors.initials: A
authors.lastname: van Geen
mesh_terms: D001151:Arsenic / Q000032:analysis*; D001459:Bangladesh; D001483:Base Sequence; D002250:Carbon Radioisotopes / Q000032:analysis*; D004247:DNA / Q000737:chemistry* / Q000235:genetics; D019015:Geologic Sediments / Q000032:analysis*; D060587:Groundwater / Q000032:analysis* / Q000382:microbiology*; D054892:Metagenome / Q000235:genetics*; D008969:Molecular Sequence Data; D010084:Oxidation-Reduction; D017422:Sequence Analysis, DNA
chemical_list: D002250:Carbon Radioisotopes; D004247:DNA; D001151:Arsenic