EVIDENCE OF MICROBIALLY MEDIATED ARSENIC MOBILIZATION FROM SEDIMENTS OF THE AQUIA AQUIFER, MARYLAND, USA

Sediments from the Aquia aquifer in coastal Maryland were collected as part of a larger study of arsenic (As) in the Aquia aquifer groundwater flow system. Previously, we demonstrated that As in Aquia groundwaters reach concentrations as high as 1072 nmol/kg, or 80 ppb, which is a factor of 8 larger than the current US EPA MCL for As in drinking water. Our previous investigation suggests that As is mobilized in the Aquia aquifer via reductive dissolution of iron (Fe) oxides within the aquifer sediments [1]. In order to test whether this As release is microbially mediated, the Aquia aquifer sediment samples were subsequently employed in a series of microcosm experiments. The microcosm experiments consisted of batch reactors prepared with aquifer sediment samples and sterilized (i.e., autoclaved), artificial groundwater (based on the major solute composition of Aquia aquifer groundwater) using 4 experimental conditions and 1 control condition. The four experimental conditions included the following scenarios: (1) aerobic; (2) anaerobic; (3) anaerobic + acetate; and (4) anaerobic + acetate + AQDS (anthraquinone-2,6-disulfonic acid). The control condition contained sterilized (i.e. killed) soil kept under anaerobic conditions with an addition of the electron shuttle, AQDS. Over the course of the 27 day microcosm experiments, dissolved As in the unamended (aerobic and anaerobic) microcosms remained constant at around ~ 2 ppb. With the addition of acetate, the amount of As released to the solution doubled reaching 4 ppb, and with the addition of acetate and AQDS, the dissolved As concentrations exceeded 6 ppb, which is similar to As concentrations in groundwaters from the aquifer sediment sampling site (i.e., up to ~10 – 12 ppb). Arsenic concentrations in the killed control experiments were generally less than 1 ppb, which we interpret to be the amount of As released from Aquia aquifer sediments owing to abiotic, surface exchange processes. Consequently, these microcosm experiments support that As release from Aquia aquifer sediments results from microbial respiration. Future investigations will include molecular analysis to examine the diversity of the microbial community in the Aquia aquifer, and probing for specific microbial enzymes (arrA, aoxB, acr3) in samples from the Aquia aquifer to determine which microbial processes predominate in the aquifer.