T178 Arsenic in Geologic Systems (Posters)

Tuesday, 2 November 2010
8:00 AM - 6:00 PM
Hall D

ARSENIC RELEASE FROM COASTAL PLAIN SEDIMENTS IN NEW JERSEY USA: BIOGEOCHEMICAL PROCESSES

Paper number: T178-8
Program: Technical Sessions
Time: 8:00 AM – 6:00 PM

Authors

Abstract

Fueled by organic carbon, microbial release of arsenic (As) from sediments to groundwater appears to have occurred in many parts of the world. Our studies of As in groundwater/surface-water interactions in the New Jersey Coastal Plain have involved two general types of geologic materials: arsenic-rich glauconitic marine-origin sediments, and arsenic-poor quartzose deltaic-origin sediments. In reducing environments where high concentrations of dissolved organic carbon (DOC) are present, As is released from the glauconitic sediments, which results in elevated As levels (6 to 89 μg/L) in shallow groundwater prior to its discharge to streams. In contrast, minor amounts of As are released from the quartzose sediments; in these sediments, however, where natural or anthropogenic inputs elevate concentrations of DOC in soil water and shallow groundwater (up to 30 mg/L), As concentrations in water entering streams can increase (from <1 to ~ 7 μg/L) presumably through microbial activity fueled by DOC.

Microbial DNA was extracted from shallow groundwater beneath a stream traversing the As-rich marine sediments, and amplification of arsenic respiratory reductase gene arrA confirmed presence of As-reducing bacteria. Cloning and sequencing indicated novel As-reducing bacteria inhabit a shallow aquifer microbial community that appears more diverse than that of the surficial streambed sediments. Results from shallow groundwater microcosms inoculated with As (V) indicated complete As reduction over 72 days, whereas inoculated streambed sediment microcosms showed that, despite reduction of dissolved As, sorbed As (V) was not reduced. Thus, most microbial reduction and subsequent release of As apparently takes place in the shallow aquifer sediments beneath the stream in microenvironments where redox conditions differ and where inputs of organic carbon stimulate microbial activity. Given conducive conditions, anthropogenic inputs of DOC to the subsurface environment could substantially increase the amount of microbially mobilized As even when the As content of the sediments is low.