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WA4 Metals in the Environment: Chemistry and Fate Issues
(351A) Bioavailability of arsenic in South Texas. What happens when the Fe hydroxide model does not work?
Herbert, B.1, Louchouarn, P.2, Lee, L.1, Markley, C.1, Moore, G.1, 1 Texas A&M University, College Station, Texas, USA2 Columbia University, Palisades, New York, USA
ABSTRACT- Iron (and manganese) oxi-hydroxides (i.e. FeOOH) are generally considered to be the dominant reactive solid-phases in soils and sediments that control the biogeochemistry and bioavailability of arsenic in oxic environments. We present data for the Nueces and San Antonio River watersheds in South Texas that suggest the Fe hydroxide model is not an appropriate conceptual model of the sequestration of arsenic in oxic environments in South Texas. We suggest that the biogeochemical cycling of arsenic in surface waters as higher molecular weight organoarsenicals can constitute a substantial and predominant sediment sink of As due to uptake by phytoplankton, especially during phytoplankton blooms under eutrophic conditions. Geogenic sources control the occurrences of arsenic and associated trace metals in Texas groundwater systems, with arsenic concentrations as high as 560 ppb. In the San Antonio and Nueces watersheds along the South Texas Coasts, arsenic and associated trace metals have been released to the environment through the leaching of volcanic ash-rich formations, including the Catahoula Formation, and extensive U mining. Sediment cores from small stock ponds and the major reservoir, Lake Corpus Christi, were characterized to evaluate whether these systems were sequestering arsenic released upstream. Low concentrations of reactive iron in South Texas sediments suggest that iron oxyhydroxides are not controlling arsenic sequestration and bioavailability. Laboratory and field experiments using a newly developed chelating resin (DOWEX M4195) technique estimated potential arsenic bioavailability in sediment in six geologic environments (i.e. lake, river, perennial stream, ephemeral stream, pond, and wetland) in the watersheds over a twenty-eight day period. Observational and modeling results suggest that arsenic and other trace metals are highly bioavailable and may pose a significant human and ecological health risk.
Key words: biogeochemistry, arsenic, Texas, water resources
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