ARSENIC GEOCHEMICAL BEHAVIOR DURING GROUND WATER-SURFACE WATER INTERACTIONS AT A CONTAMINATED SITE.
Richard Wilkin 1, Robert Ford 1, Frank Beck 1, Patrick Clark 1, Cynthia Paul 1, Joseph LeMay 2, and Robert Puls 1.
1 U.S. Environmental Protection Agency, Office of Research and Development, National Risk Management Research Laboratory, Ada, OK 74820;
2 U.S. Environmental Protection Agency, Region 1, Boston, MA 02114.

Arsenic mobility in groundwater at hazardous waste sites is often tied to redox reactions related to the geomicrobiological cycling of iron and sulfur. Important processes include adsorption or co-precipitation reactions of arsenate, arsenite, or thioarsenite species with poorly crystalline iron (oxy)hydroxides, iron monosulfides, and pyrite. Research results will be presented that address arsenic mobilization and cycling mechanisms at a Superfund site in eastern Massachusetts. The site is located in the headwaters of the Aberjona Watershed. In order to support assessments of the risk posed by off-site migration of arsenic to an adjacent downgradient wetland system, information is needed about the geochemical processes that control arsenic transport and fate on site. In addition, there is keen interest on the part of site stakeholders about the mechanisms and capacity of natural attenuation within the wetland area to prevent off-site migration of arsenic to the Aberjona River. In particular, data are needed to 1) assess the long-term assimilative capacity within the unconsolidated aquifer and the downgradient wetland, and 2) assess the potential for future mobilization of arsenic that is presently partitioned to soil/sediment solids. The study area encompasses a confined fluvial aquifer that locally discharges into a pond and wetland system. One of the challenges to assessing the impact of the discharge of arsenic contaminated ground water to surface water bodies is differentiating between the arsenic flux associated with ground-water discharge versus the arsenic flux due to dissolution of arsenic-bearing sediment components. An integrated sediment and ground water-surface water sampling strategy will be presented and monitoring results used to illustrate the method for capturing short-term temporal and spatial responses to storm events as a means to identify the sources of distinct arsenic fluxes. This presentation will also include discussion of a dynamic model of arsenic geochemical behavior that is closely tied to oxidation-reduction processes and the geobiochemical cycles of iron, sulfur, and carbon. This is an abstract of a proposed presentation and does not necessarily reflect EPA policy.