PRELIMINARY CONCEPTUAL MODEL FOR ARSENIC OCCURRENCE AND TRANSPORT IN GROUND WATER, SURFACE WATER AND POND-BOTTOM SEDIMENTS, RED COVE, PLOW SHOP POND, CENTRAL MASSACHUSETTS

Plow Shop Pond, in Ayer, Massachusetts, has had a complex industrial history and is adjacent to Shepley's Hill Landfill (SHL), a Superfund site. A small inlet is known as Red Cove, due to the ubiquitous red staining on the pond-bottom sediments. Iron (as iron oxyhydroxides) reaches percent levels in sediment, but arsenic is also present at concentrations up to several thousand mg/kg. Despite numerous environmental investigations, a clear understanding of the nature and extent of contamination in Red Cove, as well as the governing fate and transport mechanisms, has remained elusive. We present a preliminary unified conceptual model for arsenic fate and transport at Red Cove, which considers ground water flow; chemistry of ground water, surface water, sediment and sediment pore-water; and sediment toxicity.

Arsenic is naturally occurring in local soils and bedrock and is associated with iron in oxide and sulfide minerals. Previous work (Stein, et al., 2004) indicates that ground water oxidation-reduction potential (ORP) exerts considerable control on arsenic mobility, primarily through 1) precipitation of hydrous ferric oxides (HFO) and sorption of arsenic, 2) reductive dissolution of HFO and arsenic release, and 3) formation of iron and/or arsenic sulfide phases. Low ORP in the vicinity of SHL gives rise to elevated dissolved iron and arsenic concentrations. Ground water flow data from monitoring wells indicate consistent head-field gradients focused toward Red Cove. Reducing ground water, discharging to the cove from the direction of the landfill, encounters a redox boundary below the sediment-water interface, where arsenic is precipitated along with HFO and possibly sulfides in the sediments. However, arsenic is generally nondetect in surface water. A redox interface was observed at several locations at or near the cove at 1 to 2 m below the sediment surface. Preliminary estimates suggest that several Kg of arsenic are transported to Red Cove annually by these processes, and toxicity data indicate impairment. Remedial measures, if needed, must consider the geochemical and hydrologic dynamics of ground water-surface water interactions.