Arsenic behavior in the subsurface reflects redox conditions, interactions with particle surfaces, specific minerals phases present in the aquifer in the groundwater recharge dynamics. We propose field studies at two Superfund sites with As contamination, both of which will be under active groundwater treatment through the period of our investigation. The Vineland Chemical Company (ViChem) site in southern NJ, experienced dispersal of at least hundreds of tones of waste AS over five decades into surface oils, aquifer formations, groundwater surface waters, plus stream, lake, and estuary sediments far downstream from the original arsenical herbicide and fungicide production facilities. Our strategy will be to examin4e recharge dynamics of this altered flow regimen, primarily through analyzing samples from monitoring wells for tracers already present in the environment and to relate behavior of As and other redox sensitive species to those changes. As part of our investigation, we would analyze groundwater for 3H/3He, total [As], As speciation Fe, Mn, NO3, SO4, O2, PO4/3-, Si(oh)4, Ph, Eh and major element chemistry. Samples would be collected periodically, to examine critical controls on As solubility and dynamics of subsurface As transport. We will collect and analyze a series of sediment cores in Union Lake in order to reconstruct the history of As migration downstream, and to estimate the proportion of As sediment inventory mobilized upward towards the sediment surface. A similar set of groundwater measurements, directed at elucidating controls on As behavior, would be conducted at the Winthrop (ME) Landfill site. Sources of As to this strongly reducing plume are unresolved, but mobilization from surrounding natural soils appears to be a significant possibility, suggesting that similar As plumes could occur at many other landfills. Differing from the Vineland site, the hydrogeology at Winthrop is very complex due to large spatial variability of hydraulic conductivity. We propose to better constrain the groundwater flow regime by employing geophysical survey techniques (ground penetrating radar and resistivity) in addition to 3H/3He. We will study As mobilization processes by following the geochemical evolution along a flow path underneath the landfill and by tracing the compositions of a plume of re- injected water labeled with SF6. Our studies will contribute to a better understanding of the processes affecting As transport under different geochemical conditions ranging from oxidizing to highly reducing. They could potentially also contribute to an increase in efficiency of As remediation at a number of Superfund sites currently under investigation.