Highly reducing groundwaters beneath and adjacent to a closed and capped landfill site in Winthrop, ME have elevated [As] ranging from 100 to 400 ppb. In January 2001, a field pilot experiment was conducted in an attempt to significantly lower [As] via oxidant injection to oxidize and precipitate dissolved Fe(II). However, this operation did not appreciably decrease either [As] or [Fe(II)] at sampling wells as close as five meters to the injection zone. In a study described here, laboratory incubation experiments and measurements of sediment organic matter were conducted to examine possible causes for the lack of effectiveness of this in situ attempt at redox manipulation. One laboratory incubation using groundwater and sediments indicated that dissolved arsenic was primarily derived from the particles. Order of magnitude calculations demonstrated that the total As release capacity from fine-grained particles obtained at the field site (As=2.8 ppm) could result in a groundwater concentration of up to 1020 ppb under reducing conditions. A groundwater redox titration experiment and measurements of sediment organic matter indicated that subsurface solids imposed >90% of the system reducing capacity, presenting considerable resistance to manipulation of subsurface redox condition. Feasibility analysis based on these observations suggests that for groundwater systems with substantial reducing capacity, in situ oxidant injection to precipitate As and Fe would be very difficult to accomplish if appreciable aquifer volume is involved.