INTRODUCTION

Ground water contaminated with phosphate (PO₄) because of historical land disposal of treated municipal wastewater at the Massachusetts Military Reservation (MMR) is currently (2009) discharging into nearby Ashumet Pond (fig.1; McCobb and others, 2003). Unlike the transport of many contaminants in ground water beneath the MMR, phosphate transport is greatly retarded because it sorbs strongly to the aquifer sediments. Despite the removal of the wastewater source in 1995, phosphate concentrations in the ground water will remain elevated for many decades as natural flushing of the aquifer by uncontaminated ground water results in desorption of this reservoir of sorbed phosphate (Parkhurst and others, 2003).

The potential adverse effect of continued phosphate loading on the ecology of Ashumet Pond has prompted remedial actions by the Air Force Center for Engineering and the Environment (AFCEE). In August 2004, a permeable reactive barrier (PRB) was emplaced on the bottom of Ashumet Pond to reduce phosphate concentrations in ground water discharging to the pond to limit or reverse the adverse ecological effects of the phosphate. In August 2006, the U.S. Geological Survey collected pore-water samples in and near the PRB from depths below, within, and at the top of the barrier. This report presents a series of maps showing constituent concentrations and values of physical properties, layered by depth, as a visual assessment of the effects of the PRB on the geochemistry of the pore water along the shoreline of Ashumet Pond.

First posted October 2009