A plume of contaminated groundwater was formed from the disposal of secondarily-treated wastewater to rapid-infiltration sand beds on the southern portion of the Massachusetts Military Reservation (MMR), Cape Cod, Massachusetts (LeBlanc, 1984)(fig.1). Wastewater was disposed to the infiltration beds for 60 years, beginning in about 1936 and ending in December 1995. The treated-wastewater plume extends south more than 18,000 ft downgradient from the MMR towards the coastal embayments along Vineyard Sound. The plume has been defined by increases in specific conductance and pH and decreases in dissolved oxygen relative to the levels in uncontaminated groundwater, and by the presence of wastewater-related constituents, such as boron, chloride, sodium, nitrate, ammonium, detergents, and phosphorus. The eastern edge of the plume intersects Ashumet Pond, a groundwater flow-through pond about 1,700 ft southeast of the abandoned infiltration beds, in an area known as Fishermans Cove (fig.2). Phosphorus and other wastewater-related constituents discharge with the groundwater to this area of Ashumet Pond.

Elevated levels of phosphorus in groundwater upgradient of Ashumet Pond were measured in 1978-79 when the U.S. Geological Survey (USGS) first delineated the contaminant plume created from the disposal of treated wastewater (LeBlanc, 1984). Dissolved phosphorus is now (2006) present at concentrations greater than 0.1 mg/L as far as 2,500 ft downgradient from the infiltration beds and migrates more slowly in the aquifer than conservative constituents, such as boron and chloride, because of adsorption of phosphorus to the aquifer sediments (Stollenwerk, 1996; Walter and others, 1996). The USGS completed many groundwater studies near Ashumet Pond to characterize the plume. These studies found that concentrations of constituents in the wastewater-contaminated groundwater vary according to geochemical conditions, distance from the infiltration beds, and the history of bed loading. Annual plume sampling has shown that concentrations of dissolved phosphorus in the plume vary slightly in time and space. For example, the distribution of elevated dissolved phosphorus concentrations in groundwater upgradient of Ashumet Pond in July 1993 (fig. 3a) and in the summer of 1999 (fig. 3b) changed slightly over the six-year period (Walter and others, 1996; McCobb and others, 2003).

Sediment cores from different geochemical environments in the treated-wastewater plume were analyzed in the laboratory using batch experiments, core extractions, and column experiments to determine the geochemistry of phosphorus on aquifer sediments (Walter and others, 1996). The results from these tests indicate that the amount of sorbed phosphorus on the sediment is much greater than the amount of phosphorus dissolved in the groundwater.

Solute- and reactive-transport geochemical models have been developed and used to predict the movement and fate of phosphorus in the aquifer as well as estimate future fluxes of phosphorus to Ashumet Pond (Stollenwerk, 1996; Stollenwerk and Parkhurst, 1999; and Parkhurst and others, 2003). These simulations predict that desorption of phosphorus could result in phosphorus discharge to Ashumet Pond for decades.

In 1999, concern about the adverse effects of excessive loading of nutrients, particularly phosphorus, on the ecological characteristics of Ashumet Pond prompted the additional collection and analysis of groundwater immediately upgradient of the pond and the identification of discharge locations of phosphorus and other wastewater-related constituents at the pond (McCobb and others, 2003). This work was conducted by the USGS as part of the Toxic Substances Hydrology Program and in cooperation with the Air Force Center for Environmental Excellence (AFCEE).

In 2004, the AFCEE installed a geochemical barrier on the bottom of Ashumet Pond to intercept the phosphorus before it discharged to the pond. Zero-valent iron was mixed into excavated pond-bottom sediments and placed where the USGS detected dissolved phosphorus at concentrations greater than 1mg/L in pond-bottom groundwater. The USGS developed and tested devices to monitor the barrier's performance at the groundwater/surface-water interface.

The objectives of this study are to (1) better the understanding of processes that affect the movement and fate of phosphorus through naturally heterogeneous aquifers, (2) develop a better understanding of the transport of phosphorus at the groundwater/surface-water interface, (3) develop and test predictive models of the complex interactions among these processes, and (4) develop and test methods to monitor the performance of remedial actions, such as permeable reactive barriers.

The study of phosphorus in the aquifer includes detailed monitoring of groundwater quality near the abandoned wastewater-infiltration beds and at the groundwater/surface-water interface, laboratory measurements, and geochemical modeling to examine phosphorus transport processes and develop methods to monitor its remediation. Groundwater samples are collected from an array of permanent monitoring wells and multilevel samplers in and within about 2,000 feet downgradient from the infiltration beds. Groundwater samples are also collected using temporary well points driven into the bottom of Ashumet Pond. The samples are analyzed for cations, anions, nitrate, ammonium, and nitrogen isotopes. Laboratory experiments with sediment and water from the aquifer have been conducted to examine the rates and controls on phosphorus transport through the aquifer. Numerical reactive-transport groundwater models integrate the field and laboratory to increase understanding of and ability to predict the overall restoration process.