In January 1986, K-V Associates, Inc. (1991) measured the specific conductance of the groundwater beneath the pond bottom of the Fishermans Cove area of Ashumet Pond to determine the discharge area of the contaminant plume into the pond. Water samples were collected from well points driven into the pond bottom at 49 locations and analyzed for specific conductance (fig. 1). The maximum pore-water conductance was greater than 400 µS/cm, which is consistent with the maximum values observed in the center of the wastewater plume. The highest conductances were measured in the northern part of Fishermans Cove along the shoreline extending northeast of the cove.

Walter and others (1996) collected water samples from below the pond bottom from 15 drive-point samplers located in the pond about 10 ft from the shoreline. A comparison of hydraulic heads in the drive points to the pond level indicated a strong upward hydraulic gradient across the pond bottom near the shore. Elevated concentrations of phosphorus were detected in samples collected from the drive points in an area extending 400 to 800 ft north of the Fishermans Cove boat landing, with a maximum concentration of 1.9 mg/L at a site 700 ft north of the landing. Walter and others (1996) estimated that about 70 percent of phosphorus loading to the pond is from the contaminant plume and predicted that, following the end of loading to the beds in December 1995, phosphorus concentrations would increase for 8 to 30 years.

Walter and LeBlanc (1997) discussed potential remediation strategies based on the understanding of the phosphorus plume at that time. They used a groundwater-flow model and the 1995 water-quality data to estimate that about 180 kilograms of phosphorus enter the pond each year by way of the contaminant-plume discharge.

In 1999, McCobb and others (2003) delineated the discharge of wastewater-plume-related constituents to Ashumet Pond. This effort included the mapping of specific conductance (fig. 2), boron (fig. 3), phosphorus (fig. 4), nitrate (fig. 5), and ammonium (fig. 6) in the area of Fishermans Cove.

In August 2001 and June 2003, the U.S. Geological Survey delineated the discharge of wastewater-related constituents to Ashumet Pond to assess temporal changes in the discharge of the plume to the pond. Conservative species such as specific conductance and boron were present at much lower levels and were discharging over smaller areas with each subsequent sampling, reflecting the general flushing of wastewater-related contaminants from the aquifer in the 6-8 years since disposal ended. Over the 4-year period from 1999 to 2003, little change in the phosphorus concentrations and the area of plume discharge to the pond was observed.