Many groundwater studies have been completed near Ashumet Pond to characterize the plume that was formed by the discharge of secondarily treated wastewater to the aquifer. These studies found that uncontaminated groundwater in the aquifer typically is characterized by pH less than 5.5, specific conductance less than 80 µS/cm, and dissolved oxygen concentrations greater than 8 mg/L. Concentrations of iron and manganese typically are less than 0.01 mg/L, phosphorus less than 0.1 mg/L, and boron, a common component of sewage, less than 0.05 mg/L (LeBlanc, 1984; Savoie and LeBlanc, 1998). Constituent concentrations in the wastewater-contaminated groundwater vary according to geochemical conditions, distance from the infiltration beds, and the history of bed loading. In 1979, elevated phosphorus concentrations as high as 2 mg/L were recorded at locations between the infiltration beds and Ashumet Pond.

In 1993, Walter and others (1996) studied the transport of phosphorus from the wastewater-infiltration beds to Ashumet Pond. Suboxic conditions (dissolved oxygen less than 1 mg/L) were documented in the aquifer upgradient from the pond. A maximum concentration of dissolved phosphorus of 6.2 mg/L was measured in groundwater about 750 ft from the pond (Site F567, fig. 1).

Walter and others (1996) also found in laboratory experiments using sediment-core samples obtained from the aquifer that phosphorus desorbs from the sediment when uncontaminated groundwater with a low pH is introduced into the previously contaminated sediments. They concluded that adsorption of phosphorus to the sediments has created a reservoir of phosphorus on the aquifer sediments and this reservoir of phosphorus would remain a source of phosphorus in the groundwater for as long as 100 years.

In 1995, additional water samples were collected in a more detailed monitoring-well network upgradient of Ashumet Pond, particularly along the eastern side of the wastewater plume. Bussey and Walter (1996) documented the temporal and spatial variability of phosphorus in the aquifer observed during this sampling. The maximum concentration of phosphorus in the aquifer about 75 ft upgradient from Ashumet Pond (F300, fig. 1) was 1.8 mg/L, indicating little change in phosphorus concentrations near the pond between 1993 and 1995.

Annual sampling since 1993 has shown that the concentrations of phosphorus in groundwater between the infiltration beds and the pond vary with time and space. At one site near the center of the phosphorus contamination (F567, fig. 1), maximum concentrations of phosphorus varied from 6.2 mg/L in 1993 to 3.1 mg/L in 1994 and back to 6.2 mg/L in 1998. In contrast, phosphorus concentrations at a site near the pond (F300, fig. 1) increased at a steady, slow rate from 1.4 mg/L in 1993 to 2.1 mg/L in 1998 (Walter and others, 1996). Figure 2 shows vertical profiles of phosphorus, specific conductance, and dissolved oxygen from a pair of multilevel samplers located at site F567. Laboratory studies have shown that dissolved phosphorus concentrations in geochemical equilibrium with phosphorus sorbed onto sediments can vary with changes in pH and ionic strength of the groundwater. These studies also have shown that short-term variations in dissolved concentrations have little effect on the overall phosphorus mobility and the total mass of sediment-bound phosphorus (Walter and others, 1996; Stollenwerk, 1996).

McCobb and others (2003) documented the distribution of phosphorus upgradient of Ashumet Pond in 1999 using a dense network of wells and multilevel samplers (fig. 3). Section A-A', extending from the former infiltration beds to Ashumet Pond, indicates that elevated dissolved phosphorus concentrations (greater than 0.10 mg/L) are in a zone 80-100 ft thick (fig. 4a). Transverse section B-B', which extends along the pond shoreline, shows phosphorus concentrations greater than 0.1 mg/L along a shoreline width of greater than 1000 ft (fig. 4b).

In 2004, pond-bottom piezometers were installed to depths of 10, 30, 60 and 100 feet in three clusters located (1) at the pond shoreline, (2) 55 feet offshore, and (3) 110 feet offshore. These piezometers were installed to better understand the distribution of phosphorus and other nutrients prior to their discharge to the pond.