Ground-water-flow models and 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, McCobb and others, 2003, Walter and LeBlanc, 1997). By integrating field observations and laboratory measurements, numerical models have increased the understanding of and ability to predict future phosphorus transport in the aquifer.

Stollenwerk (1996) used a surface-complexation model coupled with a one-dimensional solute-transport code to simulate sorption and desorption of phosphate in laboratory column experiments. The rate of phosphate desorption in the column experiments was relatively slow. This modeling predicted that even after cessation of wastewater disposal in 1995, a large reservoir of sorbed phosphate exists on aquifer sediment and desorption of phosphate could result in contamination of Ashumet Pond for decades.

Parkhurst and others (2003) simulated the subsurface transport of phosphorus with a three-dimensional reactive-transport geochemical model (PHAST). The simulations were used to estimate the load of phosphorus transported to Ashumet Pond during operation of the wastewater-treatment plant-from 1936 to 1995-and for 60 years following cessation of wastewater disposal. The model accounted for spatial and temporal changes in water discharge from the wastewater-treatment plant, ground-water flow, transport of associated chemical constituents, and a set of chemical reactions, including phosphorus sorption on aquifer materials, dissolution and precipitation of iron- and manganese-oxyhydroxide and iron phosphate minerals, organic carbon sorption and decomposition, cation sorption, and irreversible denitrification. The modeling indicated substantial discharge of phosphorus to Ashumet Pond after about 1965. Simulations indicated that the load of phosphorus transported to Ashumet Pond decreases continuously after 2000, but the load of phosphorus remains substantial for many decades. The simulations indicated a peak in phosphorus discharge to Ashumet Pond of about 1,000 kilograms per year during the 1990s.

Walter and LeBlanc (1997) and McCobb and others (2003) used ground-water-flow models coupled with phosphorus measurements immediately upgradient of the pond and measurements of phosphorus in pond-bottom ground water to estimate the mass of phosphorus discharging to the pond from the treated-wastewater plume. Phosphorus loading estimates of 177 kg/yr (in 1995) and 316 kg/yr (in 1999) were calculated. Measurements of phosphorus in pond-bottom ground water from 1999, 2001, and 2003 yielded mass loading estimates of about 100 kg/yr.

Additional information about the PHAST model and instructions for downloading the computer program can be found at: http://wwwbrr.cr.usgs.gov/projects/GWC_coupled/phast/