Reactive-Transport Simulation of Phosphorus in the Sewage Plume at the Massachusetts Military Reservation, Cape Cod, Massachusetts
By David L. Parkhurst, Kenneth G. Stollenwerk, and John A. Colman
Water-Resources Investigations Report 03-4017
The subsurface transport of phosphorus introduced by the disposal of treated sewage effluent to ground-infiltration disposal beds at the Massachusetts Military Reservation on western Cape Cod was simulated with a three-dimensional reactive-transport model. The simulations were used to estimate the load of phosphorus transported to Ashumet Pond during operation of the sewage-treatment plant—from 1936 to 1995—and for 60 years following cessation of sewage disposal. The model accounted for spatial and temporal changes in water discharge from the sewage-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 flow and transport in the aquifer were simulated by using parameters consistent with those used in previous flow models of this area of Cape Cod, except that numerical dispersion was much larger than the physical dispersion estimated in previous studies. Sorption parameters were fit to data derived from phosphorus sorption and desorption laboratory column experiments. Rates of organic carbon decomposition were adjusted to match the location of iron concentrations in an anoxic iron zone within the sewage plume. The sensitivity of the simulated load of phosphorus transported to Ashumet Pond was calculated for a variety of processes and input parameters. Model limitations included large uncertainties associated with the loading of the sewage beds, the flow system, and the chemistry and sorption characteristics in the aquifer. The results of current model simulations indicate a small load of phosphorus transported to Ashumet Pond during 1965–85, but this small load was particularly sensitive to model parameters that specify flow conditions and the chemical process by which non-desorbable phosphorus is incorporated in the sediments. The uncertainties were large enough to make it difficult to determine whether loads of phosphorus transported to Ashumet Pond in the 1990s were greater or less than loads during the previous two decades. The model simulations indicate substantial discharge of phosphorus to Ashumet Pond after about 1965. After the period 2000–10 the simulations indicate that the load of phosphorus transported to Ashumet Pond decreases continuously, but the load of phosphorus remains substantial for many decades. The current simulations indicate a peak in phosphorus discharge to Ashumet Pond of about 1,000 kilograms per year during the 1990s; however, comparisons of simulated phosphorus concentrations with measured concentrations in 1993 indicate that the peak in phosphorus load transported to Ashumet Pond may be larger and moving more quickly in the model simulations than in the aquifer.
The results of the three-dimensional reactive-transport simulations are consistent with the loading history, experimental laboratory data, and field measurements. The results of the simulations adequately reproduce the spatial distribution of phosphorus concentrations measured in 1993, the magnitude of changes in phosphorus concentration with time in a profile near the disposal beds following cessation of sewage disposal, the observed iron zone in the sewage plume, the approximate flow of treated sewage effluent into Ashumet Valley, and laboratory-column data for phosphorus sorption and desorption.
Purpose and Scope
Sorption/Desorption Column Experiments
Fitting Surface-Sorption Parameters from Sorption/Desorption from Sorption/Desorption Experimental Data
Phosphorus Desorption Experiments
Geometry of the Model Grid
Hydrologic Boundary Conditions
Chemical Initial and Boundary Conditions
Chemical Reactions in the Sewage Plume
Fitting Surface-Sorption Parameters to Sorption/Desorption Experimental Data
Modifications to the Base-Case Model
Comparison of Measured and Simulated Phosphorus Concentrations
Transport of Phosphorus to Ashumet Pond
Limitations of the Revised Model
Summary and Conclusions
1–4. Maps showing:
1. Location of disposal beds in the study area at the Massachusetts Military Reservation sewage-treatment plant near Ashumet Pond, Massachusetts, and extent of the sewage plume in Ashumet Valley as of 1993–94
2. Areal distribution of maximum dissolved phosphorus concentrations in ground water near Ashumet Pond, August to November 1993
3. Altitude of the water table and approximate direction of ground-water flow in January 1994 near Ashumet Pond
4. Model grid with 50-meter spacing for reactive-transport model of the area near Ashumet Pond
5–7. Graphs showing:
5. Results of column experiments of phosphorus sorption and desorption on Cape Cod sediments and results of reactive-transport simulations based on fitted parameters
6. Sensitivity of phosphorus load transported to Ashumet Pond to variation in selected reactive-transport model parameters
7. Measured and simulated phosphorus concentrations for a column desorption experiment on a sediment core from the sewage-contaminated zone of the aquifer
8. Maps showing (A) measured and (B) simulated phosphorus concentrations in ground water from August to November 1993 near Ashumet Pond
9, 10. Graphs showing:
9. Measured phosphorus concentrations in July 1996 and July 1999 in multilevel sampler S469M1 and simulated phosphorus concentrations for the vertical set of nodes nearest to the location of disposal beds 5–8
10. Simulated load of phosphorus transported to Ashumet Pond and estimated load of phosphorus to disposal beds during the period 1936 to 2055, calculated with revised model parameters and revised model parameters excluding vivianite formation
1. Properties used in reactive-transport simulation of the phosphorus sorption/desorption column experiments
2. Composition of synthetic ground water used in phosphorus desorption column experiment
3. Base-case parameters for three-dimensional reactive-transport modeling
4. Estimated loading of disposal beds during the period of sewage disposal
5. Solution compositions for uncontaminated ground water, treated sewage effluent, and rainwater
6. Major chemical reactions used in reactive-transport simulations
7. Parameters derived from fitting sorption/desorption column experiments
8. Base-case and perturbed values for sensitivity simulations with the three-dimensional reactive-transport model
9. Comparison of parameters and processes for base-case and revised models
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The citation for this report, in USGS format, is as follows:
Parkhurst, D. L., Stollenwerk, K. G. and Colman, J. A., 2003, Reactive-Transport Simulation of Phosphorus in the Sewage Plume at the Massachusetts Military Reservation, Cape Cod, Massachusetts: U.S. Geological Survey Water-Resources Investigations Report 03-4017, 40 p.
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