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Phosphorus in a Ground-Water Contaminant Plume Discharging to Ashumet Pond, Cape Cod, Massachusetts, 1999

Water-Resources Investigations Report 02-4306

By TIMOTHY D. McCOBB, DENIS R. LeBLANC, DONALD A. WALTER, KATHRYN M. HESS, DOUGLAS B. KENT, and RICHARD L. SMITH

ABSTRACT

The discharge of a plume of sewagecontaminated ground water emanating from the Massachusetts Military Reservation to Ashumet Pond on Cape Cod, Massachusetts, has caused concern about excessive loading of nutrients, particularly phosphorus, to the pond. The U.S. Air Force is considering remedial actions to mitigate potentially adverse effects on the ecological characteristics of the pond from continued phosphorus loading. Concentrations as great as 3 milligrams per liter of dissolved phosphorus (as P) are in ground water near the pond's shoreline; concentrations greater than 5 milligrams per liter of phosphorus are in ground water farther upgradient. Temporary drive-point wells were used to collect water samples from 2 feet below the pond bottom to delineate concentration distributions in the pore waters of the pond-bottom sediments. Measurements in the field of specific conductance and colorimetrically determined orthophosphate concentrations provided real-time data to guide the sampling. The contaminant plume discharges to the Fishermans Cove area of Ashumet Pond as evidenced by elevated levels of specific conductance and boron, which are chemically conservative indicators of the sewage-contaminated ground water. Concentrations of nonconservative species, such as dissolved phosphorus, manganese, nitrate, and ammonium, also were elevated above background levels in ground water discharging to the pond, but in spatially complex distributions that reflect their distributions in ground water upgradient of the pond.

Phosphorus concentrations exceeded background levels (greater than 0.10 milligram per liter) in the pond-bottom pore water along 875 feet of shoreline. Greatest concentrations (greater than 2 milligrams per liter) occurred within 30 feet of the shore in an area about 225 feet long. Calculations of phosphorus flux in the aquifer upgradient of Ashumet Pond, as determined from water-flux estimates from a steady-state ground-water-flow model and phosphorus concentrations (in 1999) from multilevel samplers about 75 feet upgradient of the pond, indicate that dissolved phosphorus moves towards the pond and discharges to it with the inflowing ground water at a rate as high as about 316 kilograms per year.


CONTENTS

Abstract

Introduction

Background and Previous Investigations

Hydrogeologic Setting

Acknowledgments

Study Design and Methodology

Ground-Water Sampling upgradient of the Pond

Shallow Pond-Bottom Ground-Water Sampling

Collection and Laboratory Analysis of Selected Inorganic Solutes, Nitrate, and Ammonium

Colorimetric Field Screening for Orthophosphate and Ferrous Iron

Distribution of Phosphorus and Other Contaminant-Plume Constituents

Phosphorus in the Aquifer upgradient of Ashumet Pond

Phosphorus in Shallow Ground Water beneath the Pond Bottom

Sewage-Related Constituents in Shallow Ground Water beneath the Pond Bottom

Specific Conductance

Boron

Manganese

Nitrate and Ammonium

Flux of Phosphorus through the Aquifer towards the Pond

Summary and Conclusions

References Cited

FIGURES

1--3. Maps showing:

1. Location of the Massachusetts Military Reservation, extent of the contaminant plume, and the altitude of the water table, western Cape Cod, Massachusetts

2. Extent of the contaminant plume in the study area, locations of multilevel and well-cluster sampling sites, and area of temporary in-pond drive-point sampling, Ashumet Pond, May--October 1999

3. Distribution of specific conductance in pond-bottom ground water in 1986 as interpreted by K-V Associates, Inc., Ashumet Pond

4. Schematic showing conceptual model of the ground-water-flow system near a glacial-kettle ground-water flow-through pond

5. Map showing nearshore bathymetry in the Fishermans Cove area of Ashumet Pond and locations of temporary drive points installed in August--October 1999 to delineate the phosphorus discharge area

6. Graph showing comparison of field- and laboratory-determined dissolved phosphorus concentrations in pond-bottom ground water in the contaminant-plume discharge area, Ashumet Pond, 1999

7. Map showing areal distribution of maximum dissolved phosphorus concentrations in ground water upgradient of Ashumet Pond, Apri--July 1999

8. Cross sections showing vertical distribution of dissolved phosphorus concentrations along sections A-A' and B-B" upgradient of Ashumet Pond, April--July 1999

9--10. Maps showing areal distributions of:

9. Dissolved phosphorus in pond-bottom ground water in the contaminant-plume discharge area and maximum dissolved phosphorus concentrations measured in ground water at upgradient multilevel-sampling sites, Ashumet Pond, August--October 1999

10. Dissolved phosphorus in ground water near Ashumet Pond, Massachusetts--October 1999

11. Graph showing distribution of dissolved phosphorus, specific conductance, and dissolved oxygen at a vertical profile (3a-00-ST) at the shoreline of Ashumet Pond, August 24, 1999

12. Cross section showing vertical distribution of dissolved phosphorus concentrations along section C-C' from multilevel sampler F619 to pond-bottom location 3a-125, Ashumet Pond, April--October 1999

1--17. Maps showing areal distributions of:

13. Specific conductance in pond-bottom ground water in the contaminant-plume discharge area and maximum specific conductance measured in ground water at upgradient multilevel-sampling sites, Ashumet Pond, August--October 1999

14. Dissolved boron in pond-bottom ground water in the contaminant-plume discharge area and maximum dissolved boron concentrations measured in ground water at upgradient multilevel-sampling sites, Ashumet Pond, August--October 1999

15. Dissolved manganese in pond-bottom ground water in the contaminant-plume discharge area and maximum dissolved manganese concentrations measured in ground water at upgradient multilevel-sampling sites, Ashumet Pond, August--October 1999

16. Dissolved nitrate in pond-bottom ground water in the contaminant-plume discharge area and maximum dissolved nitrate concentrations measured in ground water at upgradient multilevelsampling sites, Ashumet Pond, August--October 1999

17. Dissolved ammonium in pond-bottom ground water in the contaminant-plume discharge area and maximum dissolved ammonium concentrations measured in ground water at upgradient multilevel-sampling sites, Ashumet Pond, August--October 1999

18. Map showing location of ground-water-flow model cells and multilevel samplers used for calculating flux of phosphorus towards Ashumet Pond

19. Cross section showing dissolved phosphorus flux towards Ashumet Pond as determined by a steady-state ground-water-flow model and the distribution of phosphorus in ground water sampled in 1999

TABLES

1. Physical and hydrologic properties of Ashumet Pond, Cape Cod, Massachusetts

2. Altitudes of the screened intervals for monitoring wells used to collect ground-water samples upgradient of Ashumet Pond, May--July 1999


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The citation for this report, in USGS format, is as follows:

McCobb, T.D., LeBlanc, D.R., Walter, D.A., Hess, K.M., Kent, D.B., and Smith, R.L., 2003, Phosphorus in a Ground-Water Contaminant Plume Discharging to Ashumet Pond, Cape Cod, Massachusetts, 1999: U.S. Geological Survey Water Resources Investigations Report 02-4306, 70 p.

For more information about USGS activities in Massachusetts-Rhode Island District, visit the USGS Massachusetts-Rhode Island Home Page.



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