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Simulated Interaction Between Freshwater and Saltwater and Effects of Ground-Water Pumping and Sea-Level Change, Lower Cape Cod Aquifer System, Massachusetts

By John P. Masterson

Scientific Investigations Report 2004-5014

 

ABSTRACT

The U.S. Geological Survey, in cooperation with the National Park Service, Massachusetts Executive Office of Environmental Affairs, Cape Cod Commission, and the Towns of Eastham, Provincetown, Truro, and Wellfleet, began an investigation in 2000 to improve the understanding of the hydrogeology of the four freshwater lenses of the Lower Cape Cod aquifer system and to assess the effects of changing ground-water pumping, recharge conditions, and sea level on ground-water flow in Lower Cape Cod, Massachusetts.

A numerical flow model was developed with the computer code SEAWAT to assist in the analysis of freshwater and saltwater flow. Model simulations were used to determine water budgets, flow directions, and the position and movement of the freshwater/saltwater interface.

Model-calculated water budgets indicate that approximately 68 million gallons per day of freshwater recharge the Lower Cape Cod aquifer system with about 68 percent of this water moving through the aquifer and discharging directly to the coast, 31 percent flowing through the aquifer, discharging to streams, and then reaching the coast as surface-water discharge, and the remaining 1 percent discharging to public-supply wells. The distribution of streamflow varies greatly among flow lenses and streams; in addition, the subsurface geology greatly affects the position and movement of the underlying freshwater/saltwater interface.

The depth to the freshwater/saltwater interface varies throughout the study area and is directly proportional to the height of the water table above sea level. Simulated increases in sea level appear to increase water levels and streamflows throughout the Lower Cape Cod aquifer system, and yet decrease the depth to the freshwater/saltwater interface. The resulting change in water levels and in the depth to the freshwater/saltwater interface from sea-level rise varies throughout the aquifer system and is controlled largely by non-tidal freshwater streams.

Pumping from large-capacity municipal-supply wells increases the potential for effects on surface-water bodies, which are affected by pumping and wastewater-disposal locations and rates. Pumping wells that are upgradient of surface-water bodies potentially capture water that would otherwise discharge to these surface-water bodies, thereby reducing streamflow and pond levels. Kettle-hole ponds, such as Duck Pond in Wellfleet, that are near the top of a freshwater flow lens, appear to be more susceptible to changing pumping and recharge conditions than kettle-hole ponds closer to the coast or near discharge boundaries, such as the Herring River.

CONTENTS

Abstract

Introduction

Geologic Setting

Depositional History

Geologic Framework

Hydrologic System

Simulation of Ground-Water Flow in the Lower Cape Cod Aquifer System

Ground-Water Recharge Areas

Water Budget

Altitude and Configuration of Water-Table Mounds

Interaction Between Ground and Surface Waters

Controls of Hydrogeologic Framework

Simulated Interaction Between Freshwater- and Saltwater-Flow Systems

Effects of Surface-Water Bodies

Effects of Ground-Water Pumping

Effects of Sea-Level Rise

Water Levels and Streamflows

Freshwater/Saltwater Interface

Pumping Wells

Simulation of Proposed Ground-Water-Pumping Scenarios

Effects on Streamflow

Effects on Water Levels in Kettle-Hole Ponds

Effects on the Movement of the Freshwater/Saltwater Interface

Simulated Effects of Local Sea-Level Change Through Removal of a Tide-Control Structure

Summary

References Cited

Appendix: Development of Ground-Water Model

FIGURES

1-3. Maps showing:

1. Location of the four flow lenses of the Lower Cape Cod aquifer system and model-calculated water-table contours, Cape Cod, Massachusetts

2. Ice recession and lobe formation in southeastern Massachusetts

3. Surficial geology of Lower Cape Cod and the depositional sequence of the Wellfleet, Truro, and Eastham outwash plains

4-6. Diagrams showing:

4. Deltaic deposits prograding into a glacial lake, including topset, foreset, and bottomset deposits

5. The Lower Cape Cod aquifer system, Cape Cod

6. Area contributing recharge to a pumping well in a simplified, hypothetical ground-water-flow system

7, 8. Maps showing:

7. The delineation of ground-water-recharge areas to public-supply wells, ponds, streams, and coastal areas for current (2002) average pumping and recharge conditions, Cape Cod

8. Model-calculated delineation of the boundary between freshwater and saltwater beneath the Lower Cape Cod aquifer system, Cape Cod

9. Model section A-A? showing the model-calculated boundary between freshwater and saltwater flow, Lower Cape Cod

10. Model section B-B? showing the model-calculated boundary between freshwater and saltwater flow, Lower Cape Cod

11. Diagram of the Lower Cape Cod aquifer system showing lateral and vertical saltwater intrusion in response to ground-water pumping

12. Map showing locations of existing (2002) and proposed public-supply wells and Traffic Analysis Zones, Lower Cape Cod

13. Profiles of natural gamma and electromagnetic (EM) geophysical logs at the Knowles Crossing well field, North Truro, measured in September 2000

14. Diagram showing lateral and vertical saltwater intrusion beneath the Knowles Crossing well field, North Truro

15, 16. Graphs showing:

15. Specific conductance in monitoring wells TSW-259 and TSW-260 beneath Knowles Crossing well number 2 (KC-2) and total pumping in 2001 at the Knowles Crossing well field, North Truro

16. Model-calculated freshwater/saltwater interface and simulated pumping from 1955-2050 at the South Hollow well field, North Truro

17. Profiles of electromagnetic (EM) logs measured in September 2000 and model- calculated changes in salt concentration for current (2002) conditions at the South Hollow well field, North Truro

18. Graph showing water-table altitude at observation well TSW-1, North Truro, 1950-2002

19. Map showing locations of long-term observation wells and the measured and model-calculated increase in the altitude of the water table with time, Lower Cape Cod

20, 21. Graphs showing:

20. Model-calculated water-table altitude from 1929 to 2050 at Sites X and Y and simulated changes in sea level A, above NGVD 29; and B, above local sea level, North Truro

21. Model-calculated altitude from 1929 to 2050 of the freshwater/saltwater interface relative to NGVD 29 beneath Sites X and Y, North Truro

22. Diagram showing a hypothetical aquifer showing ground-water discharge to a surface-water body with A, no pumping; B, pumping at a rate such that the well would capture water that would otherwise discharge to the surface-water body; and C, pumping at a higher rate so that the flow direction is reversed and the well pumps water from the surface-water body

23-25. Maps showing:

23. Location of model-calculated contributing areas to A, Hatches Creek for current (2002) conditions; B, Hatches Creek and the Roach site pumping at 0.55 million gallons per day; C, Hatches Creek and Water District G site pumping at 0.55 million gallons per day; and D, Hatches Creek, Water District G site, and the Roach site each pumping at 0.55 million gallons per day, Eastham

24. Location of model-calculated contributing areas to Hatches Creek and the proposed Water District G well pumping at 0.55 and 1.10 million gallons per day, Eastham

25. Location of A, model-calculated contributing area to Duck Pond and water-table contours for current (2002) conditions; and B, model-calculated contributing areas to Duck Pond, Coles Neck well, Boy Scout Camp site, and the Wellfleet By The Sea site, each pumping at 0.10 million gallons per day, and changes in model-calculated water levels from current (2002) conditions, Wellfleet

26. Graph showing model-calculated monthly pond-level altitudes in Duck Pond for current (2002) conditions and simulated pumping conditions of 0.10 million gallons per day at the Coles Neck well, the Boy Scout Camp site, and the Wellfleet By the Sea site, Wellfleet

27-29. Maps showing:

27. Model-calculated water-table contours and contributing areas to A, South Hollow well field and the North Truro Air Force Base wells 4 and 5 for current (2002) pumping rates; B, South Hollow well field pumping at 0.80 million gallons per day and North Truro Air Force Base wells 4 and 5 pumping at current (2002) rates; and C, South Hollow well field and the North Truro Air Force Base wells 4 and 5 for current (2002) pumping rates and North Unionfield site pumping at 0.80 million gallons per day, North Truro

28. Model-calculated water-table contours and contributing areas to A, Little Pamet River, South Hollow well field, and North Truro Air Force Base wells 4 and 5 for current (2002) pumping rates; and B, Little Pamet River, South Hollow well field, and North Truro Air Force Base wells 4 and 5 for current (2002) pumping rates and CCC-5 site pumping at 0.8 million gallons per day, Truro

29. Model-simulated boundary conditions with A, the existing Herring River tide-control structure and conditions in the Chequesset Neck area; and B, proposed tide-control structure and the variable saltwater concentrations used in simulations 1 and 4, Wellfleet

TABLES

1. Model-calculated hydrologic budget for the four flow lenses of the Lower Cape Cod aquifer system under current (2002) pumping and recharge conditions, Cape Cod, Massachusetts

2. Model-calculated changes in the altitude of the freshwater/saltwater interface in the vicinity of the Herring River tide-control structure, Wellfleet, in response to changes in simulated salt concentrations and stream stage



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

Masterson, J.P., 2004, Simulated interaction between freshwater and saltwater and effects of ground-water pumping and sea-level change, Lower Cape Cod aquifer system, Massachusetts: U.S. Geological Survey Scientific Investigations Report 2004-5014, 78 p.


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