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Simulated Water Sources and Effects of Pumping on
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The sandy sediments underlying Cape Cod, Massachusetts, compose an important aquifer that is the sole source of water for a region undergoing rapid development. Population increases and urbanization on Cape Cod lead to two primary environmental effects that relate directly to water supply: (1) adverse effects of land use on the quality of water in the aquifer and (2) increases in pumping that can adversely affect environmentally sensitive surface waters, such as ponds and streams. These considerations are particularly important on the Sagamore and Monomoy flow lenses, which underlie the largest and most populous areas on Cape Cod.
Numerical models of the two flow lenses were developed to simulate ground-water-flow conditions in the aquifer and to (1) delineate areas at the water table contributing water to wells and (2) estimate the effects of pumping and natural changes in recharge on surface waters. About 350 million gallons per day (Mgal/d) of water recharges the aquifer at the water table in this area; most water (about 65 percent) discharges at the coast and most of the remaining water (about 28 percent) discharges into streams. A total of about 24.9 Mgal/d, or about 7 percent, of water in the aquifer is withdrawn for water supply; most pumped water is returned to the hydrologic system as return flow creating a state of near mass balance in the aquifer. Areas at the water table that contribute water directly to production wells total about 17 square miles; some water (about 10 percent) pumped from the wells flows through ponds prior to reaching the wells. Current (2003) steady-state pumping reduces simulated ground-water levels in some areas by more than 4 feet; projected (2020) pumping may reduce water levels by an additional 3 feet or more in these same areas. Current (2003) and future (2020) pumping reduces total streamflow by about 4 and 9 cubic feet per second (ft3/s), corresponding to about 5 percent and 9 percent, respectively, of total streamflow.
Natural recharge varies with time, over both monthly and multiyear time scales. Monthly changes in recharge cause pond levels to vary between 1 and 2 feet in an average year; annual changes in recharge, which can be much larger than monthly variations, can cause pond levels to vary by more than 10 feet in some areas over a period of years. Streamflow, which also changes in response to changes in recharge, varies by a factor of two over an average year and can vary more over multiyear periods. On average, monthly pumping ranges from 15.8 Mgal/d in March to 45.3 Mgal/d in August. Pumping and the distribution of return flow can seasonally affect the hydrologic system by lowering ground-water and pond levels and by depleting streamflows, particularly in the summer months. Maximum drawdowns in March and August exceed 3 feet and 6 feet, respectively, for current (2003) pumping. Simulated drawdowns from projected (2020) pumping, relative to water levels representing 2003 pumping conditions, exceed 2 feet in March and 5 feet in August. Current (2003) and future (2020) pumping can decrease pond levels in some areas by more than 3 feet; drawdown generally is largest during the month of August of an average year. Over multiyear periods, seasonal pumping can lower pond levels in some areas by more than 4 feet; the effects of seasonal pumping are largest during periods of reduced recharge. Monthly streamflow depletion varies in individual streams but can exceed 2 ft3/s in some streams.
The combined effects of seasonal pumping and drought can reduce pond levels by more than 10 feet below average levels. Water levels in Mary Dunn Pond, which is in an area of large current and projected pumping, are predicted (2020) to decline during drought conditions by about 10.6 feet: about 6.9 feet from lower recharge, about 2.3 feet from current (2003) pumping, and about 1.4 feet from additional future (2020) pumping. The results indicate that pumping generally does not cause substantial streamflow depletion and that the primary effect of pumping is on water levels in ponds. Natural changes in recharge account for most of the variation in pond levels; however, pumping can cause substantial declines in the levels of ponds near pumping wells. Also, the effects of pumping and recharge can combine to cause drawdowns of more than 10 feet in some areas.
Abstract
Introduction
Purpose and Scope
Methods of Analyses
Data Collection
Numerical Models
Limitations of Analyses
Hydrogeology of the Sagamore and Monomoy Flow Lenses
Geologic History
Hydrogeologic Framework
Hydrologic System
Ground Water
Surface Water
Water Use
Simulated Water Sources to Wells and Surface Waters
Hydrologic Budgets and Source Areas to Ponds, Streams, and Coastal Boundaries
Areas Contributing Recharge to Wells
Current (2003) Conditions
Future (2020) Conditions
Limitations of Analyses
Simulated Effects of Pumping on Hydrologic System
Long-Term Average Conditions
Water Levels
Streamflow
Effects of the Upper Cape Cooperative Wells
Effects of Time-Varying Hydrologic Stresses
Time-Varying Recharge
Time-Varying Pumping
Average Monthly Conditions
Annual and Seasonal Conditions
Summary
Acknowledgments
References Cited
Appendix 1
1–3.Maps showing:
1.Location of Sagamore and Monomoy flow lenses, Cape Cod, Massachusetts
2.Location of continental ice sheets near present-day Cape Cod during the late Pleistocene
3.Surficial geology of western and central Cape Cod and location of glacial ice margins during the late Pleistocene
4. Diagram showing A, depositional model for western Cape Cod; and B, geologic section along A–A′ (fig. 3) on western Cape Cod
5. Map showing A, regional water table for central and western Cape Cod, and B, generalized vertical ground-water flow, western Cape Cod
6. Graph showing variability of precipitation at Hatchville, and water levels at well A1W230, Barnstable: A, annual averages; and B, monthly averages for precipitation for the period 1941–95, and for water levels for the period 1963–95
7–9.Maps showing:
7.Simulated heads and sources of water to wells and natural receptors— ponds, streams, and coastal estuaries—the Sagamore and Monomoy flow lenses, Cape Cod
8.Areas contributing recharge to pumped wells and associated ponds for current (2003) pumping conditions in the Sagamore and Monomoy flow lenses, Cape Cod
9.Areas contributing recharge to pumped wells and associated ponds for future (2020) pumping conditions in the Sagamore and Monomoy flow lenses, Cape Cod
10. Graph summarizing the extent to which wells are weak sinks in the regional models of the Sagamore and Monomoy flow lenses, Cape Cod
11.Map showing steady-state drawdowns associated with A, current (2003) ground-water withdrawals; and B, future (2020) ground-water withdrawals, Sagamore and Monomoy flow lenses, Cape Cod
12.Graph showing simulated streamflows for current (2003) and future (2020) pumping conditions at select sites in the Sagamore and Monomoy flow lenses, Cape Cod
13.Map showing effects of operation of the Upper Cape Cooperative wells on regional heads and streamflows for A, current (2003) pumping rates; and B, Massachusetts Department of Environmental Protection-approved pumping rates
14.Graph showing midmonthly hydrologic budgets for an average year for current (2003) pumping conditions in the Sagamore and Monomoy flow lenses, Cape Cod
15.Map showing changes in water-table altitudes arising from changes in natural recharge for A, from May to November of an average year; and B, between representative wet (1967) and dry (1973) years in the Sagamore and Monomoy flow lenses, Cape Cod
16–17.Graphs showing:
16.Changes in pond levels and streamflow because of monthly changes in recharge for an average year, Cape Cod: A, Lawrence Pond and Quashnet River; and B, Johns Pond and Johns Pond Outlet
17.Changes in pond levels and streamflow because of annual changes in recharge over a representative 40-year period, Cape Cod: A, Lawrence Pond and Quashnet River; and B, Johns Pond and Johns Pond Outlet
18–19.Maps showing:
18.Drawdowns associated with current (2003) pumping in A, March of an average year; and B, August of an average year in the Sagamore and Monomoy flow lenses, Cape Cod
19.Drawdowns associated with future (2020) pumping in A, March of an average year; and B, August of an average year in the Sagamore and Monomoy flow lenses, Cape Cod
20–26.Graphs showing:
20.Monthly changes in pond levels at Mary Dunn Pond, Cape Cod, arising from natural recharge and seasonal pumping and the combined effects of recharge and pumping
21.Monthly changes in pond levels and drawdowns at A, Mary Dunn; and B, Lawrence Ponds, Cape Cod, arising from natural recharge and current (2003) and future (2020) monthly seasonal pumping
22.Total streamflow depletion for the Sagamore and Monomoy flow lenses, Cape Cod
23.Monthly changes in streamflow and streamflow depletion for nonpumping, current (2003), and future (2020) pumping conditions at A, Mill Creek (Shawme Pond Outlet) and B, the Herring River, Cape Cod
24.Seasonal changes in pond levels and drawdowns over a 40-year period for nonpumping, current (2003), and future (2020) pumping conditions at Mary Dunn Pond, Cape Cod
25.Drawdowns associated with seasonal pumping under drought conditions for A, current (2003) pumping; and B, future (2020) pumping in the Sagamore and Monomoy flow lenses, Cape Cod
26.Seasonal changes in streamflow levels and streamflow depletion for Mill Creek (outlet to Shawme Pond) during a 40-year period for nonpumping, current (2003), and future (2020) pumping conditions, Cape Cod
1.Simulated hydrologic budgets for the Sagamore and Monomoy flow lenses for nonpumping, current (2003) pumping, and future (2020) pumping conditions, Cape Cod, Massachusetts
2.Simulated combined hydrologic budgets [current (2003) conditions] for the Sagamore and Monomoy flow lenses, Cape Cod: average budget (steady-state), budget for mid-March, and budget for mid-August
3.Simulated combined hydrologic budgets [current (2003) conditions] for the Sagamore and Monomoy flow lenses, Cape Cod: steady-state budget, budget for a typical dry year (1967), and budget for a typical wet year (1973)
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The citation for this report, in USGS format, is as follows:
Walter, D.A., and Whealan, A.T., 2005, Simulated Water Sources and Effects of Pumping on Surface and Ground Water, Sagamore and Monomoy Flow Lenses, Cape Cod, Massachusetts: U.S. Geological Survey Scientific Investigations Report 2004-5181, 85 p.
For more information about USGS activities in Massachusetts-Rhode Island District, visit the USGS Massachusetts-Rhode Island Home Page.
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