WATER RESOURCES ANALYSIS OF THE LOWER CAPE COD AQUIFER SYSTEM

Final Report:

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

Problem: 

The groundwater lenses that constitute the Lower Cape Cod Aquifer System (Nauset, Chequesset, Pamet, Pilgrim lenses, and the eastern portion of the Monomoy lens) are the sole source of drinking water for the towns of Brewster, Harwich, Orleans, Chatham, Eastham, Wellfleet, Truro, and Provincetown, and the Cape Cod National Seashore. Increased land development and population growth have created concerns regarding both the quantity and the quality of groundwater that is used for drinking water and that which discharges to surface water bodies of Lower Cape Cod. These concerns include the effects of increased groundwater withdrawals on the position of the interface between freshwater and saltwater and on the amount of freshwater discharge to ponds, streams, and coastal areas. In addition, increases in development will increase the amount of non-point source contamination in the aquifer which may limit potential locations for future water supply and may have a long-term impact on nutrient loading to freshwater streams and ponds and saltwater harbors and embayments. 

Public-water supply is the primary source of drinking water for the communities in the Monomoy Lens portion of Lower Cape Cod (Brewster, Chatham, Harwich, and Orleans), the Town of Provincetown and portions of the Town of Truro. The Town of Provincetown, located in the Pilgrim lens, receives drinking water from public-supply wells in the adjacent Pamet lens. The remainder of the residents in the Lower Cape, in the Nauset, Chequesset, and Pamet lenses, obtain drinking water from shallow, small-capacity domestic wells. 

Groundwater discharge is the primary source of water for kettle-hole ponds and streams and it is also a key component in the maintenance of the ecologically-sensitive coastal embayments of Lower Cape Cod. Declines in water levels because of increases in groundwater withdrawals could have a detrimental effect on these natural resources. Small changes in water-table altitude can result in substantial decreases in groundwater discharge to streams and coastal embayments, and can significantly affect the position of the shoreline of the many kettle-hole ponds throughout Lower Cape Cod. 

Several groundwater-flow models of the Lower Cape Cod Aquifer System have been developed as part of previous investigations to characterize subsurface hydrogeology and to address specific water-suppply issues (Wilson and Schreiber, 1981; LeBlanc, 1982; Guswa and LeBlanc, 1985; Reilly and others, 1987; Cambareri and others, 1989; Martin, 1993; Sobczak and Cambareri, 1995; Barlow, 1997a; Masterson and Barlow, 1997). These flow models have been used by the towns, the National Park Service (NPS), Barnstable County, and the State of Massachusetts for the management of the water resources of the area. Specifically, these models have been used to predict the effects of groundwater withdrawals on groundwater and surface-water levels, rates of freshwater discharge to coastal environments, and movement of the freshwater-saltwater interface that underlies Lower Cape Cod. 

Although these models have proved useful for water-resource planning and management purposes, each is limited by one or more of the following factors: (1) the method by which streams, ponds, and other groundwater discharge areas dividing the flow lenses of Lower Cape Cod, were simulated; (2) the use of groundwater recharge rates that are lower than current estimates for Cape Cod and southeastern New England (see, for example, the recent work of Bent, 1995; Barlow, 1997b; Masterson and others, 1998; and Gordon Bennett, Papadopoulos and Assoc., Inc., written commun., 1998); (3) the use of discretization schemes that do not to allow for accurate delineation of areas that contribute recharge to coastal discharge areas or of flowpaths of contaminant plumes; or (4) the use of geologic information that does not incorporate our current understanding of the depositional history of Cape Cod glacial sediments (see, for example, Masterson and others, 1997a). In addition, there is not a comprehensive groundwater-flow model of the area that simultaneously simulates groundwater flow in all of the flow lenses that constitute the Lower Cape Cod Aquifer System, and that allows for the investigation of flow conditions in the inter-lens discharge areas, particularly under changing hydrologic conditions. The proposed study will address these limitations by the development of comprehensive groundwater-flow models of Lower Cape Cod that synthesizes the most recent hydrologic and geologic information and uses the latest in computer resources. 

Objectives: 

The objectives of this proposed USGS investigation are to improve the understanding of ground- water flow in Lower Cape Cod Aquifer System and determine the effects of future groundwater pumping and waste-water disposal on (1) water-table altitudes, (2) position and movement of the freshwater-saltwater interface, (3) freshwater discharge to selected surface-water bodies, wetlands and coastal embayments, and (4) the areas that contribute recharge to natural and artificial receptors. These objectives will be accomplished by collecting additional hydrogeologic data and interpreting and analyzing these data within the framework of updated groundwater-flow models that have the capability of simulating both the freshwater and saltwater flow systems under different pumping and recharge conditions. 

Benefits: 

The groundwater-flow models developed for this investigation will provide valuable tools for water-supply managers, state and local regulators, and the NPS to use in making efficient and environmentally sound decisions for management of the groundwater resources of the Lower Cape Cod Aquifer System. More specifically, the flow models will be useful screening tools for evaluating potential water supply sites under the new source approval process required by the Massachusetts Department of Environmental Protection (MA DEP) for all new municipal wells. Model simulations will be made with changes in aquifer recharge and groundwater pumping rates to investigate the potential for saltwater intrusion, contamination from land-use activities, and adverse impacts on ponds, streams, and coastal wetlands. The models will be useful for quantifying groundwater discharge to ecologically-sensitive coastal embayments such as Wellfleet and Nauset Harbors. Finally, the models will be used to determine the areas that contribute recharge to natural receptors, the groundwater discharge rates to these receptors, and how these discharge rates may be affected by changes in groundwater pumping and changes in natural recharge rates, such as experienced during periods of extended droughts. 

References Cited:

Barlow, P.M., 1997a, Particle-tracking analysis of contributing areas of public-supply wells in simple and complex flow systems, Cape Cod, Massachusetts: U.S. Geological Survey Water-Supply Paper 2434, 66 p.

_____1997b, Dynamic models for conjunctive management of stream-aquifer systems of the 
glaciated Northeast: Storrs, Conn., unpublished dissertation, University of Connecticut, 256 p.

Bent, G.C., 1995, Streamflow, groundwater recharge and discharge, and characteristics of surficial deposits in Buzzards Bay basin, southeastern Massachusetts: U.S. Geological Survey Water- Resources Investigations Report 95-4234, 56 p.

Cambareri, Thomas, and others, 1989, Truro/Provincetown aquifer assessment and groundwater protection plan: Cape Cod Planning and Economic Development Commission Water Resources Office, Barnstable, MA.Cape Cod Commission, 1995, Orleans water table mapping project, Orleans, Massachusetts, v. 1: Barnstable, MA, 48 p.

Essaid, H.I., 1990, The computer model SHARP, a quasi-three-dimensional finite-difference model to simulate freshwater and saltwater flow in layered coastal aquifer systems: U.S. Geological Survey Water-Resources Investigations Report 90-4130, 181 p.

Guswa, J.H., and LeBlanc, D.R., 1985, Digital models of groundwater flow in the Cape Cod aquifer system, Massachusetts: U.S. Geological Survey Water-Supply Paper 2209, 112 p.
Harbaugh, A.W., and McDonald, M.G., 1996, User's documentation for MODFLOW-96, an update to the U.S. Geological Survey modular finite-difference groundwater flow model: U.S. Geological Survey Open-File Report 96-485, 56 p.

LeBlanc, D.R., 1982, Potential hydrologic impacts of groundwater withdrawal from Cape Cod National Seashore, Truro, Massachusetts: U.S. Geological Survey Open-File Report 82-438, 42 p.

LeBlanc, D.R., Guswa, J.H., Frimpter, M.H., and Lonquist, C.J., 1986, Groundwater resources of Cape Cod, Massachusetts: U.S. Geological Survey Hydrologic Investigations Atlas 692, 4 pls.

Martin, Larry, 1993, Investigation of effects of groundwater withdrawals from the Pamet and Chequesset aquifers, Cape Cod National Seashore: U.S. National Park Service Technical Report NPS/NRWRD/NRTR-93/14, 48 p.

Masterson, J.P., and Barlow, P.M., 1997, Effects of simulated groundwater pumping and recharge on groundwater flow in Cape Cod, Martha's Vineyard, and Nantucket Island Basins, Massachusetts: U.S. Geological Survey Water-Supply Paper 2447, 79 p.

Masterson, J.P., Stone, B.D., Walter, D.A., and Savoie, Jennifer, 1997a, Hydrogeologic framework of western Cape Cod, Massachusetts: U.S. Geological Survey Hydrologic Investigations Atlas 741, 1 plate.

Masterson, J.P., Walter, D.A., and Savoie, Jennifer, 1997b, Use of particle tracking to improve numerical model calibration and to analyze groundwater flow and contaminant migration, Massachusetts Military Reservation, Western Cape Cod, Massachusetts: U.S. Geological Survey Water-Supply Paper 2482, 50 p.

Masterson, J.P., Walter, D.A., and LeBlanc, D.R., 1998, Delineation of contributing areas to selected public-supply wells, western Cape Cod, Massachusetts: U.S. Geological Survey Water-Resources Investigations Report 98-4237, 45 p.

McDonald, M.G., and Harbaugh, A.W., 1988, A modular three-dimensional finite-difference ground- water flow model: U.S. Geological Survey Techniques of Water-Resources Investigations, book 6, chap. A1, 586 p.

Oldale, R.N., and Barlow, R.A., 1986, Geologic map of Cape Cod and the Islands, Massachusetts: U.S. Geological Survey Miscellaneous Investigations Series Map I-1763, 1 pl.

Pollock, D.W., 1994, User's guide for MODPATH/MODPATH-PLOT, version 3--a particle tracking post-processing package for MODFLOW, the U.S. Geological Survey finite-difference ground- water flow model: U.S. Geological Survey Open-File Report 94-464, variously paginated.

Reilly, T.E., Frimpter, M. H., LeBlanc, D.R., and Goodman, A.S.,1987, Analysis of steady-state salt- water upconing with application at Truro well field, Cape Cod, Massachusetts: Groundwater, v. 25 No. 2, p. 194-206.

Sobczak, Robert, and Cambareri, Thomas, 1995, Lower Cape Water Management Task Force interim report, fall 1995: Cape Cod Planning Commission, Barnstable, MA.

Sobczak, Robert, and Cambareri, Thomas, 1998, Water Resources of Outer Cape Cod: Final report of the Lower Cape Water Management Task Force May 1998: Cape Cod Planning Commission, Barnstable, MA

Urish, D.W., and Qanbar, E.K., 1997, Hydrologic evaluation of groundwater discharge, Nauset Marsh, Cape Cod National Seashore, Massachusetts: National Park Service Technical Report NPS/NESO-RNR/NRTR/97-07, 69 p. and appendices.

Wilson, John, and Schreiber, Robert, 1981, Analysis of groundwater development of the Truro aquifer: Camp Dresser and McKee, Inc., Boston, MA.

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