Page 1 

Introduction

The Cape Cod National Seashore (CCNS), Cape 

Cod, Massachusetts (fig. 1), is underlain by four lenses 

of fresh ground water—the Nauset, Chequesset, Pamet, 

and Pilgrim flow lenses, which are bounded laterally and 

below by saltwater (figs. 2, 3). These four flow lenses 

constitute the Lower Cape Cod aquifer system and are 

the only source of drinking water for the communities of 

Continued land development, population growth, and sea-level rise have created concerns regarding 

the supply of potable water and the quality and quantity of fresh ground water discharging to ponds, 

streams, and coastal areas on Lower Cape Cod, Massachusetts. These potential concerns include:

(1) Increased loading of the nutrients nitrogen and phosphorus, and the consequent eutrophication of 

fresh and coastal surface-water bodies.

(2) Declines in pond levels, increases in the depth to the water table beneath inland wetlands, 

reductions in streamflow, reductions in ground-water discharge to the coast, and increased 

saltwater intrusion from increased ground-water withdrawals.

(3) Increased erosion rates, damage from higher storm-surge flooding, landward intrusion of seawater 

in coastal marshes and wetlands, and a decrease in the thickness of the freshwater lens from rising 

sea levels.

Therefore, understanding how ground-water flow affects freshwater and estuarine ecosystems of the 

Cape Cod National Seashore is vital to managing and protecting these natural resources.

U.S. Department of the Interior

U.S. Geological Survey

General Information Product 13

June 2005

Eastham, Wellfleet, Truro, and Provincetown, and for the 

various National Park Service facilities throughout the 

CCNS. Also, ground-water discharge from these lenses 

provides the primary source of water for the numerous 

kettle-hole ponds, streams, and wetlands throughout 

Lower Cape Cod. This discharge also helps to maintain 

the biological diversity and productivity of estuaries and 

salt marshes. 

Potential Changes in Ground-Water Flow and their Effects on 

the Ecology and Water Resources of the Cape Cod National 

Seashore, Massachusetts

By John P. Masterson (U.S. Geological Survey) and John W. Portnoy (National Park Service)
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Pamet

flow lens

Pilgrim

flow lens

Chequesset

flow lens

Nauset

flow lens

Tide-control

structure

Griffin

Island

NADP Site

Tide-control

structure

South Hollow

Well Field

Knowles Crossing

Well Field

Tide-control

structure

Truro

Provinceto nccet etowowown

Wellfleet

Wellfleet

Center

Eastham Eastham

Cape Cod Bay

Atlantic

Ocean

Base from U.S. Geological Survey Digital Line Graphs, and topographic quadrangles,

Provincetown, Wellfleet, and Orleans, Massachusetts, 1:25,000, Polyconic projection,

NAD 1927, Zone 19

Chequesset

Neck

Bound

Brook

Island

PPamet River

Herring River

Black FFiish Creek

70o15' 70o00'

42o00'

41o52'30"

0 5 MILES

0 5 KILOMETERS

Duck

Pond

Gull

Pond

Tide-control

structure

Sewage

Infiltration Beds

P

k Pilgrim Lake Pilgrim Lak

4

6

8

2

4

6

22

4

6

2

4

6

8

2

2

ent

6

4

4

8

8

10

12

14

16

22

TSW-89

Study

area

Hatches Harbor

Figure 1. The Lower Cape Cod aquifer system in Cape Cod, Massachusetts, is composed of four flow lenses, as shown 

by the water-table contours. Ground water flows from the centers of the water-table mounds toward the coast.
Page 3 

As land development and water demand increase, 

and as wastewater continues to be returned to the aquifer system through onsite septic systems, environmental managers are becoming concerned about possible 

long-term effects of these activities on the Lower Cape 

Cod aquifer system. In response to these concerns, the 

USGS, in cooperation with the National Park Service 

(NPS), 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 Lower Cape Cod aquifer system 

and to assess possible hydrologic effects of proposed 

water-management strategies. 

This report provides a brief overview of the 

hydrogeology of the Lower Cape Cod aquifer system 

and the importance of ground-water levels and flow to 

the ecology of the Cape Cod National Seashore freshwater and estuarine ecosystems. Also, the report includes 

a discussion of the implications of future changes to the 

ground-water system and their effects on ecosystems and 

water resources of Lower Cape Cod.

For a more detailed description and analysis of groundwater flow in the Lower Cape Cod aquifer system, refer 

to U.S. Geological Survey (USGS) Scientific Investigations Report 2004-5014 (Masterson, 2004).

Hydrogeology of Lower Cape Cod

The Lower Cape Cod aquifer system is composed of 

glacially deposited sediments that range in size from clay 

to boulders. Approximately 15,000 years ago, glacial 

ice sheets covered much of New England and extended 

north and east of present-day Lower Cape Cod (Oldale, 

1992). Meltwater from these glaciers carried sediments 

that were originally trapped in the ice, and deposited 

them in large deltas in adjacent Glacial Lake Cape Cod, 

the present-day Cape Cod Bay. At that time, global sea 

levels were lower than current levels because of the large 

quantity of water contained in the enormous ice sheets. 

As the ice sheets melted and retreated, sea level rose, 

and erosion of the glacial Cape Cod shoreline began. 

Photo courtesy of John Budreski, National Park Service, Cape Cod National Seashore

Figure 2. The Pilgrim flow lens, like the other flow lenses of the Lower Cape Cod aquifer system, is bounded laterally by the 

Atlantic Ocean and Cape Cod Bay. Rainfall that reaches the water table as recharge eventually leaves the aquifer system as 

ground-water discharge along the coast. View looking southeast.
Page 4 

The remnants of the once much larger deltaic deposits, 

or outwash plains, that now constitute Lower Cape Cod 

continue to be eroded at a retreat rate of about 3 feet (ft) 

per year along the Atlantic Ocean (Zeigler and others, 

1964). These surficial deposits overlie granitic bedrock 

and range in thickness from about 500 ft in Eastham to 

over 1,000 ft in Truro (Oldale, 1992). 

 Kettle-Hole Ponds

Kettle-hole ponds are the most conspicuous freshwater feature found throughout Lower Cape Cod (fig. 4). 

These ponds are surface-water expressions of the water 

table because they directly connect with the underlying 

aquifer system. Kettle-hole ponds are also flow-through 

ponds with freshwater seeping into and exiting the pond 

in the direction of ground-water flow. The kettle holes 

were formed by blocks of ice stranded by the retreating 

ice sheets and buried by the deltas that expanded into 

Glacial Lake Cape Cod. These ice blocks were stranded 

directly atop basal till and bedrock. When the buried 

ice blocks melted, overlying and surrounding sands and 

gravels collapsed into the resulting depressions. These 

depressions eventually became ponds when they filled 

with ground water as the water table rose in response 

to the rising seas. These kettle-hole ponds are found 

throughout north Wellfleet and south Truro within the 

CCNS (fig. 1).

Pamets

Valleys, known as pamets, carved the originally flat, 

outwash-plain surfaces throughout Lower Cape Cod. 

These pamets formed possibly by “spring sapping,” a 

process in which springs migrated inland from the coast 

by gradually eroding the gently sloping land surface 

to form a series of east-west trending stream valleys 

(Oldale, 1992). These pamets sometimes are referred to 

locally as “hollows,” and can be found throughout the 

Wellfleet and Truro areas. The most notable pamet on 

Lower Cape Cod is the valley that contains the Pamet 

River in Truro (fig. 1). 

Aquifer System

The freshwater-flow lenses of the Lower Cape Cod 

aquifer system consist of four water-table mounds 

separated by surface-water discharge areas. For example, 

the Pamet River is the hydrologic boundary between the 

Pamet and Chequesset ground-water-flow lenses (fig. 1). 

Each flow lens is sustained (recharged) by precipitation 

that percolates through the ground to the water table, 

the top of the aquifer. Generally, ground water flows 

radially at a rate of about 1-2 ft per day from the highest 

point of the water table toward the coast and toward the 

streams and wetlands that form the boundaries between 

the lenses. Recharge near the center of the water-table 

Figure 3. Freshwater underlain by saltwater forms the flow lenses of the Lower 

Cape Cod aquifer system (modified from Strahler, 1972).

SALINE GROUND

WATER

SALINE GROUND

WATER

WATER TABLE

LAKE

SALINE GROUND

WATER

SALINE GROUND

WATER

BEDROCK

Schematic diagram, not to scale

FRESH GROUND

WATER
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mound travels deeper through the aquifer system than 

water that recharges near the coast (fig. 3). The water 

that travels along the deeper flow paths to the coast 

(fig. 3) can take more than 100 years to reach the coast.

All of the freshwater that flows through the Lower 

Cape Cod aquifer system is derived from aquifer 

recharge. Under current (2005) conditions, flow from 

one flow lens does not discharge to another flow lens; 

therefore, the four flow lenses are hydraulically independent of one another (Masterson, 2004) (fig. 1). 

Water Table

The elevation of the water table is typically referred 

to in terms of altitude above the mean sea-level elevation of 1929 (NGVD 29). The altitudes of the tops of 

the water-table mounds (fig. 1) range from about 16 

ft above NGVD 29 in the Nauset flow lens to about 6 

ft above NGVD 29 in the Pilgrim flow lens. Surfacewater bodies, local geology, and changing ground-water 

pumping and recharge conditions affect the altitude and 

shape of the water-table mounds. The Chequesset flow 

lens, which receives the most recharge of the four flow 

lenses, has a maximum altitude of only about 9 ft above 

NGVD 29, which is about 7 ft lower than the altitude 

of the Nauset flow lens; this difference is caused by the 

large number of streams, such as the Herring River in 

Wellfleet, that flow in the Chequesset flow lens and by 

the differences in the permeability of the glacial sediments of the Nauset and Chequesset flow lenses (Masterson, 2004).

The altitudes of the water-table mounds vary seasonally and annually because aquifer recharge rates vary 

with changes in precipitation and temperature. These 

rates also may vary from one flow lens to another 

because of differences in land use, vegetation, depth to 

the water table, and local rainfall. Water levels at observation well TSW-89 in north Truro (fig. 1) correlated 

strongly with rainfall (fig. 5) measured at the nearby 

National Atmospheric Deposition Program site in south 

Truro (NADP Site, fig. 1). Changes in precipitation, 

and therefore, aquifer recharge, also result in changes in Photo courtesy of John Portnoy, National Park Service, Cape Cod National Seashore

Figure 4. The kettle-hole ponds of Lower Cape Cod hydraulically 

connect with the underlying aquifer; therefore, they are surfacewater expressions of the water table. Ground water discharges 

into the ponds along their upgradient shores, flows through the 

ponds, and then reenters the ground-water system along the 

downgradient shore. View looking southwest.
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to determine how much of this freshwater discharges 

directly to the coast through the seabed or reaches the 

coast as streamflow (Masterson, 2004).

The total water budget for the Lower Cape Cod 

aquifer system can be subdivided by individual flow 

lenses (fig. 6). Subdividing the water budget by flow 

lens provides a better understanding of the distribution 

of flow to the various hydrologic features than can 

be obtained from the total water budget for the entire 

aquifer system. For instance, the total amount of 

ground-water discharge to streams is about 21 Mgal/d, 

or 31 percent of the total water budget for the Lower 

Cape Cod aquifer system, but nearly 60 percent of that 

streamflow is in the Herring River in Wellfleet, which 

bisects the Chequesset flow lens (fig. 1). 

Ground-water withdrawals for public supply account 

for only about 1 percent of the total water budget for 

the aquifer system; however, almost all of the pumped 

water is from the Pamet flow lens, composing about 

7 percent of its total budget (fig. 6). This pumped 

water is the primary source of drinking water for the 

town of Provincetown, parts of the town of Truro, and 

some NPS facilities in the Provincetown area. Most 

of the wastewater derived from this public supply is 

discharged into the Pilgrim flow lens as treated sewage 

at the Provincetown wastewater-treatment facility 

pond levels, streamflow, and discharge to the coast. The 

magnitudes of these changes vary across a flow lens and 

from flow lens to flow lens.

Water-level fluctuation, in response to changes in 

recharge, is greatest at the top of a water-table mound 

away from streams and the coast because water levels 

change less in areas where ground water discharges 

(Masterson, 2004). Duck Pond, which is near the top of 

the southern water-table mound in the Chequesset flow 

lens (fig. 1), had a mean annual fluctuation of about 

1.6 ft, as determined from monthly measurements from 

1994 to 1999 (Sobczak and others, 2003). In contrast, 

Gull Pond, which is close to the Herring River and is 

connected to nearby kettle ponds by a network of shallow, dug channels, had a mean annual fluctuation of 

about 0.7 ft during that same period. 

Water Budget

Precipitation is the primary source of freshwater to 

the Lower Cape Cod aquifer system. On average, the 

rainfall rate is about 42 inches per year (in/yr). It is 

assumed that about 43 percent of the water (18 in/yr) is 

removed by evaporation and plant transpiration before 

reaching the water table; the remaining water (24 in/yr) 

enters the aquifer as recharge. If a recharge rate of 24 

in/yr is assumed over the entire Lower Cape Cod land 

mass, then freshwater continually flows through and 

leaves the aquifer at a rate of about 2.5 billion gallons 

per year, or about 68 million gallons per day (Mgal/d) 

(fig. 6). Ground-water-flow simulations were made 

26%

67%

91%

9%

26%

74% 31%

51% 49%

68%

Figure 6. The hydrologic budget of the Lower Cape Cod aquifer 

system can be subdivided by individual flow lenses to show the 

relative importance of the different flow components in each 

flow lens.

PRECIPITATION

WATER LEVEL

2000 2001 2002 2003

YEARS

0

1

2

3

4

5

6

7

8

9

10

PRECIPITATION, IN INCHES

3

4

5

6

ALTITUDE OF WATER LEVEL, IN FEET 

 ABOVE NGVD 29

Figure 5. Changes in water levels are directly correlated to 

changes in precipitation as shown by the change in rainfall at the 

National Atmospheric Deposition Program site in Truro and water 

levels at observation well TSW-89 in North Truro, Cape Cod, 

Massachusetts, from 2000 to 2003.
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infiltration beds (fig. 1), with the remainder being 

discharged through onsite domestic septic systems in 

unsewered areas.

Freshwater/Saltwater Interface

The Lower Cape Cod land mass is small, thereby 

limiting freshwater recharge, aquifer area, and aquifer 

depth. Consequently, freshwater in the Lower Cape 

Cod aquifer system does not extend to the underlying 

granitic bedrock, but instead is primarily bounded 

below by saltwater (fig. 3). Depths to the boundary, 

or interface, between fresh and saltwater within the 

aquifer system differ among the flow lenses, and are 

directly proportional to the altitude of the overlying 

water table. Because of the density difference between 

fresh and saltwater, the depth to the freshwater/

saltwater interface below sea level is about 40 times 

greater than the altitude of the water table above sea 

level (Freeze and Cherry, 1979). Therefore, if the 

water table is 5 ft above sea level, the depth to the 

freshwater/saltwater interface would be about 200 

ft below sea level. Although this is only a general 

approximation of the actual relation between fresh and 

saltwater flow, it does provide reasonable estimates 

of the depths to the freshwater/saltwater interfaces 

throughout the Lower Cape Cod aquifer system.

Ground-Water Flow and its Effect on 

Ecology

In addition to being the only source of drinking water 

for the communities of Lower Cape Cod, ground water 

is also the primary source of freshwater for kettle-hole 

ponds, inland wetlands, vernal pools, streams, and 

coastal waters. These water bodies support much of the 

biodiversity found throughout the CCNS. The amount 

of and changes in ground-water discharge over time are 

essential in maintaining the freshwater and estuarine 

habitats throughout the CCNS. 

Kettle-Hole Ponds 

Because of the connection between kettle-hole 

ponds and the aquifer system, ground water discharges 

from the aquifer into the ponds primarily close to the 

shore, through the often highly vegetated zone along 

the shoreline. The shoreline plant and animal communities, often including rare species, are especially 

adapted to tolerate seasonal flooding, and depend 

on naturally high water levels to prevent displacement by less flood-tolerant species (Sorrie, 1994). 

The continual refreshing of pond water with 

ground water reduces the effects of natural salts, such 

as sea spray, and pollutants, such as from domestic wastewater systems adjacent to the ponds. This 

flushing process maintains the naturally dilute, 

clear, and slightly acidic water quality found in 

the kettle-hole ponds throughout the CCNS. 

Inland Wetlands 

Inland wetlands, such as shrub and tree swamps 

and vernal pools, are similar to kettle-hole ponds in 

that they form in low-lying areas and rely on groundwater discharge as their primary source of freshwater. 

Seasonally flooded wetlands, such as vernal pools, 

typically contain water in winter and early spring, 

when the water table is high and low-lying areas are 

temporarily flooded; they regularly are dry in midsummer (fig. 7). This regime of periodic flooding and 

drying creates habitat for a diverse and distinct group of 

plants, and particularly, vernal-pool animals, including 

mole salamanders, wood frogs, and many invertebrates, 

which could not survive or reproduce successfully in the 

presence of permanent-pond predators, especially fish 

(Colburn, 2004). 

Natural ground-water levels also sustain red maple 

and Atlantic white cedar swamps, which have their own 

distinct biotic communities (fig. 8). These wetlands 

provide important sources of water to wildlife species, 

such as box turtles, that otherwise inhabit dry upland 

landscapes during the summer. 

Streams

On Lower Cape Cod, freshwater streams are at the 

head of small estuaries, with the largest streams being 

the Herring River in Wellfleet and the Pamet River in 

Truro (fig. 1). These streams supply freshwater to coastal 

waters and provide the necessary pathway between the 

coast and the kettle-hole ponds for migratory fish like the 

anadromous blue-back herring and alewife, which spawn 

in the kettle-hole ponds; and for catadromous American 

eels, which mature in the upper estuary and kettle ponds 

but spawn in the ocean. Ground-water discharge through 

the streambed is the primary source of freshwater for 

these streams. Similar to ponds and wetlands, streams 

flow in low-lying areas where land surface intersects 

the water table. Therefore, the amount of flow in these 

streams is directly related to the altitude of the water 

table, and small changes in the altitude of the water table 

can cause large changes in streamflow, which could 

impede fish migration. 
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Figure 7. Vernal pools on Lower Cape Cod are seasonally flooded wetlands that typically 

contain water in the winter and early spring when the water table is (A) high and are 

regularly dry in the summer and early fall when the water table is (B) low. This regime of 

periodic flooding and drying is vital for the life cycles of the diverse plant and animal life that 

inhabit these areas.

Photos courtesy of John Portnoy, National Park Service, Cape Cod National Seashore

A.

B.
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Photo courtesy of Cape Cod National Seashore

Figure 8. The inland red maple (A) and white cedar (B) swamps throughout Lower Cape Cod 

depend upon the natural fluctuations in ground-water levels to support their distinct biotic communities. 

A.

B.
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Coastal Waters 

The coastal waters, including coastal marshes, that 

surround Lower Cape Cod are the ultimate discharge 

areas for freshwater from the ground-water-flow system 

(Walter and others, 2004). Freshwater enters the coastal 

waters as either streamflow or as direct ground-water 

discharge from the aquifer through the coastal seabed. 

About two-thirds of the 68 Mgal/d of freshwater discharging to the coast is direct ground-water discharge, 

and the remainder enters coastal waters as surface water 

from streams (fig. 6). Freshwater discharge to the coast 

is essential for maintaining the natural range of salinity in coastal waters and salt marshes, which sustains 

a diversity of shellfish and finfish habitats in the nearshore areas that surround Cape Cod. 

Changes in Ground-Water Flow and 

their Effects on Water Resources 

Changes on Lower Cape Cod that may affect groundwater quality and flow in the aquifer system include 

(1) increased land development and population growth, 

which could affect the quality and quantity of ground 

water for both drinking water and for the maintenance of 

freshwater and coastal ecosystems, (2) estuarine habitat 

restoration through the removal of tide-control structures 

and possible alterations to ground-water quality as a 

result of restoring natural tidal fluctuations, and (3) sealevel rise and its effect on water levels, streamflows, and 

the depth to the freshwater/saltwater interface. 

Land Development and Ground-Water Quality

The primary concerns related to increased land 

development are the increase in nonpoint-source contamination to the aquifer from septic systems and lawn 

fertilizers, and point-source loading from gasoline 

stations and municipal landfills (Godfrey and others, 

1999). Increased loading of nutrients, such as nitrogen 

and phosphorus, to the aquifer is of particular concern 

because of the potential for increased eutrophication of 

fresh and coastal surface-water bodies. Eutrophication is 

the process by which continued nutrient loading to surface-waters bodies stimulates excessive plant and algae 

growth; the resulting accumulation of organic matter 

can lead to the depletion of dissolved oxygen and to fish 

kills. In coastal waters, nutrient enrichment has caused 

nuisance algal blooms (fig. 9) and the loss of eelgrass 

beds, an important submerged aquatic habitat for shellfish and finfish (Short and Burdick, 1996). 

Figure 9. Increased discharge of nutrients to surface-water bodies has caused eutrophication and resulted in nuisance algal blooms, such as the one shown here at Salt Pond, Eastham, 

Massachusetts. These human-derived nutrients are transported through the ground-water 

system from onsite septic systems to surface-water bodies.

Photo courtesy of Barbara Dougan, National Park Service,Cape Cod National Seashore
Page 11 

Depending on the source of recharged water, many 

toxic constituents may flow with ground water and affect 

aquatic ecosystems. On Lower Cape Cod, domestic 

wastewater, which includes many compounds intended 

for household or medicinal use, is generally disposed of 

through onsite septic systems. These contaminants may 

affect both ground-water supplies and downgradient surface-water ecosystems (John Colman, U.S. Geological 

Survey, written commun., 2003), with potential chronic 

or acute effects to both individual species and entire 

ecosystems.

Land Development and Ground-Water Flow 

Presently (2004), only Provincetown and portions of 

Truro are serviced by a large capacity, municipal watersupply system that withdraws water from the Pamet flow 

lens. Other portions of Lower Cape Cod obtain drinking 

water from shallow, private wells. As land development 

increases and wastewater continues to be returned to the 

aquifer through onsite septic systems, there is a growing concern that increased amounts of nonpoint-source 

contamination in the aquifer system may degrade the 

water supply. If the water supply is adversely affected, 

it may necessitate a shift from small-capacity, private 

wells to larger, more centralized municipal systems that 

can be managed more efficiently than the current water 

supply (Sobczak and Cambareri, 2002). Federal, State, 

and local officials responsible for managing and protecting water resources are concerned that such a shift may 

cause ecologically detrimental declines in the water table 

and in pond levels, and cause decreased ground-water 

discharge to streams and coastal areas, and increased risk 

of saltwater intrusion.

A pumping well can alter ground-water discharge to 

downgradient surface-water bodies by capturing water 

that would discharge to these bodies (figs. 10A, B); and 

if the pumping rate is great enough, by reversing the 

flow direction so that the surface-water bodies contribute water directly to the pumping well (fig. 10C). This 

pumping can cause declines in pond levels, increases 

in the depth to the water table beneath inland wetlands, 

reductions in streamflow, and reductions in ground-water 

discharge to the coast. These changes can reduce the 

water available to shoreline plants, decrease the flooding 

depth and duration of vernal pools, impede migratory 

fish passage, and alter the salinity of estuaries, all of 

which alter a diverse biotic community.

Saltwater intrusion is another potentially adverse 

effect of centralized large-capacity municipal pumping 

on the ground-water-flow system. Saltwater intrudes 

into freshwater when a pumped well removes enough 

fresh ground water to cause the interface between fresh 

and saltwater to move in from the coast or up from the 

underlying saline part of the aquifer (fig. 11).

Salt-Marsh Diking and Tidal Restoration

Ongoing efforts to restore tidal flow to Lower Cape 

Cod coastal marshes, restricted for the past 150 years 

for mosquito control, marsh drainage, and flood control, 

may affect the ground-water-flow system. Tide-control 

structures (fig. 12) were built at the Herring River in 

Wellfleet, the Pamet River and East Harbor (Pilgrim 

Lake) in Truro, and Hatches Harbor in Provincetown. 

These structures restrict tidal flow, reduce high-tide 

heights, impede low-tide drainage, and thereby degrade 

Schematic diagram, not to scale

Unconfined aquifer

Unconfined aquifer

Q1

Q2

Recharge area

Land surface

Water table

C.

B.

A.

Land surface

Land surface

Surface-water

body

Surface-water

body

Surface-water

body

Unconfined aquifer

Water table

Water table

Bedrock

Bedrock

Bedrock

Recharge area

Recharge area

Figure 10. Pumping from ground water can affect discharge to 

nearby surface-water bodies. In the prepumping condition (A), 

water flows to the surface-water body. Pumping at a low rate (B) 

reduces discharge to the surface-water body. When the pumping rate is increased (C) the flow direction reverses and water 

from the surface-water body is pumped directly from the well 

(modified from Alley and others, 1999).
Page 12 

Figure 11. Overpumping from large-capacity municipal wells, such as those in the 

Pamet flow lens, can result in lateral encroachment and upconing of saltwater (modified 

from Strahler, 1972). 

nearly 2,500 acres of original salt-marsh estuaries on 

Lower Cape Cod (fig. 1) (Roman and others, 1995). As 

a result, NPS and other Federal, State, and local resource 

managers are assessing the potential hydrologic and 

ecological effects of removing tide-control structures and 

monitoring the results of ongoing restoration at Hatches 

and East Harbors to determine whether these marshes 

are returning to their natural state. 

Effects on Estuarine Habitats

With the installation of large culverts in the Hatches 

Harbor tide-control structure (Provincetown, MA) in 

1998, tidal range has been mostly restored after 68 years 

of diking. Seawater is again flooding the wetlands 

during high tides, causing the decline of exotic and 

invasive plants, especially common reed (Phragmites), 

and the recolonization of the marsh by native salt-marsh 

plants and animals. In addition, better low-tide drainage 

and tidal flushing have naturally controlled mosquitoes 

(Portnoy and others, 2003).

With partial tidal restoration at East Harbor (Pilgrim 

Lake, fig. 1) in 2002, nuisance midges and introduced 

carp have been replaced by a rich assemblage of native 

estuarine finfish, shellfish, and submerged aquatic vegetation, such as widgeon grass and eelgrass. 

This system, diked since 1868, had chronic eutrophication, summertime oxygen depletions, and nuisance insect 

problems for decades prior to the 2002 partial opening. 

The largest diked estuary on the Lower Cape is 

the Herring River (Wellfleet, MA). Restoration of 

this estuary, tide-restricted since 1909, is expected to 

eliminate water-quality problems, and also to restore up 

to 1,100 acres of native salt-marsh estuary (Roman and 

others, 1995). 

Presently, much of this marsh is constantly drained, 

leading to the leaching of acid sulfates and metals, 

which in turn stresses fish and other aquatic animals, 

and favors nuisance mosquitoes. Drainage has also 

allowed trees and shrubs to replace original salt-marsh 

plants, and has caused the original water-logged peat 

to shrink and subside. Wetland subsidence combined 

with sea-level rise may prohibit long-term wetland 

survival. Hydrologic and ecological research has shown 

that restored tidal flow can reverse these trends and 

restore native habitats (Portnoy, 1999; Spaulding and 

Grilli, 2001; Portnoy and others, 2003). Finally, tidal 

restoration of the upper Pamet River (fig. 1) marshes, 

diked since about 1869, has been considered, particularly 

after a 1991 breach of the barrier beach separating the 

Pamet River from the Atlantic Ocean (Portnoy, 2001).

SALTWATER

WATER TABLE

SALTWATER

BEDROCK

Schematic diagram, not to scale

PUMPING

WELLS

FRESHWATER
Page 13 

Effects on Ground-Water Resources

Proposed tide restorations to the upper Pamet River 

and Herring River (fig. 1) have raised concerns about 

the potential effects on nearby residents from projected 

increases in tidal exchange. Specifically, restored tidal 

flow could increase ground-water levels, compromising 

onsite septic systems, or disturb the freshwater/saltwater 

interface, affecting domestic wells in the adjacent developed uplands. The Cape Cod Commission, however, 

determined that removal of the tide-control structure on 

the Pamet River would not substantially affect the surrounding areas (Eichner and others, 1997); the NPS and 

USGS have determined a similar result for the Herring 

River (Martin, 2004; Masterson, 2004).

In the case of the Herring River, results of hydrodynamic and salinity modeling (Spaulding and Grilli, 2001) 

indicate that the most likely scenario for the removal 

of the tide-control structure at the mouth of the Herring River in Wellfleet (fig. 12) will result in increased 

tidal exchange in the river and a landward flooding of 

seawater less than 2 ft above the current mean tide. 

Ground-water-flow modeling results (Masterson, 2004), 

which include the predicted increase in stream stage and 

landward encroachment of saline surface waters, indicate 

no appreciable effect on the depth to the freshwater/saltwater interface in the areas adjacent to the river. 

Sea-Level Rise

Residents of coastal areas are increasingly concerned 

about the effects of sea-level rise. The National Oceanic 

and Atmospheric Administration (2003) reports a rising 

trend in sea level at the Boston Harbor Tide Gage, which 

has been operated since 1921, of about 0.104 in/yr or 

about 0.87 ft per 100 years. The Intergovernmental 

Panel on Climate Change (IPCC) predicts that global 

seas may rise by an additional 0.5 to 3.1 ft by the year 

2100, with a best estimate of 1.6 ft (Intergovernmental 

Figure 12. Tide-control structures open at low tide to allow freshwater to flow to the coast and close at high 

tide to prevent saltwater from flooding original salt marshes, such as in the Herring River Basin. The ecologically damaging effects of tide restrictions are prompting tide-restoration projects on Cape Cod and throughout 

the world.

Photos courtesy of Britt Argow, National Park Service, Cape Cod National Seashore;inset by Bruce Hansen, U.S. Geological Survey
Page 14 

Panel on Climate Change, 2001). This rate of rise would 

be nearly double the rate of rise observed at Boston 

Harbor over the past 80 years. Areas most susceptible to 

rising sea levels are coastal aquifers, such as Cape Cod, 

which raises concerns of potential effects of sea-level 

rise on coastal habitats and drinking-water supplies 

(Intergovernmental Panel on Climate Change, 2001; 

Moore and others, 1997).

Sea-Level Rise and Coastal Habitats

The primary concerns about the effects of sea-level 

rise on coastal habitats include increased erosion rates, 

damage from higher storm-surge flooding (Gornitz, 

1991; Theiler and Hammar-Klose, 2000), and landward 

intrusion of seawater in coastal wetlands (Donnelly and 

Bertness, 2001). Damage from increased storm-surge 

flooding is a concern on Lower Cape Cod because of 

the potentially severe effects these storms can have on 

the coastal areas along the CCNS. During the “Perfect 

Storm” of October 31, 1991, the storm surge breached 

the barrier dune separating the Pamet River from the 

Atlantic Ocean and forced seawater up to several feet 

deep onto the low-lying flood plain (fig. 13). Tidal 

restrictions near the mouth of the Pamet River (fig. 1) 

since 1869 have converted the upper Pamet River plant 

community from salt marsh to salt-intolerant wetland 

species. After the breach in 1991, seawater persisted 

for over a week in the river proper, and for months in 

the sediment. Extensive mortality of current freshwaterwetland vegetation was averted only because plants were 

dormant at that time (Portnoy, 2001).

Sea-Level Rise and Drinking-Water Supplies 

A concern for Lower Cape Cod water managers is 

the potential adverse effect of sea-level rise on the depth 

to the freshwater/saltwater interface near public-supply 

wells (fig. 11). Pumping from public-supply wells in 

coastal aquifers underlain by saltwater can lower the 

water table relative to sea level, decrease the depth to 

the freshwater/saltwater interface beneath the pumping 

well, increase the potential for saltwater intrusion, and 

thereby limit the yield of potable water from the well. 

Successive increases in the pumping rate can decrease 

the depth of the freshwater/saltwater interface until a 

critical distance between the well screen and the top of 

the interface is achieved, at which point saltwater may 

be drawn into the well (fig. 11) (Reilly and Goodman, 

1985; Reilly and others, 1987). 

In the mid-1950s, overpumping of wells in the 

Knowles Crossing well field in North Truro 

(fig. 1) caused saltwater intrusion, creating the need 

for additional water-supply wells (Masterson, 2004). 

In 1955, the Provincetown Water Department began 

pumping from the South Hollow well field in the center 

of the Pamet flow lens to meet the increased water 

demand (fig. 1).

Sea-level rise may exacerbate the effects of increased 

pumping, as was done in the 1950s, and increase the 

potential for saltwater intrusion into a pumping well. As 

sea level rises over time, the water table will rise along 

with it so that the height of the water table relative to sea 

level will not change. Under these conditions, the depth 

to the freshwater/saltwater interface below the water 

table also will not change. However, if the aquifer system were additionally stressed by a high pumping rate, 

the water table near the well may remain constant or 

decline even as sea level rises. In this case, the height of 

the water table above sea level would decrease, as would 

A.

B.

Photo courtesy of Cape Cod National Seashore

Figure 13. Overwash at Ballston Beach, Truro during the 

“Perfect Storm” of October 31, 1991. The storm surge breached 

the barrier dune (A) and forced seawater onto the flood plain to a 

depth of several feet (B). 
Page 15 

the depth to the freshwater/saltwater interface. This 

relation indicates that sea-level rise in coastal aquifers, 

such as Lower Cape Cod, may result in a decrease in 

the potential yields of a pumping well that is subject to 

saltwater intrusion, and may necessitate limiting pumping rates at well fields to prevent saltwater contamination 

of public-water-supply systems. 

Summary 

Ground water is a vital resource on Lower Cape 

Cod because it is the only source of drinking water and 

the primary source of freshwater that discharges into 

kettle-hole ponds, streams, and estuaries. It is also 

a key component in the maintenance of ecologically 

diverse habitats throughout the Cape Cod National 

Seashore. Because of the importance of this resource, 

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 Lower Cape 

Cod aquifer system and to assess possible hydrologic 

effects of proposed water-management strategies. 

This report briefly details the hydrogeology of the 

Lower Cape Cod aquifer system and discusses the 

importance of ground-water levels and flow to the 

functioning of freshwater and estuarine ecosystems of 

the Cape Cod National Seashore. It includes a discussion 

of the implications of future changes to the ground-water 

system and their potential effects on ecosystems and 

water resources of Lower Cape Cod.

Changes on Lower Cape Cod, including land 

development, population growth, and sea-level rise, 

have created concerns for potentially adverse effects 

on the ecology and ground-water flow of the Cape Cod 

National Seashore. The potential effects of increased 

loading of nutrients, such as nitrogen and phosphorus, 

include the eutrophication of fresh and coastal surfacewater bodies. The potential effects of increased 

ground-water withdrawals include declines in pond 

levels, increases in the depth to the water table beneath 

inland wetlands, reductions in streamflow, reductions 

in ground-water discharge to the coast, and increased 

saltwater intrusion. Finally, the potential effects of 

rising sea levels include increased erosion rates, damage 

from higher storm-surge flooding, landward intrusion of 

seawater in coastal marshes and wetlands, and a decrease 

in the thickness of the freshwater lens.

Acknowledgments

The authors thank Kathryn Hess of the U.S. Environmental Protection Agency, and Larry Martin and Charlie 

Roman of the National Park Service for their helpful 

reviews of this publication.

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For more information about this or other USGS investigations in Massachusetts, 

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Page 1 photo courtesy of Cape Cod National Seashore. Graphic design by Christine Mendelsohn