Science for the Stewardship of the Groundwater 

Resources of Cape Cod, Massachusetts

Introduction

Groundwater is the sole source of drinking water and 

a major source of freshwater for domestic, industrial, and 

agricultural uses on Cape Cod, Massachusetts. Groundwater 

discharged from aquifers also supports freshwater pond and 

stream ecosystems and coastal wetlands. Six hydraulically 

distinct groundwater-flow systems (lenses) have been delineated 

on Cape Cod (figs. 1 and 2). Of the approximately 450 million 

gallons per day of water that enters these lenses as recharge 

from precipitation, about 69 percent discharges directly to 

the coast, about 24 percent discharges to streams, and almost 

7 percent is withdrawn by public-supply wells (Masterson 

and Walter, 2009). In most areas, groundwater in the sand and 

gravel aquifers is shallow and susceptible to contamination 

from anthropogenic sources and saltwater intrusion. Continued 

land development and population growth on Cape Cod have 

created concerns that potable water will become less available 

and that the quantity and quality of water flowing to natural 

discharge areas such as ponds, streams, and coastal waters will 

continue to decline.

The U.S. Geological Survey (USGS) has been investigating groundwater and surface-water resources on Cape Cod for 

more than 50 years. Recent studies highlighted in this fact sheet 

have focused on the sources of water to public-supply wells, 

ponds, streams, and coastal areas; the transport and discharge 

of nitrogen derived from domestic and municipal disposal of 

wastewater; and the effects of climate change on groundwater 

and surface-water resources. Other USGS activities include 

long-term monitoring of groundwater and pond levels and field 

research on groundwater contamination at the USGS Cape Cod 

Toxic Substances Hydrology Research Site (http://ma.water.

usgs.gov/MMRCape/) near the Joint Base Cape Cod (JBCC), 

formerly the Massachusetts Military Reservation.

Sources of Groundwater to Public-Supply Wells

Cape Cod has seen rapid population growth during the 

past several decades; the year-round population more than 

doubled between 1970 and 2010. This growth has led to 

increased demands on water resources and concerns about 

declines in water quality. A sound scientific understanding of 

the groundwater hydrology of Cape Cod is essential for managing its freshwater resources. Determinations of the sources of 

groundwater to pumping wells and natural discharge areas are 

critical to this scientific understanding. In unconfined aquifer 

systems, in which the water table is the upper boundary of the 

saturated zone, the sources of groundwater are determined by 

groundwater recharge areas, which are the unique areas at the 

water table that contribute water to individual wells, streams, 

Base from U.S. Geological Survey and Massachusetts Geographic Information System, 

Massachusetts State Plane Coordinate System, mainland zone

70°30' 70°

42°

41°30'

20

40

30

10

50

60

30

10

20

5

10

5

5

Figure 3

Line of equal simulated water

level—in feet above National

Geodetic Vertical Datum of 1929.

Contour interval is variable

EXPLANATION

10

Provincetown

Figure 4

Figure 5

Figure 7

Figure 9

Figure 8

Cape Cod

National 

Seashore

Nantucket Sound

Atlantic Ocean

Buzzards Bay

Cape Cod Bay

Cape Cod aquifer

0

0 8 KILOMETERS

4 8 MILES

4

MA

73° 71°

43°

41° 0 25 50 MILES

0 25 50 KILOMETERS

Map area

CT RI

NY

VT NH

Figure 1. Location of Cape Cod, Massachusetts, and simulated 

water-table contours in the Cape Cod aquifer system for hydrologic 

and water-use conditions from 2000 through 2005. The water table 

in these unconfined sand and gravel aquifers forms a series of 

mounds that range in altitude from about 6 to 70 feet above sea 

level. Groundwater flows radially outward from the top of each 

mound toward the coast. Figure modified from Masterson and 

Walter (2009).

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2014–3067

September 2014
2

ponds, and coastal waters (fig. 2). Groundwater recharge areas are affected by pumping and 

recharge stresses on the aquifer. Changes in these 

stresses can result in changes to groundwater-flow 

rates and directions, thereby altering recharge 

areas and traveltimes and potentially affecting 

the quantity and quality of water discharging to 

surface water and public-supply wells.

A series of USGS studies has delineated 

recharge areas to public-supply wells and natural discharge areas and determined the effects of 

pumping wells on groundwater and pond levels, 

streamflows, and rates of discharge to coastal 

waters (http://ma.water.usgs.gov/projects/capegwresources/). In most of these studies, regional 

groundwater-flow models were developed to 

delineate recharge areas to these features under 

current and potential future pumping scenarios. 

These studies have demonstrated that small shifts 

in hydraulic gradients caused by changes in rates 

of groundwater pumping or recharge can significantly affect flow directions and, consequently, 

the sources of water to public-supply wells and 

natural discharge areas. The regional groundwater-flow models have proven to be useful tools 

for water-resource managers, State and local 

regulators, and other stakeholders for evaluating 

the hydrologic effects of changing water-supply 

and wastewater-disposal practices on Cape Cod. 

Local-scale models also have been developed to 

investigate public-supply wells, ponds, and contaminant plumes in specific areas of concern (fig. 

3). Many of the local-scale studies have focused 

on areas near the JBCC in support of environmental restoration efforts by the Department 

of Defense.

Cape Cod Bay

Atlantic 

Ocean

Nantucket Sound

EXPLANATION

Recharge areas to

Coastal areas

Streams

Ponds

Wells

Sagamore lens Monomoy lens

Pamet

lens

Pilgrim 

lens

Chequesset lens

Nauset

lens

0

0 8 KILOMETERS

8 MILES

70°30' 70°

42°

41°30'

Base from U.S. Geological Survey and Massachusetts Geographic Information System, 

Massachusetts State Plane Coordinate System, mainland zone

Figure 2. Simulated groundwater recharge areas on Cape Cod, Massachusetts, 

delineated by the use of numerical groundwater-flow models for average pumping 

and recharge conditions from 2000 through 2005. The sizes and shapes of these areas 

can change in response to changes in pumping and recharge conditions. Figure 

modified from Masterson and Walter (2009).

30

40

50

60

10

20

20 years

40 years

30 years

60 years

50 years

10 years

1,000 METERS

0

0

5,000 FEET 

Cape Cod Canal

70°36'42" 70°31'54"

41°42'22"

41°45'01"

EXPLANATION

20

Area of 68-percent confidence

Area of 95-percent confidence

Area of 99-percent confidence

Joint Base Cape Cod

Areas of confidence

Simulated line of equal water-table

altitude—Contour interval is 5 feet.

Datum is National Geodetic Vertical Datum of 1929

Simulated particle track

Simulated particle position

Soil contamination area

Base from U.S. Geological Survey digital data, 1991, 1:24,000

Universal Transverse Mercator projection, Zone 19

Figure 3. Simulated groundwater flow path from a known soil contamination area on the Joint Base Cape Cod 

toward a discharge area near the Cape Cod Canal, Massachusetts. Particles are used to visualize the trajectory of the 

simulated flow path. The colored bands represent confidence intervals associated with the simulated flow path. Figure 

modified from Walter and LeBlanc (2008). 
3

The USGS is currently [2014] using an extensive hydrologic dataset and the most recent generation of modeling tools 

to develop an updated regional groundwater-flow model of 

western Cape Cod. For improved development and calibration, the model also incorporates information on the locations 

of contaminant plumes on the JBCC. Simulations from this 

updated model could be used to investigate potential effects of 

plume restoration on contributing areas to public-supply wells 

in neighboring towns.

Transport and Discharge of Nitrogen Derived From 

Wastewater Disposal

Much of the densest development on Cape Cod is near the 

coast (fig. 4). As a result, coastal waters are at risk for eutrophication from excess nutrients originating onshore. Nitrogen is 

the nutrient of greatest concern for marine ecosystems. Excess 

nitrogen degrades water quality, reduces water clarity, increases 

the frequency of harmful algal blooms, and results in the loss 

of marine habitat (fig. 5). Resource managers and citizens are 

increasingly concerned about eutrophication and habitat loss in 

coastal waters, which are ecologically, recreationally, and economically important. Excess nitrogen in groundwater can also 

adversely affect drinking-water supplies.

Wastewater is the largest source of nitrogen in developed 

areas. Many towns with dense residential development rely on 

septic systems for wastewater disposal, and some of these communities are considering wastewater-management plans that 

involve sewering and centralized disposal of treated wastewater. 

The USGS has used hydrologic data and groundwater models to 

investigate nitrogen transport to public-supply wells and natural 

discharge areas at a variety of spatial scales and locations on 

Cape Cod. Regional models for central and western Cape Cod 

have been used to determine wastewater traveltimes from 

source areas to wells and sensitive ecological receptors, such as 

streams, ponds, and coastal waters, to estimate rates of wastewater discharge to these receptors, and to evaluate the effects 

of possible wastewater-disposal scenarios for addressing excess 

groundwater nitrogen. These modeling studies generally show 

that nitrogen discharge to sensitive receptors can be reduced 

by effective wastewater-disposal planning. However, because 

groundwater traveltimes between recharge and discharge areas 

can exceed 100 years on Cape Cod, a long time can elapse 

before the effects of management actions become evident.

On a local scale, studies have investigated the transport and discharge of nitrogen to individual salt marshes and 

estuaries on Cape Cod, including protected areas in the Cape 

Cod National Seashore (fig. 1). Local studies can examine in 

detail the biogeochemical reactions that increase or decrease 

nitrogen concentrations along groundwater flow paths and at 

interfaces of groundwater and surface water. The USGS and the 

U.S. Environmental Protection Agency are jointly investigating [2014] the transport of nitrogen derived from onsite septic 

systems to several saltwater embayments in Falmouth, Massachusetts (fig. 4). The objective of the study is to develop a better 

understanding of the effects of nitrogen discharge from residential wastewater-disposal systems on coastal waters.

Effects of Climate Change on Groundwater Resources

The potential effects of climate change are of particular 

concern on Cape Cod with its extensive coastline and fresh 

groundwater-flow systems underlain by saltwater (fig. 6). Current estimates are that by 2100 the sea level could be about 1 to 

6 feet higher than the current sea-level position, and precipitation rates in the northeastern United States could increase by as 

much as 20 percent (Massachusetts Executive Office of Energy 

and Environmental Affairs, 2011). Although researchers and 

other stakeholders have focused much attention on increased 

rates of shoreline erosion and flooding from severe coastal 

storms and inundation of coastal areas, less consideration has 

been given to the potentially widespread effects of changes in 

Figure 4. Coastal development in Falmouth, Massachusetts. 

Residential septic systems and lawn fertilization in this densely 

populated area have produced high concentrations of nitrogen 

in groundwater and coastal waters. Photograph by Joseph 

R. Melanson, Aerials Only (http://www.skypic.com/), used 

with permission.

Figure 5. Algal growth in Nauset Bay, Cape Cod National 

Seashore, Eastham, Massachusetts. Residential septic systems 

are a major source of excess nitrogen in the groundwater that 

discharges to the bay. Photograph by John Colman, USGS, 2005.
4

sea level and aquifer recharge on the water resources of coastal 

aquifers, including areas well inland from the coast. Potential 

effects of changes in sea level and aquifer recharge include the 

loss of potable water supplies from increased saltwater intrusion 

into the aquifer; subsurface flooding of infrastructure, including 

septic systems, from an increase in the height of the underlying water table (fig. 7); and potentially detrimental changes to 

freshwater habitats in low-lying areas.

Recent USGS studies have focused on changes in the 

groundwater-flow system and the position of the interface 

between freshwater and saltwater that can result from future 

changes in sea level. Preliminary analyses based on regional 

groundwater-flow modeling indicate that rising sea levels 

will result in increases in water-table heights inland from the 

shoreline, increases in groundwater discharge to streams, and 

saltwater intrusion into coastal aquifers. As the potential effects 

of climate change on precipitation and sea-level rise in the 

Northeast are better understood, future studies incorporating 

climate-change predictions will be able to provide more reliable 

assessments of the effects of climate change on groundwater 

resources on Cape Cod. Such assessments will be important 

for the development of strategies for adapting to the effects of 

sea-level rise.

Groundwater Contamination Research at the Cape 

Cod Toxic Substances Hydrology Research Site

The Cape Cod Toxic Substances Hydrology Research Site 

is near the former wastewater-treatment facility at the JBCC. 

A plume of treated wastewater originates from the facility’s 

infiltration beds, which were used from 1936 through 1995 (fig. 

8). The plume extends nearly 4.5 miles from the disposal site 

in the sand and gravel aquifer and contains a complex mixture 

of phosphate, nitrate, metal ions, detergents, and other organic 

Cape Cod

Bay

Atlantic Ocean

Water table

Lake

Bedrock

SCHEMATIC DIAGRAM, NOT TO SCALE

Fresh

groundwater

Sea level

-100

-200

-300

-400

-500

Altitude, in feet

SALINE

GROUNDWATER

Saline

groundwater

Figure 6. Schematic diagram of the 

groundwater-flow system on the outer 

part of Cape Cod, Massachusetts, where 

fresh groundwater is underlain by saline 

groundwater. On the inner part of Cape Cod, 

closer to the mainland, fresh groundwater 

extends to the bedrock surface, and the 

interface between freshwater and saltwater 

is confined to areas near the coast. Modified 

from Masterson and Walter (2009).

1975 1980 1985 1990 1995 2000 2005 2010 2015

2

3

4

5

6

7

8

Altitude of water table, in feet above 

National Geodetic Vertical Datum of 1929

Year

Figure 7. Water-table altitude at U.S. Geological Survey 

observation well MA–PZW 78, Provincetown, Massachusetts, from 

1975 through 2013. Water levels (shown by the blue line on the 

graph) have risen at an average rate of about 0.15 inch per year 

since 1975, possibly as a result of sea-level rise. The well is 4,300 

feet from Cape Cod Bay. The trend line is a linear fit to monthly 

groundwater-level data.
5

compounds. The wastewater plume and the surrounding aquifer 

serve as an ideal field laboratory for investigating contaminant 

transport in groundwater.

Knowledge gained from research at the Cape Cod Toxic 

Substances Hydrology Research Site is used widely by waterresource managers, regulators, educators, and scientists. 

Research results and field methods developed during more than 

30 years by the USGS and other scientists studying the wastewater plume and surrounding aquifer also have been applied to 

other groundwater investigations on Cape Cod, including investigations of the reactive transport of other contaminant plumes 

on the JBCC, the fate of pharmaceutical and personal care products in groundwater, and the discharge of nutrient-rich groundwater to freshwater ponds and saline coastal waters (fig. 9).

Long-Term Water-Level Monitoring

Long-term records of water levels have been invaluable 

for developing a better understanding of the dynamics of the 

hydrologic system on Cape Cod and documenting long-term 

trends in groundwater and pond levels. In areas not affected 

by pumping or wastewater discharge, long-term data can be 

used to determine whether groundwater levels are responding 

to changes in climate (fig. 7). Long-term records also provide 

estimates of water levels that are lower and higher than average 

that can be expected during droughts and wet periods. These 

records have helped in the proper design of septic systems. The 

historical water-level data are also a critical input in the calibration of groundwater-flow models that enable predictive analysis 

of the response of the groundwater system to future changes in 

land use and climate.

Since the mid-1960s, the USGS and its partners at the 

Cape Cod Commission (CCC), the Association to Preserve 

Cape Cod (APCC), the Massachusetts Departments of Environmental Protection and Conservation and Recreation, and the 

National Park Service have monitored water levels in wells and 

ponds on Cape Cod. The original mid-1960s network of about 

13 wells was supplemented in the mid-1970s with 12 ponds 

monitored by APCC volunteers (through about 2000) and 40 

wells monitored by the CCC (ongoing). In the early 2000s, the 

USGS began monitoring water levels in 5 additional wells and 

1 pond with support from the environmental cleanup agencies at 

the JBCC. Water levels are measured monthly to continuously 

in the wells and ponds (http://groundwaterwatch.usgs.gov/

StateMaps/MA.html). Water-level data are used widely by the 

water-resources community, including resource managers, State 

and local regulators, educators, and scientists.

Base from U.S. Geological Survey digital data

Massachusetts State Plane Coordinate System 

North American Datum of 1927

8

6

38

34

32

10

28

26

24

22

20

18

12

44

14

14

12

16

18

42

40

46

4

48

48

46

50

50

52

2

24

26

28

30

20

22

10

8

32

34

36

38

40

42

44

36

30

16

Open water or

 cranberry bog

Water-table contour—

 Altitude of simulated

 steady-state water table

 in feet above National

 Geodetic Vertical Datum

 of 1929

10

Sampling site

Nitrate concentration,

 in milligrams per liter

 as nitrogen

 0.0 to 0.4

> 0.4 to 0.8

> 0.8 to 2.0

> 2.0 to 2.8 

> 2.8 to 4.4

> 4.4 

Treated-wastewater

 infiltration bed

EXPLANATION

41°38'

41°36'

70°34' 70°32'

0 0.5 1 MILE

0 0.5 1 KILOMETER

Joint Base Cape Cod

Figure 8. The nitrate plume created by 60 years of treated 

wastewater disposal on Joint Base Cape Cod. The plume, shown 

here at 55 feet below sea level, was approximately 4.5 miles in 

length and near areas of coastal discharge in 2007. Figure modified 

from Barbaro and others (2013). 
6

Figure 9. U.S. Geological Survey (USGS) field team collecting 

samples to measure concentrations of nitrogen in shallow 

groundwater discharging to Eel Pond, Falmouth, Massachusetts. 

Photograph by Denis LeBlanc, USGS, 2013.

Acknowledgments

Major funding for the studies described in this fact sheet 

was provided by the National Park Service, the U.S. Air Force 

and Army National Guard, the Massachusetts Departments of 

Environmental Protection and Conservation and Recreation, 

and the Cape Cod Commission. The authors also recognize 

the support and interest of many local and regional agencies 

and groups.

Selected References

Barbaro, J.R., Walter, D.A., and LeBlanc, D.R., 2013, Transport 

of nitrogen in a treated-wastewater plume to coastal discharge 

areas, Ashumet Valley, Cape Cod, Massachusetts: U.S. Geological Survey Scientific Investigations Report 2013–5061, 

37 p., http://pubs.usgs.gov/sir/2013/5061/.

Colman, J.A., and Masterson, J.P., 2008, Transient simulations 

of nitrogen load for a coastal aquifer and embayment, Cape 

Cod, MA: Environmental Science & Technology, v. 42, no. 1, 

p. 207–213, http://dx.doi.org/10.1021/es070638b.

Massachusetts Executive Office of Energy and Environmental Affairs, 2011, Massachusetts climate change adaptation 

report—September 2011: Massachusetts Executive Office 

of Energy and Environmental Affairs Web page, 121 p., 

accessed June 19, 2014, at http://www.mass.gov/eea/wastemgnt-recycling/air-quality/green-house-gas-and-climatechange/climate-change-adaptation/climate-change-adaptation-report.html.

Masterson, J.P., 2004, Simulated interaction between freshwater 

and saltwater and effects of ground-water pumping and sealevel change, Lower Cape Cod aquifer system, Massachusetts: U.S. Geological Survey Scientific Investigations Report 

2004–5014, 72 p., http://pubs.usgs.gov/sir/2004/5014/.

Masterson, J.P., and Garabedian, S.P., 2007, Effects of sea-level 

rise on ground water flow in a coastal aquifer system: Ground 

Water, v. 45, no. 2, p. 209–217, http://dx.doi.org/10.1111/

j.1745-6584.2006.00279.x.

Masterson, J.P., and Walter, D.A., 2009, Hydrogeology and 

groundwater resources of the coastal aquifers of southeastern 

Massachusetts: U.S. Geological Survey Circular 1338, 16 p., 

http://pubs.usgs.gov/circ/circ1338/.

Walter, D.A., 2013, The simulated effects of wastewatermanagement actions on the hydrologic system and nitrogenloading rates to wells and ecological receptors, Popponesset 

Bay Watershed, Cape Cod, Massachusetts: U.S. Geological 

Survey Scientific Investigations Report 2013–5060, 62 p., 

http://pubs.usgs.gov/sir/2013/5060/.

Walter, D.A., and LeBlanc, D.R., 2008, Use of inverse-modeling methods to improve ground-water-model calibration 

and evaluate model-prediction uncertainty, Camp Edwards, 

Cape Cod, Massachusetts: U.S. Geological Survey Scientific 

Investigations Report 2007–5257, 57 p., http://pubs.usgs.gov/

sir/2007/5257/.

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., http://pubs.usgs.gov/sir/2004/5181/.

Office Chief

Massachusetts-Rhode Island Office

New England Water Science Center

U.S. Geological Survey

10 Bearfoot Road

Northborough, MA 01532

dc_ma@usgs.gov

or visit our Web site at

http://ma.water.usgs.gov

For additional information, contact:

By Jeffrey R. Barbaro, John P. Masterson, 

and Denis R. LeBlanc

This fact sheet was prepared with support 

from the Toxic Substances Hydrology and 

Groundwater Resources Programs of the 

U.S. Geological Survey.

Further information about the groundwater resources

of Cape Cod is available at

http://ma.water.usgs.gov/projects/capegwresources/

ISSN 2327-6916 (print)

ISSN 2327-6932 (online)

http://dx.doi.org/10.3133/fs20143067