U.S. Department of the Interior

U.S. Geological Survey

128

2

2A

93

90

95

95

1A 1

9

20

30

117

93

HOBBS BROOK

RESERVOIR

SUBBASIN

STONY BROOK

RESERVOIR

SUBBASIN

PAYSON PARK

RESERVOIR

CHARLES

RIVER CHARLES

RIVER

PIPELINE TO FRESH PON D

STONY BROOK

RESERVOIR

42 ̊

73 ̊ 72 ̊ 71 ̊

70 ̊

MASSACHUSETTS

41 ̊ 30'

42 ̊ 30'

0

0

50 KILOMETERS

50 MILES

N

0

0

10 MILES

10 KILOMETERS

CAMBRIDGE

BROOKLINE

BOSTON

NEWTON

WALTHAM

LINCOLN

LEXINGTON WINCHESTER

WESTON

SOMERVILLE

MEDFORD ARLINGTON

BELMONT

WATERTOWN

FRESH

POND

HOBBS BROOK

RESERVOIR

Study

area

highest quality water is delivered to the 

treatment plant. A key element of this plan 

is a program that combines systematic 

monitoring of the drainage basin with 

detailed investigations of the effects of 

nonpoint-source contaminants, such as 

highway-deicing chemicals, nutrients, 

oxygen-demanding organic compounds, 

bacteria, and trace metals arising from 

stormwater runoff. The U.S. Geological 

Survey (USGS) is working with the CWD 

and the Massachusetts Highway 

Department (MassHighway) to develop a 

better understanding of the sources, 

transport, and fate of many of these 

contaminants (fig. 1). This Fact Sheet 

describes source-water protection and 

water-quality investigations currently 

underway in the Cambridge drinkingIntroduction 

The Cambridge Water Department 

(CWD) supplies about 15 million gallons 

of water each day to more than 95,000 

customers in the City of Cambridge, 

Massachusetts. Most of this water is 

obtained from a system of reservoirs 

located in Cambridge and in parts of five 

other suburban-Boston communities. 

The drainage basin that contributes water 

to these reservoirs includes several 

potential sources of drinking-water 

contaminants, including major highways, 

secondary roads, areas of commercial 

and industrial development, and 

suburban residential tracts. The CWD is 

implementing a comprehensive SourceWater Protection Plan to ensure that the 

water supply system. The investigations 

are designed to complement a national 

effort by the USGS to provide water 

suppliers and regulatory agencies with 

information on the vulnerability of 

water supplies and the movement and 

fate of source-water contaminants 

(Patterson, 1997). 

The Cambridge 

Drinking-Water

Supply System 

The Cambridge, Massachusetts 

drinking-water supply system consists 

of Hobbs Brook and Stony Brook 

Reservoirs, which drain a 15,200-acre 

(6,200-hectare) basin in Lexington, 

Lincoln, Waltham, and Weston, 

USGS Fact Sheet 101-98

September, 1998 

Figure 1. U.S. Geological Survey and 

Cambridge Water Department staff collect 

water-quality information at a continuousrecord streamflow and water-quality 

monitoring station located on a tributary 

to Hobbs Brook Reservoir near Waltham, 

Massachusetts.

Figure 2. Map showing the location, extent, and components of the City of Cambridge 

drinking-water supply system, eastern Massachusetts.

Source-Water Protection and Water-Quality Investigations in the 

Cambridge, Massachusetts, Drinking-Water Supply System

Ongoing investigations of the quality of source water in the Cambridge, Massachusetts, drinking-water supply 

system will help the Cambridge Water Department (CWD) deliver high-quality drinking water at a reasonable 

cost. When completed, the investigations will provide the CWD with a better understanding of the sources and 

quantities of contaminants entering the water supply. This information will help the CWD implement costeffective watershed-management and water-quality-monitoring procedures as part of the Department’s Source 

Water Protection Plan.
Weston

Lincoln

Lexington

Waltham

Concord

Hobbs Brook

Trapelo Road

128 Cherry Brook

Stony Brook

Hobbs Brook

2A

2

95

Hobbs Brook

Reservoir

Stony Brook

Reservoir

20

Sandy

Pond

117

N

0

0

1

1

2 MILES

2 KILOMETERS

Base from U.S. Geological Survey NMD, MassGIS, Cambridge Water

Department, Harvard Design and Mapping, and Boston Edison. This

map built by USGS-WRD personnel in Marlborough, MA, April 1998. 

EXPLANATION

SUBBASINS

Hobbs Brook

Reservoir Subbasin

Stony Brook

 Reservoir Subbasin

FORESTED AND

NONFORESTED WETLANDS

BASIN BOUNDARY

SUBBASIN BOUNDARY

CONTINUOUSLY

MONITORED STATIONS

MANUALLY SAMPLED

STATIONS

71 ̊15'

42 ̊

22'

30"

71 ̊20'

42 ̊

27'

Figure 3. Hydrography, subbasin boundaries, streamflow and water-quality monitoring stations, wetlands, major highways, and 

secondary roads in the Cambridge, Massachusetts, drinking-water source area.
Lexington

Weston

Waltham

Waltham

EXPLANATION

RESERVOIR BATHYMETRYDepth in feet at full capacity

0-10

11-20

21-30

31-35

A

B

Hobbs Brook 128

Hobbs Brook

95

128

95

Hobbs Brook

Reservoir

Stony Brook

Stony Brook

Reservoir

0

0

1MILE

1 KILOMETER

N

N

0

0

1/4 MILE

.25 KILOMETER

FOREST

RESIDENTIAL

OPEN LAND

CROPLAND AND PASTURE

NON-FORESTED WETLANDS

COMMERCIAL

TRANSPORTATION

INDUSTRIAL

OTHER

WATER

EXPLANATION

128

2A

2

95

Hobbs Brook

Reservoir

Stony Brook

Reservoir

20

Sandy

Pond

117

71 ̊15'

42 ̊

21'

42 ̊

22'

30"

0

0

1

1

2 MILES

2 KILOMETERS

71 ̊20'

42 ̊

27'

Massachusetts, and Fresh Pond, a 155-

acre (63-hectare) kettle lake located in 

Cambridge (fig. 2). Hobbs Brook 

Reservoir is fed by several tributaries 

and by storm drains associated with 

State Routes 128 and 2, secondary 

roads, and commercial and industrial 

parking lots. Stony Brook Reservoir is 

fed by drainage from the Stony Brook 

subbasin, by the outflow from Hobbs 

Brook Reservoir, and by other small 

tributaries. The CWD pipes water from 

Stony Brook Reservoir to Fresh Pond, 

where it is stored prior to treatment. 

After treatment, the finished water is 

pumped to the Payson Park Reservoir in 

Belmont, Massachusetts, from which it 

flows by gravity through a 190-mile 

distribution system. Overflow from 

Stony Brook Reservoir flows into the 

Charles River near Waltham, 

Massachusetts. 

The primary source area for the 

Cambridge water supply varies 

seasonally. The larger Stony Brook 

subbasin (fig. 3) provides most of the 

water that is piped to Fresh Pond during 

periods of high flow (mainly winter 

and spring). During low-flow periods 

(summer and autumn), most of the 

supply comes from Hobbs Brook 

Reservoir. Thus differences in 

contaminant loading to streams in the 

two subbasins could result in 

seasonal differences in source-water 

quality.

Recent bathymetric surveys 

(Fugro East, Inc., 1996) of the two 

main reservoirs (fig. 4) indicate that 

the surface area of Hobbs Brook 

Reservoir, when filled to capacity, is 

580 acres (235 hectares) and that the 

average depth of the reservoir is 12.6 

feet (3.8 meters). The reservoir’s 

storage capacity is about 317 million 

cubic feet (9 million cubic meters) 

Figure 4. Bathymetric maps of (A) Hobbs 

Brook Reservoir, located in Lexington, 

Lincoln, Waltham, and Weston, 

Massachusetts, and (B) Stony Brook 

Reservoir, located in Waltham and Weston, 

Massachusetts.

Figure 5. Land use and land cover in the Cambridge, Massachusetts, drinking-water source 

area.
Cropland & pasture (5%)

Nonforested

wetlands (4%)

Water (6%)

Forest (40%)

Residential (30%)

Open Land (5%)

Commercial (3%)

Industrial (2%)

Transportation (3%) Other (2%)

(Fugro East, Inc., 1996). Stony Brook 

Reservoir has a maximum surface area 

of 66 acres (27 hectares), an average 

depth of 14.3 feet (4.4 meters), and a 

storage capacity of about 41 million 

cubic feet (1.2 million cubic meters). 

The average time required for a 

complete exchange of the water in 

Hobbs Brook Reservoir is about two 

years. In contrast, the average 

exchange time in Stony Brook 

Reservoir is only 17 to 21 days (Fugro 

East, Inc., 1996). The longer storage 

period in Hobbs Brook Reservoir may 

improve source-water quality by 

effectively trapping particle-bound 

contaminants within the reservoir and 

by providing more time for chemical 

and biological processes to remove 

dissolved contaminants from the water. 

 The drainage basin for the 

Cambridge drinking-water supply is 

located in moderately to heavily 

developed suburban areas (fig. 5). 

About 40 percent of the basin is 

forested (including forested wetlands) 

and 30 percent is residential (fig. 6). 

The residential areas consist of about 

85 percent low-density housing (lots 

larger than one-half acre), 10 percent 

medium-density housing (lots onefourth to one-half acre), and five 

percent high-density and multi-family 

housing (lots smaller than one-fourth 

acre). About three percent of the basin 

is used for commercial activities and 

two percent is considered industrial. 

Most of the industrial and commercial 

land, and all of the high-density 

housing, are concentrated in the 

eastern half of the drainage basin close 

to Hobbs Brook and Stony Brook 

Reservoirs (fig. 5). 

Parts of four major State 

highways (Routes 128, 2, 2A, and 20) 

and four interchanges, comprising 90 

lane miles, pass through the basin. (One 

lane mile is a paved road surface one 

mile long and 12 feet wide.) In addition, 

about 43 lane miles of secondary roads 

are maintained by municipalities within 

the basin. About three percent of the 

drainage basin is devoted to 

transportation, including highway 

maintenance facilities and interchanges. 

Five percent of the basin is cropland and 

pasture, six percent is water, including 

the two drinking-water supply reservoirs 

and numerous other lakes and ponds, 

and five percent is open land, including 

public parks, fields, and powerline rightof-ways. More than 6,500 acres (2,600 

hectares) of nonforested wetlands cover 

about four percent of the total basin area. 

The largest concentration of wetlands is 

in the western half of the drainage basin 

(fig. 3), which also includes most of the 

low-density residential areas. Other land 

uses, such as mining operations and 

landfills, occupy one percent or less of 

the total basin area.

Source-Water

Protection Activities

 Because the CWD owns less than 

five percent of the drainage basin that 

contributes water to Hobbs Brook and 

Stony Brook Reservoirs, protection of 

the Cambridge water supply presents a 

difficult challenge. Lack of land 

ownership and regulatory authority 

limit the range of watershedmanagement alternatives, and the 

high cost of land in the basin 

precludes future land acquisitions. In 

addition, Fresh Pond Reservation, the 

City-owned area surrounding Fresh 

Pond, is the largest and most heavily 

used recreational open space in 

Cambridge. The CWD is 

implementing an innovative Source 

Water Protection Plan to meet these 

management challenges. The plan 

includes monitoring streamflow and 

water quality in the drainage basin 

(fig. 7), tracking development and 

construction activities, advocating 

for stormwater-management 

improvements, and maintaining a 

hazardous-materials response 

program. 

 A significant part of CWD’s 

Protection Plan is an effort to build 

positive working relationships with 

communities, government agencies, 

and other parties with interests in the 

drainage basin. The CWD sponsors 

the Cambridge Watershed Advisory 

Committee, which includes 

representatives from communities in 

the basin, and has developed active 

partnerships with businesses such as 

GTE Laboratories and Polaroid 

Corporation, State and Federal 

agencies such as MassHighway and 

the USGS, and local municipal 

services such as the City of Waltham 

Fire Department. 

The CWD and the USGS have 

established partnerships with the 

Minuteman Science-Technology 

High School and Adult Career Center 

(Minuteman Tech) located within the 

basin in Lexington, Massachusetts. 

The Environmental Technology 

Program at Minuteman Tech 

prepares high school students for 

careers and advanced training in 

environmental science and 

technology, and is the first such 

program approved by the 

Massachusetts Department of 

Education. With support from the 

CWD, the Environmental 

Technology Program received a grant 

from the Massachusetts 

Environmental Trust to develop 

teaching capabilities in hydrology, 

water-quality monitoring, and 

Figure 6. Distribution of land use and land 

cover in the Cambridge, Massachusetts, 

drinking-water source area.

Figure 7. Cambridge Water Department staff 

member measuring streamflow in an 

ungaged tributary to Stony Brook Reservoir 

near Weston, Massachusetts.
geographic information systems (GIS), 

and to collect environmental data in the 

upper part of the drainage basin. USGS 

scientists regularly conduct workshops 

designed to supplement the curriculum 

with presentations and hands-on training 

in data-collection methods.

Ongoing Water-Quality 

Investigations 

The CWD’s source-water 

protection activities have been 

augmented by several technical projects 

and water-resources investigations. The 

CWD has developed an extensive GIS 

data base for the drainage basin, 

conducted bathymetric surveys of the 

reservoirs (fig. 4), and implemented a 

new water-quality monitoring network 

based on results of a limnological 

investigation of the reservoir system 

(Fugro East, Inc., 1996). The USGS is 

assisting the CWD in reviewing and 

updating the water-quality monitoring 

network and is conducting two largescale investigations of factors that may 

affect source-water quality.

Sources and Loads of Sodium, 

Chloride, and Calcium to 

the Water Supply

 In February 1997, the USGS began 

a multi-year investigation designed to 

identify sources of sodium, chloride, and 

calcium entering Hobbs Brook and 

Stony Brook Reservoirs. Sodium, which 

is a component of road-deicing salt, is 

of concern because it persists in 

treated water and may contribute 

sodium to consumers’ diets. Since 

1986, MassHighway has reduced by 

50 to 70 percent the amount of 

sodium chloride added annually to 

State highways in the drainage basin. 

After more than 10 years of such 

reductions, however, sodium 

concentrations in the reservoirs 

remain unchanged from pre-1986 

levels, indicating that additional 

sources and transport pathways may 

exist for sodium in the drainage 

basin. 

To identify sources of the 

sodium, the USGS, in cooperation 

with MassHighway, established 

continuously monitored stations on 

all major tributary streams and at the 

outlets of Hobbs Brook Reservoir and 

Stony Brook Reservoir (fig. 3). 

Stream and reservoir stage, water 

temperature, and specific 

conductance of the water are 

measured continuously at the 

monitoring stations (fig. 8), and the 

continuous records are correlated 

with periodic measurements of 

streamflow and concentrations of 

sodium, chloride, and calcium. The 

data will be used to determine annual 

loads and subbasin budgets for these 

constituents. The results of these 

analyses will help MassHighway and 

the CWD prioritize watershedmanagement activities and target 

specific subbasins for remediation. 

Nutrient, Manganese, and 

Organic-Carbon Budgets for the 

Water-Supply Reservoirs 

In September 1997, the USGS 

and CWD began an investigation of 

seasonal and annual loads of total 

nitrogen, total phosphorus, 

dissolved manganese, and dissolved 

organic carbon (DOC) to the 

reservoirs. Nitrogen and 

phosphorus, which may enter the 

water supply from nonpoint sources 

such as precipitation, bank erosion, 

fertilizer, and stormwater runoff, 

can stimulate excessive growth of 

algae, causing increased turbidity, 

depletion of dissolved oxygen, and 

mobilization of contaminants from 

reservoir sediments (Cooke and 

others, 1986). Manganese and 

DOC, derived mainly from natural 

sources in the drainage basin, may 

have additional adverse effects on 

the water supply. Dissolved 

manganese can stain laundry and 

ceramic plumbing fixtures; DOC 

can react with chlorine during 

disinfection to produce toxic 

organic by-products (Aiken and 

Cotsaris, 1995; Reckhow and 

others, 1990). 

Water samples for analysis of 

nutrients, dissolved manganese, and 

DOC are collected at each of the 

continuous-record streamflow and 

water-quality monitoring stations

and at additional ungaged stream

and reservoir sites in the drainage 

basin (fig. 9). Constituent 

concentrations measured in samples 

from these sites will be combined

with the extensive hydrologic data 

already being collected at the 

monitoring stations to determine 

annual and seasonal loads of total 

nitrogen, total phosphorus, 

dissolved manganese, and DOC 

entering the reservoirs. The loads 

will then be related to conditions in 

the reservoirs and to changing landuse activities in the drainage basin. 

This information will be used to 

design more effective operating 

strategies and long-term monitoring 

protocols for the reservoirs. 

Figure 8. U.S. Geological Survey continuous-record streamflow and water-quality monitoring 

station located on a tributary to Hobbs Brook Reservoir near Waltham, Massachusetts.
Benefits and 

Transferability

The ongoing USGS investigations 

of source-water quality in the 

Cambridge drinking-water supply 

system will help the CWD continue to 

deliver high-quality drinking water at a 

reasonable cost. When completed, the 

investigations will provide the CWD 

with a better understanding of the 

sources and quantities of contaminants 

entering the drinking-water supply. 

Contaminant loads will be determined 

for each contributing subbasin and 

related to land use, land cover, and 

other subbasin features to identify 

contaminant sources. Information on 

contaminant sources will enable the 

CWD and MassHighway to direct 

costly management and remediation 

activities to appropriate subbasins. 

Cost-effective source-water protection 

efforts applied in this way ultimately 

will lower the long-term costs of water 

treatment and treatment-plant 

maintenance. Results of the ongoing 

investigations also will provide a 

scientific basis for evaluating the 

effectiveness of watershedmanagement practices and waterquality monitoring procedures. 

Data on the spatial and seasonal 

variability of contaminant loads will be 

useful to other water-suppliers, both in 

the region and nationally. Spatial and 

seasonal differences in the types and 

amounts of DOC, for example, may be 

used to help New England water 

suppliers design operating procedures 

and reservoir-withdrawal strategies that 

minimize the formation of undesirable 

by-products of the water-treatment 

process. A better understanding of the 

relations between land use, land cover, 

and contaminant loads in the 

Cambridge drinking-water supply 

system will aid current efforts by State 

and Federal regulatory agencies to 

determine the vulnerability of the 

Nation’s source water to 

contamination. 

Literature Cited 

Aiken, George, and Cotsaris, Evangelo, 

1995, Soil and hydrology--their effect 

on NOM: Journal of the American Water 

Works Association, vol. 87, p. 36-45. 

Cooke, G.D., Welch, E.B., Peterson, S.A., 

and Newroth, P.R., 1986, Lake and 

reservoir restoration: Butterworths, 

Boston, Mass., 392 p. 

Fugro East, Inc., 1996, Limnological 

investigations of the Cambridge 

Reservoir system: impacts of watershed 

inputs and alum sludge discharge: 

Cambridge Water Department, 

Cambridge, Mass., 75 p. + 4 appendices.

 Patterson, Glenn, 1997, The U.S. 

Geological Survey Drinking Water 

Initiative: U.S. Geological Survey Fact 

Sheet 047-97, 2 p. 

Reckhow, D.A., Singer, P.C., and Malcolm, 

R.L., 1990, Chlorination of humic 

materials--by-product formation and 

chemical interpretations: Environmental 

Science and Technology, vol. 24, p. 

1655-1664.

 For more information:

District Chief 

U.S. Geological Survey 

28 Lord Road, Suite 280 

Marlborough, Massachusetts 

01752

Internet: http://mass1.er.usgs.gov

Watershed Manager

City of Cambridge 

Water Department 

250 Fresh Pond Parkway

Cambridge, 

Massachusetts 02138 

Internet: http://www.ci.cambridge.ma.us/

~Water

—Report by: 

Marcus C. Waldron, USGS, with 

contributions from Chip Norton and 

Timothy W. D. MacDonald, Cambridge 

Water Department. 

—Layout, text and graphic design by:

 Mary S. Ashman, Mark V. Bonito, 

Christine T. Mendelsohn, and 

Peter A. Steeves, USGS.

Figure 9. U.S. Geological Survey hydrologist collects water samples from Stony Brook 

Reservoir near Waltham, Massachusetts, and prepares the samples for chemical analysis.