U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2015–3030

April 2015

Prepared in cooperation with the Cambridge, Massachusetts, Water Department

Water Quality in the Cambridge, Massachusetts, 

Drinking-Water Source Area, 2005–8

Printed on recycled paper

are available to the public through the USGS New England 

Water Science Center’s Web page (http://ma.water.usgs.gov) 

and in published reports (for example, Smith, 2005). From 

2004 through 2008, the USGS and CWD characterized water 

quality during base-flow (conditions generally representing only 

groundwater discharge) and storm-runoff conditions in four 

tributaries to the Cambridge and Stony Brook Reservoirs and 

in the main stem of Stony Brook, the primary source of inflow 

to the Stony Brook Reservoir (Smith, 2013). Currently (2015), 

the USGS, in cooperation with the CWD, operates water-quality 

monitoring stations on every major tributary and on the three 

reservoirs in the drinking-water source area.

The City of Cambridge Drinking-Water Supply System

The CWD supplies approximately 15 million gallons per 

day of drinking water to more than 100,000 customers. The 

CWD obtains raw water from a serial system of three primary 

storage reservoirs—Cambridge Reservoir (also known as the 

Hobbs Brook Reservoir), Stony Brook Reservoir, and Fresh 

Pond Reservoir—in parts of Cambridge, Lexington, Lincoln, 

Waltham, and Weston, Mass. (fig. 1). These reservoirs receive 

inflow from a source area of 23.7 square miles. The Cambridge 

drinking-water source area contains major transportation corridors (Interstate 95, State Routes 2, 2A, 20, and 117) as well as 

large impervious areas of industrial, commercial, and residential 

land use (fig. 2). Only about 5 percent of this land is owned 

by Cambridge. The watershed for the Cambridge Reservoir 

includes Hobbs Brook and three unnamed tributaries that 

Abstract

During 2005–8, the U.S. Geological Survey, in cooperation 

with the Cambridge, Massachusetts, Water Department, measured concentrations of sodium and chloride, plant nutrients, 

commonly used pesticides, and caffeine in base-flow and stormwater samples collected from 11 tributaries in the Cambridge 

drinking-water source area. These data were used to characterize current water-quality conditions, to establish a baseline for 

future comparisons, and to describe trends in surface-water 

quality. The data also were used to assess the effects of 

watershed characteristics on surface-water quality and to inform 

future watershed management.

The USGS-Cambridge Water 

Department Partnership

The U.S. Geological Survey (USGS) and the Cambridge, 

Massachusetts, Water Department (CWD) have been working 

together since 1997 on programs designed to ensure that the 

Cambridge drinking-water supply is as free as possible from 

contaminants. Waldron and Bent (2001) assessed limnological 

conditions in the three serial storage reservoirs (Cambridge, 

Stony Brook, and Fresh Pond Reservoirs) and described the 

1997–98 water-quality conditions in the drinking-water source 

area. Subsequently, the USGS and CWD designed and implemented a monitoring network for the tributaries flowing into the 

drinking-water supply reservoirs. Streamflow, water-quality, and 

meteorological data collected by the USGS from this network 

Hobbs Brook

Stony Brook

Capture areas

EXPLANATION

Aqueduct

Flows 

Boston Harbor

Schematic diagram, not to scale

128

2

2A

2A

93

90

95

95

1A

145

1

9

20 16

30

117

93

2

Cambridge

Brookline

BOSTON

Newton

Waltham

Lincoln

Lexington

Winchester

Weston

Somerville

Medford

Arlington

Belmont

Watertown

Cambridge

Reservoir

Watershed

Stony Brook

Reservoir

Watershed Pason Park

Reservoir

Fresh

Pond

Charles River Charl es

River

Aqueduct to Fresh Pond

Hobbs Brook

Reservoir

Stony Brook

Reservoir

Storage

Distribution

Figure 1. Map showing 

Cambridge drinkingwater source area, 

Massachusetts.
2

EXPLANATION

Impervious

Water body

Stations

0 1 2 MILES

0 1 2 KILOMETERS

Base from U.S. Geological Survey, Massachusetts Office of Geographic 

Information, Cambridge Water Department, Harvard Design and Mapping, 

and Boston Edison digital data, April 1998

Subbasin boundary

Concentration scale,

 in milligrams per liter

 (log interval used) Sodium

Cambridge

Reservoir

Stony Brook

Reservoir

80

200

500

800

 40

44

110

275

440

 22

Chloride

71°20' 71°18' 71°16' 71°14'

42°26'

42°24'

42°22'

Figure 2. Median concentrations of chloride in water samples collected during base flow from tributaries in the Cambridge, 

Massachusetts, drinking-water supply area, eastern Massachusetts, 2005–8.
discharge directly into the reservoir. Water is discharged from 

the southern end of the Cambridge Reservoir into Hobbs Brook, 

which joins Stony Brook about 1.6 miles downstream from the 

reservoir. Stony Brook flows southward from the confluence 

with Hobb Brook for about a mile and discharges to the Stony 

Brook Reservoir. Additional water enters both reservoirs from 

tributaries and roadway and parking-lot storm drains. Water 

from Stony Brook Reservoir is piped through an aqueduct by 

the CWD directly to Fresh Pond Reservoir, where it is stored 

prior to treatment. Overflow and controlled discharges from the 

Stony Brook Reservoir flow into the Charles River in Waltham.

Measurements of Source-Area Water Quality, 2005–8

Concentrations of Sodium and Chloride Highest in 

Tributary Subbasins Containing Large Impervious Areas

High concentrations of sodium and chloride are of special 

concern in the Cambridge drinking-water source area because 

they are not readily removed by conventional treatment processes. High concentrations of chloride can affect the taste of 

finished drinking water, and high concentrations of sodium 

contribute to hypertension in susceptible individuals. Although 

these elements are common in precipitation (generally less than 

1 milligram per liter (mg/L); National Atmospheric Deposition 

Program, 2008); minerals and soils in the environment (Smith, 

2005); and in septic effluent, industrial wastes, and wastewater, 

they also are major constituents of road salt and other deicing 

compounds used within the drinking-water source area. Deicing 

compounds readily dissolve, and associated ions are transported 

to nearby streams by surface runoff and groundwater discharge. 

The contribution of deicing compounds is great because of the 

large amount of impervious area and numerous roadways in the 

drinking-water source area (fig. 2).

Concentrations of dissolved chloride and sodium in samples collected from the tributaries throughout the drinking-water 

source area varied widely, particularly in the small subbasins 

where the effects of stormwater runoff and land development 

are greater (fig. 2). Tributary concentrations of these elements 

tended to increase during the summer, when streamflow is low, 

and decrease during higher base-flow conditions and stormflows, indicating that groundwater is an important source of 

chloride and sodium. The higher concentrations of chloride 

and sodium in base flow often were greater than the U.S. 

Environmental Protection Agency (EPA) 250-mg/L secondary 

drinking-water-regulation (SDWR) standard for chloride (U.S. 

Environmental Protection Agency, 2009) and the Massachusetts 

Department of Environmental Protection (MassDEP) 20-mg/L 

drinking-water guideline for sodium (Massachusetts Department of Environmental Protection, 2009). Median concentrations of chloride in samples collected during base flow in five 

tributaries along the Interstate 95 corridor exceeded the EPA 

SDWR standards (fig. 2). Concentrations of sodium in water 

samples collected throughout the drinking-water supply area, as 

well as from the raw-water intake at the Fresh Pond Reservoir, 

routinely exceeded the MassDEP drinking-water guideline. 

Concentrations of chloride and sodium increased with the road 

density in each tributary subbasin. During 2005–8, the estimated annual mean loads of sodium and chloride to the Stony 

Brook Reservoir were about 2,600 metric tons (t) and 4,800 t, 

respectively. In a typical year, about half of the water—and thus 

half the sodium chloride—entering the Stony Brook Reservoir 

is diverted to Fresh Pond Reservoir, where it is processed as 

drinking water.

Concentrations of Nutrients Often Exceed 

Reference Values

Nutrients such as nitrogen and phosphorus are essential to 

the health and diversity of aquatic organisms. Biological and 

chemical processes can mobilize nutrients to or from the soil, 

reservoir sediments, water column, atmosphere, and organisms. 

Algae and aquatic plants take up chemical forms of each nutrient in accordance with their nutritional requirements. Generally, 

the nutrient that is in shortest supply relative to the nutritional 

needs of the plants will be the one that limits overall growth. 

Waldron and Bent (2001) determined that phosphorus was the 

limiting nutrient on plant growth in the Cambridge drinkingwater reservoirs; thus, any increases in phosphorus would be 

expected to result in potentially excessive increases in plant 

growth, a process known as eutrophication. Eutrophication can 

lead to excessive amounts of plant biomass, which in turn can 

lead to reductions in dissolved oxygen concentrations, releases 

of trace metals from sediment, and unwanted effects on other 

aquatic life.

Common sources of phosphorus include weathered rocks 

and soils, fertilizers, and domestic (septic-system) and municipal (sewer-line) wastewater. Phosphorus is somewhat insoluble 

and tends to be associated with sediment and plant matter. 

Erosion of local soils and even washoff of winter-maintenance 

sands from roads and parking lots can increase phosphorus concentrations in receiving waters. Common sources of nitrogen 

are precipitation, emissions from automobiles, degradation of 

natural organic matter, fertilizers, and domestic and municipal 

wastewater. Measured concentrations of total nitrogen and total 

phosphorus in water samples collected from subbasins in the 

drinking-water supply area ranged from 0.42 to 5.13 and from 

0.006 to 0.80 mg/L, respectively (fig. 3). Median concentrations 

for total nitrogen and total phosphorus were about 1.5 mg/L and 

0.03 mg/L, respectively. 

Concentrations of total nitrogen and total phosphorus 

generally exceeded the proposed reference concentrations of 

0.57 mg/L and 0.024 mg/L, respectively, for Level 3, Ecoregion 

59 (U.S. Environmental Protection Agency, 2000). These reference values, which represent the 25th percentile of all nutrient 

data for Ecoregion 59, are intended to protect streams and rivers 

from the adverse effects of nutrient enrichment and eutrophication. Concentrations of total nitrogen exceeded this reference 

concentration in 94 percent of all water samples collected from 

the five monitoring stations during base-flow and storm conditions and in 100 percent of all samples collected from the 

raw-water intake at the Fresh Pond Reservoir. Concentrations 

of total phosphorus exceeded this reference concentration in 56 

percent of all water samples. With few exceptions, concentrations of total phosphorus in all composite stormflow samples 

exceeded the reference value (fig. 3). Generally, concentrations of total phosphorus in samples collected during base-flow 

conditions were near or below the reference concentration. 

Concentrations of total phosphorus were similar among the four 

tributaries despite differences in land use. 

3
4

01104415

Concentration,

in milligrams per liter

0.001

0.01

0.1

1.0

01104433

Concentration,

in milligrams per liter 0.001

0.01

0.1

1.0

01104455

Concentration,

in milligrams per liter 0.001

0.01

0.1

1.0

Concentration,

in milligrams per liter 0.001

0.01

0.1

1.0

Concentration,

in milligrams per liter 0.001

0.01

0.1

1.0

EXPLANATION

Impervious

Water body

01104415

0 1 2 MILES

0 1 2 KILOMETERS

Base from U.S. Geological Survey, Massachusetts Office of Geographic 

Information, Cambridge Water Department, Harvard Design and Mapping, 

and Boston Edison digital data, April 1998

Subbasin boundary

Subbasins

Hobbs Brook Subbasin

Stony Brook Reservoir Subbasin

U.S. Geological 

Survey station 

and number

Cambridge

Reservoir

Stony Brook

Reservoir

Range

Base flow Stormflow

Base flow Stormflow

Base flow Stormflow

Base flow Stormflow

Base flow Stormflow

01104475

01104475

01104460

01104415

01104433

01104455

01104460

Figure 3. Distribution and median concentrations of total phosphorus in base-flow and stormflow samples from five U.S. Geological Survey 

stations in the drinking-water source area for Cambridge, Massachusetts, 2005–8.
Discharge measurement at U.S. Geological Survey streamgage 

(01104415) near Lexington, Mass.

Pesticides were detected at trace levels throughout 

the drinking-water source area

Pesticides, including herbicides, fungicides, and insecticides, are commonly used by homeowners to control nuisance 

plants, fungi, and insects in homes and gardens. Twenty-five 

pesticides were detected in water samples collected in the 

drinking-water source area; six pesticides also were detected 

in raw water collected from the Cambridge water-treatment 

facility intake at the Fresh Pond Reservoir (fig. 4). Most of the 

detected pesticides are in over-the-counter products used for turf 

management and the treatment of ornamental shrubs. Although 

many pesticides were detected in the drinking-water source 

area, only nine compounds were detected at a frequency greater 

than or equal to 15 percent. Imidacloprid, an insecticide, and 

norflurazon and siduron, both herbicides, were detected in about 

24 to 41 percent of all samples. 

Although the frequency of detection varied from tributary 

to tributary, concentrations of pesticides typically were less than 

1 μg/L (microgram per liter or parts per billion). These concentrations did not exceed U.S. Environmental Protection Agency 

drinking-water guidelines or other health standards and were 

several orders of magnitude less than lethal exposure levels 

established for several fish species common to the drinking-water source area (except in several samples containing elevated 

concentrations of imidacloprid that exceeded available longterm aquatic-life guidelines; fig. 4). Although imidacloprid 

was detected in many tributaries, the frequency of detection in 

samples collected at USGS station 01104475, the drainage point 

of a residential area adjacent to the Stony Brook Reservoir, was 

94 percent.

Caffeine Detected in Drinking-Water Source Area

Caffeine is commonly detected in streams and rivers 

throughout the United States and generally is considered an 

indicator of domestic or municipal wastewater contamination. 

Caffeine can also be introduced into the environment by spilled 

caffeinated beverages or leaching of caffeine-laden litter. Caffeine was detected in about 64 percent of 171 water samples 

at concentrations ranging from 0.003 to 1.82 μg/L. Although 

generally present during base-flow conditions, caffeine was 

detected more frequently in composites of stormflow. Base-flow 

concentrations of caffeine correlated significantly with parkinglot land use throughout the drinking-water supply area. The 

estimated annual load of caffeine to the Stony Brook Reservoir 

was equivalent to about 6,500 12-ounce cups of coffee, each of 

which was assumed to contain about 346 mg/L of caffeine.

Watershed Management Through Continuous 

Hydrologic and Water-Quality Monitoring

Effective management of the drinking-water source area 

requires an understanding of how streamwater quality is affected by natural and cultural watershed factors. Drought and severe 

weather are examples of natural factors, whereas the proportion 

of impervious area and application of pesticides are cultural factors. Because of the diverse land use within the drinking-water 

source area, a large number of constituents with varied transport 

mechanisms could potentially enter the drinking-water supply. 

Low-salt areas and structural best management practices could 

minimize the quantity of potentially harmful constituents entering the drinking-water source area. 

Hydrologic and water-quality monitoring is important for 

the effective management and protection of drinking-water 

supplies. Currently (2015), the USGS, in cooperation with the 

CWD, operates a network of 11 stations where both the quantity 

and the quality of water are monitored because these factors determine the physical, chemical, and biological state of the water 

supply. Without accurate information about the past and current 

condition of the water supply, effective management strategies 

cannot be implemented or evaluated.

5

 Benomyl

 Metalaxyl

Propiconazole

 2,4–D

 Diuron

 Norflurazon

 Siduron

 Imidacloprid

 Carbaryl

 Concentration, in micrograms per liter

Fungicides

10-3 10-2 10-1 100 101 102 103 104 105 106

Sample concentration

 EXPLANATION

Values to the left are estimated to be below the long-term method detection level

Concentration exceeding the EPA drinking-water maximum contaminant level 

Concentration exceeding Canadian long-term guideline for aquatic life

Concentration exceeding the 96-hour LC50 value for bluegill sunfish

Herbicieds Insecticides

Figure 4. Concentrations of selected pesticides detected in 

samples of water collected during base flow and storms at all 

monitoring stations in the Cambridge drinking-water source area, 

water years 2005–8. (The LCD50 value is the concentration of a 

constituent in water that killed 50 percent of the test animals during 

the observation period.) Figure modified from Smith (2013).
References Cited

Massachusetts Department of Environmental Protection, 2009, Standards 

and guidelines for contaminants in Massachusetts drinking waters: Massachusetts Department of Environmental Protection, Office of Research 

and Standards, accessed February 16, 2010, at http://www.mass.gov/

dep/water/dwstand.pdf.

National Atmospheric Deposition Program, 2008, NADP/NTN monitoring location MA13—2006 annual & seasonal data summary for 

Site MA13: National Atmospheric Deposition Program database, 

accessed August 26, 2007, at http://nadp.sws.uiuc.edu/sites/siteinfo.

asp?net=NTN&id=MA13.

Smith, K.P., 2005, Hydrologic, water-quality, bed-sediment, soil-chemistry, 

and statistical summaries of data for the Cambridge, Massachusetts, 

drinking-water source area, water year 2004: U.S. Geological Survey 

Open-File Report 2005–1383, 110 p.

Smith, K.P., 2013, Water-quality conditions, and constituent loads and 

yields in the Cambridge drinking-water source area, Massachusetts, 

water years 2005–07: U.S. Geological Survey Scientific Investigations 

Report 2013–5039, 73 p., http://pubs.usgs.gov/sir/2013/5039/.

U.S. Environmental Protection Agency, 2000, Ambient water quality criteria recommendations—Information supporting the development of state 

and tribal nutrient criteria—Rivers and streams in nutrient ecoregion 

XIV: Washington, D.C., U.S. Environmental Protection Agency, Office 

of Water, December 2000, EPA 822–B–00–022 [variously paginated].

U.S. Environmental Protection Agency, 2009, Drinking water 

contaminants, accessed September 24, 2013, at http://water.epa.gov/

drink/contaminants/.

Waldron, M.C., and Bent, G.C., 2001, Factors affecting reservoir and 

stream-water quality in the Cambridge, Massachusetts, drinking-water 

source area and implications for source-water protection: U.S. Geological Survey Water-Resources Investigations Report 00–4262, 89 p.

by Kirk P. Smith and Marcus C. Waldron

6

Weir at U.S. Geological Survey streamgage (01104475) near 

Weston, Mass.

Outlet pond below Stony Brook Reservoir, U.S. Geological Survey 

streamgage (01104480) near Waltham, Mass.

Director, New England Water Science Center 

U.S. Geological Survey 

10 Bearfoot Road, Northborough, MA 01532

For more information visit:

http://ma.water.usgs.gov

email dc_nweng@usgs.gov or 

call (508) 490-5000

ISSN 2327-6916 (print)

ISSN 2327-6932 (online)

http://dx.doi.org/10.3133/fs20153030