Prepared in cooperation with the 

City of Cambridge, Massachusetts, Water Department

Water-Quality Conditions, and Constituent Loads and 

Yields in the Cambridge Drinking-Water Source Area, 

Massachusetts, Water Years 2005–07

Scientific Investigations Report 2013-5039

U.S. Department of the Interior

U.S. Geological Survey
Cover. Photograph of Stony Brook Reservoir in Waltham, Massachusetts, as taken from the gate house at the southern end of the 

reservoir by Joan Whitley, U.S. Geological Survey.
Water-Quality Conditions, and 

Constituent Loads and Yields in the 

Cambridge Drinking-Water Source Area, 

Massachusetts, Water Years 2005–07

by Kirk P. Smith

Prepared in cooperation with the 

City of Cambridge, Massachusetts, Water Department

Scientific Investigations Report 2013-5039

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2013

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Suggested citation:

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/.
iii

Acknowledgments

The author wishes to thank Chip Norton, Watershed Manager, and David Kaplan, Watershed 

Protection Supervisor, Cambridge Water Department, for their help in planning and for facilitating this study. The author also wishes to thank Richard Zilinsky (retired), Jose Silva, and Robert 

Bonvie of the Cambridge Water Department for their help with the installation of the monitoring 

shelters and access maintenance for each gage throughout the year.
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v

Contents

Acknowledgments........................................................................................................................................iii

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................2

Purpose and Scope ..............................................................................................................................4

Previous Investigations........................................................................................................................4

Study Area..............................................................................................................................................4

Land Use.................................................................................................................................................4

Data Collection Methods..............................................................................................................................7

Continuous Monitoring of Streamflow and Water Quality.............................................................7

Collection of Water-Quality Samples.................................................................................................7

Quality-Assurance and Quality-Control Samples..........................................................................10

Data Analysis Methods...............................................................................................................................14

Determination of Correlations ..........................................................................................................14

Estimated Loads and Yields...............................................................................................................14

Loads Estimated from Streamflow..........................................................................................14

Loads of Major Ions Estimated from Specific-Conductance Monitoring Data ...............16

Determination of Trends ....................................................................................................................20

Water-Quality Conditions............................................................................................................................20

Water Properties.................................................................................................................................20

Specific Conductance...............................................................................................................29

Dissolved Oxygen.......................................................................................................................29

pH..................................................................................................................................................29

Turbidity .......................................................................................................................................29

Constituents.........................................................................................................................................29

Major Ions...................................................................................................................................30

Total Nutrients ............................................................................................................................30

Pesticides and Caffeine............................................................................................................31

Factors Affecting Water-Quality Properties and Constituent Concentrations.........................39

Seasonal Variability...................................................................................................................39

Stormwater runoff .....................................................................................................................42

Land Use......................................................................................................................................42

Comparison of Water-Quality Properties and Constituent Concentrations with WaterQuality Criteria and Guidelines ...........................................................................................47

Constituent Loads and Yields.....................................................................................................................54

Loads and Yields of Calcium, Chloride, Sodium, and Sulfate, Water Years 1999 to 2008........54

Loads and Yields of Total Nutrients, Water Years 2005 to 2007...................................................58

Loads and Yields of Selected Pesticides and Caffeine, Water Years 2005 to 2007..................63

Accuracy of Estimated Loads...........................................................................................................63

Chloride and Sodium Trends ......................................................................................................................64

Summary........................................................................................................................................................66

References Cited..........................................................................................................................................68
vi

Figures

1. Map showing Cambridge drinking-water source area, Massachusetts. ..........................3

2. Graph showing probability distribution of dry antecedent periods during water

years 2005–07 for U.S. Geological Survey station 01104430 in the Cambridge

Reservoir watershed, Massachusetts, in relation to the dry antecedent periods

for storms where runoff samples were collected at U.S. Geological Survey station

01104415 .........................................................................................................................................8

3. Box plot showing distribution of precipitation totals greater than 0.10 inch recorded

by the National Climatic Data Center near Boston, Massachusetts, 1967–2007;

precipitation totals for all storms recorded at the Cambridge Reservoir during

water years 2005–07; and precipitation totals for storms during which samples

were collected at each of the primary sampling stations in the Cambridge

drinking-water source area, Massachusetts, 2005–07. .......................................................9

4. Graph showing example of automated flow-proportional collection of stormflow

subsamples at U.S. Geological Survey station 01104415, Cambridge drinking-water

source area, Massachusetts, water year 2007 .....................................................................10

5. Graph showing relation of concentrations of selected constituents detected in

concurrent replicate streamflow samples, Cambridge drinking-water source area,

Massachusetts, water years 2005–07. ...................................................................................13

6. Box plots showing milliequivalent ratios of calcium, sodium, and sulfate to chloride

in sample populations from six U.S. Geological Survey monitoring stations in the

Cambridge drinking-water source area, Massachusetts, water years 2004–08 .............19

7. Graph showing example of separation of base-flow and stormflow specific

conductance at U.S. Geological Survey station 01104415, Cambridge

drinking-water source area, Massachusetts, water year 2007..........................................20

8. Graph showing concentrations of selected pesticides and caffeine detected in

samples of water collected during base flow and stormflow at all monitoring

stations in the Cambridge drinking-water source area and from the Fresh Pond

Reservoir raw-water intake, Cambridge, Massachusetts, water years 2004–08 ............40

9. Graphs showing records of flow and specific conductance during A, fall and B,

winter storm runoff at U.S. Geological Survey station 01104455, Stony Brook

Reservoir watershed, Massachusetts, water year 2007......................................................41

10. Box plots showing distributions of turbidity in base-flow and stormflow samples

collected from five U.S. Geological Survey stations in the Cambridge

drinking-water source area, Massachusetts, water years 2005–07..................................42

11. Box plots showing distributions of concentrations of A, dissolved calcium and B,

chloride in base-flow and stormflow samples from five U.S. Geological Survey

stations in the Cambridge drinking-water source area, Massachusetts, water years

2005−07. .......................................................................................................................................43

12. Box plots showing distributions of concentrations of A, dissolved sodium and B,

dissolved sulfate in base-flow and stormflow samples from five U.S. Geological

Survey stations in the Cambridge drinking-water source area, Massachusetts,

water years 2005–07. .................................................................................................................44

13. Box plots showing distributions of concentrations of A, total nitrogen and B, total

phosphorus in base-flow and stormflow samples from five U.S. Geological Survey

stations in the Cambridge drinking-water source area, Massachusetts, water years

2005−07. .......................................................................................................................................45
vii

14. Graph showing frequency of detection for selected pesticides in samples of base

flow and stormflow collected at the primary sampling stations in the Cambridge

drinking-water source area, Massachusetts, water years 2005−07..................................46

15. Graphs showing concentrations of calcium, chloride, sodium, sulfate, and caffeine

in relation to the areal percentage of State maintained roads, town roads, and

parking lots in 11 subbasins in the Cambridge drinking-water source area,

Massachusetts, water years 2005−07.....................................................................................48

16. Graph showing probability distribution of daily specific conductance values during

base flow and flow-weighted specific conductance values during runoff for U.S.

Geological Survey station 01104415, Cambridge drinking-water source area,

Massachusetts, in relation to estimated concentration values representing U.S.

Environmental Protection Agency water-quality criteria for chloride, water years

2004–08 .........................................................................................................................................52

17. Graph showing probability distribution of daily specific conductance values during

base flow and flow-weighted specific conductance values during runoff for U.S.

Geological Survey station 01104433, Cambridge drinking-water source area,

Massachusetts, in relation to estimated concentration values representing U.S.

Environmental Protection Agency water-quality criteria for chloride, water years

2004–07 .........................................................................................................................................53

18. Graph showing probability distribution of daily specific conductance values during

base flow and flow-weighted specific conductance values during runoff for U.S.

Geological Survey station 01104455, Cambridge drinking-water source area,

Massachusetts, in relation to estimated concentration values representing U.S.

Environmental Protection Agency water-quality criteria for chloride, water years

2004–08 .........................................................................................................................................53

19. Graph showing probability distribution of daily specific conductance values during

base flow and flow-weighted specific conductance values during runoff for U.S.

Geological Survey station 01104475, Cambridge drinking-water source area,

Massachusetts, in relation to estimated concentration values representing U.S.

Environmental Protection Agency water-quality criteria for chloride, water years

2004–08 .........................................................................................................................................54

20. Graphs showing relation of winter weather severity to annual sodium loads for U.S.

Geological Survey stations A, 01104415, B, 01104455, and C, 01104475, Cambridge

drinking-water source area, Massachusetts, water years 2004−08..................................65
viii

Tables

1. Names, locations, drainage areas, and periods of record for U.S. Geological Survey

monitoring and sampling stations in the Cambridge drinking-water source area,

Massachusetts, water years 1997–2008...................................................................................5

2. Areal percentages of land use for subbasins upstream from U.S. Geological Survey

monitoring/sampling stations in Hobbs Brook and Stony Brook Basins,

Massachusetts..............................................................................................................................6

3. Physical properties and constituents measured in water samples, respective

long-term method detection limits, and analytical methods. ..............................................11

4. Pearson correlation coefficients relating concentrations of major ions and total

nutrients to streamflow at five U.S. Geological Survey monitoring/sampling stations

in the Cambridge and Stony Brook Reservoir watersheds, Massachusetts, water

years 2005–07 ..............................................................................................................................15

5. Attained significance levels (p values) from Mann-Whitney tests for paired sample

sets of concentrations of selected constituents and properties collected during

base-flow and stormflow conditions in the Cambridge drinking-water source area,

Massachusetts, water years 2005–07.....................................................................................16

6. Regression equation coefficients used to estimate concentrations of calcium,

chloride, sodium, and sulfate from flow-weighted values of specific conductance

for U.S. Geological Survey monitoring/sampling stations in the Cambridge

drinking-water source area, Massachusetts, water years 2005–07..................................18

7. Summary statistics for selected water-quality properties, major ions, total

nutrients, caffeine, and pesticides in samples of base flow and stormflow collected

at five primary sampling stations in the Cambridge drinking-water source area and

at the raw water intake at Fresh Pond Reservoir, Cambridge, Massachusetts, water

years 2005–07. .............................................................................................................................21

8. Summary statistics for selected water-quality properties, major ions, total

nutrients, caffeine, and pesticides in samples of base flow collected during three

synoptic sampling rounds at the secondary sampling stations in the Cambridge

drinking-water source area, Massachusetts, water years 2006–07. ................................27

9. Characteristics of pesticides detected in samples of base flow and stormflow

collected in the Hobbs Brook and Stony Brook Reservoir watersheds,

Massachusetts, water years 2005–07.....................................................................................32

10. Frequency of detection, standards for drinking water and aquatic life, and

maximum concentrations of selected pesticides in water samples collected in

Hobbs Brook Basin and Stony Brook Reservoir watershed and in samples of raw

water collected from the intake of the water-treatment facility at Fresh Pond,

Massachusetts, water years 2005–07. ...................................................................................38

11. Pearson correlation coefficients relating percentage of land-use classes to annual

flow-weighted concentrations of selected major ions, total nutrients, caffeine, and

selected pesticides for four subbasins in the Cambridge and Stony Brook Reservoir

watersheds, Massachusetts, 2005–07 ....................................................................................49

12. Pearson correlation coefficients relating percentage of land-use classes to mean

concentrations of selected major ions and total nutrients in base flow and

maxiumum concentrations of caffeine and selected pesticides in base flow in the

subbasins of the primary and secondary sampling stations in the Cambridge and

Stony Brook Reservoir watersheds, Massachusetts, water years 2006–07 ....................50
ix

13. Mean concentrations and confidence intervals for selected major ions, total

nutrients, pesticides, and caffeine in samples of streamwater collected at five

primary sampling stations in the Cambridge drinking-water source area,

Massachusetts, water years 2005–07.....................................................................................55

14. Annual loads and yields of calcium, chloride, sodium, sulfate, total nutrients,

caffeine, and 11 pesticides for 5 primary sampling stations in the Cambridge and

Stony Brook Reservoir watersheds, Massachusetts, water years 2005–07. ..................59

15. Annual mean flow, runoff, and estimated annual loads of calcium, chloride, sodium,

and sulfate for sampling stations in the Cambridge and Stony Brook Reservoir

watersheds, Massachusetts, water years 1999–2008..........................................................61
Conversion Factors and Datum

SI to Inch/Pound

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

Acre 0.004047 square kilometer (km2

)

square mile (mi2

) 2.590 square kilometer (km2) 

Volume

gallon (gal) 3.785 liter (L) 

cubic inch (in3

) 0.01639 liter (L)

cubic foot (ft3

) 0.02832 cubic meter (m3) 

Flow rate

cubic foot per second (ft3

/s) 0.02832 cubic meter per second (m3/s)

Mass

ounce, avoirdupois (oz) 28.35 gram (g) 

pound, avoirdupois (lb) 0.4536 kilogram (kg) 

ton, short (2,000 lb) 0.9072 metric ton (MT)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32

Vertical coordinate information is referenced to North American Vertical Datum of 1988 (NAVD 

88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Altitude, as used in this report, refers to distance above the vertical datum.

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25 °C).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L). 

Abbreviations

ASTM American Society for Testing and Materials

COV coefficient of variation

CCC criterion continuous concentration

CMC criterion maximum concentration

CWD city of Cambridge, Massachusetts, Water Department

DWRL drinking-water lifetime exposure level

HPLC high-performance liquid chromatography

ISO International Organization for Standardization

MCL Maximum Contaminant Level
xi

NIST National Institute of Standards and Technology

NWQL National Water Quality Laboratory

NTRU nephelometric turbidity ratio units

NTU nephelometric turbidity units p attained significance level

RPD relative percent difference

SDWR Secondary Drinking-Water Regulation

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

WWSI winter weather severity index
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Water-Quality Conditions, and Constituent Loads and 

Yields in the Cambridge Drinking-Water Source Area, 

Massachusetts, Water Years 2005–07

By Kirk P. Smith

Abstract

The source water area for the drinking-water supply of 

the city of Cambridge, Massachusetts, encompasses major 

transportation corridors, as well as large areas of light industrial, commercial, and residential land use. Because of ongoing 

development in the drinking-water source area, the Cambridge water supply has the potential to be affected by a wide 

variety of contaminants. The U.S. Geological Survey (USGS) 

has monitored surface-water quality in the Hobbs Brook 

and Stony Brook Basins, which compose the drinking-water 

source area, since 1997 (water year 1997) through continuous 

monitoring and discrete sample collection and, since 2004, 

through systematic collection of streamwater samples during 

base-flow and stormflow conditions at five primary sampling 

stations in the drinking-water source area. Four primary 

sampling stations are on small tributaries in the Hobbs Brook 

and Stony Brook Basins; the fifth primary sampling station is 

on the main stem of Stony Brook and drains about 93 percent 

of the Cambridge drinking-water source area. Water samples 

also were collected at six secondary sampling stations, including Fresh Pond Reservoir, the final storage reservoir for the 

raw water supply. Storm runoff and base-flow concentrations of calcium (Ca), chloride (Cl), sodium (Na), and sulfate 

(SO4

) were estimated from continuous records of streamflow 

and specific conductance for six monitoring stations, which 

include the five primary sampling stations. These data were 

used to characterize current water-quality conditions, estimate loads and yields, and describe trends in Cl and Na in the 

tributaries and main-stem streams in the Hobbs Brook and 

Stony Brook Basins. These data also were used to describe 

how streamwater quality is affected by various watershed 

characteristics and provide information to guide future 

watershed management.

Water samples were analyzed for physical properties 

and concentrations of Ca, Cl, Na, and SO4

, total nitrogen 

(TN), total phosphorus (TP), caffeine, and a suite of 59 polar 

pesticides. Values of physical properties and constituent 

concentrations varied widely, particularly in samples from 

tributaries. Median concentrations of Ca, Cl, Na, and SO4

in samples collected in the Hobbs Brook Basin (39.8, 392, 

207, and 21.7 milligrams per liter (mg/L), respectively) were 

higher than those for the Stony Brook Basin (17.8, 87.7, 49.7, 

and 14.7 mg/L, respectively). These differences in major 

ion concentrations are likely related to the low percentages 

of developed land and impervious area in the Stony Brook 

Basin. Concentrations of dissolved Cl and Na in samples, and 

those estimated from continuous records of specific conductance (particularly during base flow), often were greater 

than the U.S. Environmental Protection Agency (USEPA) 

secondary drinking-water guideline for Cl (250 mg/L), the 

chronic aquatic-life guideline for Cl (230 mg/L), and the 

Commonwealth of Massachusetts, Executive Office of Energy 

and Environmental Affairs drinking-water guideline for Na 

(20 mg/L). Mean annual flow-weighted concentrations of Ca, 

Cl, and Na were generally positively correlated with the area 

of roadway land use in the subbasins. Correlations between 

mean annual concentrations of Ca and SO4 in base flow and 

total roadway, total impervious, and commercial-industrial 

land uses were statistically significant.

Concentrations of TN (range of 0.42 to 5.13 mg/L in all 

subbasins) and TP (range of 0.006 to 0.80 mg/L in all subbasins) in tributary samples did not differ substantially between 

the Hobbs Brook and Stony Brook Basins. Concentrations of 

TN and TP in samples collected during water years 2004–07 

exceeded proposed reference concentrations of 0.57 and 

0.024 mg/L, in 94 and 56 percent of the samples, respectively. 

Correlations between annual flow-weighted concentrations of 

TN and percentages of recreational land use and water-body 

area were statistically significant; however, no significant 

relation was found between TP and available land-use information. The volume of streamflow affected water-quality 

conditions at the primary sampling stations. Turbidity and 

concentrations of TP were positively correlated with streamflow. In contrast, concentrations of major ions were negatively 

correlated with streamflow, indicating that these constituents 

were diluted during stormflows. Concentrations of TN were 

not correlated with streamflow.

Twenty-five pesticides and caffeine were detected in 

water samples collected in the drinking-water source area and 
2 Water Quality in the Cambridge Drinking-Water Source Area, Massachusetts, Water Years 2005–07

in raw water collected from the Cambridge water-treatment 

facility intake at the Fresh Pond Reservoir. Imidacloprid, 

norflurazon, and siduron were the most frequently detected 

pesticides with the frequency of detections ranging from about 

24 to 41 percent. Caffeine was detected in about 64 percent 

of water samples at concentrations ranging from 0.003 to 

1.82 micrograms per liter (μg/L). Although some of the 

detected pesticides degrade rapidly, norflurazon and siduron 

are relatively stable and are able to immigrate though the 

serial reservoir system. Concentrations of 2,4-D, carbaryl, 

imazaquin, MCPA (2-methyl-4-chlorophenoxyacetic acid), 

metsulfuron-methyl, norflurazon, siduron, and caffeine were 

detected more frequently in stormflow samples than in baseflow samples. Concentrations of pesticides did not exceed 

USEPA drinking-water guidelines or other health standards 

and were several orders of magnitude less than the lethal exposure level established for several fish species common to the 

drinking-water source area. Imidacloprid, an insecticide, was 

the only pesticide with a concentration exceeding available 

long-term aquatic-life guidelines. Several pesticides correlated 

significantly with the amount of recreational, residential, and 

commercial area in the tributary subbasins. Mean annual baseflow concentrations of caffeine correlated significantly with 

parking-lot land use.

For most tributaries, about 70 percent of the annual 

loads of Ca, Cl, Na, and SO4

 were associated with base flow. 

Upward temporal trends in annual loads of Cl and Na were 

identified on the basis of data for water years 1998 to 2008 

for the outlet of the Cambridge Reservoir in the Hobbs Brook 

Basin; however, similar trends were not identified for the 

main stem of Stony Brook downstream from the reservoir. 

The proportions of the TN load attributed to base flow and 

stormflow were similar in each tributary. In contrast, more 

than 83 percent of the TP loads in the tributaries and about 

73 percent of the TP load in main stem of Stony Brook were 

associated with stormflow. 

Mean annual yields of Ca, Cl, Na, and SO4

 in the 

Stony Brook Reservoir watershed, which represents most of 

the drinking-water source area, were 14, 85, 46, and 9 metric 

tons per square kilometer, respectively. Mean annual yields 

among the individual tributary subbasins varied extensively. 

Mean annual yields for the respective constituents increased 

with an increase in roadway and parking-lot area in the tributary subbasins. Mean annual yields of TN in the tributary subbasins ranged from about 740 to more than 1,200 kilograms 

per square kilometer and exceeded the yield for the main stem 

of Stony Brook at USGS station 01104460 upstream from the 

Stony Brook Reservoir. 

Mean annual yields estimated for the herbicides 2,4-D 

and imidacloprid ranged from 34 to 310 grams per square 

kilometer (g/km2

) and 3 to 170 g/km2, respectively. Annual 

loads for 2,4-D were entirely associated with stormflow. The 

largest annual load for imidacloprid was estimated for the 

main stem of Stony Brook; however, the highest annual yield 

for this pesticide, as well as for benomyl, carbaryl, metalaxyl, 

and propiconazole, was estimated for a tributary to the Stony 

Brook Reservoir that drains largely residential and recreational 

areas. Mean annual yields for the herbicide siduron ranged 

from 6.9 to 35 g/km2 with most of the loads associated with 

stormflow. Mean annual yields for the insecticide diuron 

ranged from 2.1 to 4.4 g/km2

. Annual yields of caffeine ranged 

from 11 to 410 g/km2

. 

Introduction

The city of Cambridge, Massachusetts supplies approximately 15 million gallons per day (Waldron and Bent, 2001) 

of drinking water to more than 100,000 customers. The Cambridge Water Department (CWD) obtains raw water from a 

serial system of three primary storage reservoirs. These reservoirs receive inflow from a drainage area of 61.4 square kilometers (km2

) (henceforth, the drinking-water source area) that 

is outside Cambridge; in parts of the towns of Lexington, Lincoln, and Weston; and in part of the city of Waltham (fig. 1). 

Only about 5 percent of the land in the drinking-water source 

area is owned by the city of Cambridge. Major transportation 

corridors (Interstate 95, Routes 2, 2A, 20, and 117), as well as 

large areas of industrial, commercial, and residential land use, 

are within the drinking-water source area.

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 amount of impervious area and application of pesticides 

are cultural factors. Because of the diverse land use within the 

drinking-water source area, there is a large number of constituents with varied transport mechanisms that could potentially 

enter the drinking-water supply.

In 1997, the U.S. Geological Survey (USGS), in cooperation with the CWD, began operation of continuous waterquality monitoring stations (streamflow, water temperature, 

and specific conductance) at various locations in the drinking-water source area (Smith, 2005, 2007, 2008, and 2011; 

Socolow and others, 1999, 2000, 2001, 2002, 2003, and 2004; 

U.S. Geological Survey, 2009). In 2004, the USGS, in cooperation with the CWD, established a program to characterize 

water quality during base-flow 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. These primary sampling 

stations were identified by Waldron and Bent (2001) and by 

the CWD as draining areas that are potentially important 

sources of contaminants. Monitoring data can be used to characterize current water-quality conditions, to establish a baseline for future comparisons, and if sufficient data are available, 

to describe trends in surface-water quality. These data also can 

be used to assess the effects of various watershed characteristics on surface-water quality and to provide information to 

guide future watershed management.
Introduction 3

CAMBRIDGE

NEWTON

WALTHAM

LINCOLN

LEXINGTON WINCHESTER

WESTON

MEDFORD

ARLINGTON

BELMONT

128

2

2A

20

117

95

90

Aqueduct to Fresh PondAqueduct to Fresh Pond

Hobbs

Brook

Stony Brook Basin

Basin

PAYSON PARK

RESERVOIR

Aqueduct to Fresh Pond

STONY BROOK

RESERVOIR FRESH

POND

RESERVOIR

CAMBRIDGE

RESERVOIR

Charles River

42°26'

71°19' 71°08'

42°20'

0

0

5 MILES

5 KILOMETERS

422302071083801 

Cambridge

Reservoir

Watershed

Charles

River 42°30' Basin

42°

73° 72° 

71°

70°

MASSACHUSETTS

41°30'

0

0

50 KILOMETERS

50 MILES

MASSACHUSETTS

LINCOLN

WESTON

01104415

01104430

01104433

01104455

01104460

01104475

01104480

01104453

01104390

01104405

01104420

01104410

0

0

1

1

2 MILES

2 KILOMETERS

Base from U.S. Geological Survey,

MassGIS, Cambridge Water Department,

Harvard Design and Mapping, and Boston Edison. 

71°15'

42°22'30"

71°19'

42°27'

71°11'

Hobbs Brook 2

Cambridge

Reservoir

Stony Brook

Reservoir

Sandy

Pond

117

 20

Cherry Brook

Stony Brook

Hobbs Brook

2A

95

Trapelo Road

128

WALTHAM

Lincoln

LEXINGTON

Charles River

EXPLANATION

01104460

Hobbs Brook Basin

Stony Brook Reservoir watershed 

Wetlands

Charles River Basin

Town boundary

Basin boundary

Subbasin boundary

Continuous-monitoring station and

number

Primary sampling station

Secondary sampling station

Cambridge Reservoir watershed; 

upper Hobbs Brook Basin

Drinking-water source area boundary

Figure 1. Cambridge drinking-water source area, Massachusetts. 
4 Water Quality in the Cambridge Drinking-Water Source Area, Massachusetts, Water Years 2005–07

Purpose and Scope

The purpose of this report is to describe water-quality 

conditions in the drinking-water source area for the city of 

Cambridge during water years1

 2005–07. Loads and yields 

of selected major ions, total nutrients, polar pesticides, and 

caffeine were estimated from water samples collected during 

periods of base flow and stormwater runoff from the main 

stem of Stony Brook and four tributaries within the drinkingwater source area. Most samples were collected during water 

years 2005–07. Four stations were sampled on one occasion at 

the end of water year 2004, and USGS station 01104475 was 

sampled on one occasion at the beginning of water year 2008. 

The relations between constituent yields and land-use characteristics are discussed. Estimates of annual loads and yields 

of selected major ions for water years 1999 to 2008 at several USGS monitoring stations determined using continuous 

records of flow and mathematical relations between specific 

conductance and concentrations are presented. Finally, temporal trends in annual mean loads of chloride (Cl) and sodium 

(Na) analyzed for water years 1998 to 2008 for USGS stations 

01104430 (Hobbs Brook at the outlet of Cambridge Reservoir) 

and 01104460 (main stem of Stony Brook) are discussed. 

Previous Investigations

Since 1997, hydrologic and water-quality data were collected for various periods for tributaries in the Hobbs Brook 

and Stony Brook Basins (table 1). Waldron and Bent (2001) 

assessed limnological conditions in the three serial storage 

reservoirs and described the water-quality conditions in the 

drinking-water source area during 1997–98. They also identified sources of various constituents, including calcium (Ca), 

Cl, Na, sulfate (SO4

), total nitrogen (TN), and total phosphorus 

(TP), in the drinking-water source area and outlined a sourcewater protection program for the CWD. Subsequently the 

USGS, in cooperation with the CWD, designed and implemented a monitoring network in the drinking-water source 

area (table 1). Streamflow, water-quality, and meteorological data from this network are available to the public online 

through the USGS Massachusetts-Rhode Island Water Science 

Center on the World Wide Web page (http://ma.water.usgs.

gov) and in various reports (Smith, 2005, 2007, 2008, and 

2011; Socolow and others, 1999, 2000, 2001, 2002, 2003, and 

2004; U.S. Geological Survey, 2009). 

Study Area

The drinking-water supply area includes three serial 

storage reservoirs—Cambridge Reservoir (also known as the 

Hobbs Brook Reservoir), Stony Brook Reservoir, and Fresh 

1

 A water year is the 12-month period beginning on October 1 and ending 

September 30 of the following year. The water year is designated by the year 

in which it ends. 

Pond Reservoir (fig. 1). The Cambridge Reservoir watershed includes Hobbs Brook and three unnamed tributaries 

that flow directly into the reservoir. Water is released from 

the southern end of the Cambridge Reservoir into Hobbs 

Brook, which receives additional inflow from an unnamed 

tributary about 0.8 km (kilometers) downstream from the 

reservoir. The confluence of Hobbs Brook and Stony Brook 

is about 2.6 km downstream from the Cambridge Reservoir. 

The area above this confluence is herein referred to as the 

“Hobbs Brook Basin” and includes the drainage area of the 

Cambridge Reservoir. Two unnamed tributaries flow into 

Stony Brook about 0.4 km north of the Stony Brook Reservoir. In addition to Stony Brook, an unnamed tributary flows 

directly into the Stony Brook Reservoir on the southwestern 

side of the reservoir. Additional water enters both reservoirs 

from other minor tributaries and storm drains. Water from 

Stony Brook Reservoir is piped through an aqueduct directly 

to Fresh Pond Reservoir, which is within the city of Cambridge about 16 km east of the drinking-water source area, 

where it is stored prior to treatment. Overflow and controlled 

releases from the Stony Brook Reservoir flow into the Charles 

River. Fresh Pond Reservoir is a glacial kettle lake with 

minimal surface-water inflow. The water level in Fresh Pond 

Reservoir is generally maintained above normal groundwater 

levels, thereby, limiting groundwater inflow to about 3 percent 

of the daily water demand (City of Cambridge, 2011). 

Land Use

Land-use data were obtained, in part, from the Office of 

Geographic Information (MassGIS, 2002) and the Geographic 

Information System (GIS) developed by the CWD with data 

from MassGIS, the USGS National Mapping Division, Harvard Design and Mapping, and Boston Edison. In an effort to 

better characterize various land-use data, the areal coverages 

of commercial landscaped areas, parking lots, and roof tops 

were estimated from orthophotographs (MassGIS, 2005). The 

areas occupied by these land-use classes were subtracted from 

the more generalized land-use categories so that the aggregate of all areal coverages for each subbasin was 100 percent. 

For example, roadways, roofs, parking lots, sidewalks, and 

recreational areas that were within residential areas were not 

included in the residential category. As a result, the remaining residential land-use areas include only yards and wooded 

areas (otherwise not designated as forest) between house lots. 

Similarly, roadways, roofs, parking lots, sidewalks, or areas 

identified as landscaped were excluded from commercial 

land-use area calculations. Land-use data for each subbasin are 

listed in table 2.

Land uses in the Hobbs Brook and the Stony Brook 

Basins are similar. In general, these suburban basins are 

largely residential with relatively low percentages of commercial and light-industrial development. State and Interstate 

highways traverse each basin, and the percentages of forest and wetland areas are similar (about 32 and 11 percent, 
Introduction 

5

Table 1. Names, locations, drainage areas, and periods of record for U.S. Geological Survey monitoring and sampling stations in the Cambridge drinking-water source area, Massachusetts, water years 1997–2008.[USGS, U.S. Geological Survey; km2, square kilometer; latitude and longitude: in degrees, minutes, and seconds; (), value in parenthesis represents effective drainage area; C, continuous-monitoring station 

(streamflow and specific conductance); P, primary sampling station; S, secondary sampling station]

Station name USGS station Station type Latitude Longitude Drainage area (km2) Period of record

Hobbs Brook Basin

Stony Brook at Kendal Green 01104390 S 42 22 36 71 16 55 27.2 03/07/97 - 09/17/98; 07/18/05 - 05/15/07

Hobbs Brook at Mill Street near Lincoln 01104405 S 42 26 11 71 16 12 5.56 03/05/97 - 09/16/98; 07/18/05 - 05/15/07

Cambridge Reservoir, unnamed tributary number 1, near Lexington 01104410 S 42 26 15 71 15 53 0.88 03/05/97 - 09/16/98; 07/18/05 - 05/15/07

Cambridge Reservoir, unnamed tributary number 2, near Lexington 01104415 CP 42 26 09 71 15 38 1.06 03/06/97 - 09/22/98; 10/01/00 - 09/30/08

Cambridge Reservoir, unnamed tributary number 3, near Lexington 01104420 S 42 25 11 71 15 29 1.99 04/09/97 - 09/16/98; 07/18/05 - 05/15/07

Hobbs Brook below Cambridge Reservoir near Kendal Green 01104430 CS 42 23 53 71 16 26 17.8 03/06/97 - 09/30/08

Hobbs Brook, unnamed tributary 1 near Kendal Green 01104433 CP 42 23 28 71 16 18 0.93 (0.85) 10/23/97 - 09/15/05; 06/28/04 - 10/16/07

Stony Brook Basin

Stony Brook, unnamed tributary 2 near Weston Station 01104453 S 42 22 17 71 16 31 5.78 11/30/05 - 10/17/0

Stony Brook, unnamed tributary 1 near Waltham 01104455 CP 42 22 21 71 16 15 1.27 0/22/97 - 09/22/98; 10/01/00 - 09/30/0

Stony Brook at Route 20 at Waltham 01104460 CP 42 21 08 71 16 16 57.0 3/07/97 - 10/28/98; 05/14/02 - 09/30/08

Stony Brook Reservoir, unnamed tributary 1 near Weston 01104475 CP 42 21 16 71 16 07 2.20 1/06/97 - 09/17/98; 08/26/04 - 09/30/08

Stony Brook Reservoir at dam near Waltham 0110448 C 42 21 20 71 15 56 61.4 1/30/98 - 09/30/08

Fresh Pond Reservoir

Fresh Pond in gate house at Cambridge 422302071083801 42 23 02 71 08 38 0.00 0/01/03 - 09/30/08
6 Water Quality in the Cambridge Drinking-Water Source Area, Massachusetts, Water Years 2005–07 Table 2. Areal percentages of land use for subbasins upstream from U.S. Geological Survey monitoring/sampling stations in Hobbs Brook and Stony Brook Basins, Massachusetts. [Land-use data were obtained, in part, from the Office of Geographic Information (MassGIS, 2002) and the Geographic Information System (GIS) developed by the Cambridge Water Department with data from MassGIS, the U.S. Geological Survey National Mapping Division, Harvard Design and Mapping, and Boston Edison; total impervious area includes the sum of roadways, parking lots, roofs, compacted lands (quarry and railroad areas), and open waterbody. USGS, U.S. Geological Survey] Land-use class Percentage of subbasin area by USGS station number and basin 01104390 01104405 01104410 01104415 01104420 01104430 01104433 Hobbs Brook Basin (cumu- lative) 01104453 01104455 101104460 01104475 101104480 Agriculture 7.5 5.8 7.7 6.0 0.4 0.5 0.0 2.8 4.0 0.0 7.2 7.0 0.5 Commercial or industrial 0.4 0.0 10.8 2.5 6.7 4.8 16.3 4.2 0.6 10.8 11.8 0.0 0.5 Forest 30.9 46.5 45.9 26.2 20.7 29.9 8.2 33.6 28.2 44.0 17.5 18.8 26.3 Commercial landscaping 0.2 0.1 1.2 0.8 1.8 3.5 7.4 2.4 1.3 3.6 1.1 0.4 3.5 Parking lot 0.5 0.6 5.9 4.4 3.5 3.6 29.8 3.9 0.9 8.8 4.7 0.6 1.1 Railroad 1.2 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.4 1.9 9.7 0.0 1.1 Recreation areas 1.5 2.4 0.9 1.3 0.0 0.1 0.0 1.0 6.6 0.4 0.2 18.1 2.2 Residential 34.5 18.1 1.5 37.3 36.6 16.2 0.1 20.4 38.2 10.6 17.3 45.3 34.7 Roof (commercial) 0.3 0.1 3.5 0.9 3.2 2.4 19.5 2.5 0.6 5.6 1.6 0.2 1.5 Roof (residential) 1.5 0.9 0.1 4.4 3.7 1.2 0.0 1.5 2.0 1.7 1.5 2.1 2.3 Quarry 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 10.5 0.0 1.0 Interstate highway 0.0 0.0 0.0 7.2 1.5 2.2 6.2 1.6 0.0 6.5 0.0 0.0 4.1 State highway 0.2 0.9 0.9 1.8 1.4 0.7 0.0 0.8 0.7 1.4 1.3 0.0 0.6 Town road 2.1 1.5 3.3 5.0 7.5 3.1 4.7 3.3 2.7 4.1 2.0 2.5 3.2 Water body 4.6 1.6 2.1 0.0 0.6 28.3 0.2 12.0 3.1 0.3 6.7 1.2 16.2 Wetland 14.6 21.6 16.3 2.2 12.4 3.4 7.6 10.0 9.7 0.2 6.9 3.6 1.1 Total 100 100 100 100 100 100 100 100 100 100 100 100 100

Total roadway 2.3 2.3 4.2 13.9 10.4 6.0 10.9 5.6 3.4 12.0 3.3 2.5 7.9

Total Impervious 10.5 5.5 15.7 23.6 21.4 41.5 60.4 25.5 11.4 30.3 38.0 6.7 31.2

1Includes drainage area of the subbasin that is not otherwise included in subbasins located upstream from the basin. For example, all subbasins are located in the drainage area of station 01104480, 

but land-use values listed for this station reflect only that drainage area not included under the U.S. Geological Survey stations listed in the table. 
Data Collection Methods 7

respectively). However, the amount of impervious area, 

including roadways, parking lots, and roofs, and the area of 

open water, differs between the two basins. The Hobbs Brook 

Basin contains about twice as much impervious area and 

nearly three times as much open water as the Stony Brook 

Basin (table 2).

Data Collection Methods

The monitoring network for the Cambridge drinkingwater source area includes seven USGS stations in the 

Hobbs Brook Basin and five USGS stations in the Stony 

Brook Reservoir watershed (fig.1). During 2004–08, three 

continuous-monitoring stations (USGS stations 01104415, 

01104430, and 01104433) were operating in the Hobbs Brook 

Basin, and five additional monitoring stations were operating in the Stony Brook Reservoir watershed (USGS stations 

01104455, 01104453, 01104460, 01104475 and 01104480). 

Continuous measurements of water levels, used to estimate 

streamflow, water temperature, and specific conductance, 

were recorded at each continuous-monitoring station. During 

the present study, the USGS collected water samples during 

base-flow conditions and during rain and mixed-precipitation 

stormflow conditions at five primary sampling stations in the 

Hobbs Brook and Stony Brook Basins. Four of these primary 

sampling stations are on tributaries (USGS stations 01104415, 

01104433, 01104455, and 01104475), and one station is on the 

main stem of Stony Brook (USGS station 01104460; fig. 1) 

upstream from the Stony Brook Reservoir. The samples were 

collected primarily during water years 2005–07. Additional 

water samples were collected at the secondary sampling stations (fig.1) during base-flow conditions. Continuous streamflow records and measurements of the central tendency for 

constituent concentrations were used to estimate loads and 

yields of major ions, TN and TP, selected pesticides, and caffeine for water years 2005 to 2007. Finally, loads and yields of 

major ions also were estimated for streamflow at continuousmonitoring stations using available records of streamflow and 

constituent concentrations estimated from records of specific 

conductance. 

Continuous Monitoring of Streamflow and 

Water Quality

Streamflow data are necessary to estimate constituent loads and yields in each subbasin or watershed. At each 

continuous-monitoring station, stream stage is recorded at a 

minimum interval of 10 minutes. Streamflow at each station 

is computed with a stage-discharge relation (or rating), which 

is developed and maintained on the basis of periodic manual 

measurements of streamflow (Rantz and others, 1982). Continuous water-temperature and specific-conductance data are 

recorded concurrently with stage by water-quality monitors at 

each continuous-monitoring station. Although measurements 

of water temperature and specific conductance are generally robust, the sensors are subject to fouling from aquatic 

growth, sedimentation, and debris. In such cases, corrections 

are applied to the data to improve accuracy as described by 

Wagner and others (2006a). More detailed descriptions of the 

monitoring network are provided by Smith (2005, 2007, 2008, 

and 2011).

Collection of Water-Quality Samples

Water samples were collected from streams in all the 

subbasins in the drinking-water source area and from the rawwater intake to the treatment plant at Fresh Pond Reservoir 

during the study period (water years 2005–07); as previously 

mentioned, a few samples also were collected in water years 

2004 and 2008. At most primary sampling stations (table 1), 

15 water samples were collected manually during base-flow 

conditions, and 15 composite samples were collected with 

automatic samplers during stormflow conditions. Only 12 

samples of base-flow water and 12 composite samples of 

storm runoff were collected at USGS station 01104460 on the 

main stem of Stony Brook. Water samples also were collected 

at secondary sampling stations (USGS stations 01104390, 

01104405, 01104410, 01104420, 01104430, and 01104453) 

during three synchronous sampling rounds (Smith 2008, 2011) 

when samples of base flow were collected at primary sampling 

stations. 

Samples of base flow were collected throughout the 

year to reflect seasonal variations. Water samples were collected manually using standard USGS techniques (Wilde and 

others, 1999) during base-flow conditions preceded by a dry 

period of at least 3 days. Stormflow sampling periods were 

selected to reflect seasonal variation and variation in the length 

of antecedent dry conditions existing throughout the study 

period (fig. 2). In this study, storms are defined, in terms of 

wet precipitation, as events with one or more measurements 

of wet precipitation equal to or greater than 0.01 inch (in.; the 

minimum resolution of the rain gage) and preceded by a dry 

period of at least 6 hours. Most of the storms sampled during 

this study were within the interquartile range (between the 

25th and 75th percentiles) of precipitation totals greater than 

0.10 in. recorded from 1967 through 2007 by the National Climatic Data Center near Boston, Mass. (Granato, 2007, fig. 3), 

as well as the interquartile range of all storm volumes greater 

than 0.10 in. recorded at the Cambridge Reservoir (USGS station 01104430) during the study period. 

Stormflow samples were collected with automatic samplers controlled by a datalogger (Smith 2008, 2011). The first 

subsample was collected when flow exceeded a flow threshold 

that was marginally greater than pre-storm base flow, and 

subsequent subsamples were collected at flow-proportional 

intervals (fig. 4). Collection of subsamples continued throughout the flow recession following the peak flow. The length of 

the recession period was calculated on the basis of the size of 
8 Water Quality in the Cambridge Drinking-Water Source Area, Massachusetts, Water Years 2005–07

0

5

10

15

20

25

Storm during which water sample

 was collected at station 01104415

Empirical probability distribution

function for all antecedent periods

Antecedent dry period, in days

Percentage of storms in which the antecedent dry period was equaled or exceeded at station 01104430

0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 98 99 99.5 99.9

EXPLANATION

Figure 2. Probability distribution of 

dry antecedent periods during water 

years 2005–07 for U.S. Geological 

Survey station 01104430 in the 

Cambridge Reservoir watershed, 

Massachusetts, in relation to the dry 

antecedent periods for storms where 

runoff samples were collected 

at U.S. Geological Survey station 

01104415.

each drainage area at the location of the USGS sampling sta- in the

tion using the equation proce

labor

T A = ( ) / . 2 59 0 2., samp

 (1) tions 

where small

T is the time value, in days for the length of strea

 the recession period; solve

A is the area of the drainage basin, in square limite

 kilometers (km2 ) upstream from the samp

 sampling station; for in

2.59 is a unit conversion constant; the 2

 and diaph

 0.2 is a constant (Bedient and Huber, 2002). homo

be an

In calculating a value of T for USGS station 01104460, throu

the drainage area upstream from the Cambridge Reservoir was 0.45-

excluded from the total drainage area because the flow from caffei

the upper basin is regulated and not affected by stormwater throu

runoff. Each automatic sampler was configured to hold one ter wi

20-liter glass bottle and fitted with a pre-cleaned 1/2-in. inner- partic

diameter Teflon intake and discharge tube, and a short piece of packe

silicon pump-head tube. Water

Physical properties, including water temperature, wher

specific conductance, pH, dissolved oxygen concentration, major

and turbidity, were measured during the collection of base- and 

flow water samples. These properties, except for dissolved long 

oxygen concentration and temperature, also were measured 1998;

 composite samples of stormflow. Water samples were 

ssed in the USGS Massachusetts Water Science Center 

atory in Northborough, Mass., at the conclusion of each 

ling. For water samples collected during base-flow condiwhen the flow and depth of water in the streams were 

, the water generally was collected in the centroid of the 

m in separate bottles designated for whole water, disd constituents, and organic compounds; therefore, only 

d splitting was necessary at the laboratory. For water 

les collected during storms, subsamples to be analyzed 

organic and organic constituents were split directly from 

0-L glass bottle by transferring the water with a Teflonragm pump while the contents of the sample bottle were 

genized with a stainless-steel laboratory mixer. Water to 

alyzed for dissolved inorganic constituents was filtered 

gh a 600-cm2

 (square centimeter) capsule filter with a 

micrometer pore size. Water to be analyzed for dissolved 

ne, pesticides, and pesticide metabolites was filtered 

gh a 142-millimeter (mm) pre-combusted glass-fiber filth a nominal 0.7-mm pore diameter to remove suspended 

ulate matter. After the processing, the samples were 

d in ice and shipped overnight to the USGS National 

 Quality Laboratory (NWQL), in Lakewood, Colorado, 

e they were analyzed for concentrations of dissolved 

 ions, total nutrients, caffeine, and 59 polar pesticides 

metabolites (table 3; Patton and Kryskalla, 2003; Furand others, 2001; American Public Health Association, 

 Fishman, 1993; Fishman and Friedman, 1989). Three 
Data Collection Methods 9

0.1

1

10

Precipitation, in inches

NCDC

(1967−2007)

Cambridge Reservoir

station 01104430

(water years 2005−07)

Stormwater

samples

(2005−07)

10th percentile

25th percentile

75th percentile

90th percentile

50th percentile

Mean

Maximum

Minimum

Number of precipitation events

EXPLANATION

(x) 

(2,448) (210) (15)

Figure 3. Distribution of precipitation totals greater than 0.10 inch recorded by the National Climatic Data Center near Boston, 

Massachusetts, 1967–2007; precipitation totals for all storms recorded at the Cambridge Reservoir during water years 2005–07; and 

precipitation totals for storms during which samples were collected at each of the primary sampling stations in the Cambridge drinkingwater source area, Massachusetts, 2005–07. (NCDC, National Climatic Data Center)
10 Water Quality in the Cambridge Drinking-Water Source Area, Massachusetts, Water Years 2005–07

4/26 20:004/27 00:004/27 04:004/27 08:004/27 12:004/27 16:004/27 20:004/28 00:004/28 04:004/28 08:004/28 12:004/28 16:00

0

2

4

6

8

10

Streamflow discharge, in cubic feet per second

Date and time, water year 2007

Estimated stormflow component

Estimated base-flow component

EXPLANATION

Subsample collection

Duration after peak

Figure 4. Example of automated flow-proportional 

collection of stormflow subsamples at U.S. Geological 

Survey station 01104415, Cambridge drinking-water 

source area, Massachusetts, water year 2007.

base-flow and two storm samples collected in water years 

2004 and 2005 also were analyzed for concentrations of suspended sediment, total-recoverable metals, and polyaromatic 

hydrocarbons (Smith 2005, 2007), although these limited data 

are not included in this report.

Quality-Assurance and Quality-Control Samples

Quality-assurance samples were collected to determine 

bias and precision associated with the sample data. These 

quality-control samples include 5 field blanks, 15 replicate 

samples, and 10 field-matrix spike samples (Smith 2008, 

2011). Analytical results for these samples provided the basis 

for the interpretation of chemical data collected in the drinking-water source area. In general, the quality-assurance data 

indicate that the sample data are relatively free from contamination and analytical results are precise.

A field blank is used to test for positive bias that can 

result from contamination at any stage of sample collection, processing, or analysis. Field blanks were collected and 

processed with manual discrete sampling techniques and with 

stormflow sampling equipment. Trace amounts of Ca, Cl, Na, 

TN, TP, and caffeine were detected in at least one field blank. 

These constituents were not detected in samples of the water 

used as the source of the field blank. However, the maximum 

concentrations of Ca, Cl, Na, and TN measured in field blanks 

were one or more orders of magnitude less than the lowest 

concentrations found in all environmental samples and were 

within the precision of the analytical methods (Patton and 

Kryskalla, 2003; Fishman and Friedman, 1989). Concentrations of TP (0.01milligrams per liter (mg/L)) and caffeine 

(0.008 micrograms per liter (μg/L)) were measured in a single 

field blank. If it is assumed that the amount of phosphorus 

and caffeine measured in the field-blank water represents 

the maximum amount of contamination present in any given 

sample, the potential positive bias values for concentrations of 

TP and caffeine, compared to the mean for all concentrations 

greater than the detection limit in all samples collected during 

the study period, are 15 and 5 percent, respectively. No other 

constituents were detected in field blanks.

Replicate samples are samples that are thought to be 

identical in composition to the environmental samples. Comparisons of replicate samples provide a measure of bias and 

variability for the method of sample collection, sample processing (splitting, filtering, and preservation), and laboratory 

analysis. All replicate samples were collected concurrently 

using manual sampling techniques, paired automatic samplers, 

and concurrent manual and automatic sampling techniques 

(Smith 2008, 2011). The relative percent differences (RPDs) 

calculated for analytes detected in each pair of replicate 

samples indicated good agreement between replicates (fig. 5). 

The mean RPDs for dissolved Ca, Cl, Na, and SO4

, and TN 

were less than 3 percent. The mean RPDs for TP, caffeine, 

herbicides, insecticides, and fungicides were about 7, 9, 14, 9, 

and 8 percent, respectively.

Field-matrix spikes are quality-control samples in which 

known amounts of target compounds are added to a sample. 

The recovery of spiked analytes is used to assess the bias 

and variability associated with degradation of target analytes 

during holding and shipment to the laboratory, the limitations 

of the analytical method, and interferences contained in the 

environmental sample that mask or enhance determinations of 
Data Collection Methods 11

Table 3. Physical properties and constituents measured in water samples, respective long-term method detection limits, and 

analytical methods.—Continued

[μS/cm, microsiemens per centimeter at 25 degrees celsius; LTMDL, long-term method detection level; NTRU, nephelometric turbidity ratio units; USEPA, 

U.S. Environmental Protection Agency; mg/L, milligrams per liter; μg/L, micrograms per liter; IC, ion chromatography; ICP, inductively coupled plasma; 

ICP-AES, inductively coupled plasma-atomic emission spectrometry; GCSPE+HPLCMS, graphitized carbon-based solid-phase extraction and high-performance liquid chromatography/mass spectrometry]

Physical property or constituent LTMDL Analytical 

technique

Constituent 

detected in one 

or more 

water samples 

Reference

Turbidity, unfiltered, NTRU 0.05 USEPA 180.1 Yes Anderson, 2004

pH in standard units 0.1 Glass electrode Yes Wilde and others, 2006

Specific conductance in μS/cm 8 Wheatstone Bridge Yes Radtke and others, 2005

Calcium, filtered, mg/L 0.01 ICP-AES Yes Fishman, M.J., ed., 1993

Sodium, filtered, mg/L 0.1 ICP-AES Yes Fishman, M.J., ed., 1993

Chloride, filtered, mg/L 0.1 IC Yes Fishman and Friedman, 1989

Sulfate, filtered, mg/L 0.09 IC Yes Fishman and Friedman, 1989

Total nitrogen (nitrate + nitrite + ammonia + 

organic-nitrogen), unfiltered, mg/L

0.03 Alkaline persulfate digestion Yes Patton and Kryskalla, 2003

Phosphorus, unfiltered, mg/L 0.01 Alkaline persulfate digestion Yes Patton and Kryskalla, 2003

2,4-D methyl ester, μg/L 0.1 GCSPE+HPLCMS Yes Furlong and others, 2001

2,4-D, μg/L 0.02 GCSPE+HPLCMS Yes Furlong and others, 2001

2,4-DB, μg/L 0.01 GCSPE+HPLCMS No Furlong and others, 2001

2-Chloro-4-isopropylamino-6-amino-striazine, μg/L0.014 GCSPE+HPLCMS No Furlong and others, 2001

2-Chloro-6-ethylamino-4-amino-s-triazine 

(CEAT), μg/L

0.04 GCSPE+HPLCMS Yes Furlong and others, 2001

2-Hydroxy-4-isopropylamino-6-ethylaminos-triazine (OIET), μg/0.04 GCSPE+HPLCMS Yes Furlong and others, 2001

3-Hydroxy carbofuran, μg/L 0.004 GCSPE+HPLCMS No Furlong and others, 2001

Acifluorfen, μg/L 0.014 GCSPE+HPLCMS No Furlong and others, 2001

Aldicarb sulfone, μg/L 0.009 GCSPE+HPLCMS No Furlong and others, 2001

Aldicarb sulfoxide, μg/L 0.02 GCSPE+HPLCMS No Furlong and others, 2001

Aldicarb, μg/L 0.02 GCSPE+HPLCMS No Furlong and others, 2001

Atrazine, μg/L 0.004 GCSPE+HPLCMS Furlong and others, 2001

Bendiocarb, μg/L0.02 0.02 GCSPE+HPLCMS No Furlong and others, 2001

Benomyl, μg/L 0.011 GCSPE+HPLCMS Yes Furlong and others, 2001

Bensulfuron-methyl, μg/L 0.009 GCSPE+HPLCMS No Furlong and others, 2001

Bentazon, μg/L 0.01 GCSPE+HPLCMS No Furlong and others, 2001

Bromacil, μg/L 0.009 GCSPE+HPLCMS No Furlong and others, 2001

Bromoxynil, μg/L 0.06 GCSPE+HPLCMS No Furlong and others, 2001

Carbaryl, μg/L 0.009 GCSPE+HPLCMS Yes Furlong and others, 2001

Carbofuran, μg/L 0.008 GCSPE+HPLCMS No Furlong and others, 2001

Chloramben methyl ester, μg/L 0.012 GCSPE+HPLCMS No Furlong and others, 2001
12 Water Quality in the Cambridge Drinking-Water Source Area, Massachusetts, Water Years 2005–07

Table 3. Physical properties and constituents measured in water samples, respective long-term method detection limits, and 

analytical methods.—Continued

[μS/cm, microsiemens per centimeter at 25 degrees celsius; LTMDL, long-term method detection level; NTRU, nephelometric turbidity ratio units; USEPA, 

U.S. Environmental Protection Agency; mg/L, milligrams per liter; μg/L, micrograms per liter; IC, ion chromatography; ICP, inductively coupled plasma; 

ICP-AES, inductively coupled plasma-atomic emission spectrometry; GCSPE+HPLCMS, graphitized carbon-based solid-phase extraction and high-performance liquid chromatography/mass spectrometry]

Physical property or constituent LTMDL Analytical 

technique

Constituent 

detected in one 

or more 

water samples 

Reference

Chlorimuron-ethyl, μg/L

Chlorodiamino-s-triazine, μg/L

Clopyralid, μg/L

Cycloate, μg/L

Dacthal monoacid, μg/L

Dicamba, μg/L

Dichlorprop, μg/L

Dinoseb, μg/L

Diphenamid, μg/L

Diuron, μg/L

Fenuron, μg/L

Flumetsulam, μg/L

Fluometuron, μg/L

Imazaquin, μg/L

Imazethapyr, μg/L

Imidacloprid, μg/L

Linuron, μg/L

MCPA, μg/L

MCPB, μg/L

Metalaxyl, μg/L

Methiocarb, μg/L

Methomyl, μg/L

Metsulfuron-methyl, μg/L

N-(4-Chlorophenyl)-N’-methylurea, μg/L

Neburon, μg/L

Nicosulfuron, μg/L

Norflurazon, μg/L

Oryzalin, μg/L

Oxamyl, μg/L

Picloram, μg/L

Propham, μg/L

Propiconazole, μg/L

0.016

0.02

0.03

0.007

0.014

0.018

0.014

0.019

0.005

0.02

0.02

0.02

0.008

0.018

0.019

0.03

0.007

0.03

0.1

0.02

0.02

0.03

0.07

0.018

0.006

0.02

0.01

0.02

0.02

0.016

0.015

0.005

GCSPE+HPLCMS

GCSPE+HPLCMS

GCSPE+HPLCMS

GCSPE+HPLCMS

GCSPE+HPLCMS

GCSPE+HPLCMS

GCSPE+HPLCMS

GCSPE+HPLCMS

GCSPE+HPLCMS

GCSPE+HPLCMS

GCSPE+HPLCMS

GCSPE+HPLCMS

GCSPE+HPLCMS

GCSPE+HPLCMS

GCSPE+HPLCMS

GCSPE+HPLCMS

GCSPE+HPLCMS

GCSPE+HPLCMS

GCSPE+HPLCMS

GCSPE+HPLCMS

GCSPE+HPLCMS

GCSPE+HPLCMS

GCSPE+HPLCMS

GCSPE+HPLCMS

GCSPE+HPLCMS

GCSPE+HPLCMS

GCSPE+HPLCMS

GCSPE+HPLCMS

GCSPE+HPLCMS

GCSPE+HPLCMS

GCSPE+HPLCMS

GCSPE+HPLCMS

No

No

No

No

No

No

No

Yes

No

Yes

Yes

Yes

Yes

Yes

No

Yes

No

Yes

No

Yes

No

No

Yes

No

No

No

Yes

No

Yes

No

No

Yes

Furlong and others, 2001

Furlong and others, 2001

Furlong and others, 2001

Furlong and others, 2001

Furlong and others, 2001

Furlong and others, 2001

Furlong and others, 2001

Furlong and others, 2001

Furlong and others, 2001

Furlong and others, 2001

Furlong and others, 2001

Furlong and others, 2001

Furlong and others, 2001

Furlong and others, 2001

Furlong and others, 2001

Furlong and others, 2001

Furlong and others, 2001

Furlong and others, 2001

Furlong and others, 2001

Furlong and others, 2001

Furlong and others, 2001

Furlong and others, 2001

Furlong and others, 2001

Furlong and others, 2001

Furlong and others, 2001

Furlong and others, 2001

Furlong and others, 2001

Furlong and others, 2001

Furlong and others, 2001

Furlong and others, 2001

Furlong and others, 2001

Furlong and others, 2001
Data Collection Methods 13

Table 3. Physical properties and constituents measured in water samples, respective long-term method detection limits, and 

analytical methods.—Continued

[μS/cm, microsiemens per centimeter at 25 degrees celsius; LTMDL, long-term method detection level; NTRU, nephelometric turbidity ratio units; USEPA, 

U.S. Environmental Protection Agency; mg/L, milligrams per liter; μg/L, micrograms per liter; IC, ion chromatography; ICP, inductively coupled plasma; 

ICP-AES, inductively coupled plasma-atomic emission spectrometry; GCSPE+HPLCMS, graphitized carbon-based solid-phase extraction and high-performance liquid chromatography/mass spectrometry]

Physical property or constituent LTMDL Analytical 

technique

Constituent 

detected in one 

or more 

water samples 

Reference

Propoxur, μg/L 0.004 GCSPE+HPLCMS Yes Furlong and others, 2001

Siduron, μg/L 0.01 GCSPE+HPLCMS Yes Furlong and others, 2001

Sulfometuron-methyl, μg/L 0.03 GCSPE+HPLCMS Yes Furlong and others, 2001

Tebuthiuron, μg/L 0.013 GCSPE+HPLCMS Yes Furlong and others, 2001

Terbacil, μg/L 0.02 GCSPE+HPLCMS No Furlong and others, 2001

Triclopyr, μg/L 0.02 GCSPE+HPLCMS Yes Furlong and others, 2001

Caffeine, μg/L 0.02 GCSPE+HPLCMS Yes Furlong and others, 2001

1,000

0.001

0.01

0.1

1

10

100

1,000

Calcium, in milligrams per liter

Sodium, in milligrams per liter

Chloride, in milligrams per liter

Sulfate, in milligrams per liter

Total nitrogen, in milligrams per liter

Total phosphorus, in milligrams per liter

Caffeine, in micrograms per liter

Herbicides, in micrograms per liter

Insecticides, in micrograms per liter

Fungicides, in micrograms per liter

Constituent concentration, in measured units, for second sample

 of concurrent replicate pair

Constituent concentration, in measured units, for first sample of concurrent replicate pair

 EXPLANATION

1:1 line

0.001 0.01 0.1 1 10 100

Figure 5. Relation of concentrations of selected constituents detected in concurrent replicate 

streamflow samples, Cambridge drinking-water source area, Massachusetts, water years 2005–07. 

(Symbols showing fungicides, herbicides, and insecticides represent concentrations of individual 

compounds listed in table 7).
14 Water Quality in the Cambridge Drinking-Water Source Area, Massachusetts, Water Years 2005–07

the target analytes. Samples are routinely spiked in the laboratory to provide a measure of method performance. In general, 

laboratory spike recoveries for pesticides vary widely. For 

example, during water years 2007–08, the median value for 

the lower and upper 99-percent confidence limits for recovery 

of spiked analytes in laboratory preparation samples for the 

pesticides listed in table 3 ranged from about 30 to 140 percent 

(Smith, 2011). Recovery values for compounds in spiked environmental samples will at best fall within laboratory recovery 

criteria. Approximately 17 percent of the recoveries of spiked 

pesticides were outside of the laboratory recovery criteria; 

however, recoveries for most of the pesticides that were routinely detected met method criteria. Most recoveries that were 

outside of the method criteria tended to be low, particularly in 

water year 2007 (Smith 2011). The median RPD for replicate 

field spike samples was less than 12 percent, which indicates 

that although recoveries were low for some constituents, analytical methods were relatively precise.

Data Analysis Methods

Water-quality data were statistically analyzed to determine the relation between streamflow and land use, to calculate constituent loads and yields, and to assess temporal trends 

in selected constituents. A correlation analysis was performed 

relating concentrations of selected constituents to streamflow 

and subbasin characteristics. Concentration data, along with 

continuous records of streamflow and specific conductance, 

were used to estimate loads and yields of selected constituents 

during the study period. Trend analysis was performed for 

annual loads of Cl and Na for selected USGS streamgages 

during water years 1998–2008. 

Determination of Correlations

To examine factors affecting water quality in the drinking-water source area, a correlation analysis was performed 

relating constituent concentrations to the logarithmic values of 

streamflow and to subbasin land-use characteristics. Correlation analysis was performed for annual flow-weighted concentrations of Ca, Cl, Na, SO4

, TN, TP, caffeine, and selected 

pesticides in water samples collected at four primary USGS 

monitoring stations on the tributaries in the drinking-water 

source area during water years 2005–07. A correlation analysis 

also was performed relating subbasin land-use characteristics 

to mean concentrations of Ca, Cl, Na, SO4

, TN, and TP and to 

maximum concentrations of caffeine and selected pesticides 

in samples collected from the primary and secondary sampling stations (fig. 1), except for USGS stations 01104430 and 

1104460 where the quality of upstream water is affected by 

flow regulation, settling, and residence time in the Cambridge 

Reservoir.

The Pearson product moment correlation coefficient was 

used for the correlation analysis. This method measures the 

degree of linear relations between two variables. The correlation coefficient ranges from -1 to +1 (Helsel and Hirsch, 

2002). A negative coefficient indicates that one variable tends 

to increase as the other decreases, and a positive coefficient 

indicates that the two variables tend to increase together. 

The absolute value of the correlation coefficient indicates 

the strength of the relation between variables. Cohen (1988) 

describes correlation coefficients with an absolute value 

greater than 0.71, where the correlation between two variables 

is one in which at least 50 percent (the square of the correlation coefficient) of the variance in one variable is accounted 

for by the variance of the other variable, as strong. Furthermore, correlation coefficients with absolute values less than 

0.71 indicate a moderate to weak correlation, where less than 

one-half of the variance in one variable is accounted for by 

the other variable. In the following discussions, a correlation 

coefficient with an absolute value of 0.9 or larger indicates 

a strong linear relation, and a correlation coefficient with an 

absolute value between 0.71 and 0.90 indicates a moderately 

strong correlation. 

Estimated Loads and Yields

Loads are the quantity of a constituent transported by 

a stream during a specific time interval and are determined 

by multiplying the concentration or mass of a constituent by 

the streamflow. Yields are loads normalized by the drainage 

area of each subbasin. Loads and yields of Ca, Cl, Na, SO4

, 

TN, TP, selected pesticides, and caffeine were estimated from 

analyte concentrations in samples of base flow and stormflow 

and from streamflow data at the five primary sampling stations 

for water years 2005 to 2007. Loads and yields of Ca, Cl, 

Na, and SO4 also were estimated from continuous records of 

streamflow and specific conductance, and relations between 

dissolved major ions and specific conductance for all monitoring stations with these records. Yields were estimated using 

available data for each of the monitoring stations (table 1).

Loads Estimated from Streamflow

Various methods are available to estimate loads using 

constituent-concentration data and streamflow. In some cases, 

loads can be estimated on the basis of correlations between 

concentration data and streamflow. In this study, relations 

between discrete sample concentrations of selected constituents and instantaneous flow at the time of sample collection, 

and concentrations of selected constituents in composite samples and mean flow of each storm, were examined. Although 

the correlation between concentrations of most major ions 

and streamflow was significant (p < 0.05), the strength of the 

relations typically was not strong (less than 0.90; table 4). Correlations between concentrations of major ions and streamflow 

at USGS station 01104475 were not significant, except for Ca, 

and all of the relations were weak. The correlation between TP 

concentration and streamflow was significant for three primary 
Data Analysis Methods 15

Table 4. Pearson correlation coefficients relating concentrations of major ions and total nutrients to streamflow at five U.S. 

Geological Survey monitoring/sampling stations in the Cambridge and Stony Brook Reservoir watersheds, Massachusetts, 

water years 2005–07.

[Shaded areas indicate values that are significant at a 95-percent confidence level; data include one sample collected during water year 2004. USGS, 

U.S. Geological Survey]

USGS 

station

Number 

of

samples

Dissolved 

calcium

Dissolved 

chloride

Dissolved 

sodium

Dissolved 

sulfate

Total

phosphorus

Total

nitrogen

01104415 30 -0.70 -0.50 -0.52 -0.28 0.59 0.32

01104433 29 -0.80 -0.66 -0.64 -0.78 0.62 -0.31

01104455 30 -0.84 -0.81 -0.83 -0.91 0.80 0.35

01104460 24 -0.80 -0.48 -0.42 -0.21 0.33 0.32

01104475 30 -0.49 -0.06 -0.02 -0.29 0.04 -0.21

sampling stations (USGS stations 01104415, 01104433, and 

01104455); however, the relations were not strong. The correlation between TN concentration and streamflow was not 

significant for any of the five primary sampling stations. The 

correlation between pesticide concentrations and streamflow 

was not calculated because the frequency of detection of most 

pesticides was low and because the relations were expected 

to be weak since pesticides are applied once or twice during 

specific times each year and often are mobilized only during 

storms. In general, correlations between constituent concentrations and streamflow were neither consistently significant nor 

strong for the five primary sampling stations; however, many 

of these relations indicate that the concentrations of major ions 

tended to decrease with increasing flow and the concentration 

of TP tended to increase with increasing flow.

Because the correlations between constituent concentrations and streamflow for the five primary sampling stations 

were not sufficiently strong, loads were estimated on the basis 

of streamflow and a measure of the central tendency of each 

constituent using a stratified flow approach. Mean concentration, as opposed to medians, are particularly appropriate for 

characterizing loads because including outlier values (either 

high or low), which typically represent large flows, is important when characterizing transport of constituent masses. 

Data used to estimate loads were stratified into two 

classes (base flow and stormflow). The stratification was 

performed on the basis of the two sampling strategies and the 

separation of streamflow into a strictly base-flow component 

and a stormflow component that represent the sum of base 

flow, bank storage, and overland stormwater runoff. Continuous streamflow records for each of the five primary sampling 

stations were separated into these two components on the basis 

of the stormflow sampling algorithm described earlier. This 

method for identifying the period of the storm runoff is consistent with the fixed-base method used for hydrograph separation and described by Chow and others (1988), although in 

this study, the method was used only to identify the beginning 

and end of each storm and the associated volume of flow 

between these two points.

The Mann-Whitney test, also referred to as the rank-sum 

test, was used to determine whether constituent concentrations 

in base-flow and stormflow samples differed statistically (p

value less than 0.05). The Mann-Whitney test is a nonparametric method that makes no assumptions about the distribution 

of data and is used to determine whether groups of data come 

from the same population or alternatively whether the median 

values are different (Helsel and Hirsch, 2002). For constituents 

that were not detected or were present at levels less than the 

long-term method detection level (LTMDL; censored data), 

concentrations were set equal to a value less than the respective LTMDL. The LTMDL is the lowest detectable concentration of an analyte that can be measured and reported with 

99-percent confidence when the concentration is greater than 

zero. The chance of falsely reporting a concentration at or 

greater than the LTMDL for a sample that does not contain the 

analyte is predicted to be less than or equal to 1 percent (Childress and others, 1999). For analytes not detected in samples, 

a concentration equal to twice the LTMDL value (laboratory 

reporting limit) is reported with a “less than” (<) remark code 

in all data tables in this report. The substitution of a value less 

than the LTMDL approach is appropriate for rank-based nonparametric analysis methods for singly censored data (Helsel, 

2005). Base-flow and stormflow concentrations of pesticides 

were not compared because pesticides were infrequently 

detected and the median value typically was censored. 

With a few exceptions, concentrations of major ions and 

total nutrients in base-flow samples and stormflow composite 

samples were significantly different (table 5), and an independent mean concentration was determined for each data set. 

Where base-flow and stormflow concentrations of constituents were not significantly different, the mean concentration 

was determined from the combined dataset. For example, 

base-flow and stormflow concentrations of TN at USGS 

stations 01104415, 01104460, and 01104475 did not differ 
16 Water Quality in the Cambridge Drinking-Water Source Area, Massachusetts, Water Years 2005–07

Table 5. Attained significance levels (p values) from Mann-Whitney tests for paired sample sets of concentrations of 

selected constituents and properties collected during base-flow and stormflow conditions in the Cambridge drinkingwater source area, Massachusetts, water years 2005–07.

[USGS, U.S. Geological Survey; <, Less than; shaded areas indicate values that are significant at a 95-percent confidence interval; data 

include one sample each collected during water year 2004 at USGS stations 01104415, 01104433, 01104455, and 01104475]

Constituent or property

USGS station

01104415 01104433 01104455 01104460 01104475

Number of values

Chloride

Sodium

Calcium

Sulfate

Nitrogen, total

Phosphorus, total

pH

Turbidity

Specific Conductance

30

0.051

0.056

<0.001

0.016

0.395

<0.001

0.181

<0.001

0.038

29

<0.001

<0.001

<0.001

<0.001

0.023

<0.001

0.130

<0.001

0.001

30

<0.001

<0.001

<0.001

<0.001

0.0039

<0.001

0.427

<0.001

<0.001

24

0.355

0.371

0.019

0.023

0.977

0.002

0.048

0.204

0.215

30

0.135

0.130

<0.001

0.001

0.852

<0.001

0.018

<0.001

0.047

significantly and were considered a non-stratified dataset for 

each of the respective stations.

Concentrations of many pesticides generally were below 

the LTMDL and were reported as censored data (Smith, 2005, 

2007, 2008, and 2011). In a few cases, concentrations of TP 

and caffeine also were censored. Sample means for these analytes were estimated using the regression-on-order statistics 

(ROS) method. The ROS method is considered robust for the 

estimation of summary statistics for data sets with censored 

values (Helsel, and Cohn, 1988; Shumway and others 2002; 

Helsel, 2005) and is appropriate when 80 percent or less of the 

data are censored (Helsel, 2005). 

The distribution of many constituent concentrations, 

particularly TP in stormflow and pesticides and caffeine in 

stormflow and base flow, was lognormal rather than normal. 

For these constituents, means were estimated by assuming 

that the logarithms of the data were symmetric. This method 

provides a more reliable estimate of the population mean than 

the simple arithmetic sample mean without log transformation 

(Helsel and Hirsch, 2002). 

Loads of Major Ions Estimated from SpecificConductance Monitoring Data

Specific conductance measurements, which commonly 

are used to approximate concentrations of dissolved major 

ions (Hem, 1982, 1992; Miller and others, 1988; Church and 

others, 1996; Granato and Smith, 1999; Smith and Granato, 

2010; Smith and Breault, 2011), are used in this study to estimate concentrations of dissolved Ca, Cl, Na, and SO4

 in tributary water and to estimate concentrations and loads of these 

constituents in the drinking-water source area. Concentrations 

of dissolved Ca, Cl, Na, and SO4

 were measured in base-flow 

and stormflow samples (24–31 samples from primary sampling stations and 3 samples from secondary sampling stations 

during the study) but also were estimated from the more 

frequent, in-situ measurements of specific conductance. In 

comparison to the limited data from the water-quality samples, 

continuous records of specific conductance are available for 

all of the primary sampling stations and some secondary 

sampling stations in the drinking-water source area. These 

continuous specific conductance data are available for nearly 

every day of the study and represent a variety of hydrologic 

conditions (base flow, rain, mixed precipitation, and snowmelt 

runoff events). 

Instantaneous concentrations of Ca, Cl, Na, and SO4

were estimated from continuous measurements of specific 

conductance by using equations developed to relate specific 

conductance to concentrations of the respective ions (eq. 2 

below). These regression equations were developed using the 

MOVE.1 technique (Maintenance Of Variance-Extension, type 

1), also known as the line of organic correlation (Helsel and 

Hirsch, 1992; Hirsch, 1982), on the basis of concurrent measurements of log-transformed values of specific conductance 

and concentrations of Ca, Cl, Na, and SO4

 in samples collected from tributaries in the drinking-water source area. The 

data set for the study period includes 178 concentrations for 

these ions, where specific conductance values ranged from 67 

to 30,100 microsiemens per centimeter at 25 degrees Celsius 

(μS/cm) (Smith, 2005, 2007, and 2011; U.S. Geological Survey, 2001). The MOVE.1 technique was chosen for regression 

analysis because it minimizes errors in both the x and y directions, producing a unique equation that can be used to predict 

either variable from the other (Helsel and Hirsch, 1992).
Data Analysis Methods 17

C = ( ) Spcm ×b,

 (2)

where

 C is the concentration of the ion of interest, 

in mg/L;

 Spc is the specific conductance, in μS/cm;

m is the slope from the MOVE.1 analysis 

(table 6); and

b is the intercept from the MOVE.1 analysis 

(table 6).

These equations were optimized for each site on the basis 

of the milliequivalent ratio of each ion to Cl. Expressing concentrations in terms of milliequivalents normalized the charge 

of each ion. The mean ratios of Ca:Cl, Na:Cl, and SO4

:Cl 

were estimated for each station (fig 6). Separate equations 

were developed for each constituent and for stations where 

the ratios of the constituent to Cl were very different from 

others, based on visual inspection of boxplots of the data (for 

example, station 01104475, fig. 6). Individual equations were 

developed to relate measurements of specific conductance to 

concentrations of Ca, except for USGS stations 01104430 and 

01104455 for which a single equation was developed. One 

equation was developed to relate measurements of specific 

conductance to concentrations of Cl, Na, and SO4

 for all 

USGS stations, except USGS station 01104475 which required 

a separate equation to optimize the root mean square error. 

These equations provide reasonable estimates for dissolved 

concentrations of Ca, Cl, Na, and SO4

 on the basis of specific 

conductance measurements.

Annual loads of Ca, Cl, Na, and SO4

 were estimated for 

the period of record for all sampling sites for which streamflow and continuous specific-conductance data were available 

(except for loads previously calculated using similar methods 

for water year 1998 and reported by Waldron and Bent, 2001). 

Loads for USGS stations 01104430 and 01104460 (stations 

where flow is regulated by the CWD) were calculated by multiplying estimated daily flow-weighted concentrations of Ca, 

Cl, Na, and SO4

, in milligrams per liter, by daily flow, in liters 

per day, and added. For USGS stations 01104415, 01104433, 

011004455, and 01104475, which are primary sampling stations where both base-flow and stormflow samples were collected, annual base-flow and stormflow loads were estimated, 

in part, using the area-base hydrograph separation method 

described earlier. However, in this instance, the stormflow 

component is separated from the base-flow component as 

opposed to simply estimating the total volume of flow between 

the beginning and ending of each period of stormflow. Unlike 

the previous load estimation method that is based on the 

central tendency of constituent concentrations in base flow and 

stormflow where the concentration of a constituent of interest 

is unknown at any given point in time (except for the actual 

sample time), this method enables a flow-weighted concentration to be estimated from records of streamflow and specific 

conductance. In this case, the specific conductance in base 

flow just prior to a runoff event (rain, mixed precipitation, 

or snow melt) is assigned to the initial base-flow component 

during the event. Similar to the fixed-base method for hydrograph separation, the assigned base-flow specific conductance is extended to the peak flow as long as a mass balance 

between the base-flow specific conductance and stormflow 

specific conductance is achieved. Beginning with the point of 

peak flow, the specific conductance assigned to the base-flow 

component is adjusted over time for the remainder of the event 

on the basis of the slope derived from the starting specific conductance value and the specific conductance on the recession 

limb of the hydrograph at the end of the event (fig 7). During 

this projection, the specific conductance initially assigned to 

the base-flow component decreases with time. This is a typical 

response in the subbasins in this study because groundwater 

containing higher concentrations of major ions mixes with 

subsurface flow (rain water which has infiltrated into the 

ground) containing low concentrations of major ions (National 

Atmospheric Deposition Program, 2008) and discharges 

to the stream. The daily flow for USGS stations 01104415, 

01104433, 011004455, and 01104475, representing base flow 

and stormflow, was summed by water year. Annual base-flow 

and stormflow loads of Ca, Cl, Na, and SO4

 for USGS stations 01104415, 01104433, 011004455, and 01104475 were 

calculated by multiplying daily flow-weighted concentrations 

associated with each flow component by the respective daily 

flow, in liters per day, and summed for each water year. 

It is difficult to assess the accuracy of simplified hydrograph separation techniques because the methods to estimate 

base flow are based solely on drainage area and, therefore, 

do not include the effects of various basin features that may 

be either natural or anthropogenic. Stormflow end points 

resulting from the fixed-base method for hydrograph separation in this study generally agree with end points subjectively 

chosen in analysis of storm hydrographs at each of the USGS 

stations. Continuous measurements of specific conductance, 

which were used in other studies to aid in base-flow separation 

(Pellerin and others, 2007; Yu and Schwartz, 1999; Matsubayashi and others, 1993; Pilgrim and others, 1979; and Pinder 

and Jones, 1969), also tend to asymptotically converge with 

initial base-flow values (fig. 7) at the end points resulting from 

the separation method used in this study. Nevertheless, the 

accuracy of results from the simplified method for individual 

storms is likely variable. These methods provide an estimate 

of the volume of water and load of constituents associated 

with each component of flow, and errors in estimates of baseflow volume may balance out over long periods of time. Loads 

estimated for each component of flow are important as this 

information is useful in assessing best-management practices 

(BMPs) or land-use changes in the drinking-water source area.
18 Water Quality in the Cambridge Drinking-Water Source Area, Massachusetts, Water Years 2005–07 Table 6. Regression equation coefficients used to estimate concentrations of calcium, chloride, sodium, and sulfate from flow-weighted values of specific conductance for U.S. Geological Survey monitoring/sampling stations in the Cambridge drinking-water source area, Massachusetts, water years 2005–07. [RMSE, root mean square error. Concentration data include one sample from water year 2004 and water year 2008 for most U.S. Geological Survey stations used in this study.] Calcium Chloride Sodium Sulfate Station RMSE, RMSE, RMSE, RMSE, Slope Intercept Slope Intercept Slope Intercept Slope Intercept in percent in percent in percent in percent 01104415 0.909 0.047 9.2 01104430 0.652 0.288 4.3 01104433 1.016 0.034 13 1.112 0.121 2.5 1.105 0.069 5.2 0.637 0.232 18 01104455 0.652 0.288 27 01104460 0.733 0.206 7.9 01104475 1.065 0.042 9.3 1.482 0.011 7.0 1.229 0.028 5.1 1.188 0.022 13
Data Analysis Methods 19

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Milliequivalent ratio for calcium:chloride Milliequivalent ratio for sodium:chloride

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

U.S. Geological Survey station number 

0.00

0.05

0.10

0.15

0.20

0.25

0.30

Milliequivalent ratio for sulfate:chloride

01104415 01104430 01104433 01104455 01104460 01104475

25th percentile

75th percentile

Median

 Maximum

 Minimum

EXPLANATION

Mean

Figure 6. Milliequivalent ratios of calcium, sodium, and sulfate to 

chloride in sample populations from six U.S. Geological Survey monitoring 

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

water years 2004–08.
20 Water Quality in the Cambridge Drinking-Water Source Area, Massachusetts, Water Years 2005–07

4/27

00:00

4/27

04:00

4/27

08:00

4/27

12:00

4/27

16:00

4/27

20:00

4/28

00:00

4/28

04:00

4/28

08:00

4/28

12:00

600

800

1,000

1,200

1,400

Specific conductance, in microsiemens per centimeter at 25 degrees Celsius

Date and time

400

1,600

Streamflow discharge, in cubic feet per second

 Estimated stormflow specific conductance

 Estimated base-flow specific conductance

EXPLANATION

Point of peak flow

End of storm

Start of storm

 Streamflow discharge

0

2

4

6

8

10

12

Figure 7. Example of separation of baseflow and stormflow specific conductance 

at U.S. Geological Survey station 01104415, 

Cambridge drinking-water source area, 

Massachusetts, water year 2007.

Determination of Trends

Time trends in concentrations or loads of water-quality 

constituents can indicate long-term changes in streamwater 

quality. Statistical tests to identify time trends were performed 

for annual loads of Cl and Na estimated from records of 

flow and specific conductance at USGS monitoring stations 

01104430 and 01104460 for water years 1998 to 2008. Trend 

tests were not performed on similar data at other monitoring 

stations within the drinking-water source area nor were tests 

performed on nutrient and pesticide data because the length 

of record (about 3 years) was insufficient to produce reliable 

results from the test. Trends for each constituent at each USGS 

station were analyzed by using the seasonal Mann-Kendall test 

(Helsel and Hirsch, 2002). A trend was considered significant 

if the p value was less than or equal to 0.05.

Water-Quality Conditions

Summary statistics for measured properties (temperature, 

pH, specific conductance, and dissolved oxygen concentration) and the chemical composition of water in the Cambridge 

drinking-water source area were compared to water-quality 

criteria and used to describe water-quality conditions. In 

addition, factors, such as land use, that potentially affect 

water quality are discussed in this section. Summary statistics, including the minimum, mean, median, maximum, and 

the standard deviation for each constituent are provided in 

table 7 for the five primary sampling stations and the rawwater intake at the Fresh Pond Reservoir and in table 8 for 

base-flow samples collected at secondary sampling stations 

(Smith 2008 and 2011).

Because median values are less influenced by high or low 

concentrations (or outliers), they are more appropriate descriptors of data that are not normally distributed than are mean 

values. Medians are especially useful for summarizing small 

datasets, such as the data for the secondary sampling stations. 

However, mean values are generally more appropriate for 

characterization of loads as discussed earlier.

Water Properties

Water-quality properties include specific conductance, 

dissolved oxygen concentration, pH, and turbidity. These 

properties are general indicators of water-quality conditions 

and can be used to identify site-specific issues. Available statistics for each property are summarized in tables 7 and 8 for 

the study period.
Water-Quality Conditions 21

Table 7. Summary statistics for selected water-quality properties, major ions, total nutrients, caffeine, and pesticides in samples of base flow and stormflow collected at five primary sampling stations in the Cambridge drinking-water source area and at the raw water intake at Fresh Pond Reservoir, Cambridge, Massachusetts, water years 2005–07.—Continued[USGS, U.S. Geological Survey; n, number of samples; mg/L, milligrams per liter; μg/L, micrograms per liter; NTRU, nephelometric turbidity ratio units; μS/cm, microsiemens per centimeter at 25 degrees Celsius; <, concentration is less than value shown; --, unable to estimate value with available data; one set of samples from the end of water year 2004 and one sample from station 01104475 from water year 2008 are included]

Property or constituent

USGS station 01104415

Base flow (n=15) Stormflow (n=15)

Minimum Mean Median Maximum Standard deviation Minimum Mean Median Maximum Standard deviation

Turbidity, NTRU 0.2 1.5 1.5 2.5 0.60 6.4 40 29 180 44

Dissolved oxygen, mg/L 5.3 9.5 10.1 12.8 2.8 -- -- -- -- --

pH, standard units1 6.1 6.6 6.8 7.1 0.30 6.3 6.8 6.9 7.4 0.30

Specific conductance, μS/cm 1,490 2,090 2,140 3,160 422 356 1,300 788 3,400 1,020

Calcium, mg/L 39.4 53.8 53.2 88.0 11.5 10.5 27.0 20.8 61.3 16.9

Sodium, mg/L 227 333 329 538 74.6 48.8 204 123 534 166

Chloride, mg/L 397 611 621 970 136 80.4 368 205 1,050 315

Sulfate, mg/L 23.1 27.0 26.6 30.4 2.42 6.2 20.3 14.9 57.6 14.8

Total nitrogen, mg/L 0.97 1.50 1.57 2.02 0.29 1.09 1.76 1.54 3.14 0.58

Total phosphorus, mg/L 0.006 0.011 0.01 0.020 0.004 0.04 0.20 0.13 0.53 0.16

Caffeine, μg/L 0.008 0.033 0.014 0.234 0.058 <0.2 0.678 0.716 1.82 0.470

2,4-D, μg/L 0.017 -- <0.02 -- -- 0.143 0.655 0.143 7.92 2.01

2-Chloro-6-ethylamino-4-amino 

s-triazine (CEAT), μg/L

<0.04 -- -- -- -- <0.04 -- -- -- --

Atrazine, μg/L <0.004 -- -- -- -- <0.004 -- -- -- --

Benomyl, μg/L <0.01 -- -- -- -- <0.01 -- -- -- --

Carbaryl, μg/L <0.02 -- -- -- -- 0.009 0.018 0.011 0.051 0.015

Diuron, μg/L 0.003 0.009 0.006 0.022 0.006 <0.02 -- -- -- --

Imidacloprid, μg/L 0.008 0.011 0.021 0.032 0.069 0.019 0.375 0.104

Metalaxyl, μg/L <0.04 -- -- -- -- <0.04 -- -- -- --

Norflurazon, μg/L <0.01 -- -- -- -- <0.01 -- -- -- --Propiconazole, μg/L <0.005 -- -- -- -- <0.005 -- -- -- --Siduron, μg/L 0.007 0.012 0.011 0.015 0.005 0.007 0.071 0.016 0.696 0.175

Sulfometuron-methyl, μg/L <0.03 -- -- -- -- <0.03 -- -- -- --
22 Water Quality in the Cambridge Drinking-Water Source Area, Massachusetts, Water Years 2005–07 Table 7. Summary statistics for selected water-quality properties, major ions, total nutrients, caffeine, and pesticides in samples of base flow and stormflow collected at five primary sampling stations in the Cambridge drinking-water source area and at the raw water intake at Fresh Pond Reservoir, Cambridge, Massachusetts, water years 2005–07.—Continued [USGS, U.S. Geological Survey; n, number of samples; mg/L, milligrams per liter; μg/L, micrograms per liter; NTRU, nephelometric turbidity ratio units; μS/cm, microsiemens per centimeter at 25 degrees Celsius; <, concentration is less than value shown; --, unable to estimate value with available data; one set of samples from the end of water year 2004 and one sample from station 01104475 from water year 2008 are included] USGS station 01104433 Base flow (n=15) Stormflow (n=15) Property or constituent Standard Standard Minimum Mean Median Maximum Minimum Mean Median Maximum deviation deviation Turbidity, NTRU 1.0 4.4 4.3 8.6 2.59 14 34 28 90 21 Dissolved oxygen, mg/L 4.0 8.2 8.5 13.3 3.2 -- -- -- -- -- pH, standard units1 5.9 6.6 6.8 7.3 0.40 6.5 6.9 7.0 7.3 0.20 Specific conductance, μS/cm 1,480 1,930 1,840 2,760 355 436 858 686 1,960 466 Calcium, mg/L 61.6 77.7 74.3 112.0 13.3 19.1 28.1 26.0 48.9 8.61 Sodium, mg/L 189 259 250 374 46.0 54.2 122 88.9 309 77.2 Chloride, mg/L 387 540 509 789 112 101 232 176 569 147 Sulfate, mg/L 24.0 30.5 30 37.6 3.80 9.1 14.4 13.0 24.3 4.23 Total nitrogen, mg/L 1.02 1.90 1.97 3.12 0.49 1.05 1.54 1.44 2.47 0.43 Total phosphorus, mg/L 0.01 0.03 0.03 0.12 0.03 0.05 0.12 0.11 0.22 0.05 Caffeine, μg/L 0.021 0.193 0.140 0.678 0.183 0.129 0.476 0.206 1.690 0.541 2,4-D, μg/L <0.02 -- -- -- -- 0.050 0.071 0.018 0.379 0.108 2-Chloro-6-ethylamino-4-amino <0.04 -- -- -- -- <0.04 -- -- -- -- s-triazine (CEAT), μg/L Atrazine, μg/L <0.004 -- -- -- -- 0.014 -- <0.004 -- -- Benomyl, μg/L <0.01 -- -- -- -- 0.005 -- 0.005 0.007 -- Carbaryl, μg/L <0.02 -- -- -- -- 0.014 -- <0.02 -- -- Diuron, μg/L 0.005 0.026 0.014 0.097 0.028 0.136 -- -- -- -- Imidacloprid, μg/L 0.036 0.025 0.019 0.067 0.018 0.069 0.1012 0.018 0.788 0.203

Metalaxyl, μg/L <0.04 -- -- -- -- <0.04 -- -- -- --

Norflurazon, μg/L <0.01 -- -- -- -- <0.01 -- -- -- --

Propiconazole, μg/L <0.005 -- -- -- -- <0.005 -- -- -- --

Siduron, μg/L 0.006 0.019 0.012 0.098 0.024 0.010 0.074 0.013 0.741 0.187

Sulfometuron-methyl, μg/L <0.03 -- -- -- -- <0.03 -- -- -- --
Water-Quality Conditions 23

Table 7. Summary statistics for selected water-quality properties, major ions, total nutrients, caffeine, and pesticides in samples of base flow and stormflow collected at five primary sampling stations in the Cambridge drinking-water source area and at the raw water intake at Fresh Pond Reservoir, Cambridge, Massachusetts, water years 2005–07.—Continued[USGS, U.S. Geological Survey; n, number of samples; mg/L, milligrams per liter; μg/L, micrograms per liter; NTRU, nephelometric turbidity ratio units; μS/cm, microsiemens per centimeter at 25 degrees Celsius; <, concentration is less than value shown; --, unable to estimate value with available data; one set of samples from the end of water year 2004 and one sample from station 01104475 from water year 2008 are included]

Property or constituent

USGS station 01104455

Base flow (n=15) Stormflow (n=15)

Minimum Mean Median Maximum Standard deviation Minimum Mean Median Maximum Standard deviation

Turbidity, NTRU 0.2 0.8 0.6 2.1 0.49 11 45 34 120 30

Dissolved oxygen, mg/L 6.9 9.4 9.7 10.8 1.0 -- -- -- -- --

pH, standard units1 5.8 6.5 6.8 7.2 0.35 6.1 6.7 6.7 7.1 0.25

Specific conductance, μS/cm 852 1,180 1,180 1,610 206 67.0 334 218 1,130 310

Calcium, mg/L 31.5 42.9 38.8 70.4 10.9 3.0 10.3 8 28.6 6.7

Sodium, mg/L 121 163 164 212 26.4 7.5 46 27 159 46.1

Chloride, mg/L 216 309 307 469 69 11.4 82 52 317 88.7

Sulfate, mg/L 24.7 28.6 29.1 32.6 2.46 2.9 8.5 7.6 20.4 4.32

Total nitrogen, mg/L 1.62 2.05 1.97 2.65 0.35 0.87 1.51 1.33 2.96 0.61

Total phosphorus, mg/L 0.006 0.009 0.010 0.016 0.003 0.05 0.19 0.12 0.45 0.13

Caffeine, μg/L 0.018 -- <0.020 0.178 -- 0.082 0.184 0.167 0.583 0.118

2,4-D, μg/L <0.02 -- -- -- -- 0.042 0.077 0.027 0.380 0.104

2-Chloro-6-ethylamino-4-amino 

s-triazine (CEAT), μg/L

0.007 0.009 0.009 0.010 0.001 0.006 -- <0.040 -- --

Atrazine, μg/L 0.001 -- 0.002 0.002 -- 0.017 -- <0.004 -- --Benomyl, μg/L 0.009 0.012 0.011 0.015 0.002 <0.01 -- -- -- --Carbaryl, μg/L <0.02 -- -- -- -- <0.02 -- -- -- --Diuron, μg/L 0.006 -- <0.020 0.013 -- <0.02 -- -- -- --Imidacloprid, μg/L 0.006 0.011 0.008 0.029 0.007 0.113 -- <0.030 -- --Metalaxyl, μg/L <0.04 -- -- -- -- <0.04 -- -- -- --Norflurazon, μg/L 0.040 0.107 0.099 0.164 0.038 0.003 0.057 0.047 0.165 0.047

Propiconazole, μg/L <0.005 -- -- -- -- <0.005 -- -- -- --

Siduron, μg/L <0.02 -- -- -- -- 0.014 0.036 0.020 0.162 0.042

Sulfometuron-methyl, μg/L 0.003 0.006 0.006 0.003 0.012 <0.03 -- -- -- --
24 Water Quality in the Cambridge Drinking-Water Source Area, Massachusetts, Water Years 2005–07 Table 7. Summary statistics for selected water-quality properties, major ions, total nutrients, caffeine, and pesticides in samples of base flow and stormflow collected at five primary sampling stations in the Cambridge drinking-water source area and at the raw water intake at Fresh Pond Reservoir, Cambridge, Massachusetts, water years 2005–07.—Continued [USGS, U.S. Geological Survey; n, number of samples; mg/L, milligrams per liter; μg/L, micrograms per liter; NTRU, nephelometric turbidity ratio units; μS/cm, microsiemens per centimeter at 25 degrees Celsius; <, concentration is less than value shown; --, unable to estimate value with available data; one set of samples from the end of water year 2004 and one sample from station 01104475 from water year 2008 are included] Property or constituent USGS station 01104460 Base flow (n=12) Stormflow (n=12) Minimum Mean Median Maximum Standard deviation Minimum Mean Median Maximum Standard deviation Turbidity, NTRU 0.7 3.8 2.7 12.0 3.3 2.1 6.2 4.3 31 7.9 Dissolved oxygen, mg/L 0.1 10.6 9.8 14.3 2.8 -- -- -- -- -- pH, standard units1 6.2 6.9 7.1 7.3 0.30 6.9 7.2 7.3 7.4 0.15 Specific conductance, μS/cm 404 516 504 643 77 353 476 463 595 86 Calcium, mg/L 17.0 20.7 20.9 23.5 1.90 14.6 18.3 18.8 21.7 2.16 Sodium, mg/L 50.6 66.6 67.1 89.8 12.6 44.9 62.7 58.4 84.7 14.2 Chloride, mg/L 91.5 127 126 170 23.8 79.2 116 113 150 25.3 Sulfate, mg/L 10.9 13.5 13.3 15.9 1.66 10.2 11.8 11.6 14.1 1.24 Total nitrogen, mg/L 0.42 0.85 0.86 1.25 0.25 0.66 0.89 0.86 1.60 0.25 Total phosphorus, mg/L 0.010 0.019 0.015 0.041 0.010 0.02 0.05 0.03 0.18 0.04 Caffeine, μg/L 0.004 0.008 0.007 0.013 0.004 0.005 0.018 0.016 0.033 0.008 2,4-D, μg/L <0.02 -- -- -- -- 0.017 0.021 0.020 0.027 0.004 2-Chloro-6-ethylamino-4-amino s-triazine (CEAT), μg/L <0.04 -- -- -- -- <0.04 -- -- -- -- Atrazine, μg/L 0.001 -- 0.001 0.003 -- <0.004 -- -- -- -- Benomyl, μg/L <0.01 -- -- -- -- <0.01 -- -- -- -- Carbaryl, μg/L <0.02 -- -- -- -- <0.02 -- -- -- -- Diuron, μg/L 0.001 -- <0.02 0.014 -- 0.003 0.005 0.004 0.016 0.004 Imidacloprid, μg/L 0.005 0.011 0.010 0.021 0.005 0.010 0.017 0.013 0.050 0.013

Metalaxyl, μg/L <0.04 -- -- -- -- <0.04 -- -- -- --

Norflurazon, μg/L 0.003 0.004 0.004 0.004 0.001 0.006 0.006 0.005 0.040 0.004

Propiconazole, μg/L <0.005 -- -- -- -- <0.005 -- -- -- --

Siduron, μg/L 0.004 0.004 0.004 0.005 0.000 0.006 0.012 0.010 0.040 0.010

Sulfometuron-methyl, μg/L <0.03 -- -- -- -- <0.03 -- -- -- --
Water-Quality Conditions 25

Table 7. Summary statistics for selected water-quality properties, major ions, total nutrients, caffeine, and pesticides in samples of base flow and stormflow collected at five primary sampling stations in the Cambridge drinking-water source area and at the raw water intake at Fresh Pond Reservoir, Cambridge, Massachusetts, water years 2005–07.—Continued[USGS, U.S. Geological Survey; n, number of samples; mg/L, milligrams per liter; μg/L, micrograms per liter; NTRU, nephelometric turbidity ratio units; μS/cm, microsiemens per centimeter at 25 degrees Celsius; <, concentration is less than value shown; --, unable to estimate value with available data; one set of samples from the end of water year 2004 and one sample from station 01104475 from water year 2008 are included]

Property or constituent

USGS station 01104475

Base flow (n=16) Stormflow (n=15)

Minimum Mean Median Maximum Standard deviation Minimum Mean Median Maximum Standard deviation

Turbidity, NTRU 0.6 2.2 2.0 6.0 1.4 3.0 27 12 150 38

Dissolved oxygen, mg/L 7.4 10.7 10.4 14.6 2.3 -- -- -- -- --

pH, standard units1 6.9 7.4 7.5 7.7 0.25 5.6 7.1 7.2 7.5 0.45

Specific conductance, μS/cm 233 280 278 332 26 144 247 235 402 61

Calcium, mg/L 16.4 18.0 17.4 21.3 1.52 7.7 14.0 13.8 17.1 2.65

Sodium, mg/L 23.4 28.3 28.1 33.9 3.02 14.9 26.3 23.9 53.0 9.46

Chloride, mg/L 35.7 46.6 46.4 66.8 7.26 22.8 42.0 38.5 87.6 17.3

Sulfate, mg/L 16.1 18.8 18.6 22.1 1.98 8.0 14.5 14.8 18.8 3.35

Total nitrogen, mg/L 1.24 1.93 1.84 3.14 0.46 1.28 2.17 1.96 5.13 1.00

Total phosphorus, mg/L 0.009 0.015 0.012 0.027 0.006 0.02 0.16 0.08 0.80 0.20

Caffeine, μg/L 0.009 0.008 0.011 0.015 0.002 0.008 0.046 0.034 0.232 0.055

2,4-D, μg/L <0.02 -- -- -- -- 0.013 0.307 0.017 3.41 0.879

2-Chloro-6-ethylamino-4-amino 

s-triazine (CEAT), μg/L

<0.04 -- -- -- -- <0.04 -- -- -- --

Atrazine, μg/L 0.002 -- <0.004 -- -- <0.004 -- -- -- --

Benomyl, μg/L 0.004 0.024 0.012 0.432 0.039 0.006 0.021 0.011 0.176 0.043

Carbaryl, μg/L 0.002 0.017 0.012 0.076 0.020 0.002 0.010 0.006 0.056 0.013

Diuron, μg/L <0.02 -- -- -- -- <0.02 -- -- -- --

Imidacloprid, μg/L 0.089 0.295 0.141 1.21 0.307 0.051 0.253 0.177 1.00 0.237

Metalaxyl, μg/L 0.008 0.021 0.016 0.070 0.013 0.014 0.022 0.019 0.045 0.011

Norflurazon, μg/L <0.01 -- -- -- -- <0.01 -- -- -- --

Propiconazole, μg/L 0.017 0.055 0.032 0.178 0.048 0.013 0.081 0.033 0.544 0.137

Siduron, μg/L 0.003 -- 0.009 0.095 -- 0.003 0.024 0.012 0.073 0.024

Sulfometuron-methyl, μg/L <0.03 -- -- -- -- <0.03 -- -- -- --
26 Water Quality in the Cambridge Drinking-Water Source Area, Massachusetts, Water Years 2005–07 Table 7. Summary statistics for selected water-quality properties, major ions, total nutrients, caffeine, and pesticides in samples of base flow and stormflow collected at five primary sampling stations in the Cambridge drinking-water source area and at the raw water intake at Fresh Pond Reservoir, Cambridge, Massachusetts, water years 2005–07.—Continued [USGS, U.S. Geological Survey; n, number of samples; mg/L, milligrams per liter; μg/L, micrograms per liter; NTRU, nephelometric turbidity ratio units; μS/cm, microsiemens per centimeter at 25 degrees Celsius; <, concentration is less than value shown; --, unable to estimate value with available data; one set of samples from the end of water year 2004 and one sample from station 01104475 from water year 2008 are included] USGS station 422302071083801 Raw-water intake (n=7) Property or constituent Standard Minimum Mean Median Maximum deviation Turbidity, NTRU 0.8 1.1 1.1 1.1 0.23 Dissolved oxygen, mg/L -- -- -- -- -- pH, standard units1 6.9 7.2 7.3 7.5 0.19 Specific conductance, μS/cm 462 500 495 549 33 Calcium, mg/L 19.8 21.5 20.7 23.9 1.8 Sodium, mg/L 58.6 63.1 62.5 68.1 4.3 Chloride, mg/L 110 121 121 136 9.3 Sulfate, mg/L 12.7 13.6 13.3 15.2 1.0 Total nitrogen, mg/L 0.65 0.77 0.74 0.92 0.12 Total phosphorus, mg/L 0.006 0.008 0.008 0.010 0.002 Caffeine, μg/L 0.003 -- 0.017 0.030 -- 2,4-D, μg/L 0.017 -- <0.02 -- -- 2-Chloro-6-ethylamino-4-amino 0.003 -- -- -- -- s-triazine (CEAT), μg/L Atrazine, μg/L <0.004 -- -- -- -- Benomyl, μg/L <0.01 -- -- -- -- Carbaryl, μg/L 0.003 -- <0.020 <0.021 -- Diuron, μg/L <0.02 -- -- -- -- Imidacloprid, μg/L 0.009 -- <0.030 0.020 -- Metalaxyl, μg/L <0.04 -- -- -- Norflurazon, μg/L 0.003 -- <0.01 0.006 --

Propiconazole, μg/L <0.005 -- -- -- --

Siduron, μg/L 0.004 -- <0.020 0.005 --

Sulfometuron-methyl, μg/L <0.03 -- -- -- --

1 Mean value for pH was calculated on the basis of the hydrogen ion concentrations; standard deviation for pH was calculated using values in the sample set in standard pH units
Water-Quality Conditions 27

Table 8. Summary statistics for selected water-quality properties, major ions, total nutrients, caffeine, and pesticides in samples of base flow collected during three synoptic sampling rounds at the secondary sampling stations in the Cambridge drinking-water source area, Massachusetts, water years 2006–07. [USGS, U.S. Geological Survey; mg/L, milligrams per liter; μg/L, micrograms per liter; NTRU, nephelometric turbidity ratio units; μS/cm, microsiemens per centimeter at 25 degrees Celsius; <, less than; --, unable to estimate value with available data]

USGS station 01104390 USGS station 01104405 USGS station 01104410

Property or constituent Minimum Median Maximum Minimum Median Maximum Minimum Median Maximum

Turbidity, NTRU 2.4 3.7 4.5 3.0 3.6 6.5 2.8 3.4 8.4Dissolved oxygen, mg/L 6.8 9.2 13.8 6.8 9.8 13.7 5.9 8.5 11.0pH, standard units 7.0 7.0 7.5 6.3 6.8 7.5 6.5 6.7 7.1Specific conductance, μS/cm 231 267 287 248 288 327 890 990 1,200Calcium, mg/L 15.4 15.9 17.8 11.9 12.9 16.2 34.8 35.4 44.7Sodium, mg/L 23.7 27.6 29.9 30.3 37 40 125 138 169Chloride, mg/L 42.4 50.5 58.1 55 62.6 72 244 265 332Sulfate, mg/L 9.8 10.5 12.4 8.1 8.5 10.5 10.3 10.5 11.5Total nitrogen, mg/L 0.98 1.10 1.20 0.67 0.75 0.96 0.46 0.53 0.79Total phosphorus, mg/L 0.019 0.027 0.039 0.025 0.038 0.064 0.014 0.022 0.033Caffeine, μg/L <0.02 -- -- <0.02 <0.02 0.084 <0.02 <0.02 <0.022,4-D, μg/L <0.02 -- -- <0.02 <0.02 <0.02 <0.02 <0.02 <0.022-Chloro-6-ethylamino-4-amino-s-triazine (CEAT), μg/L <0.04 -- -- <0.04 -- -- <0.04 0.007 0.01Atrazine, μg/L <0.04 <0.04 0.002 <0.04 <0.04 0.001 <0.04 -- --Benomyl, μg/L <0.01 -- -- <0.01 -- -- <0.01 -- --Carbaryl, μg/L <0.01 -- -- <0.01 -- -- <0.01 -- --Diuron, μg/L <0.02 -- -- <0.02 -- -- <0.02 <0.02 0.016Imidacloprid, μg/L <0.03 <0.03 0.013 <0.03 -- -- <0.03 -- --Metalaxyl, μg/L <0.02 -- -- <0.02 -- -- <0.02 -- --Norflurazon, μg/L <0.02 -- -- <0.02 -- -- <0.02 <0.02 0.183Propiconazole, μg/L <0.03 -- -- <0.03 -- -- <0.03 -- --Siduron, μg/L <0.02 -- -- <0.02 <0.02 0.003 <0.02 -- --Sulfometuron-methyl, μg/L <0.03 -- -- <0.03 -- -- 0.009 <0.03 0.047
28 Water Quality in the Cambridge Drinking-Water Source Area, Massachusetts, Water Years 2005–07 Table 8. Summary statistics for selected water-quality properties, major ions, total nutrients, caffeine, and pesticides in samples of base flow collected during three synoptic sampling rounds at the secondary sampling stations in the Cambridge drinking-water source area, Massachusetts, water years 2006–07. —Continued USGS station 01104420 USGS station 01104430 USGS station 01104453 Property or constituent Minimum Median Maximum Minimum Median Maximum Minimum Median Maximum Turbidity, NTRU 2.6 3.8 9.7 6.5 7.1 7.7 1.7 2.9 4.3 Dissolved oxygen, mg/L 3.5 4.5 10.1 6.3 9.4 13.6 4.7 8.9 12.9 pH, standard units 6.4 6.6 7.2 6.5 7.1 7.7 6.6 7.0 7.4 Specific conductance, μS/cm 889 1,070 1,120 617 630 704 438 520 547 Calcium, mg/L 31.3 34.5 38.5 19.6 20.2 22.8 16.9 18.1 19.3 Sodium, mg/L 125 154 157 86.6 90.1 95.8 58.8 69.5 74 Chloride, mg/L 234 282 296 163 165 184 100 124 133 Sulfate, mg/L 6.1 11.4 18.3 11.1 11.7 12.8 12.5 14.7 16.5 Total nitrogen, mg/L 0.82 0.89 0.90 0.50 0.51 0.61 0.58 0.99 1.35 Total phosphorus, mg/L 0.018 0.039 0.079 0.011 0.014 0.014 0.019 0.027 0.037 Caffeine, μg/L <0.02 <0.02 0.006 <0.02 <0.02 0.034 <0.02 <0.02 0.006 2,4-D, μg/L <0.02 <0.02 0.018 <0.02 -- -- <0.02 <0.02 0.027 2-Chloro-6-ethylamino-4-amino-s-triazine (CEAT), μg/L <0.04 -- -- <0.04 -- -- <0.04 -- -- Atrazine, μg/L <0.04 -- -- <0.04 -- -- <0.04 <0.04 0.002 Benomyl, μg/L <0.01 -- -- <0.01 -- -- <0.01 -- -- Carbaryl, μg/L <0.01 -- -- <0.01 -- -- <0.01 <0.01 0.014 Diuron, μg/L <0.02 -- -- <0.02 <0.02 0.013 <0.02 <0.02 0.026 Imidacloprid, μg/L <0.03 <0.03 0.014 <0.03 -- -- <0.03 0.016 0.018 Metalaxyl, μg/L <0.02 -- -- <0.02 -- -- <0.02 0.004 0.007 Norflurazon, μg/L <0.02 0.034 0.037 <0.02 <0.02 0.005 <0.02 -- -- Propiconazole, μg/L <0.03 -- -- <0.03 -- -- <0.03 -- -- Siduron, μg/L <0.02 <0.02 0.034 <0.02 <0.02 0.003 <0.02 <0.02 0.006 Sulfometuron-methyl, μg/L <0.03 -- -- <0.03 -- -- <0.03 -- -- Note: less than values are less than the long-term method detection levels.
Water-Quality Conditions 29

Specific Conductance

Specific conductance is the measure of the capacity of 

water to conduct an electrical current. The conductance of 

water is affected by the type and quantity of ions in water. 

Specific conductance was measured in samples of base flow 

and composite samples of stormflow; specific conductance 

also was continuously recorded using in situ monitors at the 

primary sampling stations (fig. 1) in the Cambridge drinkingwater source area.

Specific conductance measured in water samples collected throughout the study period at all USGS monitoring 

stations ranged from 67 to 3,400 μS/cm. In contrast, recorded 

values of specific conductance, which included many more 

measurements obtained over a wide range of flow conditions, ranged from 13 to 98,500 μS/cm for the same period 

(Smith, 2005, 2007, 2008, and 2011). In-situ measurements 

of specific conductance in water released from the Cambridge 

Reservoir (USGS station 01104430) ranged from about 290 

to 1,700 μS/cm. In-situ measurements of specific conductance 

in the main stem of Stony Brook (USGS station 01104460) 

ranged from 35 to 3,420 μS/cm, although these values were 

generally between about 400 and 650 μS/cm. Because the 

quantity and quality of water in the tributaries respond rapidly 

to stormwater runoff, discrete in-situ measurements of specific 

conductance for USGS stations 01104415, 01104433, and 

01104455 on the tributaries often differed by several orders 

of magnitude from the annual mean values at each station 

(Smith 2005, 2007, 2008, and 2011). The specific conductance 

during base flow at these tributaries was typically greater 

than 1,000 μS/cm; however, specific conductance during 

runoff events ranged by as much as two orders of magnitude 

in either direction. USGS stations 01104415 and 01104455 

on two tributaries that parallel Interstate 95 had the greatest 

range in measured values. The range of in-situ measurements 

of specific conductance in the tributary to the Stony Brook 

Reservoir (USGS streamgage 01104475) that flows through 

a residential area had the smallest range of measured values 

(40–3,540 μS/cm). Annual means of specific conductance for 

this station (248–326 μS/cm) were among the lowest in the 

drinking-water source area. Median values of specific conductance in water samples collected at USGS stations 01104390, 

01104405, 01104410, 01104430, and 01104405 ranged from 

267 to 990 μS/cm (table 8). Measurements of specific conductance in water samples collected at USGS streamgage 

01104420 ranged from 889 to 1,120 μS/cm (table 8). Coincidently, the impervious area at this streamgage was greater than 

20 percent (table 2).

Dissolved Oxygen

Dissolved oxygen concentration is the amount of gaseous 

oxygen dissolved in water. Oxygen enters water by diffusion 

from the surrounding air, aeration from stream turbulence, 

and photosynthesis. Solubility in water decreases as temperature increases. Dissolved oxygen is necessary for the survival 

and growth of many aquatic organisms. Dissolved oxygen 

concentration measured during the collection of base-flow 

samples ranged from 3.5 to 14.6 mg/L in the drinking-water 

source area with a median concentration of 9.6 mg/L. With 

few exceptions, concentrations of dissolved oxygen typically 

were greater than 6.0 mg/L. Low concentrations of dissolved 

oxygen generally occurred in the small tributaries during lowflow conditions in the summer months.

pH

Measurements of pH represent the negative base-10 

log hydrogen-ion concentration, or activity. pH values near 

7 are considered neutral, below 7 are acidic, and above 7 are 

alkaline or basic. The pH of river water generally ranges from 

about 6.5 to 8.5 (Hem, 1992). The pH of water affects the 

solubility and biological availability of various chemical constituents, such as nutrients and metals. The pH of the stream 

and reservoir waters also affects water-treatment procedures. 

The pH of streamwater samples collected from the drinkingwater source area as part of this study ranged from 5.6 to 7.7 

and had a median pH of 7.0. Most pH values were within a 

range of 6.5 to 7.4. The pH of water samples collected from 

the main stem of Stony Brook (USGS station 01104460; table 

7), the largest stream in the watershed, ranged from 6.8 to 7.4, 

except for one measurement of 6.2 made in water year 2005 

(Smith, 2007). 

Turbidity

Turbidity is a measure of the scattering and absorption of 

light caused by suspended materials (such as silt, clay, and fine 

organic particles) and dissolved materials that produce color. 

Turbidity was measured in base-flow and stormflow samples. 

Turbidity values for all samples collected within the drinkingwater source area ranged from 0.2 to 180 nephelometric turbidity ratio units (NTRUs) with a median of 4.5 NTRUs. Typically, composite samples of stormflow collected at stations in 

the smaller subbasins (USGS stations 01104415, 01104433, 

01104455, and 01104475) had the highest turbidity (table 7).

Constituents

Concentrations of major ions, total nutrients, caffeine, 

and pesticides were measured in base-flow and stormflow 

samples collected at 11 sampling stations in the Cambridge 

drinking-water source area. These data were used to establish 

baseline conditions, to identify areas of concern, and to assess 

overall water quality and compliance with Federal and State 

guidelines. Concentrations of these constituents are summarized for the study period.
30 Water Quality in the Cambridge Drinking-Water Source Area, Massachusetts, Water Years 2005–07

Major Ions

The major ions, Ca, Cl, Na, and SO4, are commonly found 

in natural waters at concentrations greater than 1.0 mg/L 

(Hem, 1992) and are of special concern in the Cambridge 

drinking-water source area (Smith, 2007) because they are 

not readily removed by conventional treatment processes. 

High concentrations of Cl in drinking water can affect the 

taste of finished drinking water, and high concentrations 

of Na contribute to hypertension in susceptible individuals 

(U.S. Environmental Protection Agency, 2009b). Although 

Ca, Cl, Na, and SO4

 are common in precipitation (National 

Atmospheric Deposition Program, 2008), minerals and soils 

in the environment (Hem, 1992; Smith, 2005), septic effluent, industrial wastes, and wastewater (Mullaney and others, 

2009), these elements 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. Considering the large amounts 

of impervious area and numerous roadways in the drinkingwater source area (fig. 1 and table 2), deicing compounds are a 

potentially important source of these ions.

Concentrations of Ca, Cl, Na, and SO4

 in base-flow samples and stormflow composite samples varied widely among 

the tributaries in the drinking-water source area (tables 7 and 

8). Concentrations of these ions in water samples collected at 

USGS stations 01104415, 01104433, and 01104455, on tributaries with large amounts of impervious area in their drainage 

basins, varied by several orders of magnitude. The median 

concentrations for Ca, Cl, Na, and SO4

 in samples collected in 

the Hobbs Brook Basin were 39.8, 392, 207, and 21.7 mg/L, 

respectively, whereas median concentrations in samples from 

the tributaries in the Stony Brook Reservoir watershed were 

lower at 17.8, 87.7, 49.7, and 14.7 mg/L, respectively. Median 

major ion concentrations in samples from the main stem of 

Stony Brook (USGS station 01104460), the primary stream 

discharging to the Stony Brook Reservoir, were 19.5, 126, 

65.8, and 12.4 mg/L, respectively, indicating high-concentration waters from the Cambridge Reservoir watershed were 

diluted by inflow from tributaries draining the larger and overall less urbanized Stony Brook Reservoir watershed. 

Mean annual flow-weighted concentrations for Ca, Cl, 

Na, and SO4

 estimated from records of specific conductance 

and flow at USGS station 01104430, at the outlet of the 

Cambridge Reservoir, for water years 2004 to 2008 were 

21.5, 186, 101, and 15.3 mg/L, respectively. However, mean 

annual flow-weighted concentrations of these ions at Stony 

Brook (USGS station 01104460), estimated in the same manner and for the same period, were lower at 18.2, 111, 60.4, 

and 11.3 mg/L, respectively. Mean annual flow-weighted 

concentrations for Ca, Cl, Na, and SO4 at USGS station 

01104475, on the only other tributary that flows directly into 

the Stony Brook Reservoir (fig. 1), were 15.4, 41.6, 25.7, and 

15.8 mg/L, respectively.

Total Nutrients

Nutrients, such as nitrogen (N) and phosphorus (P), are 

essential to the health and diversity of aquatic organisms. 

Biological and chemical processes can mobilize each nutrient to or from the soil, reservoir sediments, water column, 

atmosphere, and organisms. Algae and aquatic plants consume various species of each nutrient until limited by their 

availability. In the reservoirs in the drinking-water source 

area, P was determined to be the limiting nutrient (Waldron and Bent, 2001). Thus, high concentrations of P in 

the reservoirs can result in overgrowth of plant life, algal 

blooms, reduction in oxygen levels, and decline or shift in the 

biological community. 

Common sources of P include weathering of natural 

rocks and soils, fertilizers, and leaking or failing septic tanks. 

In general, P is somewhat insoluble and tends to be associated 

with sediment and plant matter (Smith, 2002; Smith, 2005; 

Breault and others, 2005). As a result, erosion of local soils 

and even application of sand to roads and parking lots during winter maintenance activities (Smith and Granato, 2010) 

can increase TP concentrations in receiving waters. Common 

sources of nitrogen (N) are precipitation, nitrogen emissions 

from automobiles, degradation of natural organic matter, 

fertilizers, leaking sewer lines, and failing septic tanks. In a 

recent highway runoff study (Smith and Granato, 2010), the 

TN content in precipitation was estimated to account for about 

55 percent of the mean concentration of TN in all samples of 

highway runoff collected at monitoring stations along Interstate 95 near the Cambridge Reservoir. Although waterfowl 

are often considered a source of both nutrients, a previous 

USGS study (Waldron and Bent, 2001) estimated that less 

than 0.1 percent of the total input of N into the reservoirs was 

derived from waterfowl.

The concentration of TN in water samples from all subbasins ranged from 0.42 to 5.13 mg/L (tables 7 and 8). The 

median concentration for TN in all samples collected in the 

Hobbs Brook Basin was 1.48 mg/L, and the median concentration in the samples from tributary streams in the Stony Brook 

Reservoir watershed was 1.52 mg/L. Mean concentrations 

of TN in samples of base flow and stormflow collected at 

the primary sampling stations on the four tributaries ranged 

from 1.63 to 2.05 mg/L. The mean concentration of TN in 

samples of base flow and stormflow collected at USGS station 

01104460, the main stream to the Stony Brook Reservoir, was 

0.87 mg/L. 

The concentration of TP in water samples for all subbasins ranged from 0.006 to 0.80 mg/L (tables 7 and 8). The 

median concentration for TP in all samples collected in the 

Hobbs Brook Basin was about 0.04 mg/L, whereas the median 

concentration in the tributaries in the Stony Brook Reservoir 

watershed was 0.03 mg/L. Mean concentrations of TP in 

samples of base flow and stormflow collected at the primary 

sampling stations on the four tributaries ranged from 0.09 to 

0.12 mg/L. The mean concentrations of TP in samples of base 

flow and stormflow collected at USGS station 01104460, the 
Water-Quality Conditions 31

main stream to the Stony Brook Reservoir, was 0.03 mg/L. 

The low mean concentration of TP for this station is potentially the result of dilution of higher concentrations in stormflows by reservoir water containing base-flow level concentrations of TP; reservoir water is continuously released from the 

Cambridge Reservoir upstream.

Pesticides and Caffeine

Pesticides are commonly used by homeowners to control 

nuisance plants, fungi, and insects in homes and gardens. 

Pesticides, including herbicides, fungicides, and insecticides 

are used to increase the production in home gardens, prevent 

structural damage, eliminate unwanted weeds and brush, and 

protect lawns and ornamental plants. Many pesticides also 

are used on golf courses and other recreational areas, and on 

roadway right-of-ways (Racke, 1993). During 2006–07, over 

70 million pounds of pesticides were purchased in the United 

States for use in the home and garden (Fishel, 2011). As a 

result of widespread use, pesticides are frequently detected in 

groundwater and surface water in urbanized areas (Gilliom 

and others, 2006; U.S. Geological Survey, 1999).

The potential for a pesticide to reach receiving water 

bodies is a function of its physical properties, including aqueous solubility, tendency to adsorb to soils, and environmental 

persistence. Additionally, many of these properties are affected 

by the organic carbon content, pH, alkalinity, and temperature of soil and water (Hornsby and others, 1996). In general, 

pesticides with low aqueous solubility, long aerobic soil half 

life, moderate to long aqueous half life, and low soil sorption 

values have the potential to infiltrate and move though groundwater (Vogue and others, 1994). Pesticides that are strongly 

absorbed to soils are unlikely to be transported in a dissolved 

state; however, such compounds can be transported with soil 

to receiving tributaries in runoff.

Caffeine is present in the seeds and leaves of more than 

60 plant species and in a variety of beverages, food products, 

and pharmaceuticals (Burge and others, 2003). Caffeine is 

commonly detected in streams and rivers throughout the 

United States and is a generally considered an indicator of 

wastewater contamination (Lawrence and LaFontaine, 2010; 

Zaugg and others, 2007; Sando and others, 2005). Caffeine 

generally exists in the dissolved phase in many watersheds 

(Burge and others, 2003), thus sorption and sedimentation of 

the compound in the drinking-water supply is likely negligible. The detection of caffeine in base-flow samples, where 

streamflow is dominated by groundwater discharge, may 

indicate the presence of domestic (septic–system) or municipal 

(leaking sewer lines) wastewater (Seiler and others, 1999). 

Caffeine is also present in stormwater runoff from roadways 

and parking areas where caffeinated beverages were spilled 

or caffeine-laden litter is present. In a recent highway runoff study (Smith and Granato, 2010), caffeine was routinely 

detected in samples of stormwater runoff collected from the 

impervious area of five highways, including two highways that 

intersect the Cambridge Reservoir and Stony Brook Reservoir 

watershed (Interstate 95 and Route 2; fig. 1).

Twenty-five pesticides and caffeine were detected in 

water samples collected at the sampling stations in the Cambridge Reservoir and Stony Brook Reservoir watersheds and 

in raw water collected at the Cambridge water-treatment facility intake from the Fresh Pond Reservoir during this study. 

Thirty-four pesticides (table 3) were not detected. Most of 

the detected pesticides are found in over-the-counter products 

used for turf management and the treatment of ornamental 

shrubs (table 9). 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 

(table 10); the detection frequency varied substantially from 

station to station. Summary statistics for concentrations of 

pesticides that were detected in at least 25 percent of the 

samples of base flow or stormflow at one or more primary 

sampling stations are given in table 7; available statistics for 

the same pesticides detected in base-flow samples collected at 

the secondary sampling stations are in table 8. Imidacloprid, 

norflurazon, and siduron were the pesticides most frequently 

detected (table 10) in samples of water collected in the 

drinking-water source area; the frequency of detections ranged 

from about 24 to 42 percent. These compounds are highly 

mobile and are likely to occur in both stormwater runoff and 

groundwater inflow to the tributaries as a result of their chemical characteristics (table 9). Imidacloprid, siduron, and 2,4-D 

were the only pesticides detected in samples collected at all 

of the primary sampling stations (fig. 1 and table 7); 2,4-D 

was the most commonly used herbicide in the United States 

during 2006–07 (Fishel, 2011). The herbicide norflurazon was 

detected in 90 percent of the samples collected at USGS station 01104455. The herbicide siduron; the insecticides carbaryl 

and imidacloprid; and the fungicides benomyl, metalaxyl, and 

propiconazole were detected in more than 50 percent of the 

water samples collected under base-flow and stormflow conditions at USGS station 01104475. The high aqueous solubility 

of most of these pesticides explains, in part, their presence 

in base flow and stormflow. In contrast, benomyl is highly 

persistent in soils, has low solubility, and degrades quickly in 

water (table 9). As a result of these characteristics, benomyl 

is not generally found in groundwater; however, benomyl 

was detected in 81 percent of the samples collected during 

base-flow conditions at USGS station 01104475. This may be 

explained, in part, by the relatively small drainage area (2.2 

km2

) upstream from this station; thus, the time of concentration is short (less than a day). Production of benomyl ceased 

in 2001, and the sale of benomyl products ended in 2002. 

Remaining benomyl products have expiration dates ranging from January 2006 to January 2009 (U.S. Environmental 

Protection Agency, 2003). Observed decreases in benomyl 

concentrations during water years 2004–08 at USGS station 

01104475 (Smith 2005, 2007, 2008, 2011) are likely linked to 

the termination of its manufacture.

In general, the ranges in concentrations of several of 

the most frequently detected pesticides—2,4-D, norflurazon, 
32 Water Quality in the Cambridge Drinking-Water Source Area, Massachusetts, Water Years 2005–07 Table 9. Characteristics of pesticides detected in samples of base flow and stormflow collected in the Hobbs Brook and Stony Brook Reservoir watersheds, Massachusetts, water years 2005–07.—Continued [CAS, Chemical Abstracts Service; mg/L, milligrams per liter <, less than value shown; >, greater than; Koc, sorption coefficient; --, data unavailable; E, estimated] Pesticide Trade or common name(s) Type of pesti- cide

1CAS 

registry 

number

Water 

solubility 

(mg/L)

Approximate 

half life 

in soil 

(days)

Approximate 

half life in 

water 

(days)

Sorption 

coefficient 

(soil Koc)

Use References

Benomyl Agrocit, Benex, Benlate, 

Benosan, Fundazol, Fungidice 1991, and Tersan 1991

Fungicide 17804-35-2 2 90-365 <1 1,900 Used as a 

fungicide on vegetables, ornament shrubs, and turf.

U.S. Environmental Protection 

Agency (2001), Hornsby and others (1996), Extension Toxicology Network (1996a)

Metalaxyl Ridomil, Apron, Delta-Coat 

AD, Subdue 2E

Fungicide 57837-19-1 8,400 70 >28 50 Controls plant 

diseases caused by water-mold fungi. Used on crops, ornament shrubs and trees, and turf.

Wagner and others 

(2006b), Hornsby and others (1996), U.S. Environmental Protection Agency (1994), Extension Toxicology Network (1993d) 

Propiconazole Alamo, Banner, Benit, 

Break, Bumper, Desmel, Fidis, Juno, Mantis, Maxx, Novel, Practis, Orbit, Radar, Restore, Spire, Taspa, Tilt, Wocosin

Fungicide 60207-90-1 110 110 -- 650 Used on grasses 

grown for seed, many crops, conifers, hardwoods, and fruit trees.

Wagner and others 

(2006b), Extension Toxicology Network (1997), Hornsby and others (1996)

2-Chloro-6-ethylamino-4-amino-striazine (CEAT) Triazine degradation com- pound Herbicide metabolite 1007-28-9 -- -- -- -- Parent com- pound is used to control 

broadleaf and grassy weeds in crops and noncropped areas.

Wagner and others 

(2006b)

2-Hydroxy-4-isopropylamino-6-ethylamino-s-triazine 

(OIET)

Triazine degradation compound Herbicide metabolite 2163-68-0 -- -- -- -- Parent com- pound is used 

to control broadleaf and grassy weeds in crops and noncropped areas.

Wagner and others 

(2006b)
Water-Quality Conditions 33

Table 9. Characteristics of pesticides detected in samples of base flow and stormflow collected in the Hobbs Brook and Stony Brook Reservoir watersheds, Massachusetts, water years 2005–07.—Continued[CAS, Chemical Abstracts Service; mg/L, milligrams per liter <, less than value shown; >, greater than; Koc, sorption coefficient; --, data unavailable; E, estimated]

Approximate Approximate Trade 1CAS Water Sorption Type of pesti- half life half life in Pesticide or registry solubility coefficient Use References cide in soil water common name(s) number (mg/L) (soil Koc) (days) (days)

2,4-D methyl ester Aqua Kleen, Weeone, and Herbicide 1928-38-7 100 10 -- 100 Used for the con- Wagner and others 

Emulsamine trol of aquatic (2006b), Hornsby 

weeds, cropland and others (1996)and non-crop area weeds.

2,4-D “Agent White,” Bladex-B, Herbicide 94-75-7 890 10 <28 20 Used for the U.S. EnvironmenBrush Killer 64, Dicofur, control of tal Protection 

Dormon, Ipaner, Moxon, broadleaf weeds Agency (2006a), Netagrone, Pielik, Verton in agriculture, Wagner and 38, Mota Maskros, and for control others (2006b), Silvaprop 1, Agricorn D, of woody plants Hornsby and othAcme LV4, Croprider, along roadsides, ers (1996)Fernesta, Lawn-Keep, railways, and Pennamine D, Plantgard, utilities rights Tributon, Weed-B-Gon, of way.Weedatul, Agroxone, Weedar, Salvo, Green Cross Weed-No-More 80, Red Devil Dry Weed Killer, Scott’s 4XD, Weed-Rhap LV40, Weedone 100, 2,4-Dichlorophenoxyacetic acid, Weedon-2,4,-DP

Atrazine Aatrexx Herbicide 1912-24-9 33 10,000 -- 100 Used to control Wagner and others 

broadleaf and (2006b)grassy weeds in crops and noncrop areas.
34 Water Quality in the Cambridge Drinking-Water Source Area, Massachusetts, Water Years 2005–07 Table 9. Characteristics of pesticides detected in samples of base flow and stormflow collected in the Hobbs Brook and Stony Brook Reservoir watersheds, Massachusetts, water years 2005–07.—Continued [CAS, Chemical Abstracts Service; mg/L, milligrams per liter <, less than value shown; >, greater than; Koc, sorption coefficient; --, data unavailable; E, estimated] Approximate Approximate Trade 1CAS Water Sorption Type of pesti- half life half life in Pesticide or registry solubility coefficient Use References cide in soil water common name(s) number (mg/L) (soil Koc) (days) (days) Dinoseb Basanite, Caldon, Chemox, Herbicide 88-85-7 52 30 -- 30 Used in field crops Wagner and others Chemsect DNBP, Dinitro, for the selective (2006b), Hornsby Dynamyte, Elgetol, Ge- control of grass and others (1996), butox, Hel- Fire, Kiloseb, and broadleaf Extension Toxi- Nitropone, Premerge, weeds in corn; cology Network Sinox General, Subitex, also used as an (1993a) and Vertac Weed Killer. insecticide for grapes; and as a seed crop dry- ing agent. Sale discontinued in 1986. Diuron Aguton, DCMU, Di-on, Herbicide 330-54-1 42 1,000 Stable 480 Used for the con- Wagner and others Direx, Karmex, Topsite trol of weeds (2006b), Hornsby and mosses and others (1996), on turf and on Extension Toxi- many crops. cology Network (1993b) Fenuron Dybar, Beet-Kleen Herbicide 101-42-8 3,850 30 -- 42 Used on leeks, Wagner and others fruits, and nurs- (2006b), Hornsby ery trees. and others (1996) Flumetsulam Python®, Broadstrike®, Herbicide 98967-40-9 -- -- -- -- Used for selec- U.S. Environmen- Hornet®, Accent Gold® tive control of tal Protection broadleaf weeds Agency (2004b) in areas planted with soybean and field corn. Fluometuron C-2059, Ciba-2059, Co- Herbicide 2164-17-2 110 21 -- 100 Used as a selective Extension Toxicol- toran, Cotorex, Cottonex, herbicide which ogy Network Flo-Met, Higalcoton, acts on sus- (1996a)

Lanex, Pakhtaran ceptible plants 

by inhibiting photosynthesis.
Water-Quality Conditions 35

Table 9. Characteristics of pesticides detected in samples of base flow and stormflow collected in the Hobbs Brook and Stony Brook Reservoir watersheds, Massachusetts, water years 2005–07.—Continued[CAS, Chemical Abstracts Service; mg/L, milligrams per liter <, less than value shown; >, greater than; Koc, sorption coefficient; --, data unavailable; E, estimated]

Approximate Approximate Trade 1CAS Water Sorption Type of pesti- half life half life in Pesticide or registry solubility coefficient Use References cide in soil water common name(s) number (mg/L) (soil Koc) (days) (days)

Imazaquin Ala-Scept, Scepter, Squad- Herbicide 81335-37-7 60 60 -- 20 Selective, pre- and Wagner and others 

ron, Tri-Scept, Partner, postemergence (2006b), ExtenSkepter 1.5L, and Image herbicide. sion Toxicology 

1.5LC Network (1996b), 

Hornsby and others (1996)

2-methyl-4-chloro- Agritox, Agroxone, Border Herbicide 94-74-6 825 25 <35 1,000 Used for selec- Wagner and others 

phen-oxyacetic Master, Chiptox, Metax- tive control of (2006b), U.S. 

acid (MCPA) on, Rhomene, Rhonox, broadleaf weeds EnvironmenWeed-Rhap, Weedone, on turf, lawns, tal Protection 

Weedar vines, and Agency (2004b), 

rights-of-way, Hornsby and othand in forestry ers (1996), Extenapplications. sion Toxicology 

Network (1993c) 

Metsulfuron-methyl Escort®, Ally®, Allie, Herbicide 74223-64-6 9,500 30 <240 35 Used as a selec- Wagner and others 

Grooper tive pre- and (2006b), Hornsby 

postemergence and others (1996), herbicide for Extension Toxibroadleaf weeds cology Network and some an- (1993d) nual grasses.

Norflurazon Zorial, Solicam, Evital Herbicide 27314-13-2 28 E30 Stable 700 Used for the Wagner and others 

control of (2006b), U.S. 

germinating an- Environmennual grasses and tal Protection broadleaf weeds Agency (1996), in agricultural, Hornsby and othnonagricultural, ers (1996)and industrial areas.
36 Water Quality in the Cambridge Drinking-Water Source Area, Massachusetts, Water Years 2005–07 Table 9. Characteristics of pesticides detected in samples of base flow and stormflow collected in the Hobbs Brook and Stony Brook Reservoir watersheds, Massachusetts, water years 2005–07.—Continued [CAS, Chemical Abstracts Service; mg/L, milligrams per liter <, less than value shown; >, greater than; Koc, sorption coefficient; --, data unavailable; E, estimated] Approximate Approximate Trade 1CAS Water Sorption Type of pesti- half life half life in Pesticide or registry solubility coefficient Use References cide in soil water common name(s) number (mg/L) (soil Koc) (days) (days) Siduron Tupersan, Trey Herbicide 1982-49-6 18 90 120-150 420 Preemergence Wagner and others treatment for (2006b), Hornsby turf. and others (1996), United States National Library of Medicine, Toxicology Data Network (2010) Sulfometuron-methyl Oust Weed Killer and DPX Herbicide 74222-97-2 70 20 -- 78 Used to control Wagner and others 5648 annual and pe- (2006b), Hornsby rennial grasses and others (1996), and broad- Extension Toxi- leaved weeds in cology Network non-crop areas; (1994) also used to control woody tree species. Tebuthiuron Spike, Brush Bullet, EL- Herbicide 34014-18-1 2,500 360 -- 80 Used for the con- Wagner and others 103, Graslan, Perflan, trol of weeds in (2006b), Hornsby Herbec, Herbic and non-crop areas, and others (1996), Reclaim. rangelands, Extension Toxi- rights-of-way cology Network and industrial (1993f) sites. Triclopyr Access, Crossbow, Curtail, Herbicide 55335-06-3 -- 46 -- 20 Selective systemic Wagner and others ET, Garlon, Grazon, herbicide used (2006b), Hornsby Redeem, Turflon for control of and others (1996), woody and Extension Toxi- broadleaf plants cology Network in turf and (1993f) along rights-ofway.
Water-Quality Conditions 37

Table 9. Characteristics of pesticides detected in samples of base flow and stormflow collected in the Hobbs Brook and Stony Brook Reservoir watersheds, Massachusetts, water years 2005–07.—Continued[CAS, Chemical Abstracts Service; mg/L, milligrams per liter <, less than value shown; >, greater than; Koc, sorption coefficient; --, data unavailable; E, estimated]

Approximate Approximate Trade 1CAS Water Sorption Type of pesti- half life half life in Pesticide or registry solubility coefficient Use References cide in soil water common name(s) number (mg/L) (soil Koc) (days) (days)

Carbaryl Ortho Seven, Sevin Insecticide 63-25-2 120 7 10 300 Used as a broad- U.S. Environmenspectrum tal Protection 

insecticide in Agency (2004a), agriculture, Hornsby and othprofessional turf ers (1996),Extenmanagement sion Toxicology and ornamental Network (1996b)production, and residential lawns and gardens.

Imidacloprid Admire, Condifor, Gau- Insecticide 138261- 510 48-190 Stable 249-336 Used for the con- Wagner and others 

cho, Premier, Premise, 41-3 trol of sucking (2006b), ExtenProvado, Marathon, Merit insects, includ- sion Toxicology 

ing aphids, Network (2005), termites, and National Pestiturf insects in cide Information crops and turf. Center (2010)

Oxamyl Blade, DPX 1410, Oxamil, Insecticide 23135-22-0 282,000 4 -- 25 Used to control a U.S. EnvironmenOxamimidic Acid, Pratt, broad spectrum tal Protection 

Thioxamil, Vydate of insects, Agency (2006b)

mites, ticks, and roundworms on field crops, vegetables, fruits, and ornamental plants.

Propoxur Arprocarb, Bay 9010, Insecticide 114-26-1 1,800 30 -- 30 Used to control Wagner and others 

Bayer 39007, Baygon, insects in lawns, (2006b), Hornsby 

Bifex, Blattanex, Bolfo, turf, and build- and others (1996), 

Invisigard, Mobay, PHC, ings. Extension ToxiPropogon, Propyon, cology Network 

Rhoden, Sendra, Sendran, (1993e)Suncide, Tendex, UNDEN, Undene

1CAS Registry Numbers®, which is a Registered Trademark of the American Chemical Society. CAS recommends the verification of the numbers through CAS Client ServicesSM.
38 Water Quality in the Cambridge Drinking-Water Source Area, Massachusetts, Water Years 2005–07 Table 10. Frequency of detection, standards for drinking water and aquatic life, and maximum concentrations of selected pesticides in water samples collected in Hobbs Brook Basin and Stony Brook Reservoir watershed and in samples of raw water collected from the intake of the water-treatment facility at Fresh Pond, Massachusetts, water years 2005–07. [Concentration data for fish species are lethal concentrations for 50 percent of the target species (LC50) at 96 hours, in micrograms per liter. Data are median concentrations compiled from the U.S. Environ- mental Protection Agency (USEPA) ECOTOX database (U.S. Environmental Protection Agency, 2005); >, greater than or equal to; --, not available; kg, kilogram; one set of samples from the end of water year 2004 and one sample from station 01104475 from water year 2008 are included] Analyte Frequency of detection (percent) Maximum concentration (micrograms per liter) USEPA drinking-water maximum con- taminant level (micrograms per liter) Canadian long-term guideline for the protection of aquatic life (micrograms per liter) Median 96-hour LC50 values for selected aquatic organisms (micrograms per liter) Type Bluegill sunfish Carp Rainbow trout Benomyl 17 0.432 -- -- -- 2 190,000 -- Fungicide Metalaxyl 15 0.07 -- -- 139,000 >100,000 130,000 Fungicide Propiconazole 15 0.544 -- -- 4,500 46,000 6,800 Fungicide 2-Chloro-6-ethylamino-4-amino-s-triazine (CEAT) 5.8 0.01 -- -- 221,500 27,700 234,000 Herbicide-degradation compound 2-Hydroxy-4-isopropylamino-6-ethylami- no-s-triazine (OIET) 1.2 0.003 -- -- -- -- -- Herbicide-degradation compound 2,4-D methyl ester 0.6 0.041 4 -- -- -- Herbicide 2,4-D 17 7.92 70 221,500 27,700 198,500 Herbicide Atrazine 7.0 0.017 3 1.8 24,000 100,000 8,800 Herbicide Dinoseb 0.6 0.017 7 0.05 -- 3 2,000 -- Herbicide Diuron 17 0.136 1 100 -- 8,600 31,000 7,400 Herbicide Fenuron 0.6 0.036 -- -- -- -- 204,000 Herbicide Flumetsulam 2.3 1.42 -- -- 300,000 -- 300,000 Herbicide Fluometuron 0.6 0.004 1 90 -- 48,000 120,000 26,200 Herbicide Imazaquin 4.7 8.88 -- -- 420,000 -- 280,000 Herbicide 2-methyl-4-chlorophenoxyacetic acid (MCPA) 4.3 0.157 1 140 -- -- 40,000 91,000 Herbicide Metsulfuron-methyl 4.2 3.9 -- -- 16,300 -- 8,100 Herbicide Norflurazon 24 0.165 -- -- -- 8,100 -- Herbicide Siduron 36 0.741 -- -- 15,000 -- 13,000 Herbicide Sulfometuron-methyl 4.1 0.012 -- -- 81,250 -- 80,250 Herbicide Tebuthiuron 2.3 0.01 1 500 1.6 106,000 -- 143,000 Herbicide Triclopyr 1.9 1.44 -- -- 148,000 -- 7,500 Herbicide Carbaryl 16 0.076 1 400 0.2 6,200 3,080 1,470 Insecticide

Imidacloprid 42 1.21 -- 0.23 105,000 280,000 229,100 Insecticide

Oxamyl 0.6 0.003 200 -- 6,415 -- 4,450 Insecticide

Propoxur 2.9 0.246 -- -- -- 7340 8,200 Insecticide

1Drinking water equivalent level for 70-kg adult.272-hour LC50.38-hour LC50.
Water-Quality Conditions 39

siduron, imidacloprid, carbaryl—and caffeine were similar for 

the Cambridge Reservoir and Stony Brook watersheds (fig. 8). 

Diuron concentrations generally were low in the Cambridge 

Reservoir watershed, and the insecticides benomyl, metalazyl, 

and propiconazole were not detected in the Cambridge Reservoir watershed. Diuron, norflurazon, and siduron were the 

only pesticides detected in water samples collected at the outlet of the Cambridge Reservoir (Smith, 2008, 2011). Assuming 

plug flow where velocity is constant through the reservoir, 

mean retention time for water in the Cambridge Reservoir 

during water years 2006–07 was greater than 7 months. As a 

result, pesticides that have short aqueous half life may degrade 

within the reservoir. Compounds stable in the aquatic environment, particularly norflurazon and siduron, are able to advance 

though the entire reservoir system (fig. 8). The mean retention 

time for the Stony Brook Reservoir is less than about 2 weeks; 

thus, pesticides with a aqueous half life greater than a week 

often are able to persist in waters directed to the Fresh Pond 

Reservoir. Although several pesticides were detected in samples from the Fresh Pond Reservoir raw-water intake (table 7), 

carbaryl and caffeine, were the only compounds detected at 

greater than the LTMDL. The presence of carbaryl in samples 

collected from the raw-water intake may be explained in part 

by the moderate water solubility and aqueous half life of the 

compound (table 9) but also may be explained by the proximity of the outfall of a tributary containing elevated concentrations of carbaryl (USGS station 01104475; table 7) to the head 

of Stony Reservoir. Pesticides were not detected in finished 

drinking water (Smith, 2005).

Caffeine was detected in about 64 percent of 171 water 

samples at concentrations ranging from 0.003 to 1.82 μg/L 

(micrograms per liter; tables 7 and 8) and was detected in 

more than 90 percent of the samples collected at USGS 

station 01104433. Caffeine was detected in water samples 

collected at all USGS sampling stations, except for 01104390 

and 01104410.

Factors Affecting Water-Quality Properties and 

Constituent Concentrations

The quality of water in the streams and tributaries of the 

Cambridge drinking-water source area is affected by seasonal streamflow, stormwater runoff, and land-use activities. 

Streamflow volume, to some extent, correlated with waterquality properties and concentration data. Streamflow volume 

is affected by storm intensity and duration, seasonal precipitation, and evapotranspiration. Stormwater runoff can mobilize 

deicing compounds, sediment and associated constituents, and 

pesticides. In contrast, the addition of stormwater can dilute 

the concentration of constituents derived from groundwater 

inflow. The amount and type of land use often determines the 

types of constituents in tributaries and can affect the pathways 

by which these constituents reach receiving water bodies. 

Seasonal Variability

Concentrations of many constituents varied throughout 

the water year. These variations are a function of seasonal 

climatological processes, as well as seasonal anthropogenic 

activities, such as the application of deicing compounds and 

sand during the winter and the application of various pesticides at specific times during the year.

For most sampling stations in the drinking-water source 

area, concentrations of Ca, Cl, Na, and SO4

 in samples of 

tributary water are negatively correlated with the log values 

of streamflow (table 4). As a result, specific conductance and 

major ion concentrations in samples at most of the sampling 

stations tend to increase during the summer when streamflow 

is low and decrease during high base-flow conditions and periods of stormwater runoff (fig. 9), indicating that groundwater 

is an important source of many of the major ions. Although 

initial stormflow during the winter was commonly accompanied by high specific conductance measurements (fig. 9) 

indicative of high concentrations of Ca, Cl, Na, and SO4

, the 

concentrations of these ions in composite samples of stormflow were similar to, and in some cases lower than, concentrations in samples of base flow (table 7). 

Concentrations of TP were positively correlated with 

flow, especially during storms, and the relation was statistically significant at three of the five primary sampling stations (USGS stations 01104415, 01104433, and 01104455). 

Positive correlations between concentrations and streamflow 

typically occur when a constituent is strongly associated with 

suspended sediment. When the concentration of suspended 

sediment increases, as it generally does following rainfall, the 

concentrations of constituents associated with or sorbed to 

the suspended sediment also increases. Phosphorus is present 

in streambed sediment, as well as in local soil adjacent to the 

tributaries in the drinking-water source area (Smith, 2005). 

In contrast, the correlation between TN concentration and 

streamflow was poor, indicating that streamflow explains little 

of the observed variability in the TN concentrations. This lack 

of correlation is likely the result of similarities between concentrations of TN in precipitation and in groundwater inflows. 

Flow-weighted annual concentrations of TN in precipitation 

in the study area during water years 2005–07 were about 1 

mg/L (National Atmospheric Deposition Program, 2008), 

whereas mean concentrations of TN in base-flow samples 

ranged from 1.24 to 2.05 mg/L (table 7) at USGS stations 

01104415, 01104433, 01104455, and 01104475. Additional N 

from fertilizers, automobile emissions, and natural and other 

anthropogenic sources, combined with precipitation runoff, 

further reduces the difference in concentrations of TN between 

groundwater inflow and stormflow.

Many pesticides are applied once or more a year and, 

therefore, might be expected to have seasonal patterns of 

concentrations in streamwater. However, many of the pesticides were detected throughout the year in both base-flow 

and stormflow samples collected in the drinking-water source 

area. The frequent detection of many of the pesticides might 
40 Water Quality in the Cambridge Drinking-Water Source Area, Massachusetts, Water Years 2005–07

 Concentration, in micrograms per liter

Insecticides Herbicides Fungicides

Benomyl

Metalaxyl

Propiconazole

2,4-D

CEAT

Atrazine

Diuron

Norflurazon

Siduron

Sulfometuron-methyl

Carbaryl

Imidacloprid

Caffeine

0.001 0.01 0.1 1 10

( 0 )

(24)

( 0 )

Frequency of detection, in percent

( 0 )

(21)

( 0 )

( 0 )

(21)

( 0 )

(24)

(16)

( 0 )

(19)

(17)

( 0 )

(12)

(31)

(38)

(33)

(36)

(50)

(12)

(16)

(38)

(24)

(47)

(50)

(60)

(66)

(63)

( 5 ) 

( 7 )

( 0 )

( 2 ) 

(9)

( 0 )

( 5 ) 

( 4 )

( 0 )

Cambridge Reservoir watershed sample

Cambridge Reservoir outlet sample

Stony Brook Reservoir watershed and lower Hobbs Brook Basin sample

Fresh Pond Reservoir raw-water intake sample

 EXPLANATION

Values to the left are estimated to be less than the

long-term method detection level

( 0 ) Frequency of detection, by watershed, in percent

Cambridge Reservoir watershed

Stony Brook Reservoir watershed 

Fresh Pond Reservoir raw water

Figure 8. Concentrations of selected pesticides and caffeine detected in samples of water collected during base flow and 

stormflow at all monitoring stations in the Cambridge drinking-water source area and from the Fresh Pond Reservoir rawwater intake, Cambridge, Massachusetts, water years 2004–08.
Water-Quality Conditions 41

EXPLANATION

10/11

06:00

10/11

12:00

10/11

18:00

10/12

00:00

10/12

06:00

10/12

12:00

10/12

18:00

10/13

00:00

10/13

06:00

10/13

12:00

10/13

18:00

10/14

00:00

10/14

06:00

10/14

12:00

0.05

0.5

5

50

October 2006

Flow

10

100

1,000

10,000

Specific conductance

A

B

12/31

21:00

01/01

03:00

01/01

09:00

01/01

15:00

01/01

21:00

01/02

03:00

01/02

09:00

01/02

15:00

01/02

21:00

01/03

03:00

0.05

0.5

5

50

January 2007

100

1,000

10,000

100,000

 Specific conductance, in microsiemens per centimeter at 25 degrees Celsius Specific conductance, in microsiemens per centimeter at 25 degrees Celsius

Flow, in cubic feet per second Flow, in cubic feet per second

Figure 9. Records of flow and specific conductance during A, fall and B, winter storm runoff 

at U.S. Geological Survey station 01104455, Stony Brook Reservoir watershed, Massachusetts, 

water year 2007.
42 Water Quality in the Cambridge Drinking-Water Source Area, Massachusetts, Water Years 2005–07

be related to chemical properties (high solubility, long aqueous 

half life, and low soil sorption values) that allow the compounds to infiltrate groundwater and to the range in transport 

time between the location of application and the distance to 

the stream.

Stormwater runoff

Results for Mann-Whitney tests indicate that some values 

of water-quality properties and concentrations of major ions 

and nutrients in base flow were significantly different from 

those in composite samples of stormflow (table 7). The turbidity values for stormflow samples were higher than those for 

base-flow samples at four of the five primary sampling stations 

(fig. 10), and at three stations, the entire range of turbidity 

values for stormflow samples exceeded the range of turbidity 

values for base-flow samples. In many cases, the mean and 

median values of turbidity for the stormflow samples from the 

tributaries were an order of magnitude higher than the respective values for base-flow samples (table 7). 

Concentrations of TP were higher in stormflow samples 

than in base-flow samples. The interquartile range of TP concentrations in stormflow samples from the primary sampling 

stations exceeded the interquartile range of TP concentrations 

in base-flow samples. In contrast, the concentrations of major 

ions in composite stormflow samples generally were less than 

concentrations of major ions in base-flow samples, indicating 

the effects of dilution from stormflow (table 7; figs. 11–12). 

Concentrations of TN in base flow and stormflow, although 

more variable in stormflow samples, were generally similar 

(table 7 and fig. 13). The pesticides 2,4-D, carbaryl, imazaquin, MCPA, metsulfuron-methyl, norflurazon, and siduron, 

some of which have large soil sorption coefficients (table 9) 

and weakly absorb to soil, were more frequently detected 

in stormflow samples than in base-flow samples (fig. 14). 

Caffeine was detected in about 96 percent of all stormflow 

samples compared to about 44 percent for base-flow samples. 

Land Use

The land use and associated constituents in a watershed 

often affect the water quality, especially those constituents 

introduced through anthropogenic activities. Comparisons of 

concentrations of selected constituents to percentages of landuse areas can provide water-resources managers with information useful for establishing drainage-area-protection strategies, 

such as the purchase of land and restrictions on certain land 

uses. Explicit use of alternative deicing compounds, such as 

calcium chloride, to reduce Na concentrations in receiving 

water is an example of one such restriction. 

A correlation analysis was performed for mean annual 

flow-weighted concentrations of Ca, Cl, Na, SO4

, TN, TP, 

U.S. Geological Survey station number 

0.1

1

10

100

1,000

Nephelometric turbidity ratio units

Base flow

Stormflow

Base flow

Stormflow

Base flow

Stormflow

Base flow

Stormflow

Base flow

Stormflow

01104415 01104433 01104455 01104460 01104475

25th percentile

75th percentile

Median

 Maximum

 Minimum

EXPLANATION

Mean

U.S. Environmental Protection

Agency proposed guideline for 

level 3, ecoregion 59 for turbidity

Figure 10. Distributions of turbidity in base-flow and stormflow samples collected from five U.S. Geological Survey 

stations in the Cambridge drinking-water source area, Massachusetts, water years 2005–07.
Water-Quality Conditions 43

Concentration, in milligrams per liter

1

10

100

1,000

U.S. Geological Survey station number 

A Dissolved calcium

Concentration, in milligrams per liter

10

100

1,000

Base flow

Stormflow

Base flow

Stormflow

Base flow

Stormflow

Base flow

Stormflow

Base flow

Stormflow

01104415 01104433 01104455 01104460 01104475

Criterion continuous concentration

recommended for aquatic life 

Secondary drinking-water

regulation

Criterion maximum concentration

recommended for aquatic life 

B Dissolved chloride

25th percentile

75th percentile

Median

 Maximum

 Minimum

EXPLANATION

Mean

U.S. Environmental Protection

Agency criteria for chloride

Figure 11. Distributions of concentrations of A, dissolved calcium and B, chloride in base-flow and stormflow 

samples from five U.S. Geological Survey stations in the Cambridge drinking-water source area, Massachusetts, 

water years 2005−07. [U.S. Environmental Protection Agency, 2007; U.S. Environmental Protection Agency, 2009a]
44 Water Quality in the Cambridge Drinking-Water Source Area, Massachusetts, Water Years 2005–07 Concentration, in milligrams per liter

1

10

100

1,000 A Dissolved sodium

B Dissolved sulfate

Concentration, in milligrams per liter

1

10

100

Base flow

Stormflow

Base flow

Stormflow

Base flow

Stormflow

Base flow

Stormflow

Base flow

Stormflow

U.S. Geological Survey station number 

01104415 01104433 01104455 01104460 01104475

 Massachusetts drinking-

 water guideline for sodium

25th percentile

75th percentile

Median

 Maximum

 Minimum

EXPLANATION

Mean

Figure 12. Distributions of concentrations of A, dissolved sodium and B, dissolved sulfate in base-flow and 

stormflow samples from five U.S. Geological Survey stations in the Cambridge drinking-water source area, 

Massachusetts, water years 2005–07. [Data from the Commonwealth of Massachusetts, Executive Office of 

Energy and Environmental Affairs, Department of Environmental Protection, Water, Wastewater & Wetlands: 

Regulations & Standards, 2007, Massachusetts surface-water quality standards]
Water-Quality Conditions 45

Concentration, in milligrams per liter

0.1

1

10

A Total nitrogen

B Total phosphorus

U.S. Geological Survey station number 

Concentration, in milligrams per liter

0.001

0.01

0.1

1

Base flow

Stormflow

Base flow

Stormflow

Base flow

Stormflow

Base flow

Stormflow

Base flow

Stormflow

01104415 01104433 01104455 01104460 01104475

U.S. Environmental Protection

Agency proposed concentration 

guidelines for level 3, ecoregion 59

25th percentile

75th percentile

Median

 Maximum

 Minimum

EXPLANATION

Mean

Total nitrogen

Total phosphorus

Figure 13. Distributions of concentrations of A, total nitrogen and B, total phosphorus in base-flow and 

stormflow samples from five U.S. Geological Survey stations in the Cambridge drinking-water source area, 

Massachusetts, water years 2005−07. [Data from U.S. Environmental Protection Agency, 2000]
46 Water Quality in the Cambridge Drinking-Water Source Area, Massachusetts, Water Years 2005–07

 Frequency of detection, in percent

Benomyl

Metalaxyl

Propiconazole

2, 4-D methyl ester

2,4-D

Atrazine

CEAT

OIET

Diuron

Fenuron

Flumetsulam

Fluometuron

Imazaquin

MCPA

Metsulfuron-methyl

Norflurazon

Siduron

Sulfometuron-methyl

Tebuthiuron

 Triclopyr

Carbaryl

Imidacloprid

Oxamyl

Insecticides

Propoxur

Herbicides Fungicides

0 10 20 30 40 50

 Overall detection frequency

 Detection frequency in stormflow

 EXPLANATION

Figure 14. Frequency of detection 

for selected pesticides in samples of 

base flow and stormflow collected at 

the primary sampling stations in the 

Cambridge drinking-water source 

area, Massachusetts, water years 

2005−07.

caffeine, and 12 pesticides (detected in samples of base 

flow or stormflow at a frequency greater than or equal to 25 

percent) and land-use characteristics at four primary sampling 

stations (USGS stations 01104415, 01104433, 01104455, and 

01104475; fig. 1) on tributaries in the Cambridge and Stony 

Brook Reservoir watersheds (table 2). The fifth primary sampling station, USGS station 01104460, was excluded because 

its watershed encompasses most of the drinking-water source 

area, including the Cambridge Reservoir, and it is likely that 

various transformations of the constituents could result during 

the immigration of water through the reservoir system that are 

not typical in the small subbasins. Annual flow-weighted concentrations were derived from load estimates for each subbasin. A second correlation analysis was performed for the available data on concentrations (mean concentrations of major 

ions and nutrients and maximum concentrations of selected 

pesticides) in base-flow samples collected from the primary 

and secondary USGS sampling stations (table 1). Maximum 

concentrations of selected pesticide concentrations were 

used instead of mean values because of the low frequency of 

detection for most compounds and the limited data available 

from the secondary sampling stations.

Correlations between annual flow-weighted concentrations of major ions and most land-use classes in the subbasins 

of the four primary sampling stations were not significant, 

although the strength of the relation between annual flowweighted concentrations of Ca, Cl, and Na and most roadways 

categories was moderately strong to strong, particularly for 

town roadways (table 11). The lack of statistical significance 

may be due, in part, to the small number of data values (four) 

available for the analysis. In contrast, correlations between 

mean concentrations of major ions in base flow and land-use 

classes in all 11 subbasins within the drinking-water source 

area were significant for most land uses associated with paved 

surfaces, except State roadways (table 12). Relations between 

mean concentrations of Ca in base flow and commercial and 

total impervious land uses (table 12 and fig. 15) were stronger than the relation between mean concentrations of Ca in 

base flow and total roadway land uses. Commercial landscape 

area and commercial roof area are unlikely sources of these 

constituents, but commercial areas are often proportional in 
Water-Quality Conditions 47

size to the areas designated as commercial–industrial. As a 

result, these areas appear to relate to the concentration data. 

The moderately strong to strong relations between Ca and 

various commercial areas is potentially the result of winter 

applications of calcium chloride, as opposed to applications of 

sodium chloride that are restricted by the local conservation 

commissions (David Kaplan, city of Cambridge Water Department, written commun., 2011) for sidewalks and parking 

areas in the commercial areas. Liquid calcium chloride also is 

used under specific conditions on the State maintained roads 

in the drinking-water source area (Commonwealth of Massachusetts, Massachusetts Highway Department, 2008). The 

relation between concentrations of deicing constituents and 

roadway area in the drinking-water supply area is consistent 

with findings from a prior study by the USGS (Waldron and 

Bent, 2001).

Positive correlations between annual flow-weighted 

concentrations of TN in samples collected at the four primary 

sampling stations and percentages of recreational area and 

open-water area are significant (table 11); strong negative 

correlations between annual flow-weighted concentrations of 

TN and many of the classes of roadway areas also are significant. Positive correlations between mean concentrations of 

TN in base flow and commercial landscape, Interstate road, 

State maintained road, and total impervious area are significant (table 12). Correlations between annual flow-weighted 

concentrations of TP and percentages of forest and State 

roadway indicate that the relations are moderately strong; 

however, the relations are significant only at lower confidence 

intervals (about 87 percent). No significant relations between 

mean concentrations of TP in base flow and percentage of 

land-use class were found (table 12), except for commercial 

roof land use. A strong negative correlation between annual 

flow-weighted concentrations of TP and wetland land use was 

found; however, correlations are neither strong nor significant 

for mean concentrations of TP in base flow and wetland land 

use, likely because the concentrations were consistently low 

among subbasins during base-flow conditions.

Annual flow-weighted concentrations of the pesticides 

benomyl, imidacloprid, and carbaryl in samples collected at 

the four primary sampling stations correlated significantly 

with the percentages of recreational and open-water land 

uses (table 11). Mean concentrations of these pesticides in 

base flow also correlated significantly with recreational land 

use throughout the drinking-water supply area (table 12). 

Although the correlations between recreational land use 

and annual flow-weighted concentrations of benomyl and 

carbaryl were significant in both of the previous analyses, 

these pesticides were detected only in samples collected at 

two and three primary sampling stations, respectively. The 

annual flow-weighted concentration of the herbicide 2, 4-D, a 

commonly used weed killer (table 9), correlated significantly 

with residential-roof land use for the four primary sampling 

stations. Lot sizes in the drinking-water source area are 

relatively large (greater than a hectare in some towns), and 

lots often contain landscaped areas and (or) lawns that tend 

to be separated from neighboring houses by woodlots. Thus, 

residential-roof land use is not likely a source of the herbicide 

but more likely represents the mean size of a lawn or landscaped area of homes in the watershed. A significantly strong 

correlation between the insecticide diuron and parking lot land 

use was found. Relations between maximum concentrations of 

diuron in base flow and most land-use classes are weak (table 

12), except for commercial and parking lot land uses, which 

are moderately strong. Annual flow-weighted concentrations 

of the herbicide siduron for the four primary sampling stations 

correlated significantly with wetland land use (table 11). Moderately strong relations between the herbicide and commercial 

roof area and parking lots were significant only at lower confidence levels (α = 0.88). Maximum concentrations of siduron 

in base flow correlated significantly with various commercial 

land-uses, as well as, total impervious area throughout the 

drinking-water supply area (table 12). Commercial rooftop and 

parking lot land uses can represent a substantial amount of the 

total impervious area in many of the subbasins. Although it is 

unlikely that commercial rooftops and parking lots are sources 

of siduron, the area of these land-use classes may be correlated 

with commercial landscape areas. Siduron is a component of 

lawn-starter fertilizers. It is relatively persistent compared to 

many herbicides, and on the basis of its physical characteristics (table 9), the compound is likely to immigrate to groundwater or mobilize in overland stormflows.

Except for town road land use, and to a lesser extent 

total roadway, most correlation test results for annual flowweighted concentrations of caffeine in samples collected at the 

four primary sampling stations and land-use classes are weak 

(table 11). The moderately strong relation between town road 

land use and annual flow-weighted concentrations of caffeine 

(table 12 and fig. 15) were significant only at a lower confidence interval (α = 0.87). Maximum concentrations of caffeine in base flow correlated significantly with parking lot and 

commercial roof land uses throughout the watersheds; however, the relation was only moderately strong (table 12). Most 

areas in the watersheds are connected to integrated sewage 

systems, thus low-level concentrations of caffeine are potentially related to leaking sewer lines, as well as, litter or spilled 

beverages, which often are found on or near roadways and 

parking lots. In general, correlations of water-quality data and 

land use were limited by the small data set, particularly where 

concentration data are below detection limits and where landuse classes are not consistently present among the subbasins.

Comparison of Water-Quality Properties and 

Constituent Concentrations with Water-Quality 

Criteria and Guidelines

Values of water-quality properties and concentrations of 

constituents in samples from watersheds are compared to various criteria and guidelines. The Commonwealth of Massachusetts, Executive Office of Energy and Environmental Affairs, 

Department of Environmental Protection, Water, Wastewater 
48 Water Quality in the Cambridge Drinking-Water Source Area, Massachusetts, Water Years 2005–07 Concentration, in milligrams per liter Chloride Sodium Sulfate Caffeine

Percentage of subbasin state maintained roadway Percentage of subbasin town road Percentage of subbasin parking lot

Calcium

Concentration,

in milligrams per liter

Concentration,

in milligrams per liter

Concentration,

in milligrams per liter

Concentration,

in micrograms per liter

EXPLANATION

Mean annual flow-weighted concentration of ions and caffeine at primary sampling station

Mean concentration of ions in base flow or maximum concentration of caffeine in base flow 

at primary and secondary sampling station

0 2 4 6 8 10 0 2 4 6 8 0 5 10 15 20 25 30

10

25

40

55

70

85

100

10

100

1,000

10

100

1,000

5

15

25

35

45

50

0.0015

0.01

0.1

1

1.5

Figure 15. Concentrations of calcium, chloride, sodium, sulfate, and caffeine in relation to the areal percentage of State maintained 

roads, town roads, and parking lots in 11 subbasins in the Cambridge drinking-water source area, Massachusetts, water years 2005−07.
Water-Quality Conditions 49

Table 11. Pearson correlation coefficients relating percentage of land-use classes to annual flow-weighted concentrations of selected major ions, total nutrients, caffeine, and selected pesticides for four subbasins in the Cambridge and Stony Brook Reservoir watersheds, Massachusetts, 2005–07.[Shaded areas indicate values that are significant at a 90-percent confidence level]

Land use Calcium Chloride Sodium Sulfate Total nitrogen Total phos- phorus 2,4-D Benomyl Carbaryl Diuron Imidacloprid Siduron Caffeine

Agriculture -0.26 -0.07 0.01 0.87 0.64 0.26 0.81 0.62 0.62 -0.43 0.63 -0.37 0.03

Commercial/industrial 0.50 0.21 0.12 -0.68 -0.66 -0.49 -0.79 -0.64 -0.64 0.67 -0.57 0.63 0.14

Forest -0.65 -0.19 -0.13 -0.61 -0.15 0.88 0.10 -0.15 -0.15 -0.71 -0.44 -0.83 -0.28

landscaped area 0.59 0.20 0.10 -0.53 -0.57 -0.66 -0.80 -0.55 -0.55 0.77 -0.42 0.77 0.15

Railroad -0.55 -0.33 -0.32 -0.90 -0.24 0.53 -0.40 -0.22 -0.22 -0.48 -0.49 -0.55 -0.44

Recreational -0.57 -0.71 -0.68 0.57 0.99 -0.11 0.16 0.99 0.99 -0.50 0.97 -0.31 -0.62

Residential -0.44 -0.19 -0.10 0.75 0.68 0.40 0.79 0.66 0.66 -0.60 0.61 -0.55 -0.11

Roof (commercial) 0.70 0.26 0.16 -0.35 -0.52 -0.77 -0.72 -0.51 -0.51 0.87 -0.33 0.88 0.24

Roof (residential) -0.22 0.31 0.41 0.45 0.07 0.78 0.96 0.05 0.05 -0.51 -0.05 -0.62 0.33

Parking lot 0.75 0.34 0.24 -0.33 -0.57 -0.74 -0.67 -0.56 -0.56 0.90 -0.37 0.89 0.32

Interstate roadway 0.55 0.78 0.76 -0.48 -0.99 0.24 0.01 -0.99 -0.99 0.43 -0.98 0.22 0.69

State roadway -0.08 0.50 0.56 -0.22 -0.52 0.88 0.57 -0.54 -0.54 -0.33 -0.69 -0.54 0.43

Town roadway 0.76 0.92 0.89 -0.23 -0.97 0.07 0.12 -0.97 -0.97 0.62 -0.88 0.42 0.87

Open water -0.63 -0.86 -0.85 0.35 0.98 -0.22 -0.12 0.99 0.99 -0.48 0.95 -0.27 -0.79

Wetland 0.72 0.19 0.11 0.33 -0.04 -0.96 -0.37 -0.03 -0.03 0.85 0.25 0.95 0.25

State maintained roadway 0.44 0.77 0.76 -0.45 -0.95 0.41 0.14 -0.95 -0.78 0.28 -0.98 0.06 0.68

Total roadway (Interstate, 

State, and Town)

0.51 0.81 0.80 -0.41 -0.96 0.34 0.14 -0.97 -0.97 0.36 -0.97 0.14 0.73

Total impervious area 0.77 0.47 0.38 -0.42 -0.74 -0.58 -0.59 -0.72 -0.83 0.88 -0.56 0.82 0.43
50 Water Quality in the Cambridge Drinking-Water Source Area, Massachusetts, Water Years 2005–07 Table 12. Pearson correlation coefficients relating percentage of land-use classes to mean concentrations of selected major ions and total nutrients in base flow and maxiumum concentrations of caffeine and selected pesticides in base flow in the subbasins of the primary and secondary sampling stations in the Cambridge and Stony Brook Reservoir watersheds, Massachusetts, water years 2006–07. [Shaded areas indicate values that are significant at a 90-percent confidence level] Land use Calcium Chloride Sodium Sulfate Nitrogen Phosphorus 2,4-D Benomyl Carbaryl Diuron Imidacloprid Siduron Caffeine Agriculture -0.55 -0.40 -0.42 -0.56 -0.52 -0.39 -0.29 0.32 0.31 -0.30 0.24 -0.60 -0.23 Commerical/in- dustrial 0.85 0.59 0.64 0.54 0.39 0.33 -0.21 -0.33 -0.41 0.51 -0.33 0.65 0.30 Forest -0.40 -0.25 -0.27 -0.31 -0.34 -0.47 -0.20 -0.31 -0.33 -0.21 -0.29 -0.74 -0.36 Commercial landscape 0.82 0.52 0.57 0.69 0.61 0.50 -0.09 -0.23 -0.26 0.59 -0.23 0.85 0.52 Parking lot 0.87 0.56 0.62 0.63 0.53 0.55 -0.25 -0.22 -0.28 0.59 -0.26 0.89 0.66 Railroad -0.23 -0.21 -0.22 0.26 0.36 -0.44 0.23 -0.24 -0.13 0.15 -0.10 -0.36 -0.49 Recreational -0.50 -0.51 -0.53 -0.12 0.10 -0.27 0.05 0.94 0.99 -0.25 0.94 -0.27 -0.25 Residential -0.57 -0.32 -0.37 -0.27 -0.15 -0.30 0.41 0.45 0.53 -0.40 0.48 -0.37 -0.37 Roof (commer- cial) 0.84 0.51 0.56 0.58 0.51 0.60 -0.19 -0.22 -0.27 0.55 -0.24 0.91 0.62 Roof (residential) -0.11 0.26 0.20 0.08 0.02 -0.35 0.35 0.09 0.10 -0.23 0.13 -0.21 -0.23 Interstate road 0.79 0.86 0.86 0.92 0.75 -0.09 -0.30 -0.28 -0.35 0.49 -0.26 0.47 0.48 State road 0.16 0.54 0.49 0.16 -0.09 -0.42 0.14 -0.47 -0.49 0.04 -0.44 -0.30 -0.13 Town road 0.62 0.66 0.66 0.33 0.16 0.21 0.27 -0.24 -0.29 0.13 -0.22 0.48 0.06 Open water -0.65 -0.70 -0.70 -0.66 -0.51 -0.05 0.21 -0.07 0.03 -0.21 -0.05 -0.44 -0.46 Wetland -0.40 -0.49 -0.47 -0.85 -0.83 0.48 0.07 -0.33 -0.33 -0.34 -0.39 -0.13 0.04 State maintained roadway 0.75 0.88 0.87 0.87 0.67 -0.16 -0.25 -0.34 -0.41 0.45 -0.32 0.37 0.42 Total roadway 0.79 0.91 0.90 0.77 0.55 -0.04 -0.08 -0.34 -0.41 0.38 -0.32 0.46 0.33 Total Impervious 0.94 0.73 0.77 0.74 0.60 0.45 -0.19 -0.27 -0.34 0.58 -0.29 0.88 0.62
Water-Quality Conditions 51

& Wetlands: Regulations & Standards (MassEOEEA) sets 

water-quality standards for surface water on the basis of the 

intended use. Waters designated as Class A specifically include 

sources of public water supplies and their tributaries. Standards for Class A waters applicable to the data presented in 

this report pertain to pH, dissolved oxygen, and Na in drinking 

water (MassEOEEA, 2009). The U.S. Environmental Protection Agency (USEPA) has established various water-quality 

guidelines, including the criterion continuous concentration (CCC), also referred to as a chronic concentration, and 

the criterion maximum concentration (CMC) for freshwater 

aquatic life, secondary drinking-water regulations (SDWRs), 

maximum contaminant levels (MCLs), and drinking-water 

lifetime exposure levels (DWELs) for finished drinking water. 

The USEPA CCC is an estimate of the highest concentration of a constituent to which an aquatic community can be 

exposed indefinitely without adverse effects. In contrast, the 

USEPA CMC is an estimate of the highest concentration of 

a constituent to which an aquatic community can be exposed 

briefly without adverse effects. SDWRs are non-mandatory 

water-quality guidelines established by the USEPA and are 

designed to assist public water suppliers in managing aesthetic 

aspects of water, such as taste, odor, color, foaming, corrosivity, staining, scaling, and sedimentation; the regulations 

are not related to risk to human health (U.S. Environmental 

Protection Agency, 2007). Although these guidelines typically 

are applied to finished water, the presence of these constituents 

at concentrations exceeding SDWRs can result in damage to 

equipment and reduce the effectiveness of treatment for other 

constituents. Some constituents, such as Cl, Na, and N, are 

not effectively removed by the Cambridge water-treatment 

facility (Smith, 2007). MCLs and DWELs were established 

to protect human health. USEPA SDWRs and guidelines for 

drinking water are useful benchmarks used for comparison 

with constituent concentrations in the drinking-water source 

area. Turbidity and nutrient data are compared to USEPA reference values, which are considered representative of pristine 

or minimally affected waters (U.S. Environmental Protection 

Agency, 2000). Finally, pesticide concentrations are compared 

to Canadian long-term aquatic life guidelines established to 

protect the most sensitive organism during its most sensitive 

life stage from any adverse effects associated with long-term 

exposure (Canadian Council of Ministers of the Environment, 

2011).

Measurements of pH, specific conductance, and dissolved 

oxygen concentration in samples collected in the drinkingwater source area seldom exceeded available water-quality 

criteria. In about 8 percent of water samples, primarily baseflow samples, pH was less than the MassEOEEA minimum 

standard of 6.5 pH units (tables 7 and 8). Dissolved oxygen 

concentrations, which were measured only during base-flow 

conditions, were less than the MassEOEEA guideline of 5 

mg/L for warm-water fisheries in about 7 percent of the water 

samples collected at USGS stations 01104420, 01104433, 

and 01104453. The low dissolved oxygen concentrations 

occurred during periods of low streamflow. Turbidity in about 

76 percent of water samples exceeded the USEPA proposed 

reference value of 1.68 nephelometric turbidity units (NTUs, 

which is similar to NTRUs for data reported in this study) for 

level 3, ecoregion 59 (U.S. Environmental Protection Agency, 

2000, table 4a), which covers most of eastern Massachusetts, 

except for Cape Cod. Turbidity of all composite stormflow 

samples exceeded this reference value. The turbidity of only 1 

of 15 base-flow samples collected at USGS station 011104455 

exceeded the reference value. The low turbidity of samples 

from this site is likely the result of the containment of much of 

the upstream tributary in a subsurface pipe where the water is 

not exposed to sunlight, thereby, greatly limiting algal growth. 

The pH and turbidity of samples collected from the raw-water 

intake at Fresh Pond Reservoir did not exceed the aforementioned criteria.

Concentrations of dissolved Cl and Na in water samples commonly exceeded the USEPA SDWR for Cl (250 

mg/L) and the MassEOEEA drinking-water guideline for 

Na (20 mg/L). About 38 and 35 percent of concentrations of 

Cl in water samples collected in the drinking-water source 

area during 2004–07 exceeded the USEPA CCC and SDWR 

standards, respectively. During this study, Cl guidelines were 

not exceeded in water samples from USGS stations 01104390, 

01104453, 01104460 and 01104475, which are on tributaries to the Stony Brook Reservoir that drain less developed 

parts of the drinking-water source area. Concentrations of 

Cl in samples collected from the raw-water intake at Fresh 

Pond Reservoir did not exceed the guidelines (tables 7 and 

8). The concentrations of Cl in stormflow samples exceeded 

guidelines less frequently than in base-flow samples. However, the highest concentration of Cl (1,050 mg/L) measured 

in any sample of streamwater during the study period, which 

exceeded the USEPA CMC, was measured in a composite 

stormflow sample from USGS station 01104415 on a tributary 

along Interstate 95 north of the Cambridge Reservoir. 

As discussed previously, concentrations of many major 

ions tend to decrease with increasing flows. This also tends 

to be the case during winter storms despite high concentrations of deicing compounds in runoff at the onset of the storm. 

Data on Cl concentrations, as well as concentrations of other 

constituents discussed in this report, are limited by the number 

of samples collected in the tributaries of the drinking-water 

source area. However, continuous records of specific conductance are available for nearly the entire study period. These 

records of specific conductance indicate that the USEPA CCC 

and SDWR for Cl, estimated at a given level of specific conductance, were frequently exceeded during base-flow conditions at USGS stations 01104415, 01104433, and 01104455; 

the subbasins represented by these stations contain large 

paved areas (table 2; figs. 16–18). These data also indicate that 

flow-weighted values of specific conductance during runoff 

events exceed the specific conductance levels estimated at the 

concentrations for the USEPA SDWR and CCC thresholds 

for Cl less frequently than during base flow; however, some 

fraction of specific conductance during some runoff events is 

substantially greater than the maximum specific conductance 
52 Water Quality in the Cambridge Drinking-Water Source Area, Massachusetts, Water Years 2005–07

during base flow. Specific conductance values rarely exceeded 

the levels estimated at the concentrations for the SDWR and 

CCC thresholds for Cl at USGS station 01104475 in a lowdensity residential area west of the Stony Brook Reservoir 

(fig 19). About 6 percent of the daily mean values of specific 

conductance in water released from the Cambridge Reservoir 

exceeded the levels estimated at the concentrations for the 

SDWR and CCC thresholds for Cl; the estimated CMC threshold for Cl was not exceeded during water years 2004–08. 

Measurements of specific conductance in Stony Brook (USGS 

station 01104460), which receives a mixture of water from 

the Cambridge Reservoir and from the less developed areas 

of the upper Stony Brook Reservoir watershed, were rarely 

(less than 0.5 percent) greater than the specific conductance 

levels estimated at the concentrations for the SDWR and CCC 

thresholds for Cl. 

Concentrations of dissolved Na 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 Massachusetts drinking-water guideline (20 mg/L). Only in a few composite stormflow samples 

was the concentration of Na less than the guideline (table 7). 

Concentrations of Na estimated from records of specific conductance also tended to be greater than the guideline.

Concentrations of TN and TP collected in samples 

from the drinking-water source area generally exceeded the 

proposed reference concentrations of 0.57 and 0.024 mg/L, 

respectively, for level 3, ecoregion 59 (U.S. Environmental 

Protection Agency, 2000). These reference values represent 

the 25th percentile of all nutrient data for the ecoregion 59 and 

are intended to protect streams and rivers from the adverse 

effects of nutrient enrichment (U.S. Environmental Protection Agency, 2000). Concentrations of TN exceeded this 

reference concentration in 94 percent of all water samples 

and 100 percent of all samples collected from the raw-water 

intake at the Fresh Pond Reservoir. Concentrations of TP 

exceeded this reference concentration in 56 percent of all 

water samples. With few exceptions, concentrations of TP in 

all composite stormflow samples exceeded the reference value 

of 0.024 mg/L. Generally, concentrations of TP in samples 

collected during base-flow conditions were near or below the 

reference concentration.

Pesticides measured in this study were detected in less 

than one-half of the water samples collected in the watersheds during water years 2004–08. Although the frequency 

of detection varied from tributary to tributary, concentrations 

of pesticides typically were less than 1 μg/L and much less 

than drinking-water guidelines. Pesticide concentrations also 

were several orders of magnitude less than the LC50s for 

several fish species common to the watershed. The LC50 is a 

standard measurement of toxicity for exposure to an analyte 

100

1,000

10,000

Specific conductance, microsiemens per centimeter

20,000

0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 98 99 99.5 99.9

 Chloride criterion continuous concentration

Empirical probability distribution of flow-weighted

 values of specific conductance during runoff events

 Chloride criteria maximum concentration 

 Chloride secondary drinking-water standard

Empirical probability distribution of daily base-flow

 values of specific conductance

EXPLANATION

 Specific conductance value estimated for each 

 U.S. Environmental Protection Agency water-quality 

 criteria for chloride

Percentage of values in which specific conductance was equaled or exceeded 

Figure 16. Probability distribution of daily specific conductance values during base flow and flow-weighted specific conductance 

values during runoff for U.S. Geological Survey station 01104415, Cambridge drinking-water source area, Massachusetts, in relation 

to estimated concentration values representing U.S. Environmental Protection Agency water-quality criteria for chloride, water 

years 2004–08.
Water-Quality Conditions 53

100

1,000

10,000

Specific conductance, microsiemens per centimeter

Percentage of values in which specific conductance was equaled or exceeded 

 Chloride criterion continuous concentration

Empirical probability distribution of flow-weighted

 values of specific conductance during runoff events

 Chloride criteria maximum concentration 

 Chloride secondary drinking-water standard

Empirical probability distribution of daily base-flow

 values of specific conductance

EXPLANATION

 Specific conductance value estimated for each 

 U.S. Environmental Protection Agency water-quality 

 criteria for chloride

0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 98 99 99.5 99.9

Figure 17. Probability distribution of daily specific conductance values during base flow and flow-weighted specific conductance 

values during runoff for U.S. Geological Survey station 01104433, Cambridge drinking-water source area, Massachusetts, in relation 

to estimated concentration values representing U.S. Environmental Protection Agency water-quality criteria for chloride, water years 

2004–07. Specific conductance, microsiemens per centimeter

0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 98 99 99.5 99.9

10

100

1,000

10,000

100,000

Percentage of values in which specific conductance was equaled or exceeded

 Chloride criterion continuous concentration

Empirical probability distribution of flow-weighted

 values of specific conductance during runoff events

 Chloride criteria maximum concentration 

 Chloride secondary drinking-water standard

Empirical probability distribution of daily base-flow

 values of specific conductance

EXPLANATION

 Specific conductance value estimated for each 

 U.S. Environmental Protection Agency water-quality 

 criteria for chloride

Figure 18. Probability distribution of daily specific conductance values during base flow and flow-weighted specific conductance 

values during runoff for U.S. Geological Survey station 01104455, Cambridge drinking-water source area, Massachusetts, in relation 

to estimated concentration values representing U.S. Environmental Protection Agency water-quality criteria for chloride, water years 

2004–08.
54 Water Quality in the Cambridge Drinking-Water Source Area, Massachusetts, Water Years 2005–07

100

1,000

2,000

Specific conductance, microsiemens per centimeter

Percentage of values in which specific conductance was equaled or exceeded

 Chloride criterion continuous concentration

Empirical probability distribution of flow-weighted

 values of specific conductance during runoff events

 Chloride criteria maximum concentration 

 Chloride secondary drinking-water standard

Empirical probability distribution of daily base-flow

 values of specific conductance

EXPLANATION

 Specific conductance value estimated for each 

 U.S. Environmental Protection Agency water-quality 

 criteria for chloride

0.1 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 98 99 99.5 99.9

Figure 19. Probability distribution of daily specific conductance values during base flow and flow-weighted specific 

conductance values during runoff for U.S. Geological Survey station 01104475, Cambridge drinking-water source area, 

Massachusetts, in relation to estimated concentration values representing U.S. Environmental Protection Agency water-quality 

criteria for chloride, water years 2004–08.

concentration that is lethal for half of the sample population 

over a specified period.

Imidacloprid is the only pesticide detected at a concentration that exceeded the Canadian long-term aquatic-life 

guidelines (table 10). Although imidacloprid (an insecticide) 

was detected in samples from many of the sampling stations 

(table 7 and 8), the frequency of detection varied by station. 

The frequency of detection of imidacloprid in samples from 

sampling station 01104475, which drains a residential area 

adjacent to the Stony Brook Reservoir, was 94 percent with 

42 percent of the samples exceeding the Canadian long-term 

aquatic-life guideline of 0.23 μg/L. 

Constituent Loads and Yields

Load and yield estimates for the study stations provide 

information about the rates at which masses of constituents are 

transported to the reservoir. Tributaries with higher flows tend 

to have higher loads because the greater volume of water can 

carry more of the constituent (assuming similar constituent 

concentrations in each tributary) to the reservoir per unit time. 

Yields represent the constituent load per unit of drainage area 

and are calculated by dividing the estimated load for a station 

by the drainage area for the station. Yields are useful for comparison of different drainage-area sizes because the effects of 

basin size and, therefore, total streamflow volume are normalized with respect to basin area. Yields are useful for examining 

potential differences among basin properties that may affect 

water quality.

Loads for each constituent were estimated on the basis 

of the measure of the central tendency (table 13) and annual 

volumes of base flow and total flow during storms. For many 

constituents, the mean concentration of the sample set was 

expected to be similar to the population mean. However, for 

sample concentrations that appeared to follow a log-normal 

distribution, the population means were estimated with the 

assumption that the logarithms of the data were symmetric. 

This approach provides a better estimate of the population 

mean (Helsel and Hirsch, 2002). Loads for were not estimated 

for caffeine and pesticides at sites where these analytes were 

infrequently detected because data were insufficient to estimate mean concentrations.

Loads and Yields of Calcium, Chloride, Sodium, 

and Sulfate, Water Years 1999 to 2008

Loads and yields of Ca, Cl, Na, and SO4 were estimated 

for the five primary sampling stations for water years 2005–07 

on the basis of streamflow and concentration data collected 

during this study (table 14). Loads and yields for these constituents also were estimated for all monitoring stations for 
Constituent Loads and Yields 55

Table 13. Mean concentrations and confidence intervals for selected major ions, total nutrients, pesticides, and caffeine in samples of streamwater collected at five primary sampling stations in the Cambridge drinking-water source area, Massachusetts, water years 2005–07.[USGS, U.S. Geological Survey; n, number of samples; mg/L, milligrams per liter; μg/L, micrograms per liter; <, concentration is less than value shown; --, no data; one set of samples from the end of water year 2004 and one sample from station 01104475 from water year 2008 are included]

Analyte

USGS station 01104415 USGS station 01104433

Base flow (n=15) Stormflow (n=15) Base flow (n=15) Stormflow (n=15)

Mean

95-percent confidence interval Mean 95-percent confi- dence interval Mean 95-percent confi- dence interval Mean 95-percent confi- dence interval

Lower Upper Lower Upper Lower Upper Lower Upper

Calcium, mg/L 53.8 48.0 59.6 27.3 26.9 39.1 78.3 71.7 84.8 28.1 23.7 32.4

Sodium, mg/L 333 295 370 208 183 358 259 237 281 122 82.8 161

Chloride, mg/L 612 542 680 376 323 666 540 485 595 232 157 306Sulfate, mg/L 27.0 25.7 28.2 20.3 19.8 29.7 30.8 28.8 32.7 14.4 12.2 16.5

Total nitrogen, mg/L 1.63 1.46 1.80 1.63 1.46 1.80 1.95 1.69 2.20 1.54 1.33 1.76

Total phosphorus, mg/L 0.011 0.009 0.013 0.20 0.18 0.34 0.031 0.02 0.04 0.12 0.09 0.15

Caffeine, μg/L 0.026 0.018 0.065 0.850 0.602 1.997 0.193 0.100 0.286 0.476 0.203 0.750

2,4-D, μg/L <0.038 -- -- 0.645 0.122 8.501 -- -- -- -- -- --

2-Chloro-6-ethylamino-4-aminos-triazine (CEAT), μg/L <0.04 -- -- <0.04 -- -- -- -- -- -- -- --

Benomyl, μg/L <0.02 -- -- <0.02 -- -- -- -- -- -- -- --

Carbaryl, μg/L <0.02 -- -- 0.015 0.009 0.045 -- -- -- -- -- --

Diuron, μg/L 0.008 0.008 0.012 <0.04 -- -- 0.024 0.018 0.051 -- -- --

Imidacloprid, μg/L 0.086 0.027 0.567 0.009 0.007 0.011 0.024 0.022 0.038 0.135 0.033 1.265

Metalaxyl, μg/L <0.04 -- -- <0.04 -- -- -- -- -- -- -- --

Norflurazon, μg/L <0.04 -- -- <0.04 -- -- -- -- -- -- -- --

Propiconazole, μg/L <0.06 -- -- <0.06 -- -- -- -- -- -- -- --

Siduron, μg/L 0.008 0.007 0.010 0.051 0.023 0.208 0.012 0.009 0.028 0.122 0.008 6.09

Sulfometuron-methyl, μg/L <0.06 -- -- <0.06 -- -- -- -- -- -- -- --
56 Water Quality in the Cambridge Drinking-Water Source Area, Massachusetts, Water Years 2005–07 Table 13. Mean concentrations and confidence intervals for selected major ions, total nutrients, pesticides, and caffeine in samples of streamwater collected at five primary sampling stations in the Cambridge drinking-water source area, Massachusetts, water years 2005–07.—Continued [USGS, U.S. Geological Survey; n, number of samples; mg/L, milligrams per liter; μg/L, micrograms per liter; <, concentration is less than value shown; --, no data; one set of samples from the end of water year 2004 and one sample from station 01104475 from water year 2008 are included] Analyte USGS station 01104455 USGS station 01104460 Base flow (n=15) Stormflow (n=15) Base flow (n=12) Stormflow (n=12) Mean 95-percent confi- dence interval Mean 95-percent confi- dence interval Mean 95-percent confi- dence interval Mean 95-percent confi- dence interval Lower Upper Lower Upper Lower Upper Lower Upper Calcium, mg/L 42.9 37.4 48.5 10.3 7.0 13.5 20.7 19.6 21.7 18.3 17.1 19.5 Sodium, mg/L 163 150 177 46 23.1 69.8 66.6 59.5 73.8 62.7 54.7 70.7 Chloride, mg/L 309 274 344 82 37.5 127 127 113 140 116 102 130 Sulfate, mg/L 28.6 27.4 29.9 8.5 6.4 10.7 13.5 12.5 14.4 11.8 11.1 12.5 Total nitrogen, mg/L 2.05 1.87 2.23 1.51 1.20 1.82 0.85 0.70 0.99 0.89 0.75 1.03 Total phosphorus, mg/L 0.009 0.008 0.011 0.19 0.13 0.26 0.019 0.013 0.024 0.05 0.02 0.07 Caffeine, μg/L -- -- -- 0.184 0.12 0.24 0.007 0.005 0.009 0.018 0.013 0.022 2,4-D, μg/L -- -- -- 0.088 0.045 0.312 -- -- -- 2-Chloro-6-ethylamino-4-amino-s- triazine (CEAT), μg/L 0.009 0.009 0.010 -- -- -- -- -- -- -- -- -- Benomyl, μg/L 0.010 0.008 0.011 -- -- -- -- -- -- -- -- -- Carbaryl, μg/L -- -- -- -- -- -- -- -- -- -- -- -- Diuron, μg/L -- -- -- 0.005 0.004 0.008 -- -- -- 0.005 0.004 0.008 Imidacloprid, μg/L 0.011 0.010 0.017 -- -- -- 0.012 0.009 0.015 0.007 0.004 0.039 Metalaxyl, μg/L -- -- -- -- -- -- -- -- -- -- -- -- Norflurazon, μg/L 0.107 0.088 0.126 0.070 0.043 0.197 0.003 0.003 0.004 0.006 0.006 0.007 Propiconazole, μg/L -- -- -- -- -- -- -- -- -- -- -- -- Siduron, μg/L -- -- -- 0.029 0.015 0.108 -- -- -- 0.011 0.011 0.017

Sulfometuron-methyl, μg/L 0.006 0.006 0.008 -- -- -- -- -- -- -- -- --
Constituent Loads and Yields 57

Table 13. Mean concentrations and confidence intervals for selected major ions, total nutrients, pesticides, and caffeine in samples of streamwater collected at five primary sampling stations in the Cambridge drinking-water source area, Massachusetts, water years 2005–07.—Continued[USGS, U.S. Geological Survey; n, number of samples; mg/L, milligrams per liter; μg/L, micrograms per liter; <, concentration is less than value shown; --, no data; one set of samples from the end of water year 2004 and one sample from station 01104475 from water year 2008 are included]

Analyte

USGS station 01104475

Base flow (n=16) Stormflow (n=15)

Mean

95-percent confidence 

interval Mean

95-percent confidence 

interval

Lower Upper Lower Upper

Calcium, mg/L 18.0 17.2 18.8 14.0 12.7 15.3

Sodium, mg/L 28.3 26.8 29.9 26.3 21.6 31.1

Chloride, mg/L 46.6 42.9 50.2 42.0 33.2 50.7

Sulfate, mg/L 18.8 17.8 19.8 14.5 12.8 16.2

Total nitrogen, mg/L 1.93 1.69 2.16 2.17 1.67 2.68

Total phosphorus, mg/L 0.015 0.012 0.018 0.16 0.06 0.26

Caffeine, μg/L 0.008 0.008 0.010 0.045 0.038 0.082

2,4-D, μg/L -- -- -- 0.281 0.045 4.595

2-Chloro-6-ethylamino-4-amino-striazine (CEAT), μg/L -- -- -- -- -- --

Benomyl, μg/L 0.036 0.020 0.113 0.016 0.013 0.030

Carbaryl, μg/L 0.014 0.010 0.032 0.009 0.009 0.021

Diuron, μg/L -- -- -- -- -- --

Imidacloprid, μg/L 0.324 0.258 0.638 0.262 0.224 0.468

Metalaxyl, μg/L 0.024 0.023 0.038 0.022 0.022 0.028

Norflurazon, μg/L -- -- -- -- -- --

Propiconazole, μg/L 0.058 0.050 0.103 0.074 0.048 0.190

Siduron, μg/L 0.013 0.010 0.027 0.025 0.018 0.058

Sulfometuron-methyl, μg/L -- -- -- -- -- --
58 Water Quality in the Cambridge Drinking-Water Source Area, Massachusetts, Water Years 2005–07

which streamflow and continuous specific-conductance data 

were available since water year 1999 (table 15). Loads were 

estimated on the basis of the correlations between concentration data for Ca, Cl, Na, and SO4

 and values of specific 

conductance. 

Loads and yields for major ions calculated from continuous records of specific conductance were similar to those calculated on the basis of mean concentration data (tables 14 and 

15). Except for loads of Cl and Na at USGS station 01104455, 

the 95-percent confidence intervals of loads estimated using 

both of these methods overlapped. Loads and yields for Cl 

and Na estimated from concentration data for USGS station 

01104455, where the 95-percent confidence intervals of load 

estimates did not overlap, are lower than those estimated from 

continuous records of specific conductance. These differences indicate that the samples collected at this station do not 

accurately represent mean water-quality conditions at these 

stations. This is likely the result of an insufficient number of 

winter runoff samples since base-flow concentration data were 

relatively precise throughout the year. Loads and yields estimated on the basis of continuous records of specific conductance are expected to be more accurate than those estimated 

from sample concentration data because the continuous record 

estimated loads and yields represent a broader range of hydrologic conditions, including information from all runoff events, 

including snow melts.

Annual loads of Ca, Cl, Na, and SO4

 estimated on the 

basis of streamflow and mean concentration data varied from 

year to year and station to station and, as a result, were correlated with annual streamflow (table 15). Mean annual yields 

of Ca, Cl, Na, and SO4

 for the drinking-water source area (data 

from USGS stations 01104460 and 01104475) for water years 

2005 to 2007, estimated on the basis of continuous records 

of specific conductance, were 14, 85, 46, and 9 metric tons 

per square kilometer (MT/km2

), respectively. The tributary 

adjacent to Interstate 95 in the Cambridge Reservoir watershed 

(USGS station 01104415) had the highest yields for all major 

ions. Mean annual yields of major ions at this station (table 

15) were 4.3 to 8.3 times higher than the mean annual yields 

for the drinking-water source area. Mean annual yields of 

most major ions at USGS stations 01104433 and 01104455, in 

subbasins that also contain large areas of roadways and parking lots (table 2), were substantially higher than mean annual 

yields for the drinking-water source area. Yields of major 

ions for USGS station 01104475, which drains the subbasin 

with the highest percentage of residential land use and one of 

the lowest percentages of roadways, were the lowest in the 

drinking-water source area, and only 13 percent of the yield 

came from stormflow. In contrast, about 30 percent of the 

yields (estimated from records of flow and specific conductance) of each major ion for the other stations was associated 

with stormflow (table 15). The stormflow component of total 

loads estimated from sample concentration data (table 14) 

tends to be larger than that estimated from specific conductance data because the base-flow component is included in the 

stormflow volume.

Annual mean yields of Cl and Na estimated for water 

year 1999 at USGS station 01104415 (Waldron and Bent, 

2001) were similar to the respective annual mean yields 

estimated for water years 2005 to 2007. However, annual 

mean yields of Cl and Na for water years 2005 to 2007 for 

USGS station 01104460 were about twice the annual mean 

yields estimated for water year 1999. This may be the result 

of greater applications of road salt during the winters of water 

years 2005 and 2006, which were more severe than the winter 

of water year 1999.

Milliequivalent ratios of Na:Cl for mean annual yields 

estimated by both methods of calculation for water years 

2005 to 2007 indicate that sodium chloride accounts for about 

83 percent of the total Cl load at the primary sampling stations, except at USGS station 01104475 where sodium chloride accounts for about 95 percent of the total Cl load. This 

milliequivalent ratio is not necessarily indicative of the deicing 

compound matrix applied during the study period, particularly 

for ratios derived from annual loads. Although the movement 

of Cl in groundwater is conservative, Ca, Na, and other cations 

are affected though cation exchange on mineral surfaces (Millar and Turk, 1949); as a result, groundwater becomes enriched 

in Na (relative to most other common ions) as it moves along 

the flow path. The high Na:Cl milliequivalent ratio for USGS 

station 01104475 may result from the cation exchange process. 

In comparison to many of the other stations, the drainage basin 

for this station contains less impervious area; most flow occurs 

during base-flow conditions (table 15), and as a result, the flow 

path is long. The long flow path increases the potential for ion 

exchange resulting in greater concentrations of Na relative to 

other cations.

Loads and Yields of Total Nutrients, Water Years 

2005 to 2007

Loads and yields of TN and TP estimated for the primary 

monitoring stations varied by station. Mean annual yields of 

TN estimated for the primary monitoring stations were greater 

for the tributary subbasins than for the main stem of Stony 

Brook (USGS station 01104460), which represents most of 

the flow in the lower Stony Brook Reservoir watershed. Mean 

annual yields of TN for three of the tributaries (USGS stations 

01104415, 01104455, and 01104475) were greater than or 

equal to 1,200 kilograms per square kilometer (kg/km2

). The 

mean annual yield of TN at USGS station 01104433 was lower 

at 740 kg/km2

. Loads and yields of TN, as well as the other 

constituents, for this station are potentially underestimated 

because an undersized or damaged pipe upstream from the 

monitoring station diverted stormflow to an adjacent unmonitored subbasin. Base-flow and stormflow components of the 

annual TN load for primary sampling stations were similar 

(table 14). From 83 to 96 percent of the TP load in the tributaries was associated with stormflow (table 14), and 73 percent 

of the TP load in Stony Brook (USGS station 01104460) was 

associated with stormflow. Yields of TP at USGS stations 
Constituent Loads and Yields 59

Table 14. Annual loads and yields of calcium, chloride, sodium, sulfate, total nutrients, caffeine, and 11 pesticides for 5 primary sampling stations in the Cambridge and Stony Brook Reservoir watersheds, Massachusetts, water years 2005–07. [Load values estimated using a stratified flow aproach and measures of central tendancy for significantly different sample sets representing base flow and stormflow at each U.S. Geological Survey station. Insufficient information was available to calculate loads for the other 14 pesticides that were detected in samples of streamwater. USGS, U.S. Geological Survey; km2, square kilometer; 

MT, metric ton; kg, kilogram; g, gram; CI, confidence interval; --, insufficient information to estimate value; flow and therefore load and yield values for U.S. Geological Survey station 01104433 are believed to underestimate the subbasin values]

USGS 

station number Water year Calcium, MT Chloride, MT Sodium, MT Sulfate, MT Total phos- phorus, kg Total nitrogen, kg Caffeine, g 2,4-D, g 2-Chloro-6-ethyl- amino-4- amino-s-triazine, g

01104415 2005 26 320 180 16 89 1,100 380 280 --

2006 43 530 290 26 130 1,800 550 410 --2007 30 370 200 18 94 1,300 390 290 --

95-percent CI (percent) -11 to 27 -16 to 42 -16 to 42 -11 to 30 -16 to 66 ±15 -32 to 140 -83 to 1,200 --

Stormflow load percent 42 47 47 52 96 59 98 100 --

Average yield (mass/km2) 31 380 210 19 98 1,300 410 310 --

01104433Storm load 

percentage

2005 22 160 79 9.4 37 760 160 -- --2006 22 160 81 9.6 38 780 160 -- --2007 11 79 39 4.6 15 360 100 -- --

95-percent CI (percent) ±15 ±21 ±21 ±15 ±36 ±17 ±57 -- --

Stormflow load percent 33 36 39 39 83 52 79 -- --

Average yield (mass/km2) 21 160 78 9 35 740 164 -- --

01104455Storm load 

percentage

2005 16 120 65 12 95 1,300 88 42 2.42006 26 200 100 19 120 1,900 110 51 4.3

2007 17 130 69 12 87 1,300 80 38 2.7

95-percent CI (percent) ±15 ±20 ±20 ±14 ±27 ±17 ±59 -51 to 260 ±21

Stormflow load percent 26 28 29 30 97 52 100 100 0

Average yield (mass/km2) 15 120 61 11 79 1,200 73 34 2.5

01104460Storm load 

percentage

2005 820 5,130 2,730 530 1400 37,000 580 -- --2006 1,020 6,410 3,410 660 1800 46,000 710 -- --2007 820 5,100 2,710 530 1300 36,000 540 -- --

95-percent CI (percent) ±12 ±16 ±16 ±12 ±48 ±19 ±30 -- --

Stormflow load percent 50 53 53 50 73 53 72 -- --

Average yield (mass/km2) 16 97 52 10 26 700 11 -- --

01104475 2005 21 57 35 22 93 2,700 33 170 --

2006 26 71 44 27 120 3,300 43 220 --2007 17 47 29 18 76 2,200 27 130 --

95 percent CI ±12 ±17 ±16 ±13 ±62 ±20 -19 to 70 -86 to 1500 --

Stormflow load percent 36 42 42 36 92 42 76 100 --

Average yield (mass/km2) 10 26 16 10 44 1,200 16 79 --
60 Water Quality in the Cambridge Drinking-Water Source Area, Massachusetts, Water Years 2005–07 Table 14. Annual loads and yields of calcium, chloride, sodium, sulfate, total nutrients, caffeine, and 11 pesticides for 5 primary sampling stations in the Cambridge and Stony Brook Reservoir watersheds, Massachusetts, water years 2005–07.—Continued USGS station number Water year Benomyl, 

g

Carbaryl, 

g

Diuron, 

g

Imidacloprid, 

g

Metalaxyl, 

g

Norflurazon, 

g

Propiconazole, 

g

Siduron, 

g

Sulfometuronmethyl, 

g

01104415 2005 -- 6.5 2.1 27 -- -- -- 24 --

2006 -- 9.5 3.8 46 -- -- -- 36 --2007 -- 6.8 2.6 32 -- -- -- 26 --

95 percent CI (percent) -- -42 to 200 -17 to 53 -64 to 490 -- -- -- -54 to 280 --

Stormflow load percent -- 100 0 13 -- -- -- 90 --

Average yield (mass/km2

) -- 7.2 2.7 33 -- -- -- 27 --

01104433Storm load percentage

2005 -- -- 4.4 39 -- -- -- 34 --

2006 -- -- 4.5 41 -- -- -- 35 --

2007 -- -- 2.4 25 -- -- -- 22 --

95 percent CI -- -- -30 to 120 -68 to 74 -- -- -- -86 to 100 --

Stormflow load percent -- -- 0 86 -- -- -- 85 --

Average yield (mass/km2

) -- -- 4.4 41 -- -- -- 35 --

01104455Storm load percentage

2005 2.6 -- -- 2.9 -- 61 -- 14 1.6

2006 4.8 -- -- 5.3 -- 92 -- 17 2.9

2007 3 -- -- 3.3 -- 62 -- 13 1.8

95 percent CI (percent) ±20 -- -- -16 to 60 -- ±27 -- ±95 -21 to 33

Stormflow load percent 0 -- -- 0 -- 49 -- 100 0

Average yield (mass/km2

) 2.7 -- -- 3.0 -- 56 -- 12 1.7

01104460Storm load percentage

2005 -- -- 120 620 -- 140 -- 370 --2006 -- -- 140 770 -- 170 -- 460 --2007 -- -- 100 600 -- 120 -- 350 --

95 percent CI (percent) -- -- -17 to 81 -37 to 310 -- -19 to 21 -- -18 to 63 --

Stormflow load percent -- -- 100 64 -- 100 -- 76 --

Average yield (mass/km2

) -- -- 2.1 12 -- 2.5 -- 6.9 --

01104475 2005 31 20 -- 360 28 -- 86 25 --

2006 38 25 -- 440 35 -- 110 32 --

2007 25 16 -- 300 23 -- 70 21 --

95 percent CI (percent) -36 to 160 -26 to 130 -- -23 to 91 -14 to 46 -- -28 to 120 -32 to 120 --

Stormflow load percent 41 35 -- 39 44 -- 52 52 --

Average yield (mass/km2

) 14 9.2 -- 170 13 -- 40 12 --
Constituent Loads and Yields 61

Table 15. Annual mean flow, runoff, and estimated annual loads of calcium, chloride, sodium, and sulfate for sampling 

stations in the Cambridge and Stony Brook Reservoir watersheds, Massachusetts, water years 1999–2008.—Continued

[Load values estimated from continuous records of specific conductance and discharge at each U.S. Geological Survey monitoring station. USGS, 

U.S. Geological Survey; ft3/sec, cubic feet per second, MT, metric ton; MT/km2, metric ton per square kilometer; CI, confidence interval; --, unable 

to calculate as a result of regulation of discharge; E, estimated]

USGS 

station number

Water 

year

Annual mean 

discharge,

 ft3

/sec

Runoff 

percent Estimated annual load

Calcium, 

MT

Chloride, 

MT

Sodium, 

MT

Sulfate, 

MT

01104415 2004 0.77 E27 23 260 140 15.0

2005 0.82 30 31 370 200 19.0

2006 1.1 31 27 310 170 19.0

2007 0.8 27 18 190 100 13.0

2008 0.95 39 28 330 180 18.0

95-percent CI (percent) ± 35 ± 16 ± 22 ± 68

Storm load percent 30 31 31 32

Annual average yield (MT/km2

) 24 275 149 16

01104430 1999 6.1 -- 87 630 340 63.0

2000 8.6 -- 140 1,150 630 100

2001 13 -- 230 1,980 1,080 170

2002 7.0 -- 150 1,360 740 100

2003 11 -- 220 1,990 1,080 160

2004 10 -- 210 1,870 1,010 150

2005 16 -- 310 2,700 1,470 220

2006 12 -- 220 1,910 1,040 160

2007 16 -- 280 2,250 1,220 200

2008 8.1 -- 160 1,430 770 110

95-percent CI (percent) ± 20 ± 16 ± 22 ± 68

Storm load percent -- -- -- --

Annual average yield (MT/km2

) 11 97 53 8

1104433 2005 0.45 29 27 210 110 10.0

2006 0.46 22 17 120 67.0 7.6

2007 0.29 27 11 81 44.0 5.2

95-percent CI (percent) ± 48 ± 16 ± 22 ± 68

Storm load percent 26 26 26 29

Annual average yield (MT/km2

) 21 160 86 9

1104455 2004 0.94 42 21 240 130 15

2005 1.1 32 30 420 230 22

2006 1.2 -- 23 220 120 17

2007 0.87 28 16 160 88 12

2008 1.1 37 24 300 160 17

95-percent CI (percent) ± 21 ± 17 ± 23 ± 69

Storm load percent 27 35 34 27

Annual average yield (MT/km2

) 18 211 115 13
62 Water Quality in the Cambridge Drinking-Water Source Area, Massachusetts, Water Years 2005–07

Table 15. Annual mean flow, runoff, and estimated annual loads of calcium, chloride, sodium, and sulfate for sampling 

stations in the Cambridge and Stony Brook Reservoir watersheds, Massachusetts, water years 1999–2008.—Continued

[Load values estimated from continuous records of specific conductance and discharge at each U.S. Geological Survey monitoring station. USGS, 

U.S. Geological Survey; ft3/sec, cubic feet per second, MT, metric ton; MT/km2, metric ton per square kilometer; CI, confidence interval; --, unable 

to calculate as a result of regulation of discharge; E, estimated]

USGS 

station number

Water 

year

Annual mean 

discharge, 

 ft3

/sec

Runoff 

percent Estimated annual load

1104460

95-percent CI (percent)

Storm load percent

Annual average yield (MT/km2

)

1104475

95-percent CI (percent)

Storm load percent

Annual average yield (MT/km2

)

2003

2004

2005

2006

2007

2008

2005

2006

2007

2008

40

37

47

59

47

42

0.9

1.45

1.8

1.2

--

--

--

--

--

--

11

13

11

15

Calcium, 

MT

710

620

830

910

730

670

± 30

--

13

14

22

22

14

± 35

13

8

Chloride, 

MT

4,470

3,800

5,250

5,370

4,320

4,070

± 16

--

80

39

60

57

37

± 27

13

22

Sodium, 

MT

2,430

2,070

2,860

2,930

2,360

2,220

± 22

--

43

23

37

36

24

± 22

13

14

Sulfate, 

MT

430

380

510

570

450

420

± 68

--

8

14

23

22

15

± 49

13

8
Constituent Loads and Yields 63

01104415 and 01104455 were about three times greater than 

the yield for most of the drinking-water source area (USGS 

station 01104460). The high yields at these stations are likely 

the result of the tributaries receiving stormflow directly from 

large impervious areas despite weak to moderate correlations 

between TP and impervious areas at the primary sampling stations, which undoubtedly are affected by the underestimate of 

the TP yield for the drainage area of USGS station 01104433. 

Annual yields of TN estimated for water year 1998 at 

USGS stations 01104415, 01104455, 01104460, and 01104475 

(Waldron and Bent, 2001) were similar (within about 

24 percent) to the respective mean annual yields estimated 

in this study. However, yields of TP at the same USGS stations for water years 2005 to 2007 were about three or more 

times greater than the yields estimated for water year 1998. 

In the case of USGS station 01104415, the estimated mean 

annual load was about 14 times greater than the load reported 

for water year 1998. The differences between annual yields 

estimated at these USGS stations may be explained, in part, 

by differences in sample collection. In the prior study (Waldron and Bent, 2001), discrete samples of streamwater were 

collected primarily during base-flow conditions and to a lesser 

extent during stormflows. In general, concentrations of TP collected during base flow were similar in both studies. However, 

the TP concentrations in samples of base flow and stormflow 

collected as part of this study were significantly different (TP 

during stormflow was often an order of magnitude higher than 

during base flow). In the absence of detailed stormflow data, it 

is likely that the preponderance of base-flow samples collected 

during the 1998 study led to underestimated TP loads.

Loads and Yields of Selected Pesticides and 

Caffeine, Water Years 2005 to 2007

Loads and yields of 11 pesticides and caffeine were 

estimated for stations having frequent detections of these 

analytes. Data were insufficient to estimate pesticide loads 

and yields of the remaining 14 pesticides because they were 

either not detected or infrequently detected. Load and yield 

estimates for the five primary sampling stations were variable 

for pesticides and caffeine (table 14). Annual mean loads of 

individual pesticides were less than a kilogram at all stations. 

The highest yields were estimated for the herbicide 2,4-D and 

the insecticide imidacloprid, ranging from 34 to 310 g/km2 and 

3 to 170 g/km2

, respectively. The load for 2,4-D was entirely 

associated with stormflow. The largest load and yield for 2,4-D 

were estimated for USGS station 01104415 in the Cambridge 

Reservoir watershed. The highest yields for imidacloprid, 

benomyl, carbaryl, metalaxyl, and propiconazole were estimated for USGS station 01104475 on a tributary that drains 

directly into the Stony Brook Reservoir. Annual yields for 

the herbicide siduron ranged from 6.9 to 35 g/km2 with most 

of the loads associated with stormflow, during which it likely 

adsorbed to suspended sediment, based upon its low aqueous 

solubility and sorption coefficient (Koc).

 Mean annual yields for the insecticide diuron ranged 

from 2.1 to 4.4 g/km2

, and the highest yield was estimated for 

USGS station 01104433 on a tributary to Hobbs Brook south 

of the Cambridge Reservoir. Mean annual yield estimates for 

caffeine ranged from 11 to 410 g/km2

 among the five primary monitoring stations. Caffeine was primarily transported 

during stormflow. The estimated annual load of caffeine to 

the Stony Brook Reservoir (summation of loads for USGS 

station 01104460 and 01104475) is equivalent to about 6,500 

12-ounce cups of coffee, assuming an average cup of coffee 

contains about 346 mg/L of caffeine (Seiler and others, 1999).

Accuracy of Estimated Loads

The accuracy of annual load and yield estimates in 

this study is affected by errors associated with sample collection and processing techniques, analytical techniques, 

and estimation of the central tendency for each constituent 

concentration as related to the size of each dataset and the 

range of measured conditions of flow and constituent concentrations. In addition, the accuracy of annual loads estimated 

from continuous records of streamflow and specific conductance is affected by the accuracy and range of measured 

conditions of flow and specific conductance, and the accuracy of the relations between specific conductance and the 

constituent concentrations. 

Concentration errors or bias can vary from sample 

to sample on the basis of the uniformity of the constituent 

concentration throughout the stream and method of sample 

collection. Typically, less error is associated with dissolved 

constituents that tend to be more evenly distributed throughout 

the water column than with constituents such as P, which tend 

to be associated with suspended sediment. Concentration error 

also can result during sample processing from contamination, non-uniform sample splitting, and sample degradation. 

Analytical-method errors often vary with concentration of the 

constituent: smaller concentrations near the detection level of 

the method are often less accurate than concentrations greater 

than the laboratory reporting level. Concentrations of major 

ions and total nutrients measured in this study were greater 

than the laboratory reporting limits. However, many concentration values for pesticides were low and often reported as 

estimated values or were less than the analytical detection 

limit. As discussed earlier, the accuracy and precision of 

constituent concentration data are measured on the basis of the 

collection of field blank, replicate, and spike samples.

Loads are calculated from streamflow and concentration 

data. Therefore, the accuracy of streamflow records affects 

the accuracy of load estimates. The USGS rates the accuracy 

of streamflow records on the basis of the performance of the 

water-level recorder, the accuracy and range of discharge 

measurements, and the stability of the stage-discharge relation 

at a given station. Accuracy ratings assigned to streamflow 

records may vary over time or range of flow and indicate the 

difference between the computed and true values of discharge 
64 Water Quality in the Cambridge Drinking-Water Source Area, Massachusetts, Water Years 2005–07

(Kennedy, 1983). For example, a USGS rating of “good” indicates that 95 percent of the daily streamflow values are within 

5 to 10 percent of the true values. In general, most continuous 

records of streamflow collected in the drinking-water source 

area have mean errors at the 95-percent confidence limit of 

± 5 to 10 percent (U.S. Geological Survey, 2009). Base-flow 

samples were collected throughout each water year and generally represent the annual range of base-flow stream conditions. 

The stage-discharge relation for base-flow conditions typically 

was well defined; however, the stage-discharge relation often 

was extrapolated beyond the range of the highest streamflow 

measurements, and as a result, uncertainty is generally greater 

for constituent load estimates for extreme flow events. 

The relative accuracy of load data reported in tables 

14 and 15 is inclusive of available quality-control data. The 

mean-square-error propagation method (Harmel and others, 

2006) was used to estimate the 95-percent confidence interval 

about the load estimates. This method treats potential errors 

as bidirectional as opposed to additive. It combines all available estimates of error for streamflow, sample precision, and 

estimates of constituent population means or regression estimates to produce realistic estimates of overall error. For many 

constituents, the error is not normally distributed around the 

load estimate but is asymmetrical and skewed to the positive 

side as a result of the statistics of the available concentration 

data. Loads of major ions estimated from continuous records 

of flow and specific conductance, particularly loads for Cl and 

Na, are more accurate than loads estimated using the stratified 

approach. This is because the specific conductance measurements completely cover the range of field conditions, and the 

accuracy of the relation of the concentrations of major ions 

to measured specific conductance is better than the variance 

about the mean concentration data at each station (table 7).

Loads and yields for USGS station 01104433 are potentially underestimated because stormflow often was constricted 

by an undersized or damaged pipe upstream from the station. 

As a result, some portion of stormwater was diverted into an 

adjacent unmonitored subbasin. Therefore, loads reported for 

USGS station 01104433 are valid only for the location of the 

monitoring station rather than for the entire subbasin, and as 

such, yields cannot be accurately estimated for this station. 

Loads and yields are potentially underestimated by a factor of 

two on the basis of annual streamflow yields in comparison to 

those for adjacent subbasins (Smith, 2011). Because interbasin flow is primarily associated with stormflow, the bias in 

loads and yields for constituents with different concentrations 

during base flow and stormflow is potentially greater. In such 

cases, the relations between constituents and land-use classes 

potentially are stronger and significant when loads are doubled 

for this station. For example, strong significant relations result 

between total roadway area and P yields for the subbasin when 

the P yield for USGS station 01104433 is doubled.

Chloride and Sodium Trends

Intermittent measurements of Cl and Na concentrations 

over the last few decades indicate that the respective concentrations in water from Cambridge Reservoir are increasing. 

The mean concentration of Na in water released from the 

Cambridge Reservoir during 1967–83 was about 43 mg/L 

(Geotechnical Engineers, Inc., 1985 ). The mean concentration of Na in water released from the Cambridge Reservoir in 

1985 was 51 mg/L (Geotechnical Engineers, Inc., 1985 ), and 

the median Na concentration in samples collected at the outlet 

of the reservoir during 1997–98 was 64 mg/L (Waldron and 

Bent, 2001). From water year 1999 to water year 2008, annual 

flow-weighted concentrations of Na in water released from the 

Cambridge Reservoir were estimated from records of flow and 

specific conductance and ranged from 63 to 113 mg/L.

During dry years, annual mean concentrations of Cl and 

Na are likely to increase assuming that natural and anthropogenic inputs of the constituents are similar from year to year. 

In contrast, concentrations tend to decrease during wet years 

under the same circumstances. Therefore, time trends for 

annual loads of Cl and Na, which are more resistant to shifts 

related to differences in annual precipitation than are concentrations, were investigated for the 11-year period of water 

years 1998 to 2008 at USGS streamgages 01104430 at the outlet of the Cambridge Reservoir and 01104460 on Stony Brook 

above Stony Brook Reservoir. Although an upward trend in Na 

concentration since 1967 is apparent from historical sample 

data collected from the outlet of the Cambridge Reservoir, 

results of Mann-Kendall tests on available annual load data for 

water years 1998 to 2008 for Cl and Na at each USGS station 

show that the trend is not significant ( p-value >0.05). The lack 

of significance in these results is likely the consequence of 

dissimilar levels of annual input for each constituent related to 

the severity of the winters during the period of study. 

To overcome the shortcomings of the initial data used in 

the trend test, a Mann Kendall test with a LOWESS adjustment of load data using annual winter weather severity index 

(WWSI) values was performed. WWSIs are commonly used to 

relate winter weather conditions to effects on wildlife (Michigan Department of Natural Resources Wildlife Division, 2010) 

and recently were used to relate winter weather conditions to 

winter-maintenance expenditures and winter-roadway accident 

rates (Maze and others, 2007). For this study, WWSI values 

were calculated as described by Boselly and others (1993) 

using daily minimum and maximum measurements of air 

temperature made at USGS station 01104430 and annual total 

snowfall data collected in the Cambridge Reservoir watershed and obtained from the Massachusetts Department of 

Transportation (Brett Loosian, Massachusetts Department of 

Transportation, written commun., 2011). Annual snow totals 

for Boston, Mass., obtained from the National Weather Service 

(2011), were used prior to water year 2004. In general, a low 

(negative value) WWSI indicates a severe winter, and a high 

value (positive value) indicates a mild winter season. Correlations between WWSIs and annual loads of Na at USGS 
Chloride and Sodium Trends 65

-40 -30 -20 -10 0 10

0

50

100

150

200

250

300

0

50

100

150

200

250

300

0

10

20

30

40

50

60

70

95-percent confidence interval

Relation between WWSI and sodium load

EXPLANATION

Annual sodium load

R2=80

A U.S. Geological Survey station 01104415

Annual sodium load, in metric tons

B U.S. Geological Survey station 01104455

C U.S. Geological Survey station 01104475

Annual sodium load, in short tons Annual sodium load, in short tons

Winter weather severity index (WWSI)

R2=0.62

R2

=0.96

Figure 20. Relation of winter weather severity to annual 

sodium loads for U.S. Geological Survey stations A, 01104415, B, 

01104455, and C, 01104475, Cambridge drinking-water source area, 

Massachusetts, water years 2004−08.

stations 01104415 and 01104455, which received runoff from 

relatively large areas of roadway (table 2), were moderately 

strong to strong (fig. 20). Similar correlations were found 

between annual Cl loads and WWSI values. These correlations 

indicate that deicing compounds containing Cl and Na are 

applied methodically on roadway surfaces in these two subbasins. These relations also indicate that a large portion of Cl 

and Na that infiltrates to the groundwater is likely discharged 

to surface water within the same water year, at least for these 

small subbasins. Mann Kendall tests using a LOWESS adjustment (f=0.5) of load data on the basis of annual WWSIs for 

USGS station 01104430 indicated a significant upward trend 

for both Cl (p=0.02) and Na (p=0.04). Results for similar 

Mann Kendall tests on data for USGS station 01104460 were 

not significant, indicating that the bulk of Cl and Na loads 

(including those loads from the Cambridge Reservoir) transported to the Stony Brook Reservoir did not change during the 

test period.

The upward trend in Cl and Na loads at the outlet of the 

Cambridge Reservoir (USGS station 01104430) is difficult to 

explain in the absence of a more complete spatial data set for 

the Cambridge Reservoir watershed. Previous data (Waldron 

and Bent, 2001) indicate that USGS station 01104415 has the 

greatest Cl and Na yields of any subbasin within the Cambridge Reservoir watershed; however, this station accounts 

for only about 6 percent of the area and about 14 percent of 

the roadway of the entire watershed. Although the application rates for deicing compounds containing Cl and Na are 

expected to be similar for roadway surfaces throughout the 

watershed, the movement of these ions to tributaries may be 

slower in some subbasins where roadway runoff is not directly 

discharged to the receiving tributary or reservoir water. This 

delay between the time of application of deicing compounds 

and movement into the reservoir may result as roadway and 

parking lot runoff infiltrates into the ground adjacent to the 

pavement or is diverted to settling ponds or other structural 

best-management practices where it infiltrates to the groundwater. Consequently, some portion of the load of Cl and Na 

per water year estimated for the outlet of the Cambridge 

Reservoir may include loads from one or more of the other 

subbasins from prior water years.

The upward trend in Cl and Na loads estimated for the 

outlet of the Cambridge Reservoir (USGS station 01104430) 

cannot be explained on the basis of existing information from 

the single upstream monitoring station. Results from a Mann 

Kendall test indicate that there is no significant trend in winter 

severity from 1999 to 2008, thus the increase in Cl and Na 

loads is not attributed to progressively worse winter weather 

during this period. Annual Cl and Na loads for water years 

2004 to 2008 at USGS stations 01104415 and 01104455 were 

well correlated with WWSI values. This correlation indicates 

that deicing compounds were applied in a manner consistent 

with annual weather conditions and that the annual applications of the deicing compounds are likely reflected in the 

annual loads for the same water year for these small subbasins. 

In the absence of additional data, the trend may be explained, 
66 Water Quality in the Cambridge Drinking-Water Source Area, Massachusetts, Water Years 2005–07

in part, by increased loads in one or more of the unmonitored 

subbasins associated with either increased applications of salt 

or with retarded groundwater discharge of Cl and Na from 

applications of deicing compounds in previous severe winters.

Summary

The city of Cambridge, Massachusetts, Water Department 

(CWD) obtains its raw water from a system of three primary 

storage reservoirs. Except for Fresh Pond Reservoir, the 

drinking-water source area for these reservoirs is outside the 

city of Cambridge in parts of the towns of Lexington, Lincoln, 

and Weston and the city of Waltham. The Cambridge drinkingwater source area encompasses major transportation corridors, 

as well as large areas of industrial, commercial, and residential 

land use, and as a result, the drinking-water source area is 

potentially at risk from a variety of contaminants. Continuousmonitoring data collected by the U.S. Geological Survey 

(USGS) since water year 1998 and water samples collected 

during base-flow and stormflow conditions from water years 

2004 to 2008 are used to identify current water-quality conditions, estimate loads and yields, and where sufficient data are 

available, describe trends in streamwater quality. These data 

also establish a baseline for future comparisons. These data 

can be used to determine how streamwater quality is affected 

by various watershed characteristics and to provide information that can be used to guide future watershed management.

Physical properties and concentrations of calcium (Ca), 

chloride (Cl), sodium (Na), sulfate (SO4

), total nutrients, 

selected polar pesticides, and caffeine were measured in 15 to 

16 samples of base flow and 15 composite samples of stormflow collected at four USGS sampling stations on tributaries 

to the Cambridge and Stony Brook Reservoirs. An additional 

12 samples were collected from Stony Brook (USGS station 

01104460), the primary stream to the Stony Brook Reservoir 

representing the major part of the flow for the entire drinkingwater source area. These five USGS stations are the primary 

sampling stations. Three samples, analyzed for the same 

properties and constituents, were collected from each of six 

additional tributaries during base-flow conditions. Samples 

also were collected periodically from the raw-water intake 

at the Fresh Pond Reservoir in Cambridge, Massachusetts. 

These additional locations are the secondary sampling locations. Finally, concentrations of Ca, Cl, Na, and SO4

 were 

estimated on the basis of relations between continuous records 

of specific conductance and concentration data on the respective ions at six continuous-monitoring stations, including the 

five primary sampling stations and the secondary sampling 

station at the outlet of the Cambridge Reservoir (USGS 

station 01104430).

Concentrations of dissolved Ca, Cl, Na, and SO4 in 

samples collected from the tributaries throughout the drinkingwater source area varied widely, particularly in the small 

subbasins where the effects of stormwater runoff and land 

development increase. The median concentrations for Ca, Cl, 

Na, and SO4

 in samples collected in the Hobbs Brook Basin 

were 39.8, 392, 207, and 21.7 milligrams per liter (mg/L), 

respectively, and the median concentrations for the tributaries 

in the Stony Brook Reservoir watershed were lower at 17.8, 

87.7, 49.7, and 14.7 mg/L, respectively. Median concentrations in the main stem of Stony Brook were 19.5, 126, 65.8, 

and 12.4 mg/L, respectively, indicating appreciable dilution 

of high-concentration water released from the Cambridge 

Reservoir by streamflow containing low concentrations of 

dissolved ions from the less developed upper Stony Brook 

Reservoir watershed. Concentrations of Ca, Cl, Na, and SO4

in samples collected in the tributaries of the drinking-water 

supply area were negatively correlated with streamflow. As 

a result, the concentrations of major ions in tributaries tend 

to increase during the summer when streamflow is low and 

decrease during high base-flow conditions and stormflows 

(even during storms that occur in the winter and contain high 

initial concentrations of dissolved ions). This pattern indicates that groundwater is an important source for the major 

ions. The high concentrations of Cl and Na in base flow often 

were greater than the U.S. Environmental Protection Agency 

(USEPA) Secondary Drinking-Water Regulation (SDWR) for 

Cl and the Commonwealth of Massachusetts, Executive Office 

of Energy and Environmental Affairs, Department of Environmental Protection, Water, Wastewater & Wetlands: Regulations & Standards (MassEOEEA) drinking-water guideline 

for Na. Concentrations of Cl estimated from continuous 

records of specific conductance routinely exceeded the USEPA 

criterion continuous concentration (CCC) and SDWR during base-flow conditions in the tributaries at USGS stations 

01104415, 01104433, and 01104455. Flow-weighted values 

of specific conductance associated with stormflows exceeded 

specific conductance levels estimated at the concentrations for 

the USEPA SDWR and CCC thresholds for Cl less often than 

specific conductance during base flow; however, instantaneous 

measurements of specific conductance during the beginning of 

many winter runoff events were substantially greater than the 

maximum measurements of specific conductance during base 

flow. Specific conductance at USGS station 01104475, which 

drains a large residential area, rarely exceeded the specific 

conductance levels estimated at the USEPA SDWR for Cl. 

About 6 percent of the daily mean specific conductance values 

in water released from the Cambridge Reservoir were greater 

than the specific conductance levels estimated at the SDWR 

and CCC thresholds; the estimated CMC threshold was not 

exceeded during water years 2004 to 2008. Specific conductance data in the main stem of Stony Brook (USGS station 

01104460) were rarely (less than 0.5 percent) greater than the 

specific conductance levels estimated at the SDWR and CCC 

thresholds. Concentrations of dissolved Na 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 MassEOEEA drinking-water guideline. Only in a few composite samples of stormflow was the 

concentration of Na less than the guideline.
Summary 67

Concentrations of total nitrogen (TN) and total phosphorus (TP)in samples collected from the tributaries throughout 

the drinking-water source area were affected by streamflow 

volume, stormwater runoff, and land use in each subbasin. The 

median concentrations for TN and TP in all samples collected 

in the Hobbs Brook Basin were 1.48 and 0.04 mg/L, respectively, whereas the median concentrations for TN and TP in 

the tributary streams in the Stony Brook Reservoir watershed 

were 1.52 and 0.03 mg/L, respectively. Correlations between 

concentrations of TN and flow were weak, indicating that flow 

does not explain the observed variability in the TN concentrations. In contrast, concentrations of TP were positively correlated with flow, and the relation was statistically significant 

at three of the five primary sampling stations. 

Water samples collected in the drinking-water source 

area and samples of raw water collected from the Cambridge 

water-treatment facility intake at the Fresh Pond Reservoir 

were analyzed for concentrations of caffeine and 59 different 

pesticides. For all samples collected during the study, 34 of 

the 59 pesticides were not detected, 25 pesticides had at least 

one detection, and 9 of the 25 pesticides were detected at an 

overall frequency greater than 15 percent. Only 12 pesticides 

were detected in base flow and (or) stormflow samples at a 

frequency equal to or greater than 25 percent at one or more 

of the 5 primary sampling stations. Most of these pesticides 

are found in over-the-counter products used for turf management and the treatment of ornamental shrubs. Many pesticides 

were detected in both base flow and stormflow. The regular 

detection of pesticides in base flow, particularly those with 

high aqueous solubility, is an indication of local groundwater 

contamination. Imidacloprid, norflurazon, and siduron were 

the most frequently detected pesticides in samples of water 

collected in the drinking-water source area, with the number 

of detections ranging from about 24 to 41 percent. The range 

of concentrations for 2,4-D, norflurazon, siduron, imidacloprid, carbaryl, and caffeine were generally similar for Cambridge and Stony Brook Reservoir watersheds; diuron was the 

exception with concentrations generally lower in the Cambridge Reservoir watershed. Although a mixture of different 

pesticides is transported to the Cambridge Reservoir, diuron, 

norflurazon, and siduron were the only pesticides detected in 

samples collected at the outlet of the reservoir, indicating that 

other pesticides potentially degrade during the relatively long 

retention time in the reservoir (greater than 7 months). Compounds with a long aqueous half life, particularly norflurazon 

and siduron, are able to advance though the entire reservoir 

system. Compared to the Cambridge Reservoir retention time, 

the retention time for the Stony Brook Reservoir is comparatively short, less than about 2 weeks, thus even pesticides with 

a relatively short aqueous half life are able to persist in waters 

directed to the Fresh Pond Reservoir. Although six different pesticides were detected in samples from the Fresh Pond 

Reservoir raw-water intake, carbaryl was the only compound 

detected at greater than the long-term method detection level 

(LTMDL). Caffeine also was detected at greater than the 

LTMDL. Caffeine was detected in about 64 percent of 171 

water samples collected throughout the drinking-water source 

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

Annual loads and yields of major ions, total nutrients, 

selected pesticides, and caffeine were estimated for the five 

primary sampling stations using (1) a stratified flow approach 

and a measure of the central tendency of each sample set and 

(2) continuous records of streamflow and specific conductance and the relation between ion concentration and specific 

conductance. Annual loads and mean annual yields for Ca, Cl, 

Na, and SO4

 calculated from continuous records of specific 

conductance were generally similar to those calculated on 

the basis of mean concentration data. These annual loads and 

mean annual yields from continuous records were expected to 

be more accurate than those estimated from sample concentrations because specific conductance was based on a data set 

that encompasses all base-flow conditions and runoff events, 

including snow melts. Mean annual yields of Ca, Cl, Na, and 

SO4

 for the drinking-water source area for water years 2005 to 

2007, estimated on the basis of continuous records of specific 

conductance, were 14, 85, 46, and 9 metric tons per square 

kilometer, respectively. Mean annual yields of most major 

ions at USGS stations 01104415, 01104433, and 01104455 

were substantially higher than the mean annual yields for the 

drinking-water source area. These stations are in subbasins 

that contain large areas of roadways and parking lots and 

land-use classes that have moderately strong correlations with 

annual flow-weighted concentrations of Cl and Na. About 

30 percent of the loads of each major ion at each of the tributaries was associated with stormflow, except for USGS station 

01104475 where less than 30 percent of loads was attributable 

to stormflow. 

Mean annual yields of TN in the tributaries ranged from 

about 740 to more than 1,200 kilograms per square kilometer 

(kg/km2

), which were greater than the mean annual yields 

for most of the drinking-water source area. The proportion of 

the calculated annual TN load was similar in base flow and 

stormflow. In contrast, more than 83 percent of the TP load in 

the tributaries and about 73 percent of the TP load in the main 

stem of Stony Brook (USGS station 01104460) was associated 

with stormflow. Yields of TP at USGS stations 01104415 and 

01104455 were about three times greater than the yields for 

most of the drinking-water source area. Correlations between 

annual flow-weighted concentrations of TN collected at four 

primary sampling stations on the tributaries, and percentages 

of recreational area and open water, were significant. Correlations of mean concentrations of TN in base flow with commercial landscaping, interstate highway, state highway, and total 

impervious land uses were significant. Mean concentrations of 

TN in base flow were negatively correlated with wetland land 

use. Correlations between annual flow-weighted concentrations of TP and percentages of forest and State highway land 

uses were moderately strong; however, the relations were 

significant only at a lower confidence interval (p-value 0.13). 

Significant negative correlations were found between annual 

flow-weighted concentrations of TP and wetland land use. 
68 Water Quality in the Cambridge Drinking-Water Source Area, Massachusetts, Water Years 2005–07

Annual loads of individual pesticides were less than 1 

kilogram at each of the five primary sampling stations. The 

highest pesticide yields were estimated for the herbicides 

2,4-D and imidacloprid and ranged from 34 to 310 grams per 

square kilometer (g/km2

) and 3 to 170 g/km2, respectively. 

The load for 2,4-D was entirely associated with stormflow, 

and annual flow-weighted concentrations of 2,4-D correlated 

significantly with the residential roof land uses. The largest 

load and yield for 2,4-D were estimated for USGS station 

01104415 in the Cambridge Reservoir watershed, where 

the footprint area of residential land use accounts for about 

40 percent of the land uses in the subbasin. The highest yields 

for imidacloprid, benomyl, carbaryl, metalaxyl, and propiconazole were estimated for USGS station 01104475, a tributary 

to the Stony Brook Reservoir draining a largely residential 

subbasin. Significant positive correlations were found between 

recreation area land use and annual flow-weighted concentrations and maximum concentrations of benomyl, imidacloprid, 

and carbaryl in base flow. 

Yields for the herbicide siduron ranged from 6.9 to 

35 g/km2 at the five primary sampling stations, and loads were 

predominantly associated with stormflow. Positive correlations between annual flow-weighted concentrations of siduron 

were significant only for wetland land use; however, positive 

correlations between maximum concentrations of the herbicide in base flow and commercial and total impervious land 

uses were significant throughout the drinking-water supply 

area. Mean annual yields for the herbicide diuron ranged 

from 2.1 to 4.4 g/km2

, and annual flow-weighted concentrations strongly correlated with parking lot land use. The 

highest yield was measured at USGS station 01104433, a 

tributary in the Hobbs Brook Basin that contains large areas of 

commercial development. 

Yields of caffeine ranged from 11 to 410 g/km2

 for the 

five primary sampling stations. Most correlations between 

annual flow-weighted concentrations of caffeine in samples 

collected at the primary sampling stations and subbasin landuse classes were weak; however, maximum base-flow concentrations of caffeine in eight subbasins correlated significantly 

with parking lot land use throughout the drinking-water 

supply area. Although leaking sewer lines are typical sources 

of caffeine, the compound is common in various beverages, 

and associated containers are found on or near roadways and 

parking lots.

Results of Mann Kendall tests of annual Cl and Na loads 

in Hobbs Brook (USGS station 01104430; the outlet of the 

Cambridge Reservoir) for water years 1998–2008 with a 

LOWESS adjustment using the winter weather severity index 

(WWSI) indicate a significant upward trend. Results of similar 

tests of available load data for the main stem of Stony Brook 

(USGS station 01104460) were not significant, indicating no 

statistical trend in either direction. The upward trend in annual 

Cl and Na loads estimated at the outlet of the Cambridge 

Reservoir (Hobbs Book) is difficult to explain on the basis of 

existing information from the single upstream station. Results 

from a Mann Kendall test indicate that there was no significant 

trend in winter severity from 1999 to 2008, thus the increases 

in Cl and Na loads are not necessarily attributable to progressively worse winter weather during this period. Correlations 

between Na loads estimated for two tributaries that receive 

stormwater runoff from large roadway areas (USGS stations 

01104415 and 01104455) and WWSI values are moderately 

strong to strong. This correlation indicates that deicing compounds are applied in a manner consistent with annual weather 

conditions and that the annual application of the deicing 

compounds is likely reflected in the annual loads for the same 

water year for these small subbasins. In the absence of additional data, the trend may be explained, in part, by increased 

loads in one or more of the other unmonitored subbasins 

associated with either increased applications of salt or with 

retarded groundwater discharge of Cl and Na resulting from 

applications of deicing compounds in previous severe winters.

Data collection activities in the drinking-water source 

area are important for effective management and protection 

of the drinking-water supply. These data enhance the understanding of the physical, chemical, and biological state of the 

water supply. Without accurate information on the past and 

current condition of the water supply, effective preservation 

and remediation programs cannot be implemented or evaluated. The data presented in this report represent the quality of 

water throughout the drinking-water source area, with a focus 

on the base-flow and stormflow conditions of four tributaries and the primary stream to the Stony Brook Reservoir for 

water years 2005 to 2007. These data indicate the importance 

of an integrated sampling strategy for accurate assessments of 

many constituents. Load and yield data estimated as part of 

this study may serve as benchmarks for future comparisons, 

particularly pesticide data that historically have not been collected within the watersheds.

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Smith—Water Quality in the Cambridge Drinking-Water Source Area, Massachusetts, Water Years 2005–07—Scientific Investigations Report 2013-5039