U.S. Department of the Interior 

U.S. Geological Survey

Factors Affecting Reservoir and Stream-Water 

Quality in the Cambridge, Massachusetts, 

Drinking-Water Source Area and 

Implications for Source-Water Protection

By MARCUS C. WALDRON and GARDNER C. BENT

Water-Resources Investigations Report 00-4262

Prepared in cooperation with the

CITY OF CAMBRIDGE, MASSACHUSETTS, WATER DEPARTMENT

Northborough, Massachusetts

2001
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

The use of trade or product names in this report is for identification 

purposes only and does not constitute endorsement by the 

U.S. Government.

For additional information write to: Copies of this report can be purchased from:

Chief, Massachusetts-Rhode Island District U.S. Geological Survey

U.S. Geological Survey Branch of Information Services

Water Resources Division Box 25286

10 Bearfoot Road Denver, CO 80225-0286

Northborough, MA 01532

or visit our web site at

http://ma.water.usgs.gov
Contents III

CONTENTS

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

Introduction ........................................................................................................................................................................... 3

Purpose and Scope ...................................................................................................................................................... 3

Description of the Cambridge Drinking-Water Supply System.................................................................................. 5

Acknowledgments ....................................................................................................................................................... 5

Water Quality and Trophic State of Hobbs Brook Reservoir, Stony Brook Reservoir and Fresh Pond................................ 6

Reservoir Descriptions ................................................................................................................................................ 6

Methods of Data Collection ........................................................................................................................................ 6

Reservoir Sampling ........................................................................................................................................... 7

Sample Preparation and Analysis...................................................................................................................... 7

Quality Control.................................................................................................................................................. 12

Reservoir Water Quality .............................................................................................................................................. 12

Hobbs Brook Reservoir..................................................................................................................................... 12

Stony Brook Reservoir ...................................................................................................................................... 13

Fresh Pond......................................................................................................................................................... 14

Reservoir Bed-Sediment Quality................................................................................................................................. 15

Reservoir Trophic State............................................................................................................................................... 18

Effects of Drainage-Basin Characteristics on Water Quality of Tributary Streams .............................................................. 19

Description of Sampling Network and Subbasin Characteristics ............................................................................... 19

Methods of Data Collection and Analysis................................................................................................................... 22

Stage and Discharge Measurements.................................................................................................................. 22

Chemical Sampling and Analysis...................................................................................................................... 23

Event Sampling ................................................................................................................................................. 23

Loading Calculations......................................................................................................................................... 23

Constituent Concentrations, Estimated Loads, and Subbasin Yields .......................................................................... 25

Fecal Coliform Bacteria .................................................................................................................................... 26

Sodium and Chloride......................................................................................................................................... 26

Nitrogen............................................................................................................................................................. 29

Phosphorus ........................................................................................................................................................ 30

Iron and Manganese .......................................................................................................................................... 30

Dissolved Organic Carbon and Trihalomethane Formation Potential............................................................... 31

Constituent Yields in Relation to Subbasin Characteristics ........................................................................................ 31

Water and Constituent Mass Balances for Hobbs Brook Reservoir............................................................................ 33

Implications for Source-Water Protection............................................................................................................................. 38

Protection of Reservoir Quality................................................................................................................................... 38

Protection of Tributary-Stream Quality....................................................................................................................... 40

Hobbs Brook at Mill Street near Lincoln, MA (01104405).............................................................................. 40

Cambridge Reservoir, Unnamed Tributary 1, near Lexington, Mass. (01104410) ........................................... 40

Cambridge Reservoir, Unnamed Tributary 2, near Lexington, Mass. (01104415) ........................................... 40

Cambridge Reservoir, Unnamed Tributary 3, near Lexington, Mass. (01104420) ........................................... 41

Hobbs Brook, Unnamed Tributary 1, near Kendal Green, Mass. (01104433).................................................. 41

Hobbs Brook at Kendal Green, Mass. (01104440) ........................................................................................... 41

Stony Brook at Kendal Green, Mass. (01104390) ............................................................................................ 41

Stony Brook, Unnamed Tributary 1, near Waltham, Mass. (01104455) ........................................................... 42

Stony Brook at Route 20 near Waltham, Mass. (01104460)............................................................................. 42

Stony Brook Reservoir, Unnamed Tributary 1, near Weston, Mass. (01104475) ............................................. 42

Summary and Conclusions.................................................................................................................................................... 43

References Cited ................................................................................................................................................................... 46
IV Contents

Appendix A. Cambridge, Mass., Drinking-Water Source Area Water Quality Monitoring Program ................................... 81

Monitoring Objectives................................................................................................................................................. 83

Monitoring-Program Elements.................................................................................................................................... 83

Routine (Dry Weather) Surface-Water Monitoring ........................................................................................... 83

Event-Based (Wet Weather) Surface-Water Monitoring ................................................................................... 88

Continuous-Record Surface-Water Monitoring ................................................................................................ 88

Ground-Water Monitoring................................................................................................................................. 88

Data Management, Imterpretation, Reporting, and Review.............................................................................. 89

References Cited ......................................................................................................................................................... 89

FIGURES

1–2 Maps showing:

1. Location, extent, and components of the city of Cambridge drinking-water supply system, 

eastern Massachusetts ................................................................................................................................... 4

2. Bathymetry of (A) Hobbs Brook Reservoir, (B) Stony Brook Reservoir, and (C) Fresh Pond..................... 9

3–5. Graphs showing:

3. Depth profiles of temperature and concentrations of dissolved oxygen, nutrients, and other 

constituents during the 1997–98 study period at the deep hole station in Hobbs Brook Reservoir.............. 53

4. Depth profiles of temperature and concentrations of dissolved oxygen, nutrients and other 

constituents during the 1997–98 study period at the deep hole station in Stony Brook Reservoir............... 61

5. Depth profiles of temperature and concentrations of dissolved oxygen, nutrients, and other 

constituents during the 1997–98 study period at the deep hole station in Fresh Pond ................................. 68

6. Hydrography, subbasin boundaries, and streamflow and water-quality monitoring stations in the 

Cambridge, Massachusetts, drinking-water source area........................................................................................ 74

7. Graphs showing concentrations of selected chemical constituents detected at eleven monitoring stations 

in subbasins that contribute water to Hobbs Brook and Stony Brook Reservoirs, September 1997–

November 1998...................................................................................................................................................... 75

A1. Cambridge, Mass., drinking water source-area water-quality monitoring network .............................................. 84

TABLES

1. Concentrations of selected trace metals and other contaminants in surficial bed sediments of Hobbs 

Brook Reservoir and Stony Brook Reservoir, eastern Massachusetts, November 1998, and median 

concentrations of the same analytes in surficial sediments at 135 U.S. Geological Survey sampling sites 

in the lower Charles River, Boston, Massachusetts, summer 1998 ........................................................................ 16

2. Concentrations (normalized to aluminum concentrations) of selected trace metals and other contaminants 

in surficial bed sediments of Hobbs Brook Reservoir and Stony Brook Reservoir, November 1998, and 

median normalized concentrations of the same analytes in surficial sediments at 135 U.S. Geological 

Survey sampling sites in the lower Charles River, Boston, Massachusetts, summer 1998 .................................... 17

3. Median Secchi disk transparency, surface chlorophyll-a and total phosphorus concentrations, and trophic 

state indices derived from those measurements for Hobbs Brook Reservoir, Stony Brook Reservoir, and 

Fresh Pond, eastern Massachusetts, September 1997–November 1998 ................................................................. . 18

4. Names, locations, and drainage areas of monitoring stations used to assess tributary-stream quality in the 

drinking-water source area for Cambridge, Massachusetts .................................................................................... 20

5. Land use and land cover, topography, and surficial geology in subbasins that contribute water to Hobbs 

Brook and Stony Brook Reservoirs, eastern Massachusetts ................................................................................... 21

6. Ranges and median values for selected physical and chemical characteristics of water in the drinkingwater source area for Cambridge, Massachusetts, October 1997–November 1998, in relation to 

Massachusetts source-water and Federal drinking-water standards ....................................................................... 25

7. Median instantaneous loads and yields of fecal coliform bacteria and estimated annual mean loads and 

subbasin yields of selected chemical constituents during water year 1998 (October 1997 through 

September 1998) for subbasins that contribute water to Hobbs Brook and Stony Brook Reservoirs, 

eastern Massachusetts ............................................................................................................................................. 27
Contents V

8. Product moment correlation coefficients (r) relating percent areal coverage of subbasin characteristics to 

estimated annual mean yields (mass per unit area) of ten potential contaminants in subbasins of the 

 drinking-water source area for Cambridge, Massachusetts, October 1997–September 1998 ............................... 32

9. Water balance for Hobbs Brook Reservoir, October 1997–September 1998 ......................................................... 35

10. Mass balances for dissolved sodium, total nitrogen, total phosphorus, and dissolved manganese for 

Hobbs Brook Reservoir, eastern Massachusetts, October 1997–September 1998 ................................................. 37

A1. Water sources, sampling frequencies, and monitored water-quality properties and constituents, for 

water-quality monitoring stations in the drinking-water source area for Cambridge, Massachusetts.................... 86

CONVERSION FACTORS, VERTICAL DATUM, AND 

ABBREVIATED WATER-QUALITY UNITS

CONVERSION FACTORS 

VERTICAL DATUM

Sea level: In this report, “sea level” refers to the National Geodetic Vertical Datum of 1929 (NGVD

of 1929)—a geodetic datum derived from a general adjustment of the first-order level nets of the

United States and Canada, formerly called Sea Level Datum of 1929. 

Cambridge datum: Bathymetric contours are given as elevations, in feet, and are referenced to the

city of Cambridge datum, which is 10.84 feet below mean sea level.

ABBREVIATED WATER-QUALITY UNITS

Chemical concentration is given in grams per liter (g/L), milligrams per liter (mg/L), or micrograms

per liter (μg/L). Milligrams per liter is a unit expressing the concentration of chemical constituents in

solution as weight (milligrams) of solute per unit volume (liter) of water. One thousand milligrams

per liter is equivalent to one gram per liter. One thousand micrograms per liter is equivalent to one

milligram per liter. For concentrations less than 7,000 mg/L, the numerical value is the same as for

concentrations in parts per million. Specific conductance of water is expressed in microsiemens per

centimeter at 25 degrees Celsius (μS/cm). This unit is equivalent to micromhos per centimeter at 25

degrees Celsius (μmho/cm), formerly used by the U.S. Geological Survey.

Multiply By To obtain

cubic meter per second (m3/s) 35.3107 cubic foot per second

kilogram per day (kg/d) 2.2046 pound per day

kilograms per day per square kilometer (kg/d/km2) .8512 pound per day per square mile

kilogram per year (kg/yr) 2.2046 pound per year

kilometer (km) .6215 mile

liter (L) .2642 gallon

meter (m) 3.2808 foot

millimeter (mm) 0.0394 inch

square kilometer (km2) .3861 square mile

Temperature is given in degrees Celsius ( ̊C), which can be converted to degrees 

Fahrenheit ( ̊F) by use of the following equation:

 ̊F = (1.8 x  ̊C) + 32

Abstract 1

Factors Affecting Reservoir and Stream-Water 

Quality in the Cambridge, Massachusetts, 

Drinking-Water Source Area and 

Implications for Source-Water Protection

By Marcus C. Waldron and Gardner C. Bent

Abstract

This report presents the results of a study 

conducted by the U.S. Geological Survey, in cooperation with the city of Cambridge, Massachusetts, 

Water Department, to assess reservoir and 

tributary-stream quality in the Cambridge drinkingwater source area, and to use the information 

gained to help guide the design of a comprehensive 

water-quality monitoring program for the source 

area. Assessments of the quality and trophic state of 

the three primary storage reservoirs, Hobbs Brook 

Reservoir, Stony Brook Reservoir, and Fresh Pond, 

were conducted (September 1997–November 

1998) to provide baseline information on the state 

of these resources and to determine the vulnerability of the reservoirs to increased loads of nutrients 

and other contaminants. The effects of land use, 

land cover, and other drainage-basin characteristics 

on sources, transport, and fate of fecal-indicator 

bacteria, highway deicing chemicals, nutrients, 

selected metals, and naturally occurring organic 

compounds in 11 subbasins that contribute water to 

the reservoirs also was investigated, and the data 

used to select sampling stations for incorporation 

into a water-quality monitoring network for the 

source area.

All three reservoirs exhibited thermal and 

chemical stratification, despite artificial mixing 

by air hoses in Stony Brook Reservoir and Fresh 

Pond. The stratification produced anoxic or 

hypoxic conditions in the deepest parts of the reservoirs and these conditions resulted in the release of 

ammonia nitrogen orthophosphate phosphorus, and 

dissolved iron and manganese from the reservoir 

bed sediments.

Concentrations of sodium and chloride 

in the reservoirs usually were higher than the 

amounts recommended by the U.S. Environmental 

Protection agency for drinking-water sources 

(20 milligrams per liter for sodium and 250 milligrams per liter for chloride). Maximum measured 

sodium concentrations were highest in Hobbs 

Brook Reservoir (113 milligrams per liter), intermediate in Stony Brook Reservoir (62 milligrams 

per liter), and lowest in Fresh Pond (54 milligrams 

per liter). Bed sediments in Hobbs Brook and Stony 

Brook Reservoirs were enriched in iron, manganese, and arsenic relative to those in the impounded 

lower Charles River in Boston, Massachusetts. 

Trophic state indices, calculated for each reservoir based on nutrient concentrations, watercolumn transparency, and phytoplankton abundances, indicated that the upper and middle basins 

of Hobbs Brook Reservoir were moderately to 

highly productive and likely to produce algal 

blooms; the lower basin of Hobbs Brook Reservoir 

and Stony Brook Reservoir were similar and intermediate in productivity, and Fresh Pond was relatively unproductive and unlikely to produce algal 

blooms. This pattern is likely due to sedimentation 

of organic and inorganic particles in the three 
2 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection

basins of Hobbs Brook Reservoir and in Stony 

Brook Reservoir. Molar ratios of nitrogen to phosphorus ranged from 55 in Stony Brook Reservoir to 

120 in Hobbs Brook Reservoir, indicating that phytoplankton algae in these water bodies may be 

phosphorus limited and therefore sensitive to small 

increases in phosphorus loading from the drainage 

basin. Nitrogen loads were found to be less important than phosphorus to the trophic condition of the 

reservoirs.

Hobbs Brook and Stony Brook, the two principle streams draining the Cambridge drinkingwater source area, differed in their relative contributions to many of the estimated constituent loads. 

The estimated load of fecal coliform bacteria 

was more than seven times larger for the mainly 

residential Stony Brook subbasin upstream from 

Kendal Green, Mass., than it was for the more 

commercial and industrial Hobbs Brook subbasin, 

though the drainage areas of the two subbasins 

differ only by about 20 percent. The State standard 

for fecal coliform bacteria in streams in the 

Cambridge drinking-water source area (20 colony 

forming units per 100 milliliters) was exceeded at 

all sampling stations.

Estimated subbasin yields for sodium and 

chloride were significantly correlated with the percentage of the subbasin area occupied by roads, 

indicating that the application of sodium chloride in 

road salt is a significant source of the high concentrations of sodium measured in the reservoirs. The 

estimated annual mean loads of sodium and chloride produced by the Hobbs Brook subbasin were 

about three times greater than those produced by 

the Stony Brook subbasin.

The Hobbs Brook and Stony Brook subbasins produced similar estimated loads for nitrate 

nitrogen and total nitrogen. Subbasin yields of the 

two nitrogen species also were similar. In contrast, 

the estimated total phosphorus load at the mouth of 

Hobbs Brook was nearly twice that at the Stony 

Brook station.

The Hobbs Brook and Stony Brook subbasins produced similar estimated annual mean 

loads for iron. However, the estimated annual mean 

manganese load from the Hobbs Brook subbasin 

was about three times greater than that from the 

Stony Brook subbasin. Estimated annual mean 

yields for iron were greatest at stations representing 

the upper Hobbs Brook subbasins; those for 

manganese were greatest at the two stations 

downstream from Hobbs Brook Reservoir.

Both concentrations and yields of dissolved 

organic carbon were correlated with percent areal 

coverage of forested wetland in the subbasins. 

Neither concentrations nor yields of trihalomethane 

formation potential could be correlated with subbasin features such as land use, land cover, slope, or 

surficial geology. Concentrations of trihalomethane 

formation potential were similar to those reported 

in the literature for surface-water supplies in other 

parts of the country. Estimated annual mean yields 

of dissolved organic compound and trihalomethane 

formation potential were uniform, suggesting that 

no subbasin was exporting a disproportionate 

amount of either constituent on an annual basis.

The mass balance for water in Hobbs Brook 

Reservoir indicated that the time required for complete flushing of the reservoir during water year 

1998 was less than 6 months. Sodium accumulated 

during the water year as the reservoir refilled following an unusually dry summer. The reservoir 

retained much of the nitrogen and phosphorus contributed by tributary streams. Waterfowl and precipitation were insignificant as sources of nitrogen 

to the reservoir but may have been important as 

sources of phosphorus.

Based on the results obtained from these 

investigations, ten stream locations were selected 

for inclusion as primary tributary-monitoring 

stations in a source-area water-quality monitoring 

network developed jointly by the U.S. Geological 

Survey and the Cambridge Water Department. Criteria for inclusion in the network were the magnitudes of actual or potential contaminant loads and 

the proximity of the monitoring stations to the reservoirs. In addition, nine monitoring stations representative of water-quality and trophic conditions in 

Hobbs Brook Reservoir, Stony Brook Reservoir, 

and Fresh Pond were identified and incorporated 

into the network. Details of the monitoring network 

are included in an appendix to this report.
Introduction 3

INTRODUCTION

Water-quality monitoring is a critical element of 

any program designed to manage and protect drinkingwater supplies. Water-quality monitoring in this context is defined as “an integrated activity for evaluating 

the physical, chemical, and biological character of 

water in relation to human health, ecological conditions, and designated water uses” (Intergovernmental 

Task Force on Monitoring Water Quality, 1995). A 

water-quality management program includes the monitoring of streams, lakes, reservoirs, and ground-water 

resources that serve as primary sources for drinking 

water, and also may be extended to wetlands, atmospheric deposition, and surface runoff that contribute 

water to the primary source. Without accurate and 

timely information on the state of the water supply, 

effective preservation and remediation programs 

cannot be accomplished, and the effectiveness of the 

management program cannot be evaluated.

Increased development in and around source 

areas is affecting many of the Nation’s drinking-water 

supplies. Often it is impossible or impractical for a 

municipal water department to purchase and control all 

of the land that contributes to the water supply, and 

ongoing development of private property carries a risk 

of adding to contaminant loads from a variety of 

sources. Existing water-quality monitoring programs, 

however, frequently were established at a time when 

development pressures were not as great as they are 

currently (2000), and often they are either inadequate 

or not as cost-effective as they could be (Reinelt and 

others, 1988).

The U.S. Geological Survey (USGS) works 

closely with municipal water suppliers throughout the 

Nation to help address specific problems or to conduct 

detailed investigations of factors affecting source-water 

quality (Patterson, 1997). One such program, begun in 

1997 in cooperation with the city of Cambridge, Mass., 

was designed to identify sources of contaminants in the 

drinking-water source area for the city (fig. 1). The 

Cambridge Water Department (CWD) supplies about 

57 million liters of water each day to more than 95,000 

customers. Most of this water is obtained from a 

system of reservoirs located in Cambridge and in parts 

of five other suburban Boston communities. The drainage basin that contributes water to these reservoirs has 

undergone rapid development in recent years and contains major highways, secondary roads, and areas of 

residential, commercial, and industrial land uses that 

could adversely affect the water supply. Because the 

city of Cambridge owns less than 5 percent of the land 

in the basin, the CWD relies heavily on water-quality 

monitoring to ensure that the source water remains free 

from contamination.

The goals of the USGS investigation were to 

characterize current water-quality conditions in the 

drinking-water source area, to identify tributaries with 

the greatest potential for transporting contaminants to 

the reservoirs, and to provide baselines for contaminant 

loads that may be used to evaluate the effectiveness 

of watershed best-management practices. Although 

water treatment can remove many contaminants, it is 

usually better and more cost effective to prevent contamination of the water supply. There is growing recognition of the value of protecting the high-quality waters 

that are a source of drinking water as a means of reducing the cost of treatment systems required under the 

Safe Drinking Water Act (New England Interstate 

Water Pollution Control Commission, 1996; U.S. 

Environmental Protection Agency, 1998).

Purpose and Scope

The purpose of this report is to describe current 

water-quality conditions in the Cambridge, Mass., 

drinking-water source area and to use this information 

to identify tributaries and other sampling sites that 

should be monitored as part of a comprehensive 

source-water protection program. The first part of 

the report is a limnological assessment of the three primary storage reservoirs in the system, Hobbs Brook 

Reservoir, Stony Brook Reservoir, and Fresh Pond. The 

assessment, conducted during a 14-month period 

(September 1997–November 1998), includes information on water and sediment quality, and an evaluation 

of the vulnerability of the reservoirs to eutrophication. 

The second part of the report presents the results of a 

concurrent investigation of the effects of land use, land 

cover, and other drainage-basin characteristics on 

transport and fate of highway deicing chemicals, nutrients, naturally occurring organic compounds, fecalindicator bacteria, and selected trace metals within 

the source area. The third part of the report uses the 

information gained in the two investigations to identify 

reservoir and tributary-monitoring sites that are representative of source-water quality and can be used to 

account for potentially significant sources of contaminants in the area. A brief description of a water-quality 

monitoring program, designed jointly by the USGS and 

the CWD to address water-quality problems identified 

in the report, is included as an appendix.
4 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection 128

2

3

2A

1A 1

9

20

30

117

60

93

93

95

95

90

42 ̊

73 ̊ 72 ̊ 

71 ̊

70 ̊

MASSACHUSETTS

41 ̊30'

42 ̊30'

0

0 50 KILOMETERS

50 MILES

MASSACHUSETTS

CAMBRIDGE

BROOKLINE

BOSTON

NEWTON

WALTHAM

LINCOLN

LEXINGTON

WINCHESTER

WESTON

SOMERVILLE

MEDFORD

ARLINGTON

BELMONT

WATERTOWN

Study

area

CHARLES

RIVER

Hobbs Brook

Reservoir

Subbasin

Stony Brook

Reservoir

Subbasin

PAYSON PARKRESERVOIR

Aqueduct ot Fresh

Pond

STONY BROOKRESERVOIR

FRESH

POND

HOBBS BROOKRESERVOIR

CHARLES

RIV ER

42o

26'

71o00' 71o20'

42o

18'

EXPLANATION

HOBBS BROOK RESERVOIR SUBBASINSTONY BROOK RESERVOIR SUBBASINCITY OF CAMBRIDGECITY/TOWN BOUNDARIESROADSDRINKING WATER FLOW PATH

0

0

10 MILES

10 KILOMETERS

Figure 1. Location, extent, and components of the city of Cambridge drinking-water supply system, eastern Massachusetts.
Introduction 5

Description of the Cambridge 

Drinking-Water Supply System

Cambridge, Mass., is a city of about 95,000 

permanent residents and more than 60,000 university 

students. The drinking-water supply system currently 

consists of Hobbs Brook and Stony Brook Reservoirs, 

which drain a 61.4 km2 basin in Lexington, Lincoln, 

Waltham, and Weston, Mass., and Fresh Pond, a glacial 

kettle-hole lake located in Cambridge (fig. 1).

Hobbs Brook Reservoir is known locally as the 

“Cambridge Reservoir,” and is designated as such on 

the USGS 1:25,000-scale topographic quadrangle for 

Maynard, Mass. The drainage basin for the reservoir 

includes Hobbs Brook and three unnamed tributaries 

that discharge directly into the reservoir. Additional 

water enters the reservoir through other unnamed 

tributaries, storm drains associated with State Routes 2 

and 128 (Interstate-95), secondary roads, and commercial parking lots. Water is discharged from the dam 

at the lower end of Hobbs Brook Reservoir into 

Hobbs Brook, which joins Stony Brook about 2 km 

downstream. Stony Brook Reservoir is fed by Stony 

Brook, one tributary, and by storm drains from State 

Routes 128 and 20. The CWD pipes water through an 

aqueduct from Stony Brook Reservoir to Fresh Pond, 

where it is stored prior to treatment. After treatment the 

finished water is pumped to Payson Park Reservoir in 

Belmont, Mass., then flows by gravity through a 

306-kilometer long distribution system. Overflow from 

Stony Brook Reservoir flows into the Charles River 

in Waltham, Mass.

The primary source area for the water supply 

varies seasonally. During periods of high flow (mainly 

winter and spring), water from the upper Hobbs Brook 

drainage basin is used to fill Hobbs Brook Reservoir, 

and most of the water that is pumped to Fresh Pond 

comes from the larger Stony Brook drainage basin. 

During periods of low flow (mainly summer and 

autumn), the contribution from Stony Brook decreases 

considerably, and most of the water supply comes from 

releases from Hobbs Brook Reservoir.

Major water-quality concerns in the source 

area are higher-than-desired concentrations of dissolved sodium, iron, manganese, and dissolved organic 

carbon (DOC) in the reservoirs, and the potential for 

accelerated reservoir eutrophication arising from 

surface-water and ground-water inflows of nitrogen 

and phosphorus. Sodium, a component of road-deicing 

salt, is of concern because it persists in treated drinking 

water and increases consumers’ dietary intake of 

sodium. Manganese derived mainly from natural 

sources in the drainage basin occasionally appears in 

the finished water at concentrations exceeding the U.S. 

Environmental Protection Agency’s secondary maximum contaminant level (SMCL) of 50 μg/L. Problems 

associated with manganese are mainly aesthetic, such 

as discoloring of laundry and plumbing fixtures (Hem, 

1985). High concentrations of DOC are undesirable 

because some natural organic materials react with 

chlorine during water treatment to form a variety of 

potentially hazardous by-products, the most common 

of which is chloroform, a trihalomethane compound 

(Reckhow and others 1990). The propensity of source 

water to form these compounds is measured as 

trihalomethane formation potential (THMFP). Nitrogen and phosphorus, which may enter the water supply 

from nonpoint sources such as precipitation, bank erosion, fertilizer, waterfowl, and stormwater runoff, can 

stimulate excessive algal growth, causing increased turbidity, depletion of dissolved oxygen, and mobilization 

of contaminants from reservoir sediments (Cooke and 

others, 1993).

Acknowledgments

The authors wish to thank Timothy MacDonald, 

Manager of Water Operations, Edward Dowling, Water 

Quality Supervisor, and Chip Norton, Watershed Manager, Cambridge Water Department, for their help in 

planning and facilitating this study and for providing 

analyses of dissolved organic carbon, trihalomethane 

formation potential, and fecal coliform bacteria. We 

also thank Dave Velasco and Karen Zalieckas, for 

assisting in the field work and data management. Additional support for this study was provided by the 

Massachusetts Highway Department. Henry Barbaro 

and Samuel Pollack helped in facilitating this support.
6 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection

WATER QUALITY AND TROPHIC 

STATE OF HOBBS BROOK 

RESERVOIR, STONY BROOK 

RESERVOIR, AND FRESH POND

Assessments of the quality and trophic state of 

Hobbs Brook Reservoir, Stony Brook Reservoir, and 

Fresh Pond were conducted to provide baseline information on the state of these resources in support of the 

CWD’s water-quality monitoring program. Representative sampling stations were established on the three reservoirs and sampling for physical, chemical, and 

biological characteristics was carried out over a period 

of 14 months. The data were used to characterize the 

extent of vulnerability of the reservoirs to increased 

loads of nutrients and other contaminants and to help 

establish a reservoir monitoring protocol for inclusion 

in the water-quality monitoring program. Data on nutrient concentrations, water-column transparency, and 

phytoplankton abundance were used to calculate 

trophic state indices for each of the reservoirs. Reservoir bed sediments were examined once at the end of 

the study period for the presence of some trace metals 

and other constituents.

Reservoir Descriptions

Hobbs Brook Reservoir drains an area of 

17.8 km2 and has a surface area of 2.4 km2 when full 

(fig. 2A). The reservoir is divided into upper, middle, 

and lower basins by State Route 2 and Trapelo Road. 

Water flows between the three basins through unregulated submerged culverts. Except for periods of 

extreme low flow, water elevation in the three basins is 

essentially the same. The reservoir’s storage capacity is 

about 9,000,000 m3 (Fugro East, Inc., 1996). Mean 

depth at full capacity is 3.8 m.

Stony Brook Reservoir drains an area of 

61.4 km2 and has a maximum surface area of about 

0.3 km2 (fig. 2B). The narrow, steep-sided reservoir has 

a storage capacity of about 1,200,000 m3 and is divided

into two basins by State Route 128. Mean depth at full 

capacity is about 4.4 m (Fugro East, Inc., 1996).

Fresh Pond is a glacial kettle-hole lake with no 

natural surface-water inputs or outputs (fig. 2C). Water 

from Stony Brook Reservoir flows through an aqueduct 

into Fresh Pond at a rate designed to minimize groundwater inflows to the pond (Fugro East, Inc., 1996). The 

maximum surface area of Fresh Pond is 0.63 km2 and 

its maximum storage volume is 5,400,000 m3. Both 

Fresh Pond and Stony Brook Reservoir are artificially 

mixed during the spring, summer, and autumn months 

by an aeration system that bubbles air from tubes lying 

on the bottoms of the reservoirs. 

Methods of Data Collection

Water-quality sampling stations were established 

over the deepest points in Hobbs Brook Reservoir 

(fig. 2A), Stony Brook Reservoir (fig 2B), and Fresh 

Pond (fig. 2C). Additional sampling stations were 

established at the downstream ends of the middle and 

upper basins of Hobbs Brook Reservoir (fig. 2A).
Water Quality and Trophic State of Hobbs Brook Reservoir, Stony Brook Reservoir, and Fresh Pond 7

Reservoir Sampling

At intervals ranging from 4 to 13 weeks, beginning in late September 1997 and continuing through 

November 1998, depth profiles of water temperature, 

dissolved oxygen, pH, and specific conductance were 

measured at each of the deepwater sampling stations 

using a Hydrolab multiparameter water-quality 

monitoring system. Measurement intervals for the 

depth profiles were 1 m for Hobbs and Stony Brook 

Reservoirs and 2 m for Fresh Pond. Secchi disk transparency also was measured and water samples collected for determination of chlorophyll-a concentration 

(an indicator of phytoplankton biomass) and concentrations of major ions, nitrogen and phosphorus species, 

and dissolved iron and manganese. Concentrations of 

DOC and THMFP were measured, but at less frequent 

intervals than the other constituents. The water samples 

were pumped through clean Tygon tubing from a depth 

of 2 m if the water column was isothermal, or from 

three depths—0.5 m below the surface, the depth of the 

thermocline (the point of maximum rate of change in 

water temperature with depth), and 0.25 to 0.5 m above 

the bottom—if the water column was thermally stratified. Water from each sampling depth was collected 

using clean sampling protocols (Wilde and others, 

1999) into 3-liter Teflon bottles for chemical determinations and into 1-liter opaque, polyethylene bottles for 

chlorophyll-a determinations.

Samples of surficial bed sediments were collected once in November 1998 at the deepest points in 

each of the three Hobbs Brook Reservoir Basins and at 

the deep hole in Stony Brook Reservoir. Duplicate 

samples representing about 0.1 m3 were collected at 

each station with a stainless-steel Ekman dredge.

Sample Preparation and Analysis

Samples were placed in a cooler and returned 

to shore where they were prepared as required for 

each of the chemical and biological determinations. 

Chlorophyll-a samples were filtered onto glass-fiber 

filters, which were then dried in the dark for 30 minutes 

at room temperature (Godfrey and Kerr, 2000). Samples for dissolved nutrient species, major ions, and 

metals determinations were filtered through 0.45 μm 

capsule filters into polyethylene bottles and chilled or 

acidified as required. Samples for DOC determinations 

were filtered through 0.45 μm silver filters into baked 

brown-glass bottles using a stainless-steel filtration 

system. The samples then were stored on ice prior to 

analysis. Samples for determination of THMFP were 

dispensed into 1-liter baked brown glass bottles and 

stored on ice pending analysis.

Spectrophotometric chlorophyll-a analyses 

(American Public Health Association and others, 1995) 

were performed by the Environmental Analytical 

Laboratory at the University of Massachusetts in 

Amherst. DOC and THMFP determinations were conducted by the CWD laboratory in Cambridge. Analysis 

of DOC was by wet oxidation with infrared spectroscopic carbon dioxide detection (American Public 

Health Association and others, 1995).
8 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection 2,984,000 2,980,000

2,976,000

716,000 720,000

BATHYMETRIC RANGES

—Numbers

 indicate elevations, in feet, and are referenced to the city of Cambridge datum.

Greater than or equal to 180179 to 175174 to 170169 to 165164 to 160159 to 155Less than or equal to 154

BATHYMETRIC CONTOUR

—

Number indicates elevationabove datum, in feet.

RESERVOIR SAMPLING

STATION

185

EXPLANATION

Hobbs Brook Reservoir

UPPER BASIN

MIDDLE BASIN

LOWER BASIN

180 175

180

1 5 7

175

170

175

71 5

180

180

06 1

71 5

170

175

165

180

170

175

A.

State Route 2

Trapelo Road
Water Quality and Trophic State of Hobbs Brook Reservoir, Stony Brook Reservoir, and Fresh Pond 9 500 1,000 FEET 00 250 500 METERS

2,976,000

2,972,000

Base from Ocean Surveys, Inc., 1:5,400, October 19954,000-foot grid based on Massachusetts state plane coordinatesystem, 1983 North American datum 

175

175

170

165

170

165

165 1 06

1 75

61 0

1 75

61 5

61 0

160

180

165

1 65

165

1 5 5

1 60

175

1 7 5

61 5

71 5

170

Winter Street

Figure 2. Bathymetry of (

A) Hobbs Brook Reservoir, (

B) Stony Brook Reservoir, and (

C) Fresh Pond, eastern Massachusetts.
10 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection

2,958,000

2,955,000

717,000 720,000

0 500 1,000 FEET

0 250 500 METERS

BATHYMETRIC RANGES—Numbers

 indicate elevations, in feet, and are referenced

 to the city of Cambridge datum.

Greater than or equal to 70

69 to 65

64 to 60

59 to 55

54 to 50

49 to 45

Less than or equal to 44

BATHYMETRIC CONTOUR—

Number indicates elevation

above datum, in feet.

RESERVOIR-SAMPLING

 STATION

EXPLANATION

5

Base from Ocean Surveys, Inc., 1:5,400, October 1995

4,000-foot grid based on Massachusetts state plane coordinate

system, 1983 North American datum 

Stony Brook

Reservoir

65

60

60

65

70

70

70

65

60

55

65

55

50

45

65

55

128

95

B.

Figure 2. Bathymetry of (A) Hobbs Brook Reservoir, (B) Stony Brook Reservoir, and (C) Fresh Pond, eastern 

Massachusetts—Continued.
Water Quality and Trophic State of Hobbs Brook Reservoir, Stony Brook Reservoir, and Fresh Pond 11

749,250 750,750 752,250

0 500 1,000 FEET

0 250 500 METERS

2,964,000

2,965,500

2,967,000

2,968,000

2,969,000

2,963,000

BATHYMETRIC RANGES—Numbers

 indicate elevations, in feet, and are referenced

 to the city of Cambridge datum.

15 to 5

4 to -5

-6 to -15

-16 to -20

-21 to -25

-26 to -30

-31 to -35

Less than or

equal to -36

EXPLANATION

BATHYMETRIC CONTOUR—Number

indicates elevation above datum, in feet.

BATHYMETRIC CONTOUR—Number

indicates elevation below datum, in feet.

RESERVOIR-SAMPLING STATION

-5

5

Base from Ocean Surveys, Inc., 1:5,400, October 1995

4,000-foot grid based on Massachusetts state plane coordinate

system, 1983 North American datum 

C.

Fresh Pond

-30

-25

-30 -2 5

- 35

-25

-30

- 30

-20

-15

-10

-25

-25

-5

5

10

-25

-25 -20

-15

- 30

-25

-20

-20

-20

-15

-10

-5

0

0

5

10

1 5

-30

Figure 2. Bathymetry of (A) Hobbs Brook Reservoir, (B) Stony Brook Reservoir, and (C) Fresh Pond, eastern 

Massachusetts—Continued.
12 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection

Trihalomethane formation potential was measured as the sum of the concentrations of chloroform, 

bromodichloromethane, dibromochloromethane, and 

bromoform formed during chlorination of the water 

sample (American Public Health Association and 

others, 1995, Method 5710). Chlorine was added to 

buffered (pH 7.0) samples to a final concentration of 

10 mg/L and the samples were incubated for 7 days at 

25 ̊C. Quantification of the halo-organic compounds 

formed after chlorination and incubation was accomplished by purge-and-trap gas chromatography/mass 

spectrometry (American Public Health Association and 

others, 1995, Method 6232). The 10 mg/L chlorine 

dose was used to simulate conditions in the Cambridge 

water-distribution system. Residual free chlorine (Cl2) 

concentrations ranged from 0 to 6.4 mg/L with a 

mean value of 0.6 mg/L and a standard deviation of 

1.1 mg/L. Sixteen percent of the samples had no residual free chlorine; thus, analysis of THMFP concentrations may have been underestimated in these samples.

The sediment samples were subsampled so that 

only portions that had not come in contact with the 

dredge were retained. The subsamples then were combined, transferred to prelabeled polyethylene bags, and 

shipped immediately to XRAL Laboratories in Don 

Mills, Ontario, Canada, where they were digested with 

aqua regia and analyzed by inductively coupled plasma 

atomic emission spectrometry for 32 elements.

All other chemical analyses were performed by 

the USGS National Water Quality Laboratory in 

Arvada, Colo., based on analytical methods described 

in Fishman and Friedman (1989) and quality-assurance 

procedures described by Pritt and Raese (1992).

Quality Control

During each round of sampling at least one field 

blank consisting of organic-free or inorganic-free water 

was submitted for analysis. Nutrient species and metals 

were never detected in the inorganic field blanks. Small 

amounts of DOC representing a maximum of 14 percent of the mean sample concentration occasionally 

were detected in the organic blanks. THMFP in field 

blanks never exceeded 2 percent of the mean sample 

concentration.

Twenty sets of duplicate samples were collected 

at various sampling sites during the study and analyzed 

separately for DOC by the USGS National Water 

Quality Laboratory and the CWD analytical laboratory. 

Differences between duplicate DOC determinations 

ranged from 0 to 36 percent with a mean of 4.8 percent. 

THMFP analyses performed on five sets of duplicate 

samples by the CWD and a private contract laboratory 

(Camp, Dresser, and McKee, Inc., Cambridge, Mass.) 

resulted in percent differences ranging from 0.1 to 7.9 

with a mean of 3.6. Twenty-four sets of duplicate 

chlorophyll-a samples were analyzed during the study 

period by the University of Massachusetts, Amherst, 

Environmental Analytical Laboratory. The percent difference between duplicates ranged from 2.6 to 100 

with the mean percent difference of 6.8.

Reservoir Water Quality

The following sections describe the physical, 

chemical, and biological changes observed in Hobbs 

Brook Reservoir, Stony Brook Reservoir, and Fresh 

Pond during a 14-month period beginning in 

September 1997 and continuing through November 

1998.

Hobbs Brook Reservoir

Water-quality conditions in the lower basin of 

Hobbs Brook Reservoir during the study period are 

shown in figures 3A–3H (see p. 53). In late November 

1997 the water column at the deep hole reservoir-monitoring station was isothermal and exhibited uniform 

distributions of dissolved oxygen, specific conductance, and pH (fig. 3A). Water samples collected from a 

depth of 2 m contained 67 mg/L dissolved sodium and 

small amounts of dissolved manganese and total nitrogen. Concentrations of total phosphorus, ammonia 

nitrogen, and nitrate nitrogen, however, were below the 

minimum reporting limits (0.01 mg/L, 0.2 mg/L, and 

0.05 mg/L, respectively) of the analytical techniques 

used.

By January 1998, ice covered most of the reservoir. Specific conductance had increased from 

428 μS/cm to just over 600 μS/cm. Dissolved sodium 

and total nitrogen concentrations had increased from 

the November 1997 measurement (fig. 3B) and measurable amounts of total phosphorus, ammonia nitrogen, 

and nitrate nitrogen were present. The concentration 

of dissolved manganese remained low (less than 

0.2 mg/L).
Water Quality and Trophic State of Hobbs Brook Reservoir, Stony Brook Reservoir, and Fresh Pond 13

Concentrations of dissolved sodium remained 

high (greater than 80 mg/L) and were uniformly distributed with depth in early March 1998 (fig. 3C). Specific conductance also remained close to 600 μS/cm 

throughout the winter and spring. Following ice out in 

March, there were small amounts of all three nitrogen 

species present in the water column. Phosphorus, 

however, was undetectable, and dissolved manganese 

concentrations remained low. DOC and THMFP 

concentrations were low (3.8 and 0.165 mg/L, respectively) and concentrations of both organic constituents 

were uniformly distributed with depth.

With the onset of thermal stratification in early 

June 1998, dissolved oxygen concentrations began to 

decrease in the bottom layer (fig. 3D), which resulted 

in the release of phosphorus and dissolved manganese 

from bottom sediments. Concentrations of dissolved 

sodium were unchanged, but the concentration of 

ammonia nitrogen in the upper 2 m of the water 

column increased to 0.82 mg/L. This increase may 

have resulted from stormwater runoff. By June 5, DOC 

had increased to 5.4 mg/L and THMFP had increased 

to 0.315 mg/L at the surface. These increases may have 

been related to a rain storm immediately preceding the 

sampling, in which more than 4 cm of rain fell in a 

4-day period, following a 2-week dry period.

By late July 1998, the bottom 1 m of the water 

column was anoxic with high concentrations of 

total phosphorus (0.05 mg/L), ammonia nitrogen 

(0.50 mg/L), total nitrogen (0.83 mg/L), and dissolved 

manganese (2,100 μg/L) (fig. 3E). Each of these constituents is mobilized from bottom sediments under 

anoxic conditions. Nutrient concentrations in the surface layer were low and dissolved sodium concentrations were unchanged. This condition persisted through 

early August (fig. 3F). Although there was little change 

in the distribution of DOC with depth in the water column, by early August the THMFP had decreased to 

0.123 mg/L under anoxic conditions in the bottom 

layer. This decrease may indicate that the reactive components of the DOC in the bottom layer were degraded 

under anoxic conditions, or that DOC released from the 

sediments during stratification exhibited less THMFP 

than did DOC from terrestrial or water-column sources.

In early September 1998, the anoxic bottom 

layer of the water column gradually was eroded by 

nighttime convection and wind-induced surface mixing 

(fig. 3G). Phosphorus released from bottom sediments 

under anoxic conditions appeared in the upper mixed 

part of the water column. Chlorophyll-a concentrations 

(not shown) in the lower basin were low throughout the 

year (from 1.1 to 3.0 μg/L) and did not appear to 

respond to the increase in total phosphorus in the surface layers. Where anoxic conditions persisted, the 

concentration of THMFP remained low relative to the 

concentration of the upper mixed layer.

Water released from Hobbs Brook Reservoir to 

Hobbs Brook during 1997–98 was withdrawn primarily from the anoxic bottom layer with some entrainment of the oxic upper layer. This means that during 

periods of water-column stratification, most of the 

water discharged to Hobbs Brook below the reservoir 

was hypoxic and contained relatively high concentrations of total phosphorus, ammonia nitrogen, and manganese, and relatively low concentrations of THMFP. 

Evidence of this hypoxic condition and constituent 

concentrations can be seen in the black manganese 

oxide deposits on the rocks immediately downstream 

from the dam.

Stony Brook Reservoir

Water-column sampling at the deep hole station 

in Stony Brook Reservoir (fig. 2B) began in September 

1997 and continued through November 1998 (figs. 4A–

4G, see p. 61). Most of the reservoir was artificially 

destratified during spring and summer 1997 by pumping compressed air through hoses laid along the 

long axis of the basin. The basin is long and narrow 

(fig. 2B), and is substantially flushed whenever storm 

flows move through it. However, a small deep pocket 

is present in the middle of the basin, representing no 

more than 1 percent of the maximum reservoir surface 

area and 0.5 percent of the maximum reservoir volume. 

Water in this pocket became isolated in late spring 

1998 and was not flushed completely until early 

November 1998. During the period of lowest flow in 

late summer, chemical conditions in the deep hole may 

have affected as much as 30 percent of the total volume 

of the reservoir. The deep-hole water-column-sampling 

station was established at this point. However, depth 

profiles of temperature, dissolved oxygen concentration, pH, and specific conductance also were measured 

routinely at the dam and at other locations throughout 

the basin. Conditions in the main body of the reservoir 
14 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection

always were well represented by conditions in the 

upper mixed part of the water column at the deep hole 

station.

At the beginning of the study period 

(September 25, 1997), the bottom 1.5 m of the 

water column at the deep-hole station still was 

isolated from the main body of the reservoir (fig. 4A). 

The deepest 0.5 m was anoxic and exhibited increases 

in pH and specific conductance relative to the upper 

mixed layer. Concentrations of total nitrogen (3.7 

mg/L) and ammonia nitrogen (2.3 mg/L) were the 

highest measured in any of the reservoirs during the 

study period and the concentration of dissolved manganese (6.27 mg/L) was the second highest. The long 

period of reduced flows during summer 1997 apparently produced strongly reducing conditions in the 

deep hole, which favored the release of these 

constituents.

By November 1997 the entire water column was 

mixed, including the deep hole (fig. 4B). Ammonia 

nitrogen and total phosphorus were undetectable; dissolved sodium and nitrate nitrogen concentrations were 

moderately high at 51 mg/L and 0.228 mg/L, respectively; and the concentration of dissolved manganese 

was low at 0.168 mg/L. The concentration of THMFP 

at 2 m was 0.156 mg/L.

In early March 1998, the bottom 1.5 m of the 

water column began to exhibit a reduced dissolved 

oxygen concentration and an increased specific conductance (fig. 4C). All measured constituents were uniformly distributed with depth, however, indicating that 

the water column was still mixing. Concentrations of 

nitrate nitrogen were unusually high throughout the 

water column (0.548–0.566 mg/L). These were the 

highest concentrations observed in Stony Brook 

Reservoir and nearly the highest observed during the 

entire study. There were significant amounts of nitrate 

nitrogen entering the reservoir, both from the Stony 

Brook mainstem (52 ± 24 kg/d) and from a small 

tributary that enters the reservoir near the dam 

(6.5 ± 2.6 kg/d), during January through March 1998. 

Thermal and chemical isolation of the deep 

hole proceeded through spring and summer 1998. 

There was an increased total phosphorus concentration 

in the bottom water by late May (fig. 4D) and similar 

increases in the concentrations of total nitrogen, 

ammonia nitrogen, and dissolved manganese by early 

August (fig. 4E). As was the case in Hobbs Brook 

Reservoir, the concentration of THMFP decreased 

under anoxic conditions in the deep hole at Stony 

Brook Reservoir, although there was no concurrent 

decrease in the DOC concentration. In August, the 

anoxic bottom layer had the highest concentration of 

total phosphorus (0.074 mg/L) measured in any of the 

reservoirs during the study period.

By mid-September 1998, there were increased 

concentrations of ammonia nitrogen, total nitrogen, 

and dissolved manganese throughout the lower 4 m of 

the water column (fig. 4F), but by early November, 

with complete mixing of the water column, concentrations of these constituents had returned to low values 

(fig. 4G).

Fresh Pond

Water-column sampling did not begin at the 

Fresh Pond deep-hole station until October 2, 1997. 

Conditions at that time reflected the operation of 

an aeration system similar to that in Stony Brook 

Reservoir. Sampling results demonstrated that water 

temperature, dissolved oxygen, pH, and specific conductance were uniformly distributed with depth, as 

were all other measured constituents (fig. 5A, see 

p. 68). Concentrations of ammonia nitrogen, and dissolved manganese were among the lowest recorded 

in the study. The water column continued to be well 

mixed through December 3, 1997 (fig. 5B), and under 

ice cover in January (fig. 5C). In January the concentration of nitrate nitrogen increased to 0.38 mg/L 

and THMFP concentrations ranged from 0.373 to 

0.443 mg/L. By March 4, the THMFP concentrations 

had decreased to 0.159 mg/L (fig. 5D). Spring and 

summer produced few changes in water-column conditions (fig. 5E), although by late August, the dissolved 

oxygen concentration near the bottom was reduced to 

2.2 mg/L and dissolved manganese was measured at a 

concentration of 2.97 mg/L (fig. 5F). A single water 

sample collected in late August from 13.9 m at another 

deep hole located about 300 m north of the main sampling station contained the highest concentration of 

dissolved manganese (12.7 mg/L) recorded during 

the study period. In the absence of complete anoxia, 

however, THMFP concentrations remained high and 

uniformly distributed with depth throughout the 

summer.
Water Quality and Trophic State of Hobbs Brook Reservoir, Stony Brook Reservoir, and Fresh Pond 15

It is possible that manganese is released under 

hypoxic conditions from various points in the sediments of Fresh Pond in late summer. Fresh Pond has 

the typical kettle-hole morphometry with four deep 

holes reflecting the shape of the original melting glacial 

remnant (fig. 2C). If these deep areas become hypoxic 

in late summer there is a potential for release of 

reduced manganese (Mn2+) from the sediments into the 

water column. Once in solution, manganese is slow to 

reoxidize and can remain in solution for some time 

under conditions in the pond (Stumm and Morgan, 

1970). Manganese also forms soluble complexes with 

natural organic matter (Hem, 1985). Evidence is available that oxidation (and precipitation) of dissolved 

manganese in lakewaters is microbially mediated and, 

therefore, temperature dependent (Tipping, 1984). 

Thus, manganese released to the water column in late 

summer and autumn would tend to remain in solution 

as the water column cooled.

Reservoir Bed-Sediment 

Quality

Concentrations of 33 constituents in bed sediments collected in summer 1998, from Hobbs Brook 

Reservoir and Stony Brook Reservoir, together with 

median concentrations of the same analytes in bed 

sediments from 135 sampling sites in the lower 

Charles River in Boston, Mass., are shown in table 1. 

The Charles River samples were collected in June and 

July 1998 and were analyzed by the same laboratory 

(Breault and others, 2000). Breault and others (2000) 

give a detailed account of their sampling methods 

and quality-assurance procedures. Because the 

Charles River is impounded at its mouth in Boston, it 

may be regarded as a heavily urbanized reservoir for 

comparison with Hobbs Brook and Stony Brook 

Reservoirs.

For many analytes, bed-sediment concentrations in Stony Brook Reservoir were higher than 

those in Hobbs Brook Reservoir and were either 

higher than or similar to those recorded in the lower 

Charles River. The sediment phosphorus concentration in Stony Brook Reservoir was 2.7 g/kg, three 

times that of Hobbs Brook Reservoir and Fresh Pond 

and nearly twice that of the lower Charles River. 

Similarly, concentrations of aluminum, arsenic, 

cobalt, iron, manganese, titanium, and vanadium were 

appreciably higher in Stony Brook Reservoir sediments than in Hobbs Brook Reservoir, Fresh Pond, 

the lower Charles River. Concentrations of cadmium, 

copper, silver, strontium, and zinc were two to four 

times higher in Stony Brook Reservoir sediments than 

they were in those of Hobbs Brook Reservoir, but usually were much less abundant than in sediments of the 

lower Charles River. Stony Brook Reservoir and 

lower Charles River sediments contained similar 

amounts of chromium, lead, and nickel, and these 

were much higher in concentration than in Hobbs 

Brook Reservoir.

Patterns of enrichment of trace metals and other 

constituents in the Cambridge Reservoir system and 

the lower Charles River arise from the interaction of 

hydrologic features that promote deposition and the 

presence or absence of local sources. Effects of differential deposition on the observed enrichment patterns 

were clarified by normalizing elemental concentrations 

to the concentration of aluminum. Aluminum is associated with fine clay particles and is thought to be relatively conservative with respect to its rate of dissolution 

from crustal-rock sources (Horowitz, 1991). Consequently, elevated normalized concentrations (above 

background levels) at a particular site relative to those 

at another site indicate the possible presence of a 

nearby source.

Normalized bed-sediment concentrations of 

most trace elements were lower in the Cambridge 

Reservoirs than in the lower Charles River (table 2). 

Exceptions are arsenic, which was about twice as abundant in Hobbs Brook Reservoir as it was in the lower 

Charles River, and iron and manganese, which were 

higher in Stony Brook Reservoir than at any other 

station. Normalized bed-sediment concentrations of 

cadmium, chromium, copper, lead, nickel, silver, strontium, and zinc in the Cambridge Reservoirs were about 

half those in the lower Charles River. In contrast, all 

stations had similar normalized sediment concentrations of calcium, magnesium, sodium, and phosphorus. 

While there appear to be fewer potential sources of 

trace metals in the Cambridge drinking-water source 

area than in the heavily urbanized Boston area, 

phosphorus sources are similar.
16 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection

Table 1. Concentrations of selected trace metals and other contaminants in surficial bed sediments of Hobbs Brook Reservoir 

and Stony Brook Reservoir, eastern Massachusetts, November 1998, and median concentrations of the same analytes in 

surficial sediments at 135 U.S. Geological Survey sampling sites in the lower Charles River, Boston, Massachusetts, summer 

1998

[Charles River data from Breault and others (2000). MRL, minimum reporting limit. g/kg, grams per kilogram; mg/kg, milligrams per kilogram; <, actual 

value is less than value shown]

Analyte MRL

Hobbs Brook Reservoir Stony Brook 

Reservoir 

(deep hole)

Lower Charles

River Upper basin Middle basin Lower basin

Calcium (g/kg) ........................ 0.1 6.9 8.0 4.2 7.4 6.4

Magnesium (g/kg)................... .1 3.8 4.0 3.8 10.8 5.8

Sodium (g/kg) ......................... .1 0.6 .5 .3 .6 .7

Potassium (g/kg) ..................... .1 1.2 1.3 .7 2.4 2.7

Phosphorus (g/kg) ................... .1 .9 .9 .9 2.7 1.6

Aluminum (g/kg)..................... .1 10.7 12.4 12.2 29.0 18.1

Antimony (mg/kg)................... 5 <5 <5 <5 <5 <5

Arsenic (mg/kg) ...................... 3 5 4 6 14 3

Barium (mg/kg)....................... 1 86 126 67 191 192

Beryllium (mg/kg)................... .5 <.5 <.5 <.5 <.5 <.5

Bismuth (mg/kg) ..................... 5 5 <5 <5 <5 <5

Cadmium (mg/kg)................... 1 <1 1 1 3 7

Chromium (mg/kg) ................. 1 18 27 40 104 106

Cobalt (mg/kg) ........................ 1 10 13 15 25 13

Copper (mg/kg)....................... .5 23.4 54.3 22.7 81.4 270

Iron (g/kg) ............................... .1 20.2 22.5 18.1 71.7 31.5

Lead (mg/kg)........................... 2 68 178 90 651 642

Lanthanum (mg/kg)................. .5 13.8 15.2 13.0 29.1 19.2

Manganese (mg/kg)................. 2 265 320 256 1,100 466

Mercury (mg/kg)..................... 1 <1 <1 <1 <1 <1

Molybdenum (mg/kg) ............. 1 <1 <1 <1 <1 2

Nickel (mg/kg) ........................ 1 14 20 19 43 40

Scandium (mg/kg)................... .5 2.1 2.1 2.1 6.2 3.9

Silver (mg/kg) ......................... .2 <.2 <.2 <.2 .6 4.2

Strontium (mg/kg)................... .5 23.5 31.0 22.0 44.1 61.5

Tin (mg/kg) ............................. 10 <10 <10 <10 <10 <10

Titanium (g/kg) ....................... .1 .9 .8 .9 1.6 .7

Tungsten (mg/kg) .................... 10 <10 <10 <10 <10 <10

Vanadium (mg/kg)................... 2 31 46 50 127 77

Yttrium (mg/kg) ...................... .5 9.7 9.1 9.9 18.7 13

Zinc (mg/kg) ........................... .5 122 244 149 423 643

Zirconium (mg/kg).................. .5 3.4 5.7 4.3 9.3 1.9

Total organic carbon (g/kg)..... .1 27.0 67.3 32.6 77.2 109
Water Quality and Trophic State of Hobbs Brook Reservoir, Stony Brook Reservoir, and Fresh Pond 17

Table 2. Concentrations (normalized to aluminum concentrations) of selected trace metals and other contaminants in surficial 

bed sediments of Hobbs Brook Reservoir and Stony Brook Reservoir, eastern Massachusetts, November 1998, and median 

normalized concentrations of the same analytes in surficial sediments at 135 U.S. Geological Survey sampling sites in the lower

Charles River, Boston, Massachusetts, summer 1998

[Charles River data from Breault and others (2000). Analyte concentrations from table 1 are divided by the corresponding aluminum concentration and 

expressed as a percent. --, no data] 

Analyte

Hobbs Brook Reservoir Stony Brook

Reservoir

(deep hole)

Lower 

Charles

Upper basin Middle basin Lower basin River

Calcium ................................... 64.5 64.5 34.4 25.5 35.4

Magnesium.............................. 35.5 32.3 31.1 37.2 32.0

Sodium .................................... 5.6 4.0 2.5 2.1 3.9

Potassium ................................ 11.2 10.5 5.7 8.3 14.9

Phosphorus.............................. 8.4 7.2 7.4 9.3 8.8

Arsenic .................................... .05 .03 .05 .05 .02

Barium..................................... .80 1.0 .55 .66 1.1

Cadmium................................. -- .01 .01 .01 .04

Chromium ............................... .17 .22 .33 .36 .60

Cobalt...................................... .09 .11 .12 .09 .07

Copper..................................... .22 .44 .19 .28 1.5

Iron.......................................... 188.8 181.5 148.4 247.2 174.0

Lead......................................... .64 1.4 .74 2.2 3.5

Lanthanum .............................. .13 .12 .11 .10 .11

Manganese .............................. 2.5 2.6 2.1 3.8 2.6

Nickel...................................... .13 1.6 .16 .15 .22

Scandium................................. .02 .02 .02 .02 .02

Silver ....................................... -- -- -- .01 .02

Strontium................................. .22 .25 .18 .15 .34

Titanium .................................. 8.4 6.5 7.4 5.5 3.9 

Vanadium ................................ .29 .37 .41 .44 .43

Yttrium.................................... .09 .07 .08 .06 .07

Zinc ......................................... 1.1 2.0 1.2 1.5 3.6

Zirconium................................ .03 .05 .04 .03 .07
18 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection

Sediment quality of Fresh Pond was investigated 

in the early 1990s as part of a limnological study of the 

entire reservoir system (Fugro East, Inc., 1996). Concentrations of phosphorus and manganese were measured in the eastern end of the pond where bed 

sediments were only minimally affected by alum 

deposits from the treatment plant. Bed sediments at this 

site contained 0.45 g/kg phosphorus, 4.6 g/kg aluminum, and 510 mg/kg manganese. The values for aluminum and phosphorus were lower than any measured in 

Hobbs Brook Reservoir, Stony Brook Reservoir, or in 

the lower Charles River. The manganese concentration, 

however, is exceeded only by the value recorded in this 

investigation for Stony Brook Reservoir. Fresh Pond 

bed sediments may be the source of the occasional high 

concentrations of dissolved manganese encountered at 

the water-supply intake.

Reservoir Trophic State

Carlson’s trophic state index (TSI, Carlson, 

1977) was used to characterize the extent of eutrophication of the three reservoirs. The TSI is a dimensionless value ranging from 0 to 100. It may be derived 

from the mean concentration of phytoplankton chlorophyll a during the summer growing season, the annual 

mean concentration of total phosphorus, or the annual 

mean Secchi disk transparency. TSI values between 40 

and 50 indicate a moderately productive (mesotrophic) 

water body with intermediate loading of nutrients and 

organic matter (Cooke and others, 1993). TSI values 

less than 40 indicate low productivity (oligotrophic) 

and minimal external nutrient loading. A TSI greater 

than 50 corresponds to a chlorophyll-a concentration 

greater than about 6.4 μg/L, a Secchi disk transparency 

less than about 2 m, and a total phosphorus concentration greater than about 0.024 mg/L. Water bodies in 

this range are highly productive (eutrophic) and likely 

to produce algal blooms (Reckhow, 1979). Because the 

index is a continuous scale it provides a convenient 

way to compare the three reservoirs and to track 

changes over time.

TSI values calculated for the Cambridge drinking-water-supply reservoirs ranged from the high 40’s 

in Hobbs Brook Reservoir to a minimum of 27 in Fresh 

Pond (table 3). Secchi disk measurements were not 

made in the shallow upper basin of Hobbs Brook 

Reservoir, so it was not possible to calculate a TSI 

based on Secchi disk transparency. A single measurement made in the middle basin in September 1998 produced a TSI of 60, but this may not be representative of 

conditions throughout the year.

In general, the TSIs decreased with the position 

of the water body in the drainage basin. The upper and 

middle basins of Hobbs Brook Reservoir had the highest values (48–50); the lower basin of Hobbs Brook 

Reservoir and Stony Brook Reservoir were similar and 

intermediate in value (35–49), and Fresh Pond had the 

lowest values (27–42). This pattern is due partly to sedimentation of organic and inorganic particles in the 

three basins of Hobbs Brook Reservoir and in Stony 

Brook Reservoir. Sedimentation of nutrients and other 

Table 3. Median Secchi disk transparency, surface chlorophyll-a, and total phosphorus concentrations, and trophic state 

indices derived from those measurements, for Hobbs Brook Reservoir, Stony Brook Reservoir, and Fresh Pond, eastern 

Massachusetts, September 1997–November 1998

[Secchi disk transparency given in meters. Chlorophyll-a concentration given in micrograms per liter. Total phosphorus concentration given in milligrams 

per liter. TSI (SD), trophic state index calculated from secchi disk transparency; TSI (Chl), trophic state index calculated from chlorophyll-a concentrations; 

TSI (TP), trophic state index calculated from total phosphorus concentrations; TN:TP, molar ratio of total nitrogen to total phosphorus; number of samples 

used in calculation is given in parentheses; -- indicates no data]

Trophic-state indicator

Hobbs Brook Reservoir

Stony Brook 

Reservoir Fresh Pond Upper 

basin

Middle 

basin Lower basin

Secchi disk transparency ................................................. -- 1.0 (1) 2.2 (10) 2.3 (8) 3.4 (5)

Chlorophyll-a concentration............................................ 6.2 (3) 6.6 (4) 2.1 (10) 1.6 (11) 1.3 (7)

Total phosphorus concentration....................................... .024 (7) .023 (8) .012 (10) .010 (7) .005 (6)

TSI (SD) .......................................................................... -- 60 (1) 49 (10) 48 (8) 42 (5)

TSI (Chl).......................................................................... 48 (3) 49 (4) 38 (10) 35 (11) 33 (7)

TSI (TP)........................................................................... 50 (7) 49 (8) 39 (10) 41 (7) 27 (6)

Mean TN:TP.................................................................... 74 (5) 59 (5) 102 (9) 55 (5) 120 (2)
Effects of Drainage-Basin Characteristics on Water Quality of Tributary Streams 19

potential contaminants is an important feature of the 

Cambridge water-supply system; the water withdrawn 

from Stony Brook Reservoir likely is of higher quality 

than it would be in the absence of the three-basin 

cascade in Hobbs Brook Reservoir upstream.

The annual mean molar ratios of total nitrogen to 

total phosphorus were calculated for each of the reservoirs to determine which of the two nutrients might be 

limiting primary production. In inland waters, phosphorus is usually the element in shortest supply relative 

to its requirement for algal growth (Wetzel, 1993). 

Because the average molar ratio of nitrogen to phosphorus in algal biomass is about 16 to 1, a measured 

ratio greater than 20 to 1 is often considered to be evidence of phosphorus limitation of algal growth and 

ratios less than 13 to 1 are considered indicative of 

nitrogen limitation (Cooke and others, 1993). Nitrogen 

to phosphorus ratios in the Cambridge Reservoir 

system ranged from 55 in Stony Brook Reservoir to 

120 in Fresh pond, indicating that these water bodies 

may be phosphorus limited. As described by Lee and 

others (1981), these ratios do not necessarily confirm 

that a particular nutrient is growth limiting, but only 

that phosphorus is likely to be the nutrient that will be 

used up first and become limiting if growth is not limited by light or some other factor. The measured ratios 

do imply, however, that small increases in phosphorus 

loading to the reservoirs could stimulate algae to produce blooms, whereas increases in nitrogen probably 

will not.

EFFECTS OF DRAINAGE-BASIN 

CHARACTERISTICS ON WATER

QUALITY OF TRIBUTARY 

STREAMS

Sources of fecal coliform bacteria, dissolved 

sodium, dissolved chloride, nitrate nitrogen, total nitrogen, total phosphorus, dissolved iron, dissolved manganese, DOC, and THMFP entering Hobbs Brook and 

Stony Brook Reservoirs were identified and constituent 

loads were quantified by estimating mean daily loads at 

key points in the drainage system over a 1-year period. 

These load estimates then were normalized to the areas 

of the subbasins defined by the sampling stations and 

the resultant subbasin yields related statistically to land 

use, land cover, and other characteristics of the subbasins. The estimated dissolved sodium, total nitrogen, 

total phosphorus, and dissolved manganese loads were 

combined with data on stage and bathymetry of Hobbs 

Brook Reservoir and used to prepare mass balances for 

these constituents in the reservoir.

One limitation of this approach is the short time 

period in which the drainage-basin analysis was conducted. Water year 1998 (Oct. 1, 1997–September 30, 

1998) was dryer than normal. During calendar year 

1997, the National Weather Service observation station 

at Bedford, Mass., less than 3 km north of the study 

area, received 1,004 mm of precipitation, 133 mm 

less than the 30-year normal (National Oceanic and 

Atmospheric Administration, 1998). The monthly 

departure from normal from May 1997 through July 

1997 and from September 1997 through December 

1997 was negative. Precipitation for 1998 was above 

normal, due largely to a 233 mm positive departure 

from the monthly normal in June 1998.

Description of Sampling 

Network and Subbasin 

Characteristics

In 1997, as part of a study designed to identify 

sources of sodium, calcium, and chloride to the reservoirs, the USGS, in cooperation with the CWD and the 

Massachusetts Highway Department (MassHighway), 

established eight stations in the drainage basin, seven 

equipped for continuous monitoring of stream stage 

and temperature-compensated specific conductance 

and one manually sampled (partial-record) station. The 

continuous-record stations were located on Hobbs 

Brook, upstream from Hobbs Brook Reservoir, on the 

three unnamed tributaries to Hobbs Brook Reservoir, 

on Hobbs Brook at the outlet of the reservoir and just 

upstream from its confluence with Stony Brook, and on 

Stony Brook upstream from Stony Brook Reservoir 

near the intersection of State Route 20 and State Route 

128 (Interstate-95) (fig. 6, see p. 74). The partial-record 

station was located on Stony Brook just upstream from 

its confluence with Hobbs Brook.

In April 1998, the USGS and the CWD installed 

one additional continuous-record monitoring station on 

a small unnamed tributary draining a moderately 

sloped and heavily developed area along State Route 

128 (fig. 6). Additional partial-record stations were 

established on an unnamed tributary that enters Hobbs 

Brook between Hobbs Brook Reservoir and the Stony 

Brook confluence, and on an unnamed tributary that 

enters Stony Brook Reservoir near the Stony Brook 
20 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection

dam. Most of these stations had been monitored previously by the CWD and were part of their water-quality 

monitoring network.

The stations in downstream order with both 

the USGS and the CWD station numbers are listed in 

table 4. Characteristics of each of the station subbasins 

in terms of percent areal coverage of 21 land use/land 

cover categories, minimum, maximum, and mean, 

slope, and surficial geology are given in table 5. 

Drainage-basin characteristics were extracted from 

a Geographic Information System (GIS) developed 

by the CWD with data from MassGIS, the USGS 

National Mapping Division, Harvard Design and 

Mapping, and Boston Edison. Subbasin slopes were 

derived from digital elevation models (DEMs) at a 

1:24,000 scale.

The four subbasins that discharge water directly 

into Hobbs Brook Reservoir drain approximately 

9.5 km2, which represents about 61 percent of the 

Hobbs Brook Reservoir drainage basin exclusive of 

the mean reservoir surface area (fig. 6). The subbasins 

are located in moderately to heavily developed suburban areas with small amounts (0-18 percent) of commercial land use and no industrial land use (table 5). 

The subbasin defined by the upper Hobbs Brook station 

(station 01104405) is more than 68 percent forested. 

Land use is about 18 percent low-density residential 

(lots greater than one-half acre), with no commercial or 

industrial land. Twenty-two percent of the subbasin is 

wetland, mostly in the form of a large red maple 

swamp in the Hobbs Brook headwater area. In contrast, 

the subbasin defined by station 01104415 is only 16 

percent forested and is only a little over 2 percent 

wetland. Thirty eight percent of the subbasin is 

medium density residential (lots one-fourth to one-half 

acre) land.

The upper Hobbs Brook subbasins include 

parts of three State-maintained highways (State Routes 

2, 2A, and 128), three major interchanges, and numerous locally maintained roads (fig. 6). These are some 

of the most heavily traveled roads in Massachusetts. 

MassHighway operates a storage depot for highway 

deicing salt in the station 01104410 subbasin. Although 

the depot currently is covered and paved, prior to 1996 

the salt was stored uncovered on bare ground. Subbasins for stations 01104410 and 01104420 are situated 

mainly to the east of State Route 128 and parts of 

the drainages are routed under the highway through 

culverts.

Downstream from Hobbs Brook Reservoir are 

two continuous-record stations and one partial-record 

station. The partial-record station (station 01104433) 

drains a small subbasin with the heaviest concentrations of commercial (22 percent) and industrial (53 percent) land use of the area defined by any of the stations. 

The area immediately upstream from the station is a 

small forested wetland representing about 7 percent of 

the total area of the subbasin. The continuous-record 

stations are station 01104430, which is located just 

downstream from the dam and receives only the regulated discharge from Hobbs Brook Reservoir, and station 01104440, which is located about 2 km farther 

downstream at the confluence of Hobbs Brook and 

Stony Brook at Kendal Green, Mass. (fig. 6).

Table 4. Names, locations, and drainage areas of monitoring stations used to assess tributary-stream quality in the drinking-water 

source area for Cambridge, Massachusetts

[USGS, U.S. Geological Survey; CWD, Cambridge Water Department; No., number; km2, square kilometers; --, not applicable]

Station name USGS

Station No.

CWD

Station No.

Drainage area 

(km2)

Hobbs Brook at Mill Street near Lincoln ................................................................... 01104405 HB4.50C1+2 5.59 

Cambridge Reservoir, unnamed tributary 1, near Lexington...................................... 01104410 SDB0.15S .91

Cambridge Reservoir, unnamed tributary 2, near Lexington...................................... 01104415 LB0.01C 1.06

Cambridge Reservoir, unnamed tributary 3, near Lexington...................................... 01104420 TL0.30C 1.89

Hobbs Brook below Cambridge Reservoir near Kendal Green.................................. 01104430 HB1.60S 17.8

Hobbs Brook, unnamed tributary 1, near Kendal Green ............................................ 01104433 IB0.05S 1.08

Hobbs Brook at Kendal Green.................................................................................... 01104440 HB0.05S 21.9

Stony Brook at Kendal Green..................................................................................... 01104390 SB2.10S 26.9

Stony Brook, unnamed tributary 1, near Waltham ..................................................... 01104455 -- 1.2

Stony Brook at Rt. 20 near Waltham .......................................................................... 01104460 SB1.00S 57.0

Stony Brook Reservoir, unnamed tributary 1, near Weston........................................ 01104475 SS0.10S 2.2
Effects of Drainage-Basin Characteristics on Water Quality of Tributary Streams 21

Table 5. Land use and land cover, topography, and surficial geology in subbasins that contribute water to Hobbs Brook and Stony Brook Reservoirs, eastern Massachusetts[All subbasin characteristics are expressed as percentages of the total subbasin area. Maximum, mean and minimum slopes are percent vertical change per unit horizontal change. No., number; --, not applicable]

Subbasin characteristic

Subbasin station No. Drainage 

basin

totals 1104405 1104410 1104415 1104420 1104430 1104433 1104440 1104390 1104455 1104460 1104475

Cropland.......................................... 3.9 2.9 2.3 0 1.7 0 1.6 5.9 0 3.5 0 3.3Pasture............................................. 3.2 3.7 0 0 1.2 0 1.0 2.1 0 1.4 0 1.3Forest............................................... 69 49 16 27 42 8.8 40 43 35 42 19.0 40Forested wetland ............................. 21 10 0.7 12 9.5 7.1 8.7 11 .2 9.7 3.0 13Non-forested wetland...................... 1.3 7.1 1.5 1.8 1.4 .6 1.6 4.8 0 3.3 .6 3.1Open land ........................................ .1 11 3.2 0 1.4 0 1.6 1.7 .1 1.6 7.2 2.0Water ............................................... .1 0 0 0 13 0 11 3.2 0 5.7 .6 5.8Participatory recreation ................... .1 0 0 0 0 0 0 .2 0 .2 19 .9Spectator recreation......................... .9 0 .8 0 .4 0 .5 .2 0 .4 0 .4Water-based recreation.................... .0 0 0 0 0 0 0 .03 0 .02 0 .01Urban open...................................... .7 4.0 2.3 .7 .8 0 .9 .5 0 1.1 2.1 2.0State-maintained roads.................... 1.0 .9 8.1 2.8 2.4 4.3 2.2 .04 6.7 1.1 0 1.1Locally maintained roads................ 1.4 3.5 5.0 7.0 3.1 5.5 3.2 2.2 4.5 2.7 2.5 2.6Commercial..................................... 0 18 6.4 16 8.3 22 7.9 .4 9.8 3.7 0 3.5Industrial ......................................... 0 0 0 0 .2 53 3.5 0 20 1.8 0 1.7Waste disposal................................. 0 0 0 0 0 0 0 .03 0 .02 0 .01Mining............................................. 0 0 0 0 .1 0 .1 0 0 .6 0 .6Multi-family residential .................. .2 0 0 0 .5 0 .4 .3 0 .3 0 .3High-density residential .................. 0 0 0 15 2.8 0 2.3 .2 6.5 1.1 0 1.3Medium-density residential............. 0 0 38 17 8.7 0 7.0 .7 8.4 3.4 0 3.2Low-density residential................... 18 0 9.1 10 10 0 13 35 0 25 49 27Other................................................ .4 0 7.5 3.6 1.9 5.9 2.3 0 9.0 1.2 0 .2Minimum slope ............................... 0 0 0 0 0 0 0 0 0 0 0 --Maximum slope............................... 95 87 72 92 148 136 148 181 189 234 96 --Mean slope ...................................... 17 19 15 20 18 21 20 18 37 20 24 --Sand and gravel............................... 52 30 69 28 11 30 51 48 21 51 32 --Till or bedrock................................. 44 58 28 72 89 69 49 51 79 48 68 --Fine-grained deposits ...................... 1.3 0 0 0 0 0 0 0 0 0 0 --Floodplain alluvium ........................ 2.7 13 2.5 0 .3 .4 0 1.1 0 1.4 0 --
22 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection

The subbasin defined by the partial-record 

station on Stony Brook just upstream from the Hobbs 

Brook confluence (station 01104390) drains 26.9 km2 

in parts of Lincoln and Weston, Mass., and effectively 

integrates conditions in that part of the drainage basin 

(fig. 6). The subbasin is 43 percent forested and about 

16 percent is wetland. Little commercial and no industrial land use is present. Nearly 35 percent of the land 

use in the subbasin is low-density residential; however, 

the landfill and transfer station for the town of Weston 

is immediately adjacent to the sampling station. Roads 

occupy about 2.2 percent of the subbasin, and these 

roads are predominantly locally maintained. State 

Routes 117 and a small section of State Route 20 

cross the subbasin.

Station 01104455 drains a 1.2 km2 subbasin with 

a large amount (6.7 percent) of State-maintained roads 

and large amounts of commercial (9.8 percent) and 

industrial (20 percent) land uses. The monitoring station is located at the opening of a large culvert that 

runs under State Route 128.

The station on Stony Brook at the intersection of 

State Routes 20 and 128 (station 01104460) includes 

all of the Hobbs Brook and most of the Stony Brook 

drainage. Stony Brook enters the upper basin of Stony 

Brook Reservoir approximately 0.2 km downstream 

from the station. 

The subbasin defined by station 01104475 contains nearly 50 percent low-density residential land and 

has no other type of residential land use, no commercial or industrial land use, no State-maintained roads, 

and only 2.5 percent of its area consists of locallymaintained roads. The stream discharges directly into 

Stony Brook Reservoir not far from the intake for the 

aqueduct to Fresh Pond.

The topography of the subbasins ranges from 

gently rolling hills in the Stony Brook subbasin to 

fairly steep along the extreme eastern edge of the drainage area. Mean subbasin slopes are range from 17 to 20 

percent (vertical change per unit horizontal change), 

with the exception of the subbasin defined by station 

01104455, which has a mean slope of 37 percent. 

Surficial geology of the subbasins consists of 

sand-and-gravel deposits and glacial till or bedrock. 

There are numerous exposures of bedrock throughout 

the drainage basin.

Methods of Data 

Collection and Analysis

Data collection and analysis involved a large 

number of techniques and procedures, some of which 

had to be modified to accomodate problems unique to 

specific sampling stations. These modifications are 

presented in detail in the appropriate sections below.

Stage and Discharge 

Measurements

Discharge was measured at all stations with 

current meters using standard USGS procedures 

(Rantz and others, 1982). Stage was converted to discharge based on measured stage-discharge relations 

(Rantz and others, 1982). Stage and discharge were 

determined simultaneously at intervals of about 4 

weeks during the 13-month study period. Additional 

discharge measurements were obtained at high and 

low flows. Daily mean discharges at three of the continuous-record gaging stations (stations 01104410, 

01104420, and 01104460) were estimated for part or 

all of October 1997 because the stations were not fully 

operational for the entire month. This estimation was 

done using hydrograph comparison methods with 

nearby continuous streamflow-gaging stations. At station 01104455, daily mean discharges were estimated 

from October 1997 through mid-April 1998 using 

hydrograph comparison methods.

Streamflow measurements at the partial-record 

stations were correlated with concurrent daily mean 

discharges from at least six nearby continuous-record 

streamflow-gaging stations. A scatter plot of logtransformed instantaneous streamflow at the partialrecord station and same-day log transformed daily 

mean discharges at each of the six nearby continuous 

streamflow-gaging stations was made to determine 

the nature and quality of the relation among the 
Effects of Drainage-Basin Characteristics on Water Quality of Tributary Streams 23

stations. When the scatter plots indicated a log-linear 

relation, the maintenance of variance extension, type 1 

(MOVE.1) technique (Hirsch, 1982), was used to provide an equation that relates streamflow at the partialrecord station to that at the continuous streamflowgaging station. The daily mean discharges for the 1998 

water year at the continuous streamflow-gaging station 

were substituted into the equation to obtain the corresponding daily mean discharges for the partial-record 

stations. Retransformation of the daily mean discharge 

data (that is, taking the antilog) can introduce a bias. 

In this study, the Duan’s smearing method (Duan, 

1983) was used to calculate the bias-correction factor 

for each equation between a partial-record station and 

continuous-streamflow-gaging station (U.S. Geological 

Survey, 1992). Each continuous streamflow-gaging 

station estimate of daily mean discharge at the partialrecord stations was then weighted based on the calculated mean squared error from each stations equation to 

obtain a weighted estimate of daily mean discharge at 

the partial-record station.

Estimates of daily mean discharge at the partialrecord station on Stony Brook at Kendal Green, Mass. 

(station 01104390), were based on hydrographs for 

continuous USGS streamflow-gaging stations on the 

Assabet River at Maynard, Mass. (station 01097000, 

about 16 km west of the study area); on Nashoba Brook 

near Acton, Mass. (station 01097300, about 16 km 

northwest of the study area); and on the Charles River 

at Dover, Mass. (station 01103500, about 10 km south 

of the study area). Estimates of daily mean discharge at 

the partial-record station on the unnamed tributary to 

Stony Brook Reservoir (station 01104475) were based 

on the upper Hobbs Brook continuous streamflowgaging stations 01104405 and 01104420. Attempts 

to estimate daily mean discharges at the remaining 

partial-record station (station 01104433) resulted in 

unit runoff values (discharge per unit area) that were 

inconsistent with those obtained for the other 10 stations. Therefore, constituent loads and yields could not 

be calculated for station 01104433.

Chemical Sampling and 

Analysis

All 11 stream-monitoring stations were sampled 

every 4 to 6 weeks from October 1997 through 

September 1998. Water samples for chemical analysis 

were collected at stream and reservoir sampling stations using clean-sampling protocols (Wilde and 

others, 1999) for all aspects of sample collection, preservation, and transport. Samples were collected by 

combining volumes of water proportional to the 

amount of discharge at 10–12 equally spaced points 

along a stream cross section (Edwards and Glysson, 

1999). Filterable fractions of metals, nutrients, and 

common ions were obtained by filtration through 

acid-cleaned, in-line capsule filtration units with 

0.45-micrometer pore-size filters. Sample preparation, 

analysis, and quality assurance were as described 

previously for the reservoir investigations.

Event Sampling

Sampling for nutrients, major ions, dissolved 

manganese and dissolved iron was conducted during 

eight precipitation, snowmelt, or salt-application 

events. Sampling was conducted during rainstorms in 

November 1997, February 1998, March 1998, and June 

1998, and during snowstorms in December 1997 and 

January 1998. Stations usually were sampled three to 

four times during the course of an event. However, usually only 4 or 5 of the 11 stations could be sampled in 

this way during a single rain or snow storm.

Loading Calculations

Annual mean loads for nitrate nitrogen, total 

nitrogen, total phosphorus, dissolved iron, dissolved 

manganese, DOC, and THMFP were computed for 

water year 1998 at 10 of the stations (excluding station 

01104433) using the USGS program ESTIMATOR.93. 

This program calculates annual and monthly mean constituent loads based on the relation between the logarithm of the sampled constituent concentration and the 

concurrent logarithm of streamflow, and applies the 

relation to the daily mean discharge record for the 
24 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection

period of interest (Cohn and others, 1989). Constituent 

concentration data and discharge records were evaluated for each station to determine the validity of the 

model used. For most stations and constituents, the 

model:

ln [C] = B0 + B1ln[Q] + e

was used, where ln[ ] denotes the natural logarithm 

function, C = constituent concentration, Q = discharge, 

B0 is a constant, B1 is a coefficient estimated from the 

data, and e is an independent random error.

Subbasin loads for dissolved sodium, dissolved 

chloride, and fecal coliform bacteria could not be computed using ESTIMATOR.93 because transport of 

these constituents in the streams within the basin was 

not always a simple function of stream discharge. 

Increased concentrations of dissolved sodium and chloride usually occurred during road-salt applications, 

when small amounts of highly saline slush entered the 

streams with little or no change in discharge. 

Continuous concentrations of sodium and 

chloride were calculated for seven of the eight continuous streamflow-gaging stations (all except station 

01104455) based on relations between measured 

concentrations of sodium and chloride and concurrent 

continuous specific conductance records. Coefficients 

of determination (R2) for these relations ranged from 

0.962 to 0.999 for sodium and from 0.957 to 1.000 for 

chloride. The MOVE.1 technique (Hirsch, 1982) was 

used to develop equations to represent these relations, 

and the Duan’s (1983) smearing method was used to 

determine the bias correction factor (U.S. Geological 

Survey, 1992). The continuous specific conductance 

record (15- or 20-minute values) for the 1998 water 

year were substituted into the equation to obtain the 

corresponding continuous sodium and chloride concentration records. The estimated continuous sodium and 

chloride concentration records then were multiplied by 

the corresponding continuous streamflow records and a 

unit conversion factor to calculate a continuous sodium 

and chloride discharge record (15- or 20-minute 

values) at the seven continuous streamflow-gaging 

stations for the 1998 water year. The continuous 

sodium and chloride discharges then were multiplied 

by the time interval between values (15- or 20-minutes) 

and summed for the day to determine daily sodium and 

chloride load.

To calculate sodium and chloride loads at the 

partial-record station on Stony Brook at Kendal Green, 

Mass. (station 01104390), an estimate of daily mean 

specific conductance first was developed based on the 

relation between instantaneous specific conductance 

(collected concurrently with sodium and chloride concentration data) and concurrent instantaneous streamflow. Specific conductance was negatively related to 

streamflow with R2 equal to 0.916. The MOVE.1 technique was used to develop an equation to represent the 

relation between specific conductance and flow, and the 

Duan’s smearing method was used to determine the 

bias correction factor. The estimated daily mean discharge record for the 1998 water year was substituted 

into the equation to obtain the corresponding daily 

mean specific conductance. Estimates of daily mean 

sodium and chloride concentration then were developed based on the relation between the instantaneous 

measured sodium and chloride concentrations and concurrent instantaneous specific conductance values. The 

R2 for this positive relation between instantaneous 

sodium concentration and concurrent instantaneous 

specific conductance was 0.912, and R2 for the positive 

relation between instantaneous chloride concentration 

and concurrent instantaneous specific conductance was 

0.944. The MOVE.1 technique was used to develop 

equations to represent the relations between specific 

conductance and sodium and chloride, respectively, 

and the Duan’s smearing method was used to determine the bias correction factor. The estimated daily 

mean specific conductance record for the 1998 water 

year was substituted into the equation to obtain the 

corresponding daily mean sodium and chloride concentrations. Next, the daily mean sodium and chloride 

concentrations were multiplied by the daily mean discharge and a unit conversion factor to determine the 

daily mean sodium and chloride discharge, and this 

value then was multiplied by the number of seconds in 

a day to determine the daily sodium and chloride loads.

Median instantaneous sodium and chloride loads 

for stations 01104455 and 01104475 were determined 

by multiplying each measured sodium and chloride 

concentration by the corresponding instantaneous discharge and then finding the median value for the water 

year. The same approach was used to compute median 

instantaneous fecal coliform bacteria loads for all 

stations except station 01104433. 
Effects of Drainage-Basin Characteristics on Water Quality of Tributary Streams 25

Constituent Concentrations, 

Estimated Loads, and 

Subbasin Yields

Concentrations of fecal coliform bacteria, dissolved sodium, dissolved chloride, nitrate nitrogen, 

ammonia nitrogen, total nitrogen, total phosphorus, 

orthophosphate phosphorus, dissolved iron, dissolved 

manganese, DOC, and THMFP in samples collected 

from the 11 subbasin monitoring stations in the 

Cambridge drinking-water source area are presented 

as truncated box plots in figures 7A–7L (see p. 75). Box 

plots provide information on both the central value 

(median concentration) of each constituent and the 

variability and skewness of the data. In truncated box 

plots, the highest and lowest 10 percent of the data are 

not represented, and whiskers are drawn to the 10th and 

90th percentiles of the data. Truncation allows the 

majority of the data to be plotted without compressing 

the scale of the box to show extreme outliers; maximum measured concentrations will be reported in the 

discussion that follows.

The Commonwealth of Massachusetts sets minimum quality standards for surface water based on the 

intended uses of the water. Inland waters that are 

sources of drinking water are designated “Class A.”

Applicable standards for Class A streams and reservoirs in the Cambridge drinking-water source area are 

presented in table 6, together with the ranges and 

median values for selected water-quality characteristics 

recorded in this study. 

Estimated annual mean daily loads (in kilograms 

per day) and subbasin yields (in kilograms per square 

kilometer per day) for dissolved sodium, dissolved 

chloride, nitrate nitrogen, total nitrogen, total phosphorus, dissolved iron, dissolved manganese, DOC, and 

aCriterion for warm water fisheries. bArithmetic mean of a set of representative samples.

cArithmetic mean of all determinations.

dExcept where background conditions are lower.

Table 6. Ranges and median values for selected physical and chemical characteristics of water in the drinking-water source 

area for Cambridge, Massachusetts, October 1997–November 1998, in relation to Massachusetts source-water and Federal 

drinking-water standards

[DWEL, drinking water equivalent level; MCL, maximum contaminant level; SMCL, secondary maximum contaminant level; TTHM, total trihalomethanes 

formed during disinfection with chlorine; USEPA, U.S. Environmental Protection Agency. Median values for this study are given in parentheses following 

the ranges. mg/L, milligrams per liter; μg/L, micrograms per liter; CFU/100 mL, colony forming units per 100 milliliters;  ̊C, degrees Celsius; <, actual value 

is less than reported value; ≥, actual value is greater than or equal to reported value; --, no standard has been established or not measured]

Constituent

Massachusetts

Class “A” surface 

water standards

(raw water)

Cambridge drinking-water 

source area USEPA

 Tributary streams Reservoir

Drinkingwater

MCL

Drinkingwater

SMCL

DWEL

(guidelines)

pH .............................................. 6.5–8.3 4.1–8.7 (6.7) 5.8–8.1 (6.9) -- -- --

Temperature ( ̊C) ....................... a< 28.3 0–26.1 (8.8) 0.2–27.7 (13.4) -- -- --

Fecal coliform bacteria 

(CFU/100 mL) ....................... b20 0–1,700 c(48) -- -- -- --

Sodium (mg/L) .......................... -- 12–11,200 (66) 15–113 (59.5) -- -- 20

Chloride (mg/L)........................ -- 20–21,000 (120) 35–200 (110) -- 250 --

Nitrate (mg/L as N) .................. -- 0.05–2.13 (0.478) 0.05–0.79 (0.20) 10 -- --

Nitrite (mg/L as N) .................... -- 0.01–0.057 (0.014) 0.01–0.055 (0.014) 1 -- --

Sulfate (mg/L as SO4) ............... -- 4.5–190 (16) 0.15–32 (13) -- 250 --

Iron (μg/L)................................. -- 3.6–1,200 (165) 3–17,300 (115) -- 300 --

Manganese (μg/L) ..................... -- 1.7–1,420 (124) 1.5–12,700 (103) -- 50 --

Dissolved oxygen (mg/L) .......... d≥ 6.0 0.9–15 (10.1) 0–14.7 (8.1) -- -- --

TTHM (μg/L) ............................ -- 53–370 (200) 55–366 (281) 80 -- --
26 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection

THMFP for ten of the eleven subbasin monitoring stations (excluding station 01104433) are presented in 

table 7. Also presented in table 7 are median instantaneous loads and subbasin yields for fecal coliform bacteria. Error estimates based on the agreement between 

the measured and calculated constituent concentration 

data are provided for loads and yields determined with 

ESTIMATOR.93. Standard errors for discharge records 

used to calculate loads in this study ranged from 10 to 

18 percent for continuous-record stations and from 21 

to 56 percent for partial-record stations, with an overall 

median of 13 percent. Those for specific conductance 

ranged from 5.1 to 8.8 percent with a median of 6 

percent.

Fecal Coliform Bacteria

Most concentrations of fecal coliform bacteria in 

water samples collected at the 11 monitoring stations 

were less than 200 CFU/100 mL (colony forming units 

per 100 milliliters of sample water) (fig. 7A). Event 

sampling (precipitation and road-salt application), 

however, produced much higher maximum concentrations (up to 1,700 CFU/100 mL at station 01104420), 

especially when the event was preceded by at least 3 

days of dry weather. The water samples with the lowest 

maximum concentrations were collected at stations 

01104430 (100 CFU/100 mL) and 01104440 

(140 CFU/100 mL), both of which receive most of their 

flow from Hobbs Brook Reservoir, and 01104475 

(50 CFU/100 mL), which drains a small, predominantly low-density residential subbasin. Median 

concentrations determined during the study period 

ranged from 20 CFU/100 mL for samples collected at 

stations 01104475 and 01104390 to 200 CFU/100 mL 

for samples collected at station 01104410. The State 

standard for fecal coliform bacteria in streams in the 

Cambridge drinking-water source area is 20 CFU/100 

mL, based on the arithmetic mean of a representative 

set of samples (table 6). This standard was met only 

during the winter months at most stations and was not 

met at all during the study period at station 01104455.

Median instantaneous fecal coliform bacteria 

loads ranged from 3,200 CFU/s (colony forming units 

per second) at station 01104475 to 800,000 CFU/s at 

station 01104390 (table 7). The median instantaneous 

load at station 01104390, which integrates most of the 

Stony Brook subbasin, was only slightly larger than 

that at station 01104440 at the mouth of Hobbs Brook, 

which drains an area similar in size but more intensely 

developed and with greater road density than the Stony 

Brook subbasin.

Subbasin yields (load per unit area) provide a 

more direct comparison among subbasins that differ 

in size. Median instantaneous yields of fecal coliform 

bacteria (CFU/km2/s [colony forming units per 

square kilometer per second]) varied widely among 

subbasins, but most were in the range of 4,000 to 

23,000 CFU/km2/s (table 7). Exceptions were station 

01104455, which produced an estimated yield of 

29,000 CFU/km2/s and station 01104475, where the 

yield was 1,500 CFU/km2/s. 

Sodium and Chloride

Concentrations of dissolved sodium and chloride 

varied in similar ways at each of the monitoring stations (figs. 7B and 7C). Highest maximum concentrations (5,220 mg/L for sodium and 9,050 mg/L for 

chloride) were measured at station 01104415 during a 

March 1998, road-salt application event. Highest 

median concentrations (258 mg/L for sodium and 

463 mg/L for chloride) also were obtained at station 

01104415. Concentrations measured at stations 

01104415, 01104420, and 01104455 had the largest 

ranges.

Whereas no State or Federal standards are set 

for sodium and chloride in surface waters, the U.S. 

Environmental Protection Agency (USEPA) has established a secondary maximum contaminant level 

(SMCL) for chloride of 250 mg/L and a drinking-water 

equivalent level guideline for sodium of 20 mg/L 

(table 6). Because these constituents are difficult to 

remove during conventional drinking-water treatment, 

comparison of finished water standards with measured 

concentrations in the streams and reservoirs is relevant 

(table 6). All 11 sampling stations had dissolved 

sodium concentrations in excess of the guideline at 

least once during the study and nine of the stations 

had dissolved sodium concentrations in excess of the 

guideline at least 50 percent of the time (that is, median 

dissolved sodium concentrations were greater than 

20 mg/L). Six of the stations had dissolved chloride 

concentrations in excess of the SMCL at least once 

during the study and one station had dissolved chloride 

concentrations in excess of the SMCL at least 50 

percent of the time.
Effects of Drainage-Basin Characteristics on Water Quality of Tributary Streams 27

Table 7. Median instantaneous loads and yields of fecal coliform bacteria and estimated annual mean loads and subbasin yields of selected chemical constituents during water year 1998 (October 1997 through September 1998) for subbasins that contribute water to Hobbs Brook and Stony Brook Reservoirs, eastern Massachusetts

—Continued

Subbasin properties

Subbasin station No.

01104405 01104410 01104415 01104420 01104430 01104440 01104390 01104455 01104460 01104475

Hydrologic characteristics

Subbasin area (km2)........................................... 5.59 0.9 1.07 1.9 17.77 21.95 27.06 1.25 57.14 2.19

Annual mean discharge (m3/s)........................... .12 .022 .026 .054 .362 .496 .773 .037 1.43 .057

Discharge per unit area (m3/km2/s).................... .021 .025 .024 .029 .02 .023 .029 .03 .025 .026

Fecal coliform bacteria

Load (CFU/s) ..................................................... 25,000 8,500 7,900 32,000 59,000 510,000 580,000 37,000 800,000 3,200Yield (CFU/km2/s) ............................................ 4,500 9,400 7,400 17,000 3,300 23,000 21,000 29,000 14,000 1,500

Sodium

Load (kg/d)......................................................... 270 150 380 360 2,100 2,500 850 192 4,200 43Yield (kg/km2/d) ................................................ 48 170 360 190 120 110 32 150 73 20

Error (%) ............................................................ 30 9.2 11.6 14 4.8 6.4 14 -- 11 --

Chloride

Load (kg/d)......................................................... 470 280 670 640 3,800 4,700 1,500 330 7,400 59Yield (kg/km2/d) ................................................ 84 300 630 340 210 220 54 270 130 27

Error (%) ............................................................ 20 5.2 7.6 13 5.8 7.0 13 -- 8.4 --

Dissolved manganese

Load (g/d)........................................................... 560 300 360 440 7,200 7,800 2,600 370 15,000 100Yield (g/km2/d) .................................................. 100 330 340 230 400 360 95 290 270 46

Error (%) ............................................................ 64 39 37 23 116 43 40 31 26 76

Dissolved iron

Load (g/d)........................................................... 3,900 800 590 1,100 3,900 5,800 7,700 190 22,000 700Yield (g/km2/d) .................................................. 700 880 560 570 220 260 290 150 380 320

Error (%) ............................................................ 55 72 124 35 97 63 49 52 40 84

Nitrate nitrogen

Load (kg/d)......................................................... 1.9 .24 2.2 2.9 1.3 10 16 3.7 40 4.3Yield (kg/km2/d) ............................................... .33 .27 2.1 1.6 .07 .44 .58 3.0 .70 2.0

Error (%) ............................................................ 59 42 35 103 45 64 19 30 44 42

Table 7. Median instantaneous loads and yields of fecal coliform bacteria and estimated annual mean loads and subbasin yields of selected chemical constituents 

during water year 1998 (October 1997 through September 1998) for subbasins that contribute water to Hobbs Brook and Stony Brook Reservoirs, eastern Massachusetts

[01104455 and 01104475: Tabled values are median instantaneous loads and yields. Error is two times the standard error of the estimated load expressed as a percent. No., number; CFU/s, colony forming 

units per second; CFU/km2/s, colony forming units per square kilometer per second; g/d, grams per day; g/km2/d, grams per square kilometer per day; kg/d; kilograms per day; kg/km2/d, kilograms per 

square kilometer per day; km2, square kilometers; m3/km2/s, cubic meters per square kilometer per second; m3/s, cubic meters per second; %, percent; --, not determined]
28 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection Total nitrogen Load (kg/d)......................................................... 7.3 .81 3 5 13 23 29 5 82 5.9 Yield (kg/km2/d) ................................................ 1.3 .89 2.8 2.6 .71 1.1 1.1 4.0 1.4 2.7 Error (%) ............................................................ 20 22 35 60 14 29 12 22 21 31 Total phosphorus Load (kg/d)......................................................... .22 .01 .02 .06 .42 .63 .33 .07 1.4 .07 Yield (kg/km2/d) ................................................ .04 .01 .02 .03 .02 .03 .01 .06 .02 .03 Error (%) ............................................................ 71 80 90 80 46 48 61 194 79 100 Dissolved organic carbon Load (kg/d)......................................................... 140 11 8 30 180 270 260 16 740 26 Yield (kg/km2/d) ................................................ 26 12 7.5 16 10 12 10 13 13 12 Error (%) ............................................................ 43 36 40 38 13 22 18 63 15 25 Trihalomethane formation potential Load (kg/d)......................................................... 2 .34 .36 .95 7.7 11 8.1 .36 29 1.1 Yield (kg/km2/d) ............................................... .36 .37 .33 .50 0.43 .50 .30 .29 .52 .51 Table 7. Error (%) ............................................................ 17 25 45 26 16 19 22 33 18 42 Median instantaneous loads and yields of fecal coliform bacteria and estimated annual mean loads and subbasin yields of selected chemical constituents during water year 1998 (October 1997 through September 1998) for subbasins that contribute water to Hobbs Brook and Stony Brook Reservoirs, eastern Massachusetts

—Continued

Subbasin properties

Subbasin station No.

01104405 01104410 01104415 01104420 01104430 01104440 01104390 01104455 01104460 01104475
Effects of Drainage-Basin Characteristics on Water Quality of Tributary Streams 29

Estimated loads of dissolved sodium and chloride from the Hobbs Brook subbasin were much larger 

than those from the Stony Brook subbasin (table 7), 

despite the fact that the two subbasins occupy similar 

areas. The estimated loads at the mouth of Hobbs 

Brook above the Stony Brook confluence (station 

01104440) were 2,500 kg/d for dissolved sodium and 

4,700 kg/d for dissolved chloride. About 84 percent 

of the dissolved sodium load and 81 percent of 

the dissolved chloride load were accounted for by the 

discharge from Hobbs Brook Reservoir. In contrast, 

the estimated loads at the Stony Brook station just 

upstream from the Hobbs Brook confluence station 

01104390) were 850 kg/d for dissolved sodium and 

1,500 kg/d for dissolved chloride. The sum of the 

sodium loads from the Hobbs Brook subbasin, the 

Stony Brook subbasin upstream from Kendal Green, 

Mass., and the unnamed tributary (station 01104455) 

that enters Stony Brook upstream from station 

01104460, indicates that another 658 kg/d entered 

Stony Brook in the area between these stations and 

station 01104460, and subsequently was discharged 

to Stony Brook Reservoir.

Estimated subbasin yields for dissolved sodium 

and chloride were largest at stations 01104410, 

01104415, 01104420, and 01104455 (table 7), all of 

which are heavily affected by State-maintained roads 

(table 5). Subbasins upstream from stations 01104415 

and 01104455 have the highest areal percentages of 

State-maintained roads and station 01104420 has the 

highest areal percentage of locally maintained roads of 

any station in the monitoring network (table 5). Estimated sodium yields at these stations ranged from 

150 kg/km2/d at station 01104455 to 360 kg/km2/d at 

station 01104415. Subbasins from the remaining upper 

Hobbs Brook station (station 01104405), together with 

those for the Stony Brook integrator station at Kendal 

Green (station 01104390) and the unnamed tributary 

that discharges directly into Stony Brook Reservoir 

(station 01104475), produced estimated dissolved 

sodium yields ranging from 20 to 48 kg/km2/d. These 

subbasins contained little or no State-maintained road 

area (table 5).

Nitrogen

Median concentrations of nitrate nitrogen varied 

among the monitoring stations, as did the concentration 

ranges (fig. 7D). Most samples from the station just 

downstream from Hobbs Brook Reservoir (station 

01104430) had nitrate nitrogen concentrations that 

were at or below the minimum reporting level (MRL) 

of 0.05 mg/L. These results produced a median concentration for that station equal to the MRL. Median concentrations determined for most of the other stations 

were 0.5 mg/L or lower. Median concentrations at 

two of the stations, stations 01104415 and 01104455, 

were higher at 1.22 and 1.52 mg/L, respectively. These 

stations also had the widest ranges of nitrate nitrogen 

concentrations, with maximum concentrations of 1.72 

and 2.13 mg/L at stations 01104415 and 01104455, 

respectively. Although no State surface-water standard 

for nitrate nitrogen is available, the USEPA’s MCL 

(maximum contaminant level) for finished drinking 

water is 10 mg/L (table 6).

Highest median concentrations of ammonia 

nitrogen were determined for stations 01104415 

(0.12 mg/L), 01104420 (0.12 mg/L), and 01104433 

(0.16 mg/L) (fig. 7E). Subbasins for these stations also 

had high areal percentages of commercial and industrial land use (table 5). Highest maximum concentrations were in samples collected at stations 01104420 

(1.69 mg/L) and 01104455 (1.23 mg/L) during runoff 

and road-salt application events. Both these stations 

receive most of their flow from culverts extending 

underneath State Route 128.

The pattern of total nitrogen concentrations in 

water samples collected at the 11 subbasin-monitoring 

stations reflected the high variability of the nitrate 

nitrogen concentrations in the same samples (fig. 7F), 

as most nitrogen in the samples was nitrate. As with the 

nitrate nitrogen concentrations, median and maximum 

concentrations of total nitrogen were highest at stations 

01104415 and 01104455.

The Hobbs Brook subbasin (station 01104440) 

and the Stony Brook subbasin upstream from Kendal 

Green (station 01104390) produced similar estimated 

loads for both nitrate nitrogen and total nitrogen 

(table 7). Subbasin yields of the two nitrogen species 

also were similar, and were comparable in magnitude 

to those determined for the combined drainage basin 

upstream from station 01104460. The estimated subbasin yield for nitrate nitrogen at this station was 

0.70 kg/km2/d and that for total nitrogen was 

1.4 kg/km2/d. However, appreciably larger yields 

ranging from 1.6 to 3.0 kg/km2/d for nitrate nitrogen 

and from 2.6 to 4.0 kg/km2/d for total nitrogen were 

obtained at stations 01104415, 01104420, 01104455, 

and 01104475. The higher yields at station 01104475 
30 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection

may be indicative of fertilizer applications or septicsystem leachate, as land use in this subbasin is 

primarily low-density residential (table 5).

Phosphorus

Median total phosphorus concentrations at 

most stations were equal to the MRL (0.01 mg/L) 

(fig. 7G). Highest median total phosphate concentrations were about twice the MRL. Maximum concentrations ranged from 0.024 mg/L in a sample collected 

at the Stony Brook integrator site near Kendal Green, 

Mass. (station 01104390) to 0.133 mg/L in a sample 

collected at station 01104420 in the upper Hobbs 

Brook subbasin. There was little station-to-station 

variability in median orthophosphate phosphorus concentrations (fig. 7H), although the highest median 

concentration (0.016 mg/L, at station 01104410) was 

four times greater than the lowest median concentration (0.004 mg/L, at station 01104460). Maximum 

concentrations also were similar, ranging only from 

0.016 mg/L at stations 01104475 and 01104430 to 

0.026 mg/L at station 01104415. Orthophosphate phosphorus is the form of phosphorus that is most readily 

available to stimulate growth of phytoplankton algae 

(Wetzel, 1993). 

The estimated total phosphorus load at the mouth 

of Hobbs Brook (station 01104440, 0.63 kg/km2/d) 

was nearly twice that at the Stony Brook integrator station (station 01104390, 0.33 kg/km2/d). The largest 

yield was from station 01104455, but a high standard 

error was associated with that estimate. In general, 

phosphorus yields were small (less than 0.6 kg/km2/d).

Iron and Manganese

Iron and manganese enter streams primarily 

from natural geologic sources. Both metals become 

soluble under anoxic conditions and so water discharging from wetland soils frequently is enriched with 

them. Under aerobic conditions, iron rapidly is oxidized and precipitates as a ferric oxyhydroxide. Manganese may remain in solution even under aerobic 

conditions and both metals may form complexes with 

dissolved organic matter (Hem, 1985). Both metals are 

undesirable in drinking-water supplies because they 

tend to deposit as oxides on plumbing fixtures. Manganese oxides are common on rocks in the Hobbs Brook 

Reservoir tailrace and throughout the Stony Brook 

Reservoir Basin.

In the Cambridge Reservoir system, median concentrations of dissolved iron and manganese varied 

greatly from station to station, depending on sources 

and proximity of the station to anoxic inflows. Samples 

from stations 01104410, 01104415, 01104420, and 

01104433 produced the highest median concentrations 

for both metals (200 to 280 μg/L for iron and 145 to 

286 μg/L for manganese) (figs. 7I and 7J). All four 

statiions receive ground water rich in iron and manganese during base flow. Samples from tributary stations 

01104455 and 01104475 and Hobbs Brook station 

01104430 produced the lowest median concentrations 

(35–50 μg/L for iron and 5.3–54 μg/L for manganese). 

Station 01104430 receives discharge from Hobbs 

Brook Reservoir and, although there was considerable 

dissolved iron and manganese in the discharge during 

summer stratification, this did not result in elevated 

median concentrations.

Maximum concentrations of iron ranged from 

130 μg/L in samples from station 01104475 to 

1,200 μg/L in samples from station 01104405. Maximum concentrations of manganese ranged from 22 

in samples from station 01104475 to 1,420 μg/L in 

samples from station 01104433. The USEPA has set 

secondary MCLs for iron and manganese in finished 

drinking water at 300 μg/L and 50 μg/L, respectively 

(table 6).

The Hobbs Brook and Stony Brook subbasins 

(defined by stations 01104440 and 01104390, respectively) produced similar estimated annual mean 

loads for iron (5,800 and 7,700 g/d, respectively) 

(table 7). However, the estimated annual mean 

manganese load from the Hobbs Brook subbasin 

was greater (7,800 g/d ± 43%) than that from the 

Stony Brook subbasin (2,600 g/d ± 40%). Estimated 

annual mean yields for iron were greatest at stations 

representing the upper Hobbs Brook subbasins, stations 01104405 (700 g/km2/day) and 01104410 

(880 g/km2/day). Those for manganese were greatest 

at the two stations downstream from Hobbs Brook 

Reservoir, stations 01104430 (400 g/km2/day) and 

01104440 (360 g/km2/day). These higher dissolved 

manganese loads likely were due to releases from 

Hobbs Brook Reservoir bed sediments during periods 

when the reservoir was thermally stratified and the 

hypolimnion was anoxic.
Effects of Drainage-Basin Characteristics on Water Quality of Tributary Streams 31

Dissolved Organic Carbon and

Trihalomethane Formation Potential

The median concentration of DOC was highest 

(8.5 mg/L) in samples from the upper Hobbs Brook 

station (station 01104405), which drains a large red 

maple swamp, and lowest (2.7 mg/L) in samples from 

station 01104455, which drains a small, moderately 

sloped subbasin with large amounts of paved area and 

almost no wetland area (fig. 7K). The highest maximum concentration (19 mg/L) also was measured at 

station 01104405.

Median concentrations of THMFP ranged from 

0.085 mg/L at station 01104455 to 0.273 mg/L at 

the mouth of Hobbs Brook (station 01104440). Maximum concentrations were more uniform, ranging from 

0.204 mg/L at station 01104455 to 0.370 mg/L at 

station 01104390. These values are similar to the concentration range (0.144 to 0.421 mg/L) reported by 

Krasner and others (1994) for the San Joaquin and 

Sacramento Rivers in central California. The USEPA 

drinking-water standard for THMFP is 0.080 mg/L 

(table 6).

Estimated annual mean yields of DOC and 

THMFP were remarkably uniform, suggesting that no 

subbasin was exporting a disproportionate amount of 

either constituent on an annual basis. One exception 

was station 01104405, where the estimated DOC yield 

was two to three times greater than those of the other 

stations.

Constituent Yields in 

Relation to Subbasin 

Characteristics

To examine relations between subbasin characteristics and constituent yields in more detail, a correlation analysis was performed relating percent areal 

coverage of 28 land-use, land-cover, topographic, and 

geologic features to the estimated annual mean yields 

of selected water-quality constituents in 10 of the 

Cambridge subbasins. Station 01104433 was excluded 

from the analysis because yields were not estimated 

for that station. Product-moment correlation coefficients (r) showing the relative degree of association 

between each basin characteristic and each constituent 

yield are presented in table 8. The product-moment 

correlation coefficient can range in value from -1.00 to 

+1.00. A correlation coefficient of r = +1.00 signifies a 

perfect positive linear relation; a correlation coefficient 

of r = -1.00 indicates a perfect negative or inverse 

linear relation between the two variables. A correlation 

coefficient of r = 0 indicates no relation between the 

respective values of the two variables. The square of 

the correlation coefficient (r2) indicates the proportion 

of variance in one of the variables accounted for by the 

variance in the other variable (Kachigan, 1986). No 

assumptions regarding cause and effect are made in 

correlation analysis. Rather, the analysis determines the 

extent to which the observed variation in one variable 

is coincident with the observed variation in another 

variable. However, the analysis can be useful in selecting relations for more detailed investigation. In the following discussion, a significant correlation between 

two variables is defined as one in which at least 50 percent of the variance in one variable is accounted for by 

the variance in the other variable; that is, the correlation coefficient (r) is less than or equal to -0.710, or is 

greater than or equal to + 0.710. 

By the criteria used, there were few significant 

correlations between estimated 1998 subbasin constituent yields and the subbasin characteristics selected 

for analysis. There were no significant correlations 

between subbasin median instantaneous fecal coliform 

bacteria yields and any of the 28 subbasin characteristics tested (table 8). Also, no significant correlations 

between THMFP yields and any subbasin characteristics were present, although there was a significant positive correlation (r = 0.729) was determined between 

the estimated subbasin DOC yield and the percent 

coverage of forested wetlands in the subbasins.

The estimated subbasin yield of nitrate nitrogen 

was negatively correlated with pasture (r = -0.753) and 

forested wetland (r = -0.759). Total nitrogen yield was 

negatively correlated with pasture (r = -0.730). Total 

phosphorus yield was negatively correlated with nonforested wetland (r = -0.742) and positively correlated 

(r = 0.780) with the mean slope of the subbasins. Manganese yield was negatively correlated (r = -0.774) 

with the percentage of low-density residential land use.

The apparent positive correlation (r = 0.774) 

between total phosphorus yield and industrial land use 

is due to the relatively large amount of industrial land 

use (20 percent) and the large total phosphorus yield 

(0.057 kg/km2/d) in the subbasin defined by station 

01104455. If this subbasin is omitted from the analysis, 

then no correlation is present. Similarly, the correlation 

between subbasin yield of dissolved iron and percent 

areal coverage of floodplain alluvium is due entirely to 
32 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection

the disproportionately large yield (880 g/km2/day) and 

floodplain-alluvium coverage (13 percent) in the subbasin defined by station 01104410, and the apparent 

correlation between DOC yield and percent areal coverage of fine-grained deposits is due to the disproportionately large yield of DOC (26 kg/km2/d) from the 

subbasin defined by station 01104405.

The most consistent relations to emerge from the 

analysis were those between dissolved sodium and 

chloride yields and the percent areal coverage of roads 

in the subbasins. Dissolved sodium yields were 

positively correlated with percent areal coverage of 

State-maintained roads (r = 0.835), locally maintained 

roads (r = 0.726), and all roads (r = 0.882). Dissolved 

chloride yield also was positively correlated with 

sodium yield (r = 0.998) and similarly was correlated 

with State-maintained roads (r = 0.812), locally maintained roads (r = 0.713), and all roads (r = 0.860). A 

positive correlation between dissolved sodium and 

chloride yields and percent areal coverage of roads 

in the subbasins is expected because of the winter 

application of sodium chloride (road salt) as a road 

deicing agent.

Table 8. Product moment correlation coefficients (r) relating percent areal coverage of subbasin characteristics to estimated 

annual mean yields (mass per unit area) of ten potential contaminants in subbasins of the drinking-water source area for 

Cambridge, Massachusetts, October 1997–September 1998

[Fecal coliform bacteria expressed as median instantaneous yield. DOC, dissolved organic carbon; THMFP, trihalomethane formation potential]

Subbasin characteristic

Fecal 

coliform 

bacteria

Sodium Chloride Nitrate 

nitrogen

Total 

nitrogen

Total 

phosphorus

Dissolved 

iron

Dissolved 

manganese

DOC THMFP

Cropland .................................... -0.042 -0.289 -0.272 -0.604 -0.633 -0.578 0.232 -0.222 0.079 -0.388

Pasture ....................................... -.177 -.362 -.331 -.753 -.730 -.386 .588 -.083 .410 -.254

Forest ......................................... -.074 -.510 -.490 -.679 -.612 .025 .307 -.078 .669 -.229

Forested wetland........................ .041 -.481 -.457 -.759 -.673 -.181 .444 -.271 .729 .093

Non-forested wetland ................ .147 -.073 -.043 -.544 -.581 -.742 .539 .067 -.181 -.145

Open land................................... -.371 .055 .076 -.122 -.188 -.462 .486 .001 -.300 .062

Water.......................................... -.011 -.236 -.205 -.553 -.582 -.208 -.520 .498 -.278 .374

Participatory recreation.............. -.516 -.343 -.358 .311 .260 .103 -.176 -.573 -.097 .391

Spectator recreation ................... -.169 .159 .174 -.327 -.303 -.094 .141 .106 .284 -.108

Water-based recreation .............. .269 -.423 -.427 -.246 -.300 -.429 -.237 -.337 -.211 -.149

Urban open ................................ -.272 .326 .351 -.155 -.205 -.557 .668 .158 -.281 .050

State-maintained roads .............. .144 .835 .812 .657 .658 .347 -.131 .503 -.289 -.410

Locally maintained roads .......... .528 .726 .713 .512 .547 .130 .090 .388 -.246 .083

All roads .................................... .318 .882 .860 .669 .685 .296 -.055 .511 -.303 -.253

Commercial ............................... .420 .523 .539 .029 .065 -.057 .369 .644 -.160 .042

Industrial.................................... .217 .065 .033 .621 .633 .774 -.479 .201 -.010 -.373

Waste disposal ........................... .269 -.423 -.427 -.246 -.300 -.429 -.237 -.337 -.211 -.149

Mining ....................................... .431 -.224 -.219 -.207 -.204 -.129 -.136 .122 -.042 .423

Multi-family residential............. .006 -.425 -.397 -.694 -.705 -.234 -.433 .278 -.044 .253

High-density residential............. .606 .232 .216 .341 .424 .384 -.089 .146 .135 .226

Medium-density residential ....... .163 .915 .906 .439 .431 -.071 .056 .422 -.383 -.145

Low-density residential ............. -.317 -.615 -.625 -.027 -.084 -.178 -.279 -.774 -.036 .340

Maximum slope ......................... .400 -.437 -.446 -.062 -.085 .143 -.637 .099 -.158 .019

Mean slope................................. .084 -.120 -.158 .686 .689 .780 -.497 -.031 -.016 -.197

Sand and gravel.......................... .130 .227 .235 -.041 -.069 -.295 .251 -.185 .028 -.097

Till or bedrock ........................... -.094 -.253 -.267 .119 .146 .375 -.422 .146 -.052 .153

Fine-grained deposits................. -.297 -.291 -.292 -.275 -.181 .293 .396 -.411 .894 -.201

Floodplain alluvium................... -.116 .183 .209 -.340 -.346 -.443 .786 .195 .018 -.249
Effects of Drainage-Basin Characteristics on Water Quality of Tributary Streams 33

Dissolved sodium and chloride yields also were 

significantly correlated with percent areal coverage of 

medium-density residential land use in the subbasins. 

The correlation coefficient for the relation with dissolved sodium was 0.915 and that for dissolved chloride was 0.906 (table 8). Harper and others (1992) 

found that medium density residential land use was one 

of three factors that were significant in determining 

sodium concentrations in drinking-water supply wells. 

In residential areas, salt is applied to driveways, side 

walks, and other walk ways. It is unlikely, however, 

that medium-density residential land was an important 

source of sodium and chloride in the Cambridge drinking-water source area because there were no significant 

correlations with other residential land-use categories. 

Rather, the correlation probably was due to the significant positive correlations between medium-density 

residential land and State-maintained roads (r = 0.830) 

and between medium-density residential land and all 

roads (r = 0.857). 

The ratio of sodium ions to chloride ions in water 

samples collected during the study gives another indication of the sources of these constituents. The mean 

ratio of sodium concentration to chloride concentration 

(both in milligrams per liter) in 266 water samples collected at the 11 monitoring stations was 0.564 + 0.008 

(99 percent confidence bounds), and the median ratio 

was 0.558. Based on their atomic weights, the ratio 

of the two ions in sodium chloride (NaCl) salt should 

be 0.648. Granato and others (1995), working at a 

study site in southeastern Massachusetts, found mean 

sodium to chloride ratios of about 0.63 for direct highway runoff and 0.55 for ground water directly down 

gradient from the highway. Thus, the water samples 

collected in the Hobbs Brook Reservoir Basins may 

be more indicative of ground-water discharge than of 

stormwater runoff. Many of the samples were collected 

during baseflow, when streamflow is essentially 

ground-water discharge. The use of calcium chloride 

(CaCl2) as a partial substitute for sodium chloride on 

State-maintained roads, an alternative deicing practice 

in use in the Cambridge drinking-water source area 

since 1986, also would lower the sodium-to-chloride 

ratio.

It is likely that State-maintained roads had a 

greater impact than locally maintained roads on the 

yields of sodium and chloride in the source area. The 

estimated annual mean loads of dissolved sodium 

and chloride for the Hobbs Brook subbasin (station 

01104440) were about three times greater than those 

for the Stony Brook subbasin upstream from Kendal 

Green, Mass., although the drainage areas differ only 

by about 20 percent (21.9 km2 for the Hobbs Brook 

subbasin versus 26.0 km2 for the Stony Brook subbasin, table 7). The areal coverage of locally maintained 

roads is 3.2 percent for the Hobbs Brook subbasin and 

2.2 percent for the Stony Brook subbasin (table 5). 

However, the percent coverage of State-maintained 

roads is much higher in the Hobbs Brook subbasin (4.3 

percent) than it is in the Stony Brook subbasin (0.4 percent). The Hobbs Brook subbasin also contains more 

commercial and industrial land than the Stony Brook 

subbasin, but these land uses were not correlated with 

dissolved sodium and chloride yields.

Water and Constituent Mass 

Balances for Hobbs Brook 

Reservoir

Mass balances were determined for water and 

selected chemical constituents for Hobbs Brook 

Reservoir for water year 1998. The water balance, 

which defines the balance between water gains (inflow 

components) and losses (outflow components) over a 

given period of time, is a useful tool for general management decisions. The water balance determined for 

Hobbs Brook Reservoir during water year 1998 can be 

considered a generalized approximation of the overall 

water availability. Constituent balances can be used to 

identify critical sources and sinks for potential contaminants entering the reservoir. Because only surfacewater inputs and outputs were measured, and because a 

large part (39 percent) of the reservoir drainage basin 

was ungaged, the mass balances are only approximate. 

Large errors may arise from failure to measure groundwater inflows and outflows and their constituent loads 

(Winter, 1981).

Monthly water balances were determined for 

water year 1998 based on the equation

∆S = SWin + GWin + P - SWout - GWout - E ± R,

where ∆S is the change in storage (change in total 

reservoir volume); SWin is the sum of all tributary 

inflows; GWin is ground-water inflow; P is direct 

precipitation; SWout is the outflow from the reservoir;

GWout is ground-water outflow, E is evaporation from 

the reservoir surface, and R is an error term.
34 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection

The change in storage, ∆S, was calculated from 

measured stage data and stage-capacity curves developed from available bathymetric contour maps for each 

of the three Hobbs Brook Reservoir Basins (Fugro 

East, Inc., 1996) (fig. 2A). SWin was determined using 

data from the USGS continuous-record monitoring 

stations on upper Hobbs Brook (station 01104405) 

and three unnamed tributaries (stations 01104410, 

01104415, and 01104420). Daily mean streamflows 

(in meters per second) for each stream (Socolow and 

others, 1999) were multiplied by the number of seconds in one day and then summed to obtain monthly 

total volumes (in cubic meters). The monthly streamflow totals then were plotted against subbasin drainage 

area and the resulting equations used to estimate 

monthly runoff from the ungaged part of the drainage 

basin. Daily precipitation totals were obtained from the 

National Atmospheric Deposition Program/National 

Trends Network (NADP/NTN) monitoring site 

MA13 (East) located about 2 km east of the reservoir 

(National Atmospheric Deposition Program (NRSP-3)/ 

National Trends Network, 1999). With very few exceptions, these daily precipitation totals were similar to 

those measured at a USGS precipitation-monitoring 

station located at Hanscom Field near Bedford, Mass. 

(station 01100568), about 3 km northwest of the reservoir. Direct inflows of ground water to the reservoir 

(GWin) were not measured.

SWout was determined from the Hobbs 

Brook station downstream from the reservoir (station 

01104430). E was estimated from monthly mean 

class-A pan evaporation data reported by the National 

Weather Service for the period 1956–70 for their 

observation station at Rochester, Mass., about 85 km 

southwest of the reservoir (Farnsworth and others, 

1982). 

GWout was not measured. Because ground-water 

inflows and outflows were not measured during the 

study, there was no way of determining the relative 

importance of these flows in the water balance for 

Hobbs Brook Reservoir. Many water-balance analyses 

assume that ground-water contributions are accounted 

for in the residual term. Winter (1981) has shown, however, that the errors associated with the other terms in 

the water balance can be very large and may misrepresent the ground-water contribution by more than 

100 percent.

Results of the water-balance analysis are presented in table 9. The total surface-water inflow to 

Hobbs Brook Reservoir during water year 1998 was 

estimated to be 14,400,000 m3. Tributary inflows were 

proportional to drainage area and were highest in May 

and June and lowest in September and October. Precipitation was estimated to contribute 2,200,000 m3. 

Monthly precipitation inputs were small (61,000 to 

83,200 m3) during the first 3 months of the water year, 

and varied considerably during the remaining 9 months 

with a maximum precipitation input of 746,000 m3 in 

June 1998.

Outflow was regulated by the CWD such that the 

reservoir gained water from November 1997 through 

June 1998 and then lost water from July through 

September 1998 (table 9). The 1998 total surface-water 

outflow was 11,400,000 m3 and the estimated evaporative water loss was 1,350,000 m3. The estimated 

change in reservoir storage, 1,640,000 m3, was only 

60,000 m3 less than the difference between the inputs 

and outputs, which is the residual.

Because precipitation in the Cambridge drinking-water source area was below normal during the 

second half of 1997, water levels in the reservoir were 

extremely low during the first 3 months (October 

through December) of the study period. The upper 

basin and part of the middle basin of Hobbs Brook 

Reservoir were reduced to small streams that presumably consisted mainly of ground-water discharges. 

The net increase in water storage for the year 

(1,640,000 m3) reflects this slow refilling of the 

reservoir.

Hydraulic detention time can be defined as the 

time it would take for the reservoir to empty out if all 

inputs of water to the reservoir ceased (Thomann and 

Mueller, 1987). Dividing the mean reservoir volume 

for water year 1998 (5,280,000 m3) by the total outflow 

for the year (12,700,000 m3) produces an estimated 

hydraulic detention time of 0.42 years. Previous 

detention-time estimates for Hobbs Brook Reservoir on 

the order of 2 years (Fugro East, Inc., 1996) may have 

been based on an empirical model relating hydraulic 

detention time to the ratio of drainage area to reservoir 

surface area (Thomann and Mueller, 1987) and not to 

any estimate of the actual outflow. The much shorter 

detention time determined in this study indicates that, 

for water year 1998 at least, the flushing rate was much 

higher. Various studies have indicated that, at detention 

times of less than 60 to 100 days, primary productivity 

begins to be limited by washout of the phytoplankton 

(Kimmel and others, 1990). The detention time of 

Hobbs Brook Reservoir during water year 1998 

(150 days) was close to this threshold.
Effects of Drainage-Basin Characteristics on Water Quality of Tributary Streams 35

Table 9. Water balance for Hobbs Brook Reservoir, eastern Massachusetts, October 1997

–September, 1998

[All units in cubic meters. SWin, surface-water input; SWout, surface-water output; U.S. Geological Survey station numbers in parentheses]

Month

Measured and estimated inputs Measured and estimated outputs Summary budget

SWin

(01104405)

SWin

(01104410)

SWin

(01104415)

SWin

(01104420)

SWin

(ungaged)

Precipitation SWout (01104430) Evapor- ation Change in reservoir storage Input Output Residual

October ................. 2,890 3,280 5,900 19,800 104,000 62,300 1,610,000 92,600 -1,300,000 198,000 1,700,000 202,000November.............. 133,000 37,200 42,600 128,000 616,000 83,200 726,000 44,200 108,000 1,040,000 770,000 -162,000December .............. 127,000 35,900 31,900 73,900 160,000 61,000 602,000 18,000 36,100 489,000 620,000 167,100January .................. 402,000 75,600 84,200 180,000 504,000 162,000 344,000 9,980 875,000 1,410,000 353,000 -182,000February ................ 497,000 88,300 118,000 203,000 599,000 193,000 273,000 10,800 1,260,000 1,700,000 284,000 -156,000March .................... 573,000 102,000 125,000 216,000 673,000 209,000 231,000 38,100 2,290,000 1,900,000 270,000 660,000April ...................... 316,000 55,900 58,700 132,000 362,000 132,000 724,000 79,100 316,000 1,060,000 803,000 59,000May ....................... 626,000 107,000 110,000 242,000 703,000 157,000 474,000 157,000 1,690,000 1,950,000 631,000 371,000June ....................... 891,000 151,000 176,000 389,000 972,000 746,000 1,160,000 240,000 1,350,000 3,320,000 1,400,000 -570,000July........................ 171,000 33,700 35,300 64,200 185,000 125,000 1,810,000 264,000 -1,300,000 615,000 2,080,000 165,000August ................... 32,200 10,500 13,000 36,300 151,000 153,000 1,970,000 227,000 -2,300,000 397,000 2,200,000 -497,000September.............. 12,800 3,740 9,640 32,000 158,000 120,000 1,430,000 165,000 -1,300,000 335,000 1,600,000 -35,000

Water year 

1998 totals ...... 3,780,000 705,000 810,000 1,720,000 5,190,000 2,200,000 11,400,000 1,350,000 1,640,000 14,400,000 12,700,000 -60,000
36 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection

Mass balances for dissolved sodium, total nitrogen total phosphorus, and dissolved manganese entering and leaving Hobbs Brook Reservoir during water 

year 1998 were calculated based on measured and 

estimated constituent loads using the equation

∆M = MSWin + MP + MWF - MSWout ± R,

where ∆M is the change in a constituent mass in the 

reservoir; MSWin is the input of a constituent from 

tributary inflows; MP is input of a constituent from 

direct precipitation; MWF is input of a constituent from 

waterfowl; MSWout is output of a constituent from the 

reservoir; and R is the residual of the mass-balance 

equation.

Annual (water year 1998) mass balances for each 

constituent were calculated by summing the massbalance terms from seven shorter measurement periods 

ranging in length from 5 to 13 weeks. ∆M for each constituent was calculated by multiplying the changes in 

reservoir volume determined for each of the seven 

measuring periods by constituent concentrations measured at various depths at the beginning and end of the 

measurement periods. MSWin was calculated for each 

gaged tributary inflow (stations 01104405, 01104410, 

01104415, and 01104420) by summing the estimated 

daily mean loads to obtain measuring-period totals for 

each constituent. Subbasin yields (loads per unit area) 

were determined for each constituent and the loads 

entering the reservoir from ungaged parts of the drainage basin were estimated by multiplying the ungaged 

area by the mean of the corresponding subbasin yields.

MP for sodium was obtained from weekly 

measurements of dissolved sodium concentrations in 

precipitation at the NADP/NTN monitoring site just 

east of the reservoir. These data were multiplied by 

the corresponding precipitation volumes entering the 

reservoir and summed over each of the seven measuring periods to obtain measuring-period totals. MP

for total nitrogen was obtained by summing concentrations of nitrate and ammonia nitrogen reported for 

the same precipitation samples (National Atmospheric 

Deposition Program (NRSP-3)/National Trends 

Network, 1999). Because data on total phosphorus concentrations in precipitation were not available for the 

1998 water year, the mean total phosphorus concentration in wet precipitation samples collected at a USGS 

precipitation-monitoring station near Concord, Mass. 

(station 422630071201301), about 6 km northwest of 

the reservoir, during water year 1999 (October 1998–

September 1999), was applied to the precipitation volumes for water year 1998. No attempt was made to 

estimate masses of constituents entering the reservoir 

in dry deposition.

MWF was determined based on published information and weekly counts of Canada geese, mallard 

ducks, herring gulls, cormorants, herons, and other 

waterfowl (mainly other duck species), made at three 

observation points on Hobbs Brook Reservoir during 

water year 1999. Daily per capita rates of excretion of 

total phosphorus and total nitrogen obtained from the 

work of Manny and others (1994), for Canada geese, 

and from Marion and others (1994), for mallards, herring gulls, cormorants, herons, American black ducks, 

and scaups, were applied to the 1999 counts and used 

to estimate water year 1998 nutrient loads from waterfowl.

The residual of the constituent-mass-balance 

equation includes ground-water inflow, ground-water 

outflow, and storage and release of constituents from 

bottom sediments. It also includes constituent masses 

entering the reservoir in dry deposition. Because these 

components were not measured there is no way to 

determine their relative importance as sources. The 

residual represents the net balance of all the unmeasured inputs and outputs plus the net error associated 

with all the measured and estimated terms. Winter 

(1981) and LaBaugh and Winter (1984) provide general estimates of the errors associated with similar 

water and constituent mass balances.

Sodium accumulated in the reservoir water 

column during water year 1998. The estimated total 

increase in dissolved sodium mass (based on measured 

changes in volume and concentration) was 79,000 kg 

(table 10). However, the difference between the total 

surface-water input (880,000 kg) and the total surfacewater output (750,000 kg) was 130,000 kg, producing a 

residual of -51,000 kg. The subbasins defined by stations 01104415 and 01104420 together contributed 31 

percent (270,000 kg) of the dissolved sodium entering 

the reservoir. In contrast, precipitation contributed less 

than 0.1 percent (460 kg). 
Effects of Drainage-Basin Characteristics on Water Quality of Tributary Streams 37

The reservoir acted as a trap for total nitrogen 

during water year 1998. An estimated 15,000 kg 

entered the reservoir, with the contributions from the 

tributary subbasins roughly proportional to their drainage areas (table 10). The amount entering in precipitation (730 kg) was more than twice that from the 

subbasin defined by station 01104410 (300 kg). Waterfowl contributed an estimated 9 kg, which was less 

than 0.1 percent of the total input. Output of total nitrogen in surface water was 4,600 kg, less than one-third 

of the total input, but very little of the remainder 

(630 kg) appeared in storage in the water column, leaving a residual of -9,770 kg. This result indicates that a 

large part of the residual total nitrogen was trapped as 

particulate nitrogen in the reservoir bed sediments.

The reservoir also was a trap for total phosphorus during water year 1998. Only about half of the total 

input from the drainage basin (250 kg) was accounted 

for in the surface-water output (130 kg) (table 10). 

Loads from tributary subbasins were proportional to 

drainage area. However, the total-phosphorus contributions from precipitation (7 kg) and waterfowl (4 kg) 

were of similar magnitude to those from stations 

01104410 (5 kg) and 01104415 (9 kg), indicating that 

precipitation and waterfowl may be at least as significant as some of the tributary sources of total phosphorus. There was a small decrease in the estimated 

storage of total phosphorus (-10 kg) and a large negative residual (-130 kg), which includes the net effects 

of both ground-water discharge and internal loading 

and sedimentation. Data on late summer hypolimnetic 

phosphorus concentrations (fig. 3) indicate that internal 

loading from reservoir bed sediments could be an 

important source. It is unclear, however, from this analysis, how the mass of phosphorus released from the 

sediments compares with that from the other sources. 

Dry deposition, which also was not measured in this 

study, is another potentially important source of phosphorus, especially in oligotrophic lakes (Cole and 

others, 1990).

In contrast to the other constituents for which 

mass balances were determined, the water-year 1998 

output of dissolved manganese (2,600 kg) was about 

twice that of the input (1,200 kg) and this difference, 

together with the small increase in storage (79 kg), produced a positive residual of 1,479 kg (table 10). The 

residual term represents the net load of manganese in 

ground water as well as the net movement into and out 

of reservoir sediments.

Table 10. Mass balances for dissolved sodium, total nitrogen, total phosphorus, and dissolved manganese for Hobbs Brook 

Reservoir, eastern Massachusetts, October 1997–September 1998

[SWin, surface-water input; SWout, surrface-water output. U.S. Geological Survey station numbers in parentheses. --, mass assumed to be negligible; all 

units in kilograms]

Constituent

Measured and estimated inputs

SWin

(01104405)

SWin

(01104410)

SWin

(01104415)

SWin

(01104420)

SWin

(ungaged) Precipitation Waterfowl

Dissolved sodium ................... 97,000 54,000 140,000 130,000 460,000 460 --

Total nitrogen.......................... 2,700 300 1,100 1,800 8,600 730 9

Total phosphorus..................... 79 5 9 23 120 7 4

Dissolved manganese ............. 210 110 130 160 590 -- --

Constituent

Measured

output SWout

(01104430)

Summary

Change in

reservoir storage Input Output Residual

Dissolved sodium ............................... 750,000 79,000 880,000 750,000 -51,000

Total nitrogen...................................... 4,600 630 15,000 4,600 -9,770

Total phosphorus................................. 130 -10 250 130 -130

Dissolved manganese ......................... 2,600 79 1,200 2,600 1,479
38 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection

IMPLICATIONS FOR SOURCEWATER PROTECTION

A major purpose of this report is to use the data 

and results obtained in the previously described investigations of reservoir and tributary stream quality in the 

Cambridge drinking-water source area to identify reservoir and tributary sampling stations that should be 

monitored as part of a comprehensive source-water 

protection program. Monitoring of conditions in 

drinking-water source areas is an important part of a 

water-quality management program. The data such a 

monitoring program provides are both a baseline and 

an ongoing mechanism for tracking the effects of 

changing drainage-basin activities. These data also can 

be used to evaluate the effectiveness of watershed best 

management practices (BMPs) at ensuring long-term 

protection of the water supply. Although water treatment can remove most contaminants, it is usually better 

and more cost effective to prevent contamination of the 

water supply. There is growing recognition of the value 

of protecting the high-quality waters that are a source 

of drinking water as a means of reducing the cost of 

treatment systems required under the Safe Drinking 

Water Act (New England Interstate Water Pollution 

Control Commission, 1996; U.S. Environmental 

Protection Agency, 1998).

Protection of Reservoir 

Quality

Hobbs Brook Reservoir, Stony Brook Reservoir, 

and especially Fresh Pond are the primary sources of 

water in the Cambridge drinking-water supply system. 

Whereas the quality of the water supply is in most 

respects excellent, the CWD is concerned about higher 

than desired concentrations of dissolved sodium and 

chloride, a potential for eutrophication arising from 

increased inputs of phosphorus and nitrogen, and occasionally high concentrations of dissolved iron and manganese. Also, because there are numerous storm drains 

discharging highway runoff directly to Hobbs Brook 

and Stony Brook Reservoirs, there is a constant concern over the possibility of chemical spills and other 

components of highway runoff.

Median concentrations of dissolved sodium measured during the study period exceeded the USEPA’s 

DWEL guideline of 20 mg/L in all three reservoirs, 

ranging from 64 mg/L in the upper basin of Hobbs 

Brook Reservoir to 44 and 47 mg/L in Stony Brook 

Reservoir and Fresh Pond, respectively. Median concentrations of dissolved chloride never exceeded the 

USEPA’s drinking-water SMCL of 250 mg/L, but 

were higher in Hobbs Brook Reservoir (110 mg/L) 

than in Stony Brook Reservoir (75 mg/L) or Fresh 

Pond (84 mg/L). 

There was a distinct seasonal pattern in the 

reservoir sodium concentrations. In Hobbs Brook 

Reservoir, the highest measured sodium and chloride 

concentrations (82 mg/L sodium and 150 mg/L chloride) occurred in early spring and the lowest (38 mg/L 

sodium and 100 mg/L chloride) occurred in late summer. In Fresh Pond the highest measured sodium 

and chloride concentrations (54 mg/L sodium and 

100 mg/L chloride) occurred in mid-winter and the 

lowest concentrations (15 mg/L sodium and 68 mg/L 

chloride) occurred in mid-summer. These results are 

consistent with the conclusion that winter and spring 

applications of road salt are the most important source 

of dissolved sodium and chloride in the drainage basin, 

that Hobbs Brook Reservoir is more severely affected 

than Stony Brook Reservoir, and that dilution by water 

from the Stony Brook subbasin ameliorates the 

problem to some extent.

The higher median dissolved sodium concentrations in the Hobbs Brook Reservoir Basins are 

consistent with mass-balance estimates suggesting 

that station 01104415, which discharges into the upper 

basin, and station 01104420, which discharges into the 

middle basin, together contributed 31 percent of the 

dissolved sodium entering the reservoir, while contributing only about 18 percent of the water (tables 9 and 

10). The estimated dissolved sodium yield from Hobbs 

Brook (110 kg/km2/d), which drains the reservoir, 

was more than three times that from Stony Brook 

(32 kg/km2/d), which drains the western half of the 

drainage basin (table 7). Thus, it is likely that dilution 

by water from Stony Brook and other tributaries supplying Stony Brook Reservoir resulted in lower dissolved sodium concentrations in the two reservoirs 

downstream from the Hobbs Brook confluence.

Because sodium and chloride are difficult and 

expensive to remove during conventional water treatment, it will be necessary to monitor concentrations of 

these ions in the water supply to determine if they are 

increasing and how they may be responding to management practices, particularly those related to highway 
Implications for Source-Water Protection 39

deicing activities, in the drainage basin. This can be 

readily accomplished by the use of specific conductance as an indicator of dissolved sodium and chloride 

concentrations. The most effective location for these 

monitors would be at the dams on Stony Brook and the 

lower Hobbs Brook Basin, and near the intake in Fresh 

Pond. Specific conductance data collected at frequent 

intervals at these three stations could be converted to 

concentrations of dissolved sodium and chloride concentrations by means of relations obtained in this investigation. Both short-term and long-term trends could be 

identified and steps quickly taken to isolate the source 

of any increases. Tributary loads of dissolved sodium 

and chloride also need to be monitored closely for the 

same reasons. Loads from direct precipitation appear 

not to be an important source of sodium entering the 

reservoir.

Trophic state indices based on median phytoplankton chlorophyll-a concentrations, median Secchi 

disk transparencies, and median total phosphorus concentrations indicate that all three reservoirs should be 

classified as mesotrophic, or moderately productive, 

and, therefore, susceptible to cultural eutrophication. 

The three Hobbs Brook Reservoir Basins, with TSIs 

ranging from 38 to 60, boarder on the eutrophic, or 

highly productive, trophic state (table 3). Stony Brook 

Reservoir and Fresh Pond, with TSIs ranging from 27 

to 48, boarder on the oligotrophic, or unproductive, 

trophic state. More significantly, mean molar ratios of 

total nitrogen to total phosphorus are extremely high 

(55 to102), indicating that phytoplankton growth in the 

reservoirs is much more likely to be limited by the 

availability of phosphorus and then by the availability 

of nitrogen. Thus, small increases in phosphorus loading to the reservoirs could stimulate phytoplankton 

algae to produce noxious blooms, whereas increases 

in nitrogen loading probably will not.

Median concentrations of orthophosphate phosphorus in the three reservoirs were only slightly higher 

than the minimum reporting level of 0.001 mg/L. 

Median concentrations of total phosphorus, an indicator mainly of living and nonliving particulate phosphorus, decreased from a high of 0.032 mg/L in the 

middle basin of Hobbs Brook Reservoir to a low of 

0.014 mg/L in Fresh Pond. This pattern probably is due 

to sedimentation of organic and inorganic particles in 

the three basins of Hobbs Brook Reservoir and in 

Stony Brook Reservoir. Median concentrations of 

nitrate nitrogen and nitrite nitrogen were significantly 

(usually more than 10 percent) lower than the USEPA 

drinking-water MCLs of 10 and 1 mg/L, respectively.

Waterfowl feeding and roosting on the reservoirs 

are of concern because they are a source of fecal bacteria and also because their feces are rich in phosphorus. 

The estimated total phosphorus load to Hobbs Brook 

Reservoir from waterfowl during the study period was 

similar to those from some of the smaller tributaries 

and from direct precipitation (table 10). In contrast, the 

estimated total nitrogen load from waterfowl was less 

than 1 percent of that from other sources. Given the 

potential sensitivity of the reservoirs to increased phosphorus loading, it may be necessary to keep track of 

waterfowl abundances on the reservoirs and take steps 

to discourage their presence if populations increase.

Median concentrations of dissolved iron rarely 

exceeded the USEPA drinking water SMCL of 

300 μg/L. In contrast, the USEPA drinking-water 

SMCL of 50 μg/L for dissolved manganese was 

exceeded in Hobbs and Stony Brook Reservoirs but not 

in Fresh Pond. Naturally occurring iron and manganese 

is mobilized under anoxic conditions and transported in 

ground water to streams, where both metals tend to 

precipitate as oxyhydroxides. Natural organic compounds can act as chelators holding iron and manganese atoms in solution, even under oxidizing conditions 

(Hem, 1985), and transporting them to the reservoirs. 

During periods of summer stratification in the reservoirs, manganese goes into solution in the hypolimnion 

earlier than iron, but is precipitated later than iron when 

turnover occurs (Stumm and Morgan, 1970). Thus, 

manganese is more likely to be lost in reservoir outflows and to enter drinking-water intakes than is iron.

On four occasions, surveys of the spatial variability of dissolved oxygen concentration, pH, temperature, and specific conductance were conducted and the 

data compared with that from the deep hole stations. 

No consistent spatial trends were observed, indicating 

that a single depth profile of measurements at the deep 

hole station could be considered indicative of conditions throughout the reservoir. It should be noted, however, that these comparisons were done only during 

periods when there was no ice cover. During the winter 

deicing season, and especially under ice cover, there 

may be horizontal variations in sodium and chloride 

concentrations related to the discharge of highway 

runoff directly to the reservoirs. The deep hole stations 

in the three main basins, together with stations on the 
40 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection

upper and middle basins of Hobbs Brook Reservoir, 

can be considered as secondary reservoir-monitoring 

stations in addition to the three primary stations 

described earlier. Additional secondary reservoirmonitoring stations should be established in the upper 

and middle basins of Hobbs Brook Reservoir.

Protection of Tributary-Stream

Quality

A second level of monitoring for protection of 

the Cambridge drinking-water supply involves the tributaries that contribute water directly or indirectly to 

the reservoirs. The investigation of tributary-stream 

quality previously described estimated contaminant 

loads for water year 1998 at 10 tributary-monitoring 

stations and examined the effects of land use and other 

drainage-basin features on contaminant yields from 

the subbasins defined by these stations. These analyses 

provided the information needed to justify incorporation of the stations into a permanent water-qualitymonitoring network. In the following discussion, each 

tributary-monitoring station is characterized in terms of 

its relative contribution to the total loads of contaminants entering the water supply and its proximity to the 

reservoir. The 10 stations can be considered as primary 

tributary-monitoring stations, indicative of the quality 

of water entering the reservoirs.

Hobbs Brook at Mill Street near 

Lincoln, Mass. (01104405)

Hobbs Brook is one of three tributaries that 

convey water directly to the upper basin of Hobbs 

Brook Reservoir. The subbasin defined by station 

01104405 (Hobbs Brook at Mill Street, near Lincoln, 

Mass.), at 5.59 km2, is by far the largest of the three. 

During water year 1998, the subbasin was a major 

source of phosphorus, dissolved iron, and DOC to the 

water supply. Median concentrations of DOC were 

higher at station 01104405 than at any other station in 

the drainage basin. The subbasin also produced high 

yields of total phosphorus, dissolved iron, DOC, and 

THMFP, relative to the other subbasins in the source 

area (table 7). These high concentrations and yields 

probably are related to the relatively large percent wetland cover in the subbasin (table 5). Wetlands are a 

source of DOC, which increases the solubility of iron 

by forming organic-iron complexes, and anoxic water, 

which impedes formation of insoluble inorganicphosphorus complexes. Phosphorus is the most important constituent in this tributary with respect to its 

potential effect on the Hobbs Brook Reservoir.

Cambridge Reservoir, Unnamed 

Tributary 1, near Lexington, Mass. 

(01104410)

The subbasin defined by station 01104410 

(Cambridge Reservoir, Unnamed Tributary 1, near 

Lexington, Mass.) contributed more dissolved iron per 

square kilometer than any subbasin in the source area. 

In addition, the highest median concentrations of fecal 

coliform bacteria, orthophosphate phosphorus, and 

dissolved iron were measured at the station. The percentage of floodplain alluvium in the subbasin is 

more than five times that of any other subbasin in the 

source area (table 5) and this may account for the high 

median concentrations of orthophosphate phosphorus 

and iron, since a high proportion of streamflow in the 

tributary enters as anoxic ground water rich in these 

constituents. 

Cambridge Reservoir, Unnamed 

Tributary 2, near Lexington, Mass. 

(01104415)

The subbasin defined by station 01104415 

(Cambridge Reservoir, Unnamed Tributary 2, near 

Lexington, Mass.) was the greatest source of contamination in the drainage basin during water year 1998. 

Median concentrations of dissolved sodium and chloride, total nitrogen, and dissolved manganese were 

higher at that station than at any other station in the 

source area, and median concentrations of ammonia 

nitrogen, nitrate nitrogen, and total nitrogen exceeded 

national background concentrations reported by the 

USGS National Water-Quality Assessment (NAWQA) 

Program (U.S. Geological Survey, 1999) for undisturbed stream sites. Subbasin yields of sodium and 

chloride were larger at station 01104415 than at any 

other subbasin and the subbasin also produced relatively large yields of dissolved manganese, dissolved 

iron, nitrate nitrogen, and total nitrogen (table 7). 

Although the subbasin contributed less than 6 percent 

of the water entering Hobbs Brook Reservoir during 

water year 1998, it contributed nearly 16 percent of the 

dissolved sodium (table 10).
Implications for Source-Water Protection 41

It is likely that much of the contamination is 

related to highway runoff. More than 13 percent of the 

subbasin is covered by roads, the highest coverage of 

any subbasin in the source area (table 5). The subbasin 

includes a major highway interchange connecting State 

Routes 2A and 128 and a salt storage area managed by 

MassHighway. The subbasin also contains a large proportion of medium-density residential land use, but 

most of this land is not directly associated with the tributary. State highways occupy twice as much of the subbasin as they do in any other subbasin (table 5) and are 

located in close proximity to the station, the tributary, 

and the reservoir (fig. 6). Inclusion of this station in a 

water-quality monitoring program is essential because 

of the serious potential for increased contributions of 

sodium and other contaminants to the water supply.

Cambridge Reservoir, Unnamed 

Tributary 3, near Lexington, Mass. 

(01104420)

Station 01104420 (Cambridge Reservoir, 

Unnamed Tributary 3, near Lexington, MA) is on a 

small unnamed tributary that enters the middle basin of 

Hobbs Brook Reservoir. The tributary receives runoff 

from State Routes 2 and 128 and from a commercial 

parking lot and also drains a wetland area east of Route 

128 (fig. 6). Maximum concentrations of fecal coliform 

bacteria measured at the station were the highest of any 

in the source area and median concentrations of ammonia nitrogen and total nitrogen exceeded national background concentrations reported by the USGS National 

Water-Quality Assessment (NAWQA) Program (U.S. 

Geological Survey, 1999) for undisturbed stream sites. 

Hobbs Brook, Unnamed Tributary 1, 

near Kendal Green, Mass. (01104433)

Station 01104433 (Hobbs Brook, Unnamed 

Tributary 1, near Kendal Green, MA) is on a small tributary that enters Hobbs Brook about 1 km downstream 

from the dam. The subbasin drains a small forested 

wetland (fig. 6) and has the greatest densities of commercial and industrial land use of any subbasin in 

the source area (table 5). Median concentrations of 

ammonia nitrogen and total nitrogen measured at the 

station exceeded national background concentrations 

(U.S. Geological Survey, 1999) and the median concentration of dissolved manganese was the highest 

measured in the source area. Because there were problems estimating daily mean discharges at this station, 

no constituent loads or subbasin yields were calculated. 

Consequently, direct comparisons with other subbasins 

are not possible. Nonetheless, there appears to be a 

potential for significant contaminant transport from this 

subbasin. However, the wetland in the subbasin and 

a small ponded area downstream from the station probably reduce the potential for serious contamination 

of Hobbs Brook. The tributary should be monitored, 

especially as commercial and industrial development 

continues in the subbasin.

Hobbs Brook at Kendal Green, Mass. 

(01104440)

Station 01104440 (Hobbs Brook at Kendal 

Green, Mass.) is important because it integrates water 

and constituent loads from the entire Hobbs Brook subbasin. The station is located just upstream from the 

confluence of Hobbs Brook and Stony Brook and 

affords useful comparisons with monitoring data collected at station 01104390 on Stony Brook (Stony 

Brook at Kendal Green, Mass.) nearby. Streamflow 

and water quality at the Hobbs Brook station reflected 

the operation and condition of the reservoir upstream 

during water year 1998. That is, the water quality 

generally was very good, and yields of most measured 

constituents were consistent with those from the other 

smaller subbsins and from the larger Stony Brook 

subbasin (table 7). Exceptions were the estimated 

subbasin yields of fecal coliform bacteria, dissolved 

manganese, and THMFP, each of which were larger 

than most other subbasins.

Stony Brook at Kendal Green, Mass. 

(01104390)

Station 01104390 (Stony Brook at Kendal 

Green, Mass.) is located on Stony Brook just upstream 

from its confluence with Hobbs Brook (fig. 6). As such, 

water-quality data from the station integrates and represents conditions in a subbasin comprising more than 

half the total source area. The subbasin area is similar 

to that above station 01104440. Land use and land 

cover, however, are appreciably different in the two 

integrator subbasins. The Stony Brook subbasin contains significantly less commercial and industrial land 

(0.4 percent for Stony Brook and 8.4 percent for Hobbs 

Brook), far smaller amounts of State-maintained roads 

(0.4 and 2.2 percent, respectively), and a larger amount 

of low-density residential land use (35 versus 13 percent) (table 5). Both subbasins produced relatively 
42 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection

large estimated yields for fecal coliform bacteria 

(table 7). However, estimated subbasin yields for 

sodium, chloride, manganese, total phosphorus, and 

THMFP all were larger in the Hobbs Brook subbasin 

than they were in the Stony Brook subbasin. Nonetheless, continued monitoring at station 01104390 will 

provide a good indication of any major changes in land 

use and management in the upstream subbasin.

Stony Brook, Unnamed Tributary 1, 

near Waltham, Mass. (01104455)

Station 01104455 (Stony Brook, Unnamed 

Tributary 1, near Waltham, Mass.) discharges through 

a small nonforested wetland to Stony Brook about 

0.7 km upstream from Stony Brook Reservoir (fig. 6). 

The subbasin defined by the station contains significant 

amounts of State and locally-maintained roads, and 

commercial and industrial land use (table 5). Much of 

the lower part of the subbasin is paved and this part of 

the stream is routed through culverts that directly drain 

State Route 128 and the interchange connecting State 

Routes 20 and 128. Also a large amount of high-density 

and multi-family residential land use relative to other 

subbasins is present in the source area (table 5).

During water year 1998, median concentrations 

of ammonia nitrogen, nitrate nitrogen, and total nitrogen exceeded national background concentrations 

(U.S. Geological Survey, 1999) and median concentrations of dissolved sodium and orthophosphate phosphorus were high relative to those of other subbasins in 

the source area. The State standard for fecal coliform 

bacteria in Class A streams (20 CFU/100mL) was 

exceeded every time the stream was sampled during 

water year 1998.

The subbasin produced the largest yields of fecal 

coliform bacteria, nitrate nitrogen, total nitrogen, and 

total phosphorus of any subbasin in the source area 

(table 7). Because of its proximity to Stony Brook 

Reservoir, constituent loads from this subbasin may 

have affected the quality and trophic state of the reservoir. Furthermore, the large amount of impermeable 

surface area in the subbasin near the monitoring station 

resulted in very rapid increases in discharge in 

response to precipitation and stormwater runoff. For 

these reasons, monitoring at this station would benefit 

from installation of continuous stage and specific conductance recorder and a system for automatically 

sampling the stream when stage or specific conductance change significantly. Sampling for additional 

constituents indicative of contamination from highway 

runoff (total petroleum hydrocarbons, total suspended 

solids, trace metals, and certain volatile organic carbon 

compounds) would provide a more complete picture of 

the effects of the highway on this tributary.

Stony Brook at Route 20 near 

Waltham, Mass. (01104460)

Station 01104460 (Stony Brook at Route 20 near 

Waltham, Mass.) effectively integrates the main part of 

the source area upstream from Stony Brook Reservoir 

(fig. 6). Most of the water that enters Stony Brook 

Reservoir flows past this station. Median concentrations and estimated subbasin yields of nearly all constituents measured during water year 1998 were either 

low or at most moderate relative to other subbasins in 

the source area (table 7). Regular monitoring at this station will provide important information on the quality 

of water entering Stony Brook Reservoir and help 

guide management of the reservoir.

Stony Brook Reservoir, Unnamed 

Tributary 1, near Weston, Mass. 

(01104475)

Station 01104475 (Stony Brook Reservoir, 

Unnamed Tributary 1, near Weston, Mass) is located on 

a small, unnamed tributary that discharges directly into 

Stony Brook Reservoir near the dam (fig. 6). Land use 

in the subbasin defined by the station is considerably 

different from that in most other subbasins in the 

source area (table 5). The subbasin contains relatively 

little forest compared to the other subbasins, and there 

are no State-maintained roads, no commercial or industrial land use. The predominant land use is low-density 

residential. The tributary’s contribution to the total flow 

of water through the system was small during water 

year 1998 (table 7), but the location of its outfall close 

to the water-supply intake makes it an important station 

to monitor. During water year 1998, median concentrations of fecal coliform bacteria, sodium, chloride, 

ammonia nitrogen, and dissolved iron and manganese 

were the lowest of any in the source area. Subbasin 

yields of fecal coliform bacteria, sodium, chloride, and 

manganese were much smaller than those of the other 
Summary and Conclusions 43

subbasins. During the same period, however, median 

concentrations of nitrate nitrogen and total nitrogen 

were higher than published national background 

concentrations for undisturbed streams (U.S. Geological Survey, 1999), and subbasin yields of nitrate nitrogen, total phosphorus, and THMFP were larger than 

those of most other subbasins (table 7). The high concentrations and large yields of nutrients probably 

reflect fertilizer use in the largely residential subbasin, 

and may be related to the presence of a golf course. 

Thus, the station can serve as a control for the effects of 

road deicing on sodium and chloride loads, but should 

be considered an important source of nutrients to Stony 

Brook Reservoir and possibly to Fresh Pond.

SUMMARY AND 

CONCLUSIONS

Assessments of reservoir and tributary-stream 

quality were conducted in the Cambridge, Mass., 

drinking-water source area to help guide development 

of a comprehensive water-quality monitoring program 

for the source area. Limnological assessments of 

the three primary storage reservoirs, Hobbs Brook 

Reservoir, Stony Brook Reservoir, and Fresh Pond, 

were conducted to provide baseline information on 

the state of these resources and to determine the vulnerability of the reservoirs to increased loads of nutrients and other contaminants. The effects of land use, 

land cover, and other drainage-basin characteristics on 

transport and fate of highway deicing chemicals, nutrients, naturally occurring organic compounds, fecalindicator bacteria, and selected metals within the 

source area were examined and ranked statistically. 

Results of these investigations were used to select 

monitoring stations to be included in a long-term 

water-quality monitoring network for the source area.

Representative sampling stations were established on the three reservoirs and sampling for physical, chemical, and biological characteristics was 

carried out over a period of 14 months, beginning in 

September 1997 and continuing through November 

1998. Reservoir bed sediments were examined once 

at the end of the study period for the presence of 27 

trace metals and other constituents, and data on 

nutrient concentrations, water-column transparency, 

and phytoplankton abundance were used to calculate 

trophic state indices for each of the reservoirs.

Sources of fecal coliform bacteria, dissolved 

sodium and chloride, nitrate nitrogen, total nitrogen, 

total phosphorus, dissolved iron, dissolved manganese, 

DOC, and THMFP entering Hobbs Brook and Stony 

Brook Reservoirs were identified and quantified by 

estimating mean daily loads at key points in the drainage system during water year 1998 (October 1997–

September 1998). These load estimates were then normalized to the areas of the subbasins defined by the 

sampling stations and the resultant subbasin yields 

related statistically to land use, land cover and other 

characteristics of the subbasins. The estimated sodium, 

total nitrogen, total phosphorus, and dissolved manganese loads were combined with data on stage and 

bathymetry of Hobbs Brook Reservoir and used to 

determine mass balances for these constituents in the 

reservoir.

Data and other information gained in the assessments of reservoir and tributary quality were used to 

select reservoir and tributary monitoring stations for 

inclusion in a water-quality monitoring program 

designed to ensure long-term protection of the water 

supply and delivery of high quality water for treatment 

or distribution.

Seasonal thermal and chemical stratification 

were observed in all three reservoirs during the study 

period. Hobbs Brook Reservoir was thermally stratified 

from early June through September 1998, and the 

hypolimnion was anoxic below a depth of 4 m from 

late July through September 1998. Under these conditions, ammonia nitrogen, orthophosphate phosphorus, 

and dissolved manganese were released from the sediments and accumulated in the hypolimnion. The main 

basin of Stony Brook Reservoir was artificially mixed 

during the study period. However, a small deep hole 

resulted that was thermally and chemically isolated 

from the rest of the water column in September 1997, 

and from early March through November 1998. Water 

samples collected near the bottom at this station in 

September 1997 contained the highest concentrations 

of total nitrogen (3.7 mg/L) and ammonia nitrogen 

(2.3 mg/L) and the second highest concentration of dissolved manganese (6.27 mg/L) measured in any of the 

reservoirs during the study period. Fresh Pond also was 
44 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection

artificially mixed during most of the study period. 

Despite the mixing, however, there were losses of dissolved oxygen in late summer 1998 in the deepest parts 

of the pond, and these were accompanied by releases of 

dissolved manganese. A sample of this hypolimnetic 

water collected in late August 1998 contained the highest concentration of dissolved manganese (12.7 mg/L) 

measured duruing the study. Nutrients released in the 

hypolimnia of the reservoirs during summer stratification have the potential to stimulate algal growth when 

the water column mixes at turnover, and manganese 

released under hypoxic conditions in the deep holes of 

Fresh Pond is likely to remain in solution.

Median concentrations of dissolved sodium measured during the study period exceeded the USEPA’s 

DWEL guideline of 20 mg/L in all three reservoirs and 

generally were higher in Hobbs Brook Reservoir than 

Stony Brook Reservoir or Fresh Pond. Median concentrations of dissolved chloride never exceeded the 

USEPA’s drinking-water SMCL of 250 mg/L, but were 

higher in Hobbs Brook Reservoir than in Stony Brook 

Reservoir or Fresh Pond. There was a distinct seasonal 

pattern of sodium and chloride concentrations, with 

highest concentrations of both ions appearing in the 

winter and spring, and lowest concentrations appearing 

in summer and autumn. These results are consistent 

with the conclusion that winter and spring applications 

of road salt are the most significant sources of dissolved sodium and chloride in the drainage basin, that 

Hobbs Brook Reservoir is more severely affected than 

Stony Brook Reservoir, and that dilution by water from 

the Stony Brook subbasin ameliorates the problem to 

some extent.

Dissolved organic carbon concentrations 

increased somewhat during the summer months and 

decreased in the autumn and winter. However, even 

during the summer period of summer stratification, the 

DOC was uniformly distributed with depth. In contrast, 

THMFP decreased in the anoxic hypolimnion during 

summer stratification. The range of THMFP concentrations measured in the reservoirs (0.055 to 0.366 mg/L) 

was similar to that reported in the literature for surfacewater supplies in North Carolina and Arkansas (Singer 

and others, 1981; Pomes and others, 1999). 

Concentrations of trace metals in Stony Brook 

Reservoir bed sediments were higher than those in 

Hobbs Brook Reservoir and were either higher than or 

similar to median concentrations of the same metals 

measured at 135 bed-sediment sampling stations in the 

lower Charles River Basin. The concentration of phosphorus in Stony Brook sediments was three times that 

of the other reservoirs in the Cambridge drinking-water 

supply system and nearly twice that of the lower 

Charles River Basin. When sediment trace metal concentrations were normalized to the concentration of 

aluminum to remove the effects of differential deposition rates, the normalized concentrations generally 

were higher in the lower Charles River Basin than in 

either Hobbs Brook or Stony Brook Reservoirs. Normalized concentrations of arsenic, iron, and manganese, however, were higher in the three Cambridge 

Reservoirs than in the lower Charles River Basin, indicating possible localized sources for these metals. Normalized total phosphorus concentrations were similar 

in all samples.

Trophic state indices calculated for the reservoirs 

ranged from the high 40’s in Hobbs Brook Reservoir to 

a minimum of 27 in Fresh Pond. In general, the reservoirs in the upper part of the water-supply system were 

more eutrophic than those in the lower part of the system. The upper and middle basins of Hobbs Brook 

Reservoir had the highest TSI values, indicating that 

these basins are moderately to highly productive and 

likely to produce algal blooms; the lower basin of 

Hobbs Brook Reservoir and Stony Brook Reservoir 

were similar and intermediate in TSI value, and Fresh 

Pond had the lowest values, ranging from moderately 

productive to unproductive, and unlikely to produce 

algal blooms. This pattern is due mainly to sedimentation of organic and inorganic particles in the three 

basins of Hobbs Brook Reservoir and in Stony Brook 

Reservoir. Nitrogen-to-phosphorus ratios in the reservoirs ranged from 55 in Stony Brook Reservoir to 120 

in Hobbs Brook Reservoir, indicating that these water 

bodies may be phosphorus limited and that small 

increases in phosphorus loading to the reservoirs could 

stimulate algae to produce noxious blooms.

Concentrations of fecal coliform bacteria in 

water samples collected at 11 tributary-monitoring 

stations in the Cambridge drinking-water source 

area frequently exceeded the State standard of 

20 CFU/100 mL for Class A streams. Concentrations 

were highest (up to 1,700 CFU/100 mL) in samples 

collected during storms. Estimated subbasin yields for 
Summary and Conclusions 45

fecal coliform bacteria were largest in the subbasin 

defined by station 01104455 and for the two stations 

representing integrator stations for the Stony Brook 

(station 01104390) and Hobbs Brook (station 

01104440) subbasins. 

Concentrations of dissolved sodium and chloride 

were highest at stations 01104410, 01104415, 

01104420, and 01104455, all of which are heavily 

affected by State-maintained roads. It is likely that 

State-maintained roads had a greater effect than locally 

maintained roads on yields of sodium and chloride in 

the source area. The estimated annual mean loads of 

sodium and chloride for the Hobbs Brook integrator 

station (station 01104440) were about three times 

greater than those for the Stony Brook integrator station (station 01104390), although the drainage areas 

differ only by about 21 percent. The areal coverage of 

locally maintained roads is 3.2 percent for the Hobbs 

Brook subbasin and 2.2 percent for the Stony Brook 

subbasin. The percent coverage of State-maintained 

roads, however, is much higher in the Hobbs Brook 

subbasin (4.3 percent) than it is in the Stony Brook 

subbasin (0.4 percent). The Hobbs Brook drainage also 

contains more commercial and industrial land than the 

Stony Brook drainage, but these land uses were not 

correlated with sodium and chloride yields.

The Hobbs Brook subbasin and the Stony Brook 

subbasin produced similar estimated annual mean 

loads for nitrate nitrogen and total nitrogen. Subbasin 

yields of the two nitrogen species were similar, and 

were comparable in magnitude to those determined for 

the combined drainage basin upstream from station 

01104460. Little station-to-station variability resulted 

in median orthophosphate phosphorus concentrations. 

The estimated total phosphorus load at the Hobbs 

Brook integrator station was nearly twice that at the 

Stony Brook integrator station. In general, however, 

phosphorus yields were small throughout the drainage 

basin.

The Hobbs Brook and Stony Brook subbasins 

produced similar estimated annual mean loads for iron. 

The estimated annual mean manganese load from the 

Hobbs Brook subbasin, however, was greater than that 

from the Stony Brook subbasin. 

The median concentration of DOC was highest 

in samples from station 01104405, which drains a large 

red maple swamp, and lowest in samples from station 

01104455, which drains a small subbasin with large 

amounts of paved area and almost no wetland area. 

Median concentrations of THMFP ranged from 

0.085 mg/L at station 01104455 to 0.273 mg/L at the 

mouth of Hobbs Brook (station 01104440). Maximum 

concentrations were more uniform, ranging from 

0.204 mg/L at station 01104455 to 0.370 mg/L at station 01104390. The USEPA drinking-water standard is 

0.080 mg/L. Estimated annual mean yields of DOC and 

THMFP were remarkably uniform, suggesting that no 

subbasin was exporting a disproportionate amount of 

either constituent on an annual basis. One exception 

was station 01104405, where the estimated DOC yield 

was two to three times greater than those of the other 

stations.

A hydraulic detention time of 0.42 years was 

determined for Hobbs Brook Reservoir during water 

year 1998. Previous detention-time estimates for 

Hobbs Brook Reservoir were on the order of 2 years. 

The much shorter detention time determined in this 

study suggests that, for water year 1998 at least, the 

flushing rate was much higher and therefore, the capability of the reservoir to assimilate nutrient loads was 

greater than previously determined.

Sodium accumulated in Hobbs Brook Reservoir 

during water year 1998. Larger amounts of sodium 

were contributed by the subbasins represented by stations 01104415 and 01104420 than would be expected 

based on their drainage areas and contributions to the 

water balance. In contrast, precipitation, which 

accounted for 31 percent of water entering the reservoir, contributed only about 0.05 percent of the sodium.

The reservoir was a trap for total nitrogen during 

water year 1998. An estimated 15,000 kg entered the 

reservoir, with the contributions from the tributary subbasins roughly proportional to their drainage areas. The 

amount entering in precipitation was more than twice 

that from the subbasin defined by station 01104410. 

Waterfowl contributed an estimated 9 kg, which was 

less than 0.1 percent of the total input. Output of total 

nitrogen in surface water was 4,600 kg, less than onethird of the total input, but very little of the remainder 

(630 kg) appeared in storage in the water column. It is 

possible that a large part of the residual total nitrogen 

was trapped as particulate nitrogen in the reservoir 

sediments.
46 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection

The reservoir also apparently was a trap for 

total phosphorus during water year 1998. Only about 

half of the total input from the drainage basin (250 kg) 

was accounted for in the surface-water output (130 kg). 

Loads from tributary subbasins were proportional 

to drainage area. However, the total-phosphorus 

contributions from precipitation (7 kg) and waterfowl 

(4 kg) were of similar magnitude to those from stations 

01104410 (5 kg) and 01104415 (9 kg), indicating 

that precipitation and waterfowl may be at least as 

significant as some of the tributary sources of total 

phosphorus.

Based on the results obtained from these investigations, 10 of the 11 tributary-monitoring stations were 

selected for inclusion as primary tributary-monitoring 

stations in a source-area water-quality monitoring network developed jointly by the USGS and the CWD. 

These stations represent streams that contribute water 

either directly to the reservoirs and major tributaries, or 

integrate large areas of the drainage basin. Criteria for 

inclusion in the network were the magnitudes of actual 

or potential contaminant loads and the proximity of the 

monitoring stations to the reservoirs. In addition, eight 

monitoring stations representative of water-quality and 

trophic conditions in Hobbs Brook Reservoir, Stony 

Brook Reservoir, and Fresh Pond were identified and 

incorporated into the network. Details of the monitoring network are included in an appendix to this report.

REFERENCES CITED

American Public Health Association, American Water Works 

Association, and Water Pollution Control Association, 

1995, Standard methods for the examination of water 

and wastewater (19th ed.): Washington, D.C., American 

Public Health Association, variously paginated.

Bingham, D.R., 1998, Watershed management in U.S. urban 

areas, in Reimold, R.J. (ed.), Watershed management. 

practices, policies, and coordination: New York, 

McGraw-Hill, p. 169–201

Carlson, R.E., 1977, A trophic state index for lakes: 

Limnology and Oceanography, v. 22, no. 2, p. 361–369.

Church, P.E., Armstrong, D.S., Granato, G.E., Stone, V.J., 

Smith, K.P., and Provencher, P.L., 1996, Effectiveness 

of highway-drainage systems in preventing 

contamination of ground water by road salt, Route 25, 

southeastern Massachusetts—description of study area, 

data collection programs, and methodology: U.S. 

Geological Survey Open-File Report 96-317, 72 p.

Cohn, T.A., Caulder, D.L., Gilroy, E.J., Zynjuk, L.D., and 

Summers, R.M, 1992, The validity of a simple 

statistical model for estimating fluvial constituent 

loads—An empirical study involving nutrient loads 

entering Chesapeake Bay: Water Resources Research, 

v. 28, no. 9, p. 2353–2363.

Cohn, T.A., DeLong, L.L., Gilroy, E.J., Hirsch, R.M., and 

Wells, D.K., 1989, Estimating constituent loads: Water 

Resources Research, v. 25, no. 5, p. 937–942.

Cole, J.J, Caraco, N.F., and Likens, G.E., 1990, Short-range 

atmospheric transport–a significant source of 

phosphorus to an oligotrophic lake: Limnology and 

Oceanography, v. 35, no. 6, p. 1230–1237.

Colman, J.A., and Breault, R.F., 2000, Sampling for mercury 

at subnanogram per litre concentrations for load 

estimation in rivers: Canadian Journal of Fisheries and 

Aquatic Sciences, v. 57, p. 1073–1079.

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

P.R., 1993, Restoration and management of lakes and 

reservoirs (2d ed.): New York, Lewis Publishers, 548 p.

Duan, Naihua, 1983, Smearing estimate—A nonparametric 

retransformation method: Journal of the American 

Statistical Association, v. 78, p. 605–610.

Edwards, T.K., and Glysson, G.D., 1999, Field methods for 

measurement of fluvial sediment: U.S. Geological 

Survey Techniques of Water-Resources Investigations, 

book 3, chap. C2, 89 p.

Farnsworth, R.K., Thompson, E.S., and Peck., E.L., 1982, 

Evaporation atlas for the contiguous 48 United States: 

Washington, D.C., National Weather Service, Office of 

Hydrology, 26 p.

Fishman, M.J., and Friedman, L.C., eds., 1989, Methods for 

determination of inorganic substances in water and 

fluvial sediments: U.S. Geological Survey Techniques 

of Water-Resources Investigations, book 5, chap. A1, 

545 p.

Fugro East, Inc., 1996, Limnological investigations of the 

Cambridge Reservoir system—Impacts of watershed 

inputs and alum sludge discharge: Cambridge, Mass., 

Cambridge Water Department, 77 p. plus Appendixes.

Geotechnical Engineers, Inc., 1985, Hobbs Brook Reservoir 

sodium chloride study: Geotechnical Engineers, Inc., 

variously paginated.
References Cited 47

Gilroy, E.J., Hirsch, R.M., and Cohn, T., 1990, Mean square 

error of regression-based constituent transport 

estimates: Water Resources Research, v. 26, no. 9, 

p. 2069–2077.

Godfrey, P.J., and Kerr, P.A., 2000, Preliminary evaluation of 

a promising forced air-drying technique for preserving 

chlorophyll on glass-fiber filters: Lake and Reservoir 

Management, v. 16, no. 3, p. 222–234.

Granato, G.E., 1995, Deicing chemicals as source of 

constituents of highway runoff: Transportation 

Research Record 1533, p. 50–58. 

Granato, G.E. Church, P.E., and Stone, V.J., 1995, 

Mobilization of major and trace constituents of 

highway runoff in groundwater potentially caused by 

deicing chemical migration: Transportation Research 

Record 1483, p. 92–104.

Harper, C.R., Goetz, W.J., and Willis, C.E., 1992, Groundwater protection in mixed land-use aquifers: 

Environmental Management, v. 16, no. 6, p. 777–783.

Hem, J.D., 1992, Study and interpretation of the chemical 

characteristics of natural water (3d ed.): U.S. 

Geological Survey Water-Supply Paper 2254, 263 p.

Hirsch, R.M., 1982, A comparison of four streamflow record 

extension techniques: Water Resources Research, v. 18, 

no. 4, p. 1081–1088.

Horowitz, A.J., 1991, A primer on sediment-trace element 

chemistry (2d ed.), Chelsea, Mich., Lewis Publishers, 

136 p.

Intergovernmental Task Force on Monitoring Water Quality, 

1995, The strategy for improving water-quality 

monitoring in the United States, Final report of the 

Intergovernmental Task Force on Monitoring Water 

Quality: Reston, Va., U.S. Geological Survey, Office of 

Water Data Coordination, 25 p. plus Appendixes.

Kachigan, S.K., 1986, Statistical analysis. An 

interdisciplinary introduction to univariate and 

multivariate methods: New York, Radius Press, 599 p. 

Kennedy, E.J., 1984, Stage measurements at discharge 

stations. U.S. Geological Survey Techniques of WaterResources Investigations, book 3, chap. A10, 59 p.

Kimmel, B.L., Lind, O.T., and Paulson, L.J., 1990, Reservoir 

primary production, in Thornton, K.W., Kimmel, B.L., 

and Payne, F.E. (eds.), Reservoir limnology—

Ecological perspectives: New York, John Wiley and 

Sons, p. 133–193.

Krasner, S.W., Sclimenti, M.J., and Means, E.G., 1994, 

Quality degradation—Implications for DBP formation: 

Journal of the American Water Works Association, June 

1994, p. 34–47.

LaBaugh, J.W., 1985, Uncertainty in phosphorus retention, 

Williams Fork Reservoir, Colorado: Water Resources 

Research, v. 21, no. 11, p. 1684–1692.

LaBaugh, J.W., and Winter, T.C., 1984, The impact of 

uncertainties in hydrologic measurements on 

phosphorus budgets and empirical models for two 

Colorado Reservoirs: Limnology and Oceanography, 

v. 29, no. 2, p. 322–339.

Lee, G.F., Jones, R.A., and Rast, W., 1981, Index to evaluate 

lake restoration: Journal of the Environmental 

Engineering Division ASCE, v. 107, p. 1334–1336.

Manny, B.A., Johnson, W.C., and Wetzel, R.G., 1994, 

Nutrient additions by waterfowl to lakes and 

reservoirs—their effects on productivity and water 

quality: Hydrobiologia, v. 279/280, p. 121–132.

Marion, Loic, Clergeau, Phillipe, Brient, Luc, and Bertu, 

Georges, 1994, The importance of avian-contributed 

nitrogen (N) and phosphorus (P) to Lake Grand-Lieu, 

France: Hydrobiologia, v. 279/280, p. 133–147.

Massachusetts Highway Department, 1997, Draft 

environmental impact report—Volumes I, II, and III, 

Hobbs Brook and Stony Brook watersheds highway 

drainage improvement project EOEA number 8263, 

variously paginated.

Mattson, M.D., and Godfrey, P.J., 1994, Identification of 

road salt contamination using multiple regression and 

GIS: Environmental Management: v. 18, no. 5, p. 767–

773.

Moore, R.A., and Barbaro, H.L., 1998, Role of highways in 

watershed planning, in Reimold, R.J. (ed.), Watershed 

management. practice, policies, and coordination: New 

York, McGraw-Hill, 391 p.

National Atmospheric Deposition Program (NRSP3)/National Trends Network, 1999, NADP Program 

Office, Illinois State Water Survey: Champaign, Ill., 

accessed December 10, 1999, at URL

http://nadp.sws.uiuc.edu/nadpdata/siteinfo.asp?id=MA

13&net=NADP.

National Oceanic and Atmospheric Administration, 1998, 

Climatological data annual summary—New England 

1997: Ashville, N.C., National Climatic Data Center, 

v. 109, no. 13.

National Oceanic and Atmospheric Administration, 1999, 

Climatological data annual summary—New England 

1998: Ashville, N.C., National Climatic Data Center, 

v. 110, no. 13.
48 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection

New England Interstate Water Pollution Control 

Commission, 1996, Source protection—A guidance 

manual for small surface water supplies in New 

England: New England Interstate Water Pollution 

Control Commission, 77 p. plus Appendixes.

Patterson, Glenn, 1997, The U.S. Geological Survey 

Drinking Water Initiative: U.S. Geological Survey Fact 

Sheet 047-97, 2 p.

Pomes, M.L., Green, W.R., Thurman, E.M., Orem, W.H., 

and Lerch, H.E., 1999, DBP formation potential of 

aquatic humic substances: Journal of the American 

Water Works Association, v. 91, no. 3, p.103–115.

Pritt, J.W., and Raese, J.W., 1992, Quality assurance/quality 

control manual. National Water Quality Laboratory: 

U.S. Geological Survey Open-File Report 92-495, 33 p. 

Rantz, S.E., and others, 1982, Measurement and 

computation of streamflow—Volume 1. Measurement 

of stage and discharge—Volume 2. Computation of 

discharge: U.S. Geological Survey Water-Supply Paper 

2175, 631 p.

Reckhow, K.H., 1979, Quantitative techniques for the 

assessment of lake quality: Washington, D.C., U.S. 

Environmental Protection Agency, Office of Water 

Planning and Standards, Report No. EPA-440/5-79-

015, 146 p.

Reckhow, D.A., Singer, P.C., and Malcolm, R.L., 1990, 

Chlorination of humic materials—Byproduct formation 

and chemical interpretations: Environmental Science 

and Technology, v. 24, no. 11, p. 1655–1664.

Reinelt, L.E., Horner, R.R., and Mar, B.W., 1988, Nonpoint 

source pollution monitoring program design: Journal of 

Water Resources Planning and Management, v. 114, 

no. 3, p. 335–352.

Singer, P.C., Barry, J.J., Palen, G.M., and Scrivner, A.E., 

1981, Trihalomethane formation in North Carolina 

drinking waters: Journal of the American Water Works 

Association, v. 73, no. 8, p. 392–401.

Socolow, R.S., Comeau, L.Y., Zanca, J.L., and Ramsbey, 

L.R., 1999, Water-resources data for Massachusetts and 

Rhode Island, water year 1998: U.S. Geological Survey 

Water-Data Report MA-RI-98-1, 438 p.

Stephens, T.L., and Shiller, A.M., 1999, Temperaturedependent microbial control of dissolved manganese in 

the lower Mississippi River: American Society of 

Limnology and Oceanography, 1999 Aquatic Sciences 

Meeting, Abstracts Book, p. 170.

Stumm, W., and Morgan J.J., 1970, Aquatic chemistry—An 

introduction emphasizing chemical equilibria in natural 

waters: New York, Wiley-Interscience, 583 p.

Thomann, R.V., and Mueller, J.A., 1987, Principles of 

surface water quality modeling and control: New York, 

Harper Collins, 644 p.

Todd, D.K., 1980, Groundwater hydrology (2d ed.): New 

York, John Wiley and Sons, Inc., 535 p.

Tipping, E., 1984, Temperature dependence of MN(II) 

oxidation in lakewaters—A test of biological 

involvement: Geochimica et Cosmochimica Acta, v. 48, 

p. 1353–1356.

U.S. Environmental Protection Agency, 1998, Clean water 

action plan—Restoring and protecting America’s 

waters: U.S. Environmental Protection Agency, EPA840-R-98-001, 89 p.

U.S. Geological Survey, 1992, Recommendation for use of 

retransformation methods in regression models used to 

estimate sediment loads (“the bias correction 

problem”): U.S. Geological Survey, Office of Surface 

Water Technical Memorandum No. 93.08, 13 p.

U.S. Geological Survey, 1999, The quality of our Nation’s 

waters—Nutrients and pesticides: U.S. Geological 

Survey Circular 1225, 82 p.

Wetzel, R.G., 1993, Limnology (2d ed.): Philadelphia, W.B. 

Saunders Co., 743 p.

Wetzel, R.G., and Likens, G.E., 1991, Limnological analysis 

(2d ed.): Philadelphia, W.B. Saunders Co., 391 p.

Wilde, F.D., Radtke, D.B., Gibs, Jacob, and Iwatsubo, R.T., 

1999, Collection of water samples, National field 

manual for the collection of water-quality data: U.S. 

Geological Survey Techniques of Water-Resources 

Investigations, book 9, chap. A4, 103 p.

Winter, T.C., 1981, Uncertainties in estimating the water 

balance of lakes: Water Resources Bulletin, v. 17, 

p. 82–115.

GLOSSARY

Algal bloom—The rapid proliferation of passively floating, 

simple plant life in and on a body of water.

Anoxic—The absence of oxygen; anaerobic.

Atmospheric deposition—The transfer of substances from 

the atmosphere to the surface of the Earth or to objects 

on its surface. Transfer can be either by wet-deposition 

processes (rain, snow, dew, fog, frost, hail) or by dry 

deposition (gases, aerosols, fine to coarse particles) in 

the absence of water.

Bed sediment —The material that temporarily is stationary 

in the bottom of a stream or other water body.

Colony-forming units (CFU)—Unit of bacterial population 

size referring to the colonies that appear on a nutrient-
Glossary 49

agar plate following inoculation of the plate with a 

sample of water. Each colony may arise from a single 

bacterial cell or from a small cluster of cells; hence, 

the colony is reported as a CFU and the bacterial 

population density is reported as the number of CFUs 

per unit volume (usually 100 milliliters) of water.

Contamination—Change of water quality by the addition of 

constituents as a result of human activity or natural 

processes.

Constituent—A compound such as a chemical species or 

biological population whose magnitude in water, 

sediment, biota, or other matrix is determined by an 

analytical method.

Correlation coefficient—A statistic that can be used to 

measure the strength of a relation between two 

variables.

Discharge (hydraulics)—Rate of flow, especially fluid flow; 

a volume of liquid passing a point per unit of time, 

commonly expressed in cubic feet per second, million 

gallons per day, or liters per second.

Dissolved oxygen (DO)—Oxygen dissolved in water; one 

of the most important indicators of the condition of a 

water body. Dissolved oxygen is necessary for the life 

of fish and most other aquatic organisms.

Drainage basin—Land area drained by a river or stream; 

watershed.

Epilimnion—Warm, oxygen-rich, upper layer of water in a 

lake or other body of water, usually seasonal. See also 

Metalimnion, Hypolimnion

Eutrophic—Term applied to a body of water with a high 

degree of nutrient enrichment and high productivity.

Eutrophication—Process by which water becomes enriched 

with plant nutrients, most commonly phosphorus and 

nitrogen.

Fecal coliform bacteria—Group of several types of bacteria 

that are found in the alimentary tract of warm-blooded 

animals. The bacteria commonly are used as an 

indicator of animal and fecal contamination of water.

Ground water—In the broadest sense, all subsurface water, 

as distinct from surface water; as more commonly used, 

that part of the subsurface water in the saturated zone. 

See also Surface water.

Hypolimnion—Cold, oxygen-poor, deep layer of water in a 

lake or other water body. See also Epilimnion, 

Metalimnion

Hypoxic—The near absence of oxygen.

Kettle-hole lake—Glacially-formed lake with no surfacewater inflows or outflows.

Limnology—Scientific discipline dealing with the physics, 

chemistry, and biology of inland waters such as lakes, 

ponds, reservoirs, streams, and wetlands.

Load—Material that is moved or carried by streams, 

reported as the weight of the material transported 

during a specific time period, such as kilograms per day 

or tons per year.

Main stem—The main trunk of a river or stream.

Maximum contaminant level (MCL)—Maximum 

permissible level of a contaminant in water that is 

delivered to any user of a public water system, 

established by a regulatory agency such as the U.S. 

Environmental Protection Agency. See also Secondary 

maximum contaminant level.

Mean—The arithmetic average obtained by dividing the 

sum of a set of quantities by the number of quantities in 

the set.

Median—The middle or central value in a distribution of 

data ranked in order of magnitude. The median also is 

known as the 50th percentile.

Mesotrophic—Term applied to a body of water with 

intermediate nutrient content and intermediate 

productivity.

Metalimnion—Transition zone between the warm upper 

layer and the cold deep layer of a lake or other water 

body, characterized by rapidly decreasing temperature 

with increasing depth. See also Epilimnion, 

Hypolimnion.

Minimum reporting limit (MRL)—The lowest measured 

concentration of a constituent that can be reported 

reliably using a given analytical method.

Monitoring station—A site on a stream, canal, lake, or 

reservoir used to observe systematically the chemical 

quality and discharge or stage of water.

Nutrient—An element or compound essential for animal 

and plant growth. Common nutrients in fertilizer 

include nitrogen, phosphorus, and potassium.

Oligotrophic—Term applied to a body of water low in 

nutrients and in productivity.

pH—The logarithm of the reciprocal of the hydrogen ion 

concentration of a solution; a measure of the acidity 

(pH less than 7) or alkalinity (pH greater than 7) of a 

solution; a pH of 7 is neutral.

Phytoplankton algae—Free-floating, mostly microscopic 

aquatic plants.

Phytoplankton chlorophyll a—Primary light-trapping 

pigment in most phytoplankton algae. Concentration 

can be used as an indirect indicator of the abundance of 

phytoplankton algae in a lake or other water body.
50 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection

Runoff—That part of precipitation that appears in surface 

streams. It is equivalent to streamflow unaffected by 

artificial diversions, storage, or other human works in or 

on the stream channel.

Secondary maximum contaminant level (SMCL)—

Maximum recommended level of a contaminant in 

water that is delivered to any user of a public water 

system. These contaminants affect the esthetic quality 

of the water such as odor or appearance; therefore, the 

levels are intended as guidelines. See also Maximum 

contaminant level.

Specific conductance—A measure of the ability of a sample 

of water to conduct electricity.

Subbasin—Drainage basin or watershed defined by a 

specific monitoring station and representing the land 

area that contributes water to that station.

Surface water—An open body of water, such as a stream or 

lake. See also Ground water.

Swamp—A forested wetland that has standing water during 

most or all of the growing season.

Thermal stratification—Seasonal division of a lake or 

other water body into a warm upper layer and a cold 

deep layer that is no longer in contact with the 

atmosphere. In some lakes, thermal stratification can 

result in a loss of oxygen in the deep layer and 

subsequent chemical stratification.

Trihalomethane formation potential (THMFP)—

Tendency of naturally occurring organic compounds in 

a water supply to form toxic trihalomethanes during 

water treatment.

Trophic state—The extent to which a body of water is 

enriched with plant nutrients. See also Eutrophic, 

Mesotrophic, Oligotrophic.

Trophic state index (TSI)—A numerical index indicating 

the degree of nutrient enrichment of a body of water.

Turbidity—The opaqueness or reduced clarity of a fluid due 

to the presence of suspended matter.

Water year—The continuous 12-month period, October 1 

through September 30, in U.S. Geological Survey 

reports dealing with the surface-water supply. The 

water year is designated by the calendar year in which it 

ends and which includes 9 of the 12 months. Thus, the 

year ending September 30, 1998, is referred to as the 

“1998” water year.

Wetlands—Lands that are inundated or saturated by surface 

or ground water at a frequency and duration sufficient to 

support, and that under normal circumstances do 

support, a prevalence of vegetation typically adapted for 

life in saturated soil conditions.

Yield—The weight of material transported during any given 

time divided by unit drainage area, such as kilograms 

per day per square kilometer or tons per year per square 

mile.
FIGURES 3–7
Figure 3 53

0

0

5 10 15 20 25 30

TEMPERATURE,

 IN DEGREES CELSIUS

0 2 4 6 8 10 12 14 0 20 40 60 80 100 0 0.01 0.02 0.03 0.04 0 0.5 1.0 1.5

CONCENTRATION, IN MILLIGRAMS PER LITER

200 400 600 800

DEPTH, IN METERS DISSOLVED OXYGEN CONCENTRATION, IN MILLIGRAMS PER LITER

CONCENTRATION, IN MILLIGRAMS PER LITER

567890 0 0.2 0.4 0.6 0.8 1 0 0.1 0.2 0.3 0.4 0.8 1.6 2.4

Sodium Total phosphorus

Nitrate nitrogen

Total nitrogen

Ammonia nitrogen Manganese

SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER

CENTIMETER, AT 25o C

pH, IN STANDARD

UNITS

A. Hobbs Brook Reservoir, November 25, 1997

1234560123456

0

DEPTH, IN METERS

123456

0123456

BELOW MINIMUM REPORTING LIMIT

EXPLANATION

Figure 3. Depth profiles of temperature and concentrations of dissolved oxygen, nutrients, and other constituents during the 1997–98 study period at the 

deep hole station in Hobbs Brook Reservoir, eastern Massachusetts.
54 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection

0 0 0 5 10 15 20 25 30 2 4 6 8 10 12 14 0 20 40 60 80 100 0 0.01 0.02 0.03 0.04 0.5 1.0 1.5

400 600 800

DEPTH, IN METERS

DEPTH, IN METERS

5 6 7 8 9 0 0.2 0.4 0.6 0.8 1 0 0.1 0.2 0.3 0.4 0 0.8 1.6 2.4

Sodium Total phosphorus

Nitrate nitrogen

Total nitrogen

Ammonia nitrogen Manganese

0123456

200

01234560123456

0123456

TEMPERATURE,

 IN DEGREES CELSIUS

CONCENTRATION, IN MILLIGRAMS PER LITER DISSOLVED OXYGEN

CONCENTRATION,

 IN MILLIGRAMS PER LITER

CONCENTRATION, IN MILLIGRAMS PER LITER SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER

CENTIMETER, AT 25o CELSIUS

pH, IN STANDARD

UNITS

B. Hobbs Brook Reservoir, January 28, 1998

Figure 3. Depth profiles of temperature and concentrations of dissolved oxygen, nutrients, and other constituents during the 1997

–98 study period at the 

deep hole station in Hobbs Brook Reservoir, eastern Massachusetts

—Continued.
Figure 3 55

0 5 10 15 20 25 30 0 2 4 6 8 10 12 14 0 20 40 60 80 100 0 0.01 0.02 0.03 0.04 0.5 1.0 1.5

200 400 600 800 5 6 7 8 9 0 0.2 0.4 0.6 0.8 1 0 0.1 0.2 0.3 0.4 0 0.1 0.2 0.3 0.4

0

0 0.8 1.6 2.4

Sodium Total phosphorus

Nitrate nitrogen

Total nitrogen

Ammonia nitrogen Manganese

0123456

0123456

0123456

0123456

DEPTH, IN METERS

DEPTH, IN METERS 0 4 8 12 16 DOC TEMPERATURE, IN DEGREES CELSIUS CONCENTRATION, IN MILLIGRAMS PER LITER

CONCENTRATION, IN MILLIGRAMS PER LITER

DISSOLVED OXYGEN

CONCENTRTION,

 IN MILLIGRAMS PER LITER

SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER

CENTIMETER, AT 25o CELSIUS

pH, IN STANDARD

UNITS

C. Hobbs Brook Reservoir, March 5, 1998

THMFP

DOC—DISSOLVED ORGANIC CARBON

—BELOW MINIMUM REPORTING LIMIT

EXPLANATION

THMFP—TRIHALOMETHANE FORMATION POTENTIAL

Figure 3. Depth profiles of temperature and concentrations of dissolved oxygen, nutrients, and other constituents during the 1997–98 study period at the deep hole 

station in Hobbs Brook Reservoir, eastern Massachusetts—Continued.
56 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection TEMPERATURE, IN DEGREES CELSIUS CONCENTRATION, IN MILLIGRAMS PER LITER DISSOLVED OXYGEN CONCENTRATION, IN MILLIGRAMS PER LITER CONCENTRATION, IN MILLIGRAMS PER LITER SPECIFIC CONDUCTANCE, IN MICROSIEMENS PER CENTIMETER AT 25o CELSIUS pH, IN STANDARD UNITS D. Hobbs Brook Reservoir, June 4, 1998 0 5 10 15 20 25 30 0 2 4 6 8 10 12 14 0 20 40 60 80 100 0 0.01 0.02 0.03 0.04 0 0.5 1.0 1.5 200 400 600 800

DEPTH, IN METERS

5 6 7 8 9 0 0.2 0.4 0.6 0.8 1 0 0.1 0.2 0.3 0.4 0 0.1 0.2 0.3 0.4 0 0.8 1.6 2.4

Sodium Total phosphorus Total nitrogen

Ammonia nitrogen Manganese

0123456

DEPTH, IN METERS

0123456

0123456

0123456

0123456

Nitrate nitrogen

0 4 8 12 16

DOC

THMFP

DOC

—DISSOLVED ORGANIC CARBON

—BELOW MINIMUM REPORTING LIMIT

EXPLANATION

THMFP

—TRIHALOMETHANE FORMATION POTENTIAL

Figure 3. Depth profiles of temperature and concentrations of dissolved oxygen, nutrients, and other constituents during the 1997

–98 study period at the deep hole 

station in Hobbs Brook Reservoir, eastern Massachusetts

—Continued.
Figure 3 57

0.05

TEMPERATURE,

 IN DEGREES CELSIUS CONCENTRATION, IN MILLIGRAMS PER LITER DISSOLVED OXYGEN CONCENTRATION,

 IN MILLIGRAMS PER LITER

CONCENTRATION, IN MILLIGRAMS PER LITER SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER

CENTIMETER AT 25o CELSIUS

pH, IN STANDARD

UNITS

E. Hobbs Brook Reservoir, July 23, 1998

0123456

01234560123456

0 5 10 15 20 25 30 0 2 4 6 8 10 12 14 0 20 40 60 80 100 0 0.01 0.02 0.03 0.04 0 0.5 1.0 1.5

0123456

200 400 600 800

DEPTH, IN METERS

DEPTH, IN METERS

5 6 7 8 9 0 0.2 0.4 0.6 0.8 1 0 0.1 0.2 0.3 0.4 0 0.8 1.6 2.4

Sodium Total phosphorus

Nitrate nitrogen

Total nitrogen

Ammonia nitrogen Manganese

*

BELOW MINIMUM REPORTING LIMIT

EXPLANATION

Figure 3. Depth profiles of temperature and concentrations of dissolved oxygen, nutrients, and other constituents during the 1997–98 study period at the deep hole station in Hobbs Brook Reservoir, eastern Massachusetts—Continued.
58 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection 0 5 10 15 20 25 30 0 2 4 6 8 10 12 14 0 20 40 60 80 100 0 0.01 0.02 0.03 0.04 0.5 1.0 1.5 200 400 600 800 5 6 7 8 9 0 0.2 0.4 0.6 0.8 1 0 0.1 0.2 0.3 0.4 0 0.1 0.2 0.3 0.4

0

0 0.8 1.6 2.4

Sodium Total phosphorus

Nitrate nitrogen

Total nitrogen

Ammonia nitrogen Manganese

0123456

0123456

0123456

0123456

DEPTH, IN METERS

DEPTH, IN METERS 0 4 8 12 16 DOC TEMPERATURE, IN DEGREES CELSIUS CONCENTRATION, IN MILLIGRAMS PER LITER

CONCENTRATION, IN MILLIGRAMS PER LITER

DISSOLVED OXYGEN

CONCENTRTION,

 IN MILLIGRAMS PER LITER

SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER

CENTIMETER, AT 25o CELSIUS

pH, IN STANDARD

UNITS

C. Hobbs Brook Reservoir, March 5, 1998

THMFP

DOC

—DISSOLVED ORGANIC CARBON

—BELOW MINIMUM REPORTING LIMIT

EXPLANATION

THMFP

—TRIHALOMETHANE FORMATION POTENTIAL

Figure 3. Depth profiles of temperature and concentrations of dissolved oxygen, nutrients, and other constituents during the 1997

–98 study period at the deep hole 

station in Hobbs Brook Reservoir, eastern Massachusetts

—Continued.
Figure 3 59

0

0 5 10 15 20 25 30 0 2 4 6 8 10 12 14 0 20 40 60 80 100 0 0.01 0.02 0.03 0.04 0 0.5 1.0 1.5

123456

0123456

0123456

0123456

200 400 600 800

DEPTH, IN METERS

DEPTH, IN METERS

5 6 7 8 9 0 0.2 0.4 0.6 0.8 1 0 0.1 0.2 0.3 0.4 0 0.1 0.2 0.3 0.4 0 0.8 1.6 2.4

Sodium Total phosphorus

Nitrate nitrogen

Total nitrogen

Ammonia nitrogen Manganese

TEMPERATURE,

 IN DEGREES CELSIUS

CONCENTRATION, IN MILLIGRAMS PER LITERCONCENTRATION, IN MILLIGRAMS PER LITER

DISSOLVED OXYGEN

CONCENTRATION,

 IN MILLIGRAMS PER LITER

SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER

CENTIMETER AT 25o CELSIUS

pH, IN STANDARD

UNITS

G. Hobbs Brook Reservoir, September 1, 1998

THMFP

0 4 8 12 16

 DOC 

DOC—DISSOLVED ORGANIC CARBON

—BELOW MINIMUM REPORTING LIMIT

EXPLANATION

THMFP—TRIHALOMETHANE FORMATION POTENTIAL

Figure 3. Depth profiles of temperature and concentrations of dissolved oxygen, nutrients, and other constituents during the 1997–98 study period at the deep hole 

station in Hobbs Brook Reservoir, eastern Massachusetts—Continued.
60 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection 0 5 10 15 20 25 30 0 2 4 6 8 10 12 14 0 20 40 60 80 100 0 0.01 0.02 0.03 0.04 0 0.5 1.0 1.5 0 4 8 12 16

0123456

01234560123456

0123456

DEPTH, IN METERS

DEPTH, IN METERS

200 400 600 800 5 6 7 8 9 0 0.2 0.4 0.6 0.8 1 0 0.1 0.2 0.3 0.4 0 0.8 1.6 2.4

Sodium Total phosphorus

Nitrate nitrogen

Total nitrogen

Ammonia nitrogen Manganese

 DOC

TEMPERATURE,

 IN DEGREES CELSIUS

CONCENTRATION, IN MILLIGRAMS PER LITER DISSOLVED OXYGEN

CONCENTRATION,

 IN MILLIGRAMS PER LITER

CONCENTRATION, IN MILLIGRAMS PER LITER SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER

CENTIMETER AT 25o CELSIUS

pH, IN STANDARD

UNITS

H. Hobbs Brook Reservoir, November 9, 1998

THMFP

0 0.1 0.2 0.3 0.4

DOC

—DISSOLVED ORGANIC CARBON

—BELOW MINIMUM REPORTING LIMIT

EXPLANATION

THMFP

—TRIHALOMETHANE FORMATION POTENTIAL

Figure 3. Depth profiles of temperature and concentrations of dissolved oxygen, nutrients, and other constituents during the 1997

–98 study period at the deep hole 

station in Hobbs Brook Reservoir, eastern Massachusetts

—Continued.
Figure 4 61

0 0.5 1.0 1.5

0 0.8 1.6 2.4

10

02468

12

024681012

0 5 10 15 20 25 30 0 2 4 6 8 10 12 14 0 20 40 60 80 100 0 0.01 0.02 0.03 0.04

10

12

02468

024681012

200 400 600

DEPTH, IN METERS

DEPTH, IN METERS

5 6 7 8 9 0 0.2 0.4 0.6 0.8 1 0 0.1 0.2 0.3 0.4

A. Stony Brook Reservoir, September 25. 1997

Nitrate nitrogen Ammonia nitrogen Manganese

Sodium Total phosphorus Total nitrogen

800

TEMPERATURE,

 IN DEGREES CELSIUS CONCENTRATION, IN MILLIGRAMS PER LITER DISSOLVED OXYGEN CONCENTRATION,

 IN MILLIGRAMS PER LITER

CONCENTRATION, IN MILLIGRAMS PER LITER SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER

CENTIMETER AT 25o CELSIUS

pH, IN STANDARD

UNITS

DOC—DISSOLVED ORGANIC CARBON

—BELOW MINIMUM REPORTING LIMIT

EXPLANATION

THMFP—TRIHALOMETHANE FORMATION POTENTIAL

2.3

3.7

6.27

Figure 4. Depth profiles of temperature and concentrations of dissolved oxygen, nutrients, and other constituents during the 1997–98 study period at the deep hole station in Stony Brook Reservoir, eastern Massachusetts.
62 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection 0 0.5 1.0 1.5 0 0.8 1.6 2.4 0 5 10 15 20 25 30 0 2 4 6 8 10 12 14 0 20 40 60 80 100 0 0.01 0.02 0.03 0.04 1012

02468

200 400 600

DEPTH, IN METERS

DEPTH, IN METERS

5 6 7 8 9 0 0.2 0.4 0.6 0.8 1 0 0.1 0.2 0.3 0.4 0 0.1 0.2 0.3 0.4

B. Stony Brook Reservoir, November 26, 1997

Nitrate nitrogen Ammonia nitrogen Manganese

Sodium Total phosphorus Total nitrogen

800

TEMPERATURE,

 IN DEGREES CELSIUS CONCENTRATION, IN MILLIGRAMS PER LITER DISSOLVED OXYGEN CONCENTRATION,

 IN MILLIGRAMS PER LITER

CONCENTRATION, IN MILLIGRAMS PER LITER SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER

CENTIMETER AT 25o CELSIUS

pH, IN STANDARD

UNITS

DOC

THMFP

0 4 8 12 16

10

02468

12

024681012

024681012

DOC

—DISSOLVED ORGANIC CARBON

—BELOW MINIMUM REPORTING LIMIT

EXPLANATION

THMFP

—TRIHALOMETHANE FORMATION POTENTIAL

Figure 4. Depth profiles of temperature and concentrations of dissolved oxygen, nutrients, and other constituents during the 1997

–98 study period at the deep hole 

station in Stony Brook Reservoir, eastern Massachusetts

—Continued.
Figure 4 63

0 0.5 1.0 1.5

0 0.8 1.6 2.4

0 5 10 15 20 25 30 0 2 4 6 8 10 12 14 0 20 40 60 80 100 0 0.01 0.02 0.03 0.04

10

12

02468

024681012

200 400 600

DEPTH, IN METERS

DEPTH, IN METERS

5 6 7 8 9 0 0.2 0.4 0.6 0.8 1 0 0.1 0.2 0.3 0.4 0 0.1 0.2 0.3 0.4

C. Stony Brook Reservoir, March 6, 1998

Nitrate nitrogen Ammonia nitrogen Manganese

Sodium

Total phosphorus

Total nitrogen

800

TEMPERATURE,

 IN DEGREES CELSIUS CONCENTRATION, IN MILLIGRAMS PER LITER DISSOLVED OXYGEN CONCENTRATION,

 IN MILLIGRAMS PER LITER

CONCENTRATION, IN MILLIGRAMS PER LITER SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER

CENTIMETER AT 25o CELSIUS

pH, IN STANDARD

UNITS

0 4 8 12 16

DOC

THMFP

10

02468

12

024681012

0.566

0.563

0.548

DOC—DISSOLVED ORGANIC CARBON

—BELOW MINIMUM REPORTING LIMIT

EXPLANATION

THMFP—TRIHALOMETHANE FORMATION POTENTIAL

Figure 4. Depth profiles of temperature and concentrations of dissolved oxygen, nutrients, and other constituents during the 1997–98 study period at the deep hole 

station in Stony Brook Reservoir, eastern Massachusetts—Continued.
64 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection 0.048 0 0.5 1.0 1.5 0 0.8 1.6 2.4 0 5 10 15 20 25 30 0 2 4 6 8 10 12 14 0 20 40 60 80 100 0 0.01 0.02 0.03 0.04 1012

02468

200 400 600

DEPTH, IN METERS

DEPTH, IN METERS

5 6 7 8 9 0 0.2 0.4 0.6 0.8 1 0 0.1 0.2 0.3 0.4

D. Stony Brook Reservoir, May 28, 1998

Nitrate nitrogen Ammonia nitrogen Manganese

Sodium Total phosphorus Total nitrogen

800

TEMPERATURE,

 IN DEGREES CELSIUS

CONCENTRATION, IN MILLIGRAMS PER LITER DISSOLVED OXYGEN CONCENTRATION,

 IN MILLIGRAMS PER LITER

CONCENTRATION, IN MILLIGRAMS PER LITER SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER

CENTIMETER AT 25o CELSIUS

pH, IN STANDARD

UNITS

024681012

10

02468

12

024681012

DOC

—DISSOLVED ORGANIC CARBON

—BELOW MINIMUM REPORTING LIMIT

EXPLANATION

THMFP

—TRIHALOMETHANE FORMATION POTENTIAL

Figure 4. Depth profiles of temperature and concentrations of dissolved oxygen, nutrients, and other constituents during the 1997

–98 study period at 

the deep hole station in Stony Brook Reservoir, eastern Massachusetts

—Continued.
Figure 4 65

0 0.5 1.0 1.5

0 0.8 1.6 2.4

0 5 10 15 20 25 30 0 2 4 6 8 10 12 14 0 20 40 60 80 100 0 0.01 0.02 0.03 0.04

10

12

02468

200 400 600

DEPTH, IN METERS

5 6 7 8 9 0 0.2 0.4 0.6 0.8 1 0 0.1 0.2 0.3 0.4

E. Stony Brook Reservoir, August 5, 1998

Nitrate nitrogen Ammonia nitrogen Manganese

Sodium Total phosphorus Total nitrogen

800

TEMPERATURE,

 IN DEGREES CELSIUS

CONCENTRATION, IN MILLIGRAMS PER LITER DISSOLVED OXYGEN

CONCENTRATION,

 IN MILLIGRAMS PER LITER

CONCENTRATION, IN MILLIGRAMS PER LITER SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER

CENTIMETER AT 25o CELSIUS

pH, IN STANDARD

UNITS

1.57

4.36

0.074

0 0.1 0.2 0.3 0.4

0 4 8 12 16

DOC

THMFP

024681012

DEPTH, IN METERS

024681012

10

02468

12

DOC—DISSOLVED ORGANIC CARBON

—BELOW MINIMUM REPORTING LIMIT

EXPLANATION

THMFP—TRIHALOMETHANE FORMATION POTENTIAL

Figure 4. Depth profiles of temperature and concentrations of dissolved oxygen, nutrients, and other constituents during the 1997–98 study period at the deep hole 

station in Stony Brook Reservoir, eastern Massachusetts—Continued.
66 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection 0 0.5 1.0 1.5 0 0.8 1.6 2.4 0 5 10 15 20 25 30 0 2 4 6 8 10 12 14 0 20 40 60 80 100 0 0.01 0.02 0.03 0.04 1012

02468

200 400 600

DEPTH, IN METERS

5 6 7 8 9 0 0.2 0.4 0.6 0.8 1 0 0.1 0.2 0.3 0.4 0 0.1 0.2 0.3 0.4

Nitrate nitrogen Ammonia nitrogen Manganese

Sodium Total phosphorus Total nitrogen

800

TEMPERATURE,

 IN DEGREES CELSIUS CONCENTRATION, IN MILLIGRAMS PER LITER DISSOLVED OXYGEN CONCENTRATION,

 IN MILLIGRAMS PER LITER

CONCENTRATION, IN MILLIGRAMS PER LITER SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER

CENTIMETER AT 25o CELSIUS

pH, IN STANDARD

UNITS

0 4 8 12 16

DOC

THMFP

2.52 0.068

3.105.30 1.56

F. Stony Brook Reservoir, September 10, 1998

024681012

DEPTH, IN METERS

024681012

10

02468

12

DOC

—DISSOLVED ORGANIC CARBON

—BELOW MINIMUM REPORTING LIMIT

EXPLANATION

THMFP

—TRIHALOMETHANE FORMATION POTENTIAL

Figure 4. Depth profiles of temperature and concentrations of dissolved oxygen, nutrients, and other constituents during the 1997

–98 study period at the deep hole 

station in Stony Brook Reservoir, eastern Massachusetts

—Continued.
Figure 4 67

0 0.5 1.0 1.5

0 0.8 1.6 2.4

0 5 10 15 20 25 30 0 2 4 6 8 10 12 14 0 20 40 60 80 100 0 0.01 0.02 0.03 0.04

10

12

02468

024681012

200 400 600

DEPTH, IN METERS

DEPTH, IN METERS

5 6 7 8 9 0 0.2 0.4 0.6 0.8 1 0 0.1 0.2 0.3 0.4 0 0.1 0.2 0.3 0.4

G. Stony Brook Reservoir, November 6, 1998

Nitrate nitrogen Ammonia nitrogen Manganese

Sodium Total phosphorus Total nitrogen

800

TEMPERATURE,

 IN DEGREES CELSIUS CONCENTRATION, IN MILLIGRAMS PER LITER DISSOLVED OXYGEN CONCENTRATION,

 IN MILLIGRAMS PER LITER

CONCENTRATION, IN MILLIGRAMS PER LITER SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER

CENTIMETER AT 25o CELSIUS

pH, IN STANDARD

UNITS

0 4 8 12 16

DOC

THMFP

10

02468

12

024681012

DOC—DISSOLVED ORGANIC CARBON

—BELOW MINIMUM REPORTING LIMIT

EXPLANATION

THMFP—TRIHALOMETHANE FORMATION POTENTIAL

Figure 4. Depth profiles of temperature and concentrations of dissolved oxygen, nutrients, and other constituents during the 1997–98 study period at the deep hole 

station in Stony Brook Reservoir, eastern Massachusetts—Continued.
68 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection 0.0024 0.0015 0 5 10 15 20 25 30 0 2 4 6 8 10 12 14 0 20 40 60 80 100 0 0.01 0.02 0.03 0.04 0 0.5 1.0 1.5 200 400 600 800

DEPTH, IN METERS

DEPTH, IN METERS

5 6 7 8 9 0 0.2 0.4 0.6 0.8 1 0 0.1 0.2 0.3 0.4 0 0.8 1.6 2.4

Sodium Total phosphorus

Nitrate nitrogen

Total nitrogen

Ammonia nitrogen Manganese

TEMPERATURE,

 IN DEGREES CELSIUS

CONCENTRATION, IN MILLIGRAMS PER LITER

CONCENTRATION, IN MILLIGRAMS PER LITER

DISSOLVED OXYGEN

CONCENTRATION,

 IN MILLIGRAMS PER LITER

SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER

CENTIMETER AT 25o CELSIUS

pH, IN STANDARD

UNITS

A. Fresh Pond, October 2, 1997

10121416

2468

10121416

002468

2468

10121416

0246810121416

0

DOC

—DISSOLVED ORGANIC CARBON

—BELOW MINIMUM REPORTING LIMIT

EXPLANATION

THMFP

—TRIHALOMETHANE FORMATION POTENTIAL

Figure 5. Depth profiles of temperature and concentrations of dissolved oxygen, nutrients, and other constituents during the 1997

–98 study period at 

the deep hole station in Fresh Pond, Cambridge, Massachusetts.
Figure 5 69

0.025

0 5 10 15 20 25 30 0 2 4 6 8 10 12 14 0 20 40 60 80 100 0 0.01 0.02 0.03 0.04 0 0.5 1.0 1.5

200 400 600 800

DEPTH, IN METERS

DEPTH, IN METERS

5 6 7 8 9 0 0.2 0.4 0.6 0.8 1 0 0.1 0.2 0.3 0.4 0 0.8 1.6 2.4

Sodium Total phosphorus

Nitrate nitrogen

Total nitrogen

Ammonia nitrogen Manganese

TEMPERATURE,

 IN DEGREES CELSIUS

CONCENTRATION, IN MILLIGRAMS PER LITER

CONCENTRATION, IN MILLIGRAMS PER LITER

DISSOLVED OXYGEN

CONCENTRATION,

 IN MILLIGRAMS PER LITER

SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER

CENTIMETER AT 25o CELSIUS

pH, IN STANDARD

UNITS

B. Fresh Pond, December 3, 1998 DOC—DISSOLVED ORGANIC CARBON

—BELOW MINIMUM REPORTING LIMIT

EXPLANATION

THMFP—TRIHALOMETHANE FORMATION POTENTIAL

10121416

02468

10121416

02468

0246810121416

0246810121416

Figure 5. Depth profiles of temperature and concentrations of dissolved oxygen, nutrients, and other constituents during the 1997–98 study period at the deep hole station in Fresh Pond, Cambridge, Massachusetts—Continued.
70 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection 0.021 0.026 0 5 10 15 20 25 30 0 2 4 6 8 10 12 14 0 20 40 60 80 100 0 0.01 0.02 0.03 0.04 0 0.5 1.0 1.5 200 400 600 800

DEPTH, IN METERS

DEPTH, IN METERS

5 6 7 8 9 0 0.2 0.4 0.6 0.8 1 0 0.1 0.2 0.3 0.4 0 0.1 0.2 0.3 0.4 0 0.8 1.6 2.4

Sodium Total phosphorus

Nitrate nitrogen

Total nitrogen

Ammonia nitrogen Manganese

TEMPERATURE,

 IN DEGREES CELSIUS

CONCENTRATION, IN MILLIGRAMS PER LITER

CONCENTRATION, IN MILLIGRAMS PER LITER

DISSOLVED OXYGEN

CONCENTRATION,

 IN MILLIGRAMS PER LITER

SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER

CENTIMETER AT 25o CELSIUS

pH, IN STANDARD

UNITS

THMFP

0 4 8 12 16

 DOC 

10121416

02468

10121416

02468

0246810121416

0246810121416

C. Fresh Pond January 30, 1998 DOC

—DISSOLVED ORGANIC CARBON

—BELOW MINIMUM REPORTING LIMIT

EXPLANATION

THMFP

—TRIHALOMETHANE FORMATION POTENTIAL

0.443

Figure 5. Depth profiles of temperature and concentrations of dissolved oxygen, nutrients, and other constituents during the 1997

–98 study period at the deep hole 

station in Fresh Pond, Cambridge, Massachusetts

—Continued.
Figure 5 71

0.018

0.018

0 5 10 15 20 25 30 0 2 4 6 8 10 12 14 0 20 40 60 80 100 0 0.01 0.02 0.03 0.04 0 0.5 1.0 1.5

200 400 600 800

DEPTH, IN METERS

5 6 7 8 9 0 0.2 0.4 0.6 0.8 1 0 0.1 0.2 0.3 0.4 0 0.1 0.2 0.3 0.4 0 0.8 1.6 2.4

Sodium Total phosphorus

Nitrate nitrogen

Total nitrogen

Ammonia nitrogen Manganese

TEMPERATURE,

 IN DEGREES CELSIUS

CONCENTRATION, IN MILLIGRAMS PER LITERCONCENTRATION, IN MILLIGRAMS PER LITER

DISSOLVED OXYGEN

CONCENTRATION,

 IN MILLIGRAMS PER LITER

SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER

CENTIMETER AT 25o CELSIUS

pH, IN STANDARD

UNITS

THMFP

0 4 8 12 16

 DOC 

101214

16

02468

10121416

02468

DEPTH, IN METERS

0246810121416

0246810121416

D. Fresh Pond, March 4, 1998 DOC—DISSOLVED ORGANIC CARBON

—BELOW MINIMUM REPORTING LIMIT

EXPLANATION

THMFP—TRIHALOMETHANE FORMATION POTENTIAL

Figure 5. Depth profiles of temperature and concentrations of dissolved oxygen, nutrients, and other constituents during the 1997–98 study period at the deep hole 

station in Fresh Pond, Cambridge, Massachusetts—Continued.
72 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection E. Fresh Pond, May 21, 1998 0.464 0.437 0 5 10 15 20 25 30 0 2 4 6 8 10 12 14 0 20 40 60 80 100 0 0.01 0.02 0.03 0.04 0 0.5 1.0 1.5 200 400 600 800

DEPTH, IN METERS

DEPTH, IN METERS

5 6 7 8 9 0 0.2 0.4 0.6 0.8 1 0 0.1 0.2 0.3 0.4 0 0.1 0.2 0.3 0.4 0 0.8 1.6 2.4

Sodium Total phosphorus

Nitrate nitrogen Ammonia nitrogen Manganese

TEMPERATURE,

 IN DEGREES CELSIUS

CONCENTRATION, IN MILLIGRAMS PER LITER

CONCENTRATION, IN MILLIGRAMS PER LITER

DISSOLVED OXYGEN

CONCENTRATION,

 IN MILLIGRAMS PER LITER

SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER

CENTIMETER AT 25o CELSIUS

pH, IN STANDARD

UNITS

THMFP

0 4 8 12 16

 DOC 

10121416

02468

0246810121416

10121416

02468

0246810121416

Total nitrogen

DOC

—DISSOLVED ORGANIC CARBON

—BELOW MINIMUM REPORTING LIMIT

EXPLANATION

THMFP

—TRIHALOMETHANE FORMATION POTENTIAL

Figure 5. Depth profiles of temperature and concentrations of dissolved oxygen, nutrients, and other constituents during the 1997

–98 study period at the deep hole 

station in Fresh Pond, Cambridge, Massachusetts

—Continued.
Figure 5 73

2.97

0.415

0 5 10 15 20 25 30 0 2 4 6 8 10 12 14 0 20 40 60 80 100 0 0.01 0.02 0.03 0.04 0 0.5 1.0 1.5

200 400 600 800

DEPTH, IN METERS

DEPTH, IN METERS

5 6 7 8 9 0 0.2 0.4 0.6 0.8 1 0 0.1 0.2 0.3 0.4 0 0.1 0.2 0.3 0.4 0 0.8 1.6 2.4

Sodium Total phosphorus

Nitrate nitrogen

Total nitrogen

Ammonia nitrogen Manganese

TEMPERATURE,

 IN DEGREES CELSIUS

CONCENTRATION, IN MILLIGRAMS PER LITERCONCENTRATION, IN MILLIGRAMS PER LITER

DISSOLVED OXYGEN

CONCENTRATION,

 IN MILLIGRAMS PER LITER

SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER

CENTIMETER AT 25o CELSIUS

pH, IN STANDARD

UNITS

THMFP

0 4 8 12 16

 DOC 

10121416

02468

10121416

02468

02468101214160246810121416

F. Fresh Pond, August 27, 1998 DOC—DISSOLVED ORGANIC CARBON

—BELOW MINIMUM REPORTING LIMIT

EXPLANATION

THMFP—TRIHALOMETHANE FORMATION POTENTIAL

Figure 5. Depth profiles of temperature and concentrations of dissolved oxygen, nutrients, and other constituents during the 1997–98 study period at the deep 

hole station in Fresh Pond, Cambridge, Massachusetts—Continued.
74 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection

01104430

Weston

Lincoln

Lexington

Concord

Waltham

01104410

01104405 01104415

01104420

01104440

01104433

01104390 01104455

01104460

01104475

Hobbs Brook

Cherry Brook

Stony Brook

Hobbs Brook

Hobbs Brook

Reservoir

Stony Brook

Reservoir

Sandy

Pond

128

2A

2

95

20

117

0

0

1

1

2 MILES

2 KILOMETERS

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

Department, Harvard Design and Mapping, and Boston Edison. 

EXPLANATION

BASIN BOUNDARY

SUBBASIN BOUNDARY

STREAMFLOW AND WATERQUALITY MONITORING

STATION AND NUMBER

Continuous record

Partial record

71 ̊15'

42 ̊

22'

30"

71 ̊20'

42 ̊

27'

Figure 6. Hydrography, subbasin boundaries, and streamflow and water-quality monitoring stations in the 

Cambridge, Massachusetts, drinking-water source area.
Figure 7 75

FECAL COLIFORM BACTERIA, IN COLONY 

FORMING UNITS PER 100 MILLILITERS

0

200

400

600

800

1,000

1,200

1,400

0110440501104410011044150110442001104430011044330110444001104390011044550110446001104475

SODIUM CONCENTRATON, IN MILLIGRAMS PER LITER

0

250

500

750

1,000

1,250

1,500

3,000

3,500

(13)

(9)

(11)

(12)

(9)

(29) (25)

(36)

(34)

(26)

(11)

26) (27)

(17)

(28) (8)

(9)

(8)

(8)

(10)

(11)

(6)

A.

B.

U.S. GEOLOGICAL SURVEY STATION NUMBER

EXPLANATION

(11) NUMBER OF

MEASUREMENTS

90th percentile

75th percentile

Median

25th percentile

10th percentile

Figure 7. Concentrations of selected chemical constituents detected at eleven monitoring stations in subbasins that 

contribute water to Hobbs Brook and Stony Brook Reservoirs, eastern Massachusetts, September 1997–November 1998.
76 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection CHLORIDE CONCENTRATON, IN MILLIGRAMS PER LITER

0

500

1,000

1,500

2,000

2,500

3,000

6,000

0110440501104410011044150110442001104430011044330110444001104390011044550110446001104475

NITRATE-NITROGEN CONCENTRATON,

IN MILLIGRAMS PER LITER

0

0.5

1

1.5

2

2.5

(29)

(25)

(36)

(35)

(26)

(11) 

(26) (27)

(17)

(28) (8)

(9)

(17)

(19)

(15)

(15)

(12)

(16)

(24)

(25)

(16) (20)

C.

D.

U.S. GEOLOGICAL SURVEY STATION NUMBER

EXPLANATION

(8) NUMBER OF

MEASUREMENTS

90th percentile

75th percentile

Median

25th percentile

10th percentile

Figure 7. Concentrations of selected chemical constituents detected at eleven monitoring stations in subbasins that 

contribute water to Hobbs Brook and Stony Brook Reservoirs, eastern Massachusetts, September 1997–November 1998—

Continued.
Figure 7 77

AMMONIA-NITROGEN CONCENTRATON,

IN MILLIGRAMS PER LITER

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0110440501104410011044150110442001104430011044330110444001104390011044550110446001104475

TOTAL NITROGEN CONCENTRATON,

IN MILLIGRAMS PER LITER

0

0.5

1.0

1.5

2.0

2.5

E.

F.

(20)

(16)

(25)

(24)

(16)

(12)

(15)

(15)

(19)

(17)

(9)

(9)

(17)

(19)

(15)

(15)

(12)

(17)

(24)

(25)

(16) (18)

U.S. GEOLOGICAL SURVEY STATION NUMBER

EXPLANATION

(9) NUMBER OF

MEASUREMENTS

90th percentile

75th percentile

Median

25th percentile

10th percentile

Figure 7. Concentrations of selected chemical constituents detected at eleven monitoring stations in subbasins that 

contribute water to Hobbs Brook and Stony Brook Reservoirs, eastern Massachusetts, September 1997–November 

1998—Continued.
78 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection

G.

H.

U.S. GEOLOGICAL SURVEY STATION NUMBER

01104405

0

0.005

0.01

0.015

0.02

0.025

ORTHO-PHOSPAHTE PHOSPHORUS CONCENTRATON,

IN MILLIGRAMS PER LITER TOTAL PHOSPHORUS CONCENTRATON, IN MILLIGRAMS PER LITER

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

(20)

(20)

(16)

(25)

(24)

(16)

(12)

(15)

(15)

(19)

(17)

(9)

(16)

(25)

(24)

(16) (12) (15)

(15)

(19)

(17)

(9)

0110441001104415011044200110443001104433011044400110439001104455

0110446001104475

EXPLANATION

(9) NUMBER OF

MEASUREMENTS

90th percentile

75th percentile

Median

25th percentile

10th percentile

Figure 7. Concentrations of selected chemical constituents detected at eleven monitoring stations in subbasins that 

contribute water to Hobbs Brook and Stony Brook Reservoirs, eastern Massachusetts, September 1997–November 1998—

Continued.
Figure 7 79

DISSOLVED IRON CONCENTRATON,

IN MICROGRAMS PER LITER

(18)

(13)

(25)

(23)

(14)

(11)

(14)

(14)

(15)

(15)

(9)

0

100

200

300

400

500

600

I.

J.

U.S. GEOLOGICAL SURVEY STATION NUMBER

0

100

200

300

400

500

600

700

800

01104405

(15)

(9)

(15)

(14) (14)

(11)

(14)

(23)

(25)

(13)

(18)

0110441001104415011044200110443001104433

0110444001104390011044550110446001104475

DISSOLVED MANGANESE CONCENTRATION,

IN MICROGRAMS PER LITER

EXPLANATION

(15) NUMBER OF

MEASUREMENTS

90th percentile

75th percentile

Median

25th percentile

10th percentile

Figure 7. Concentrations of selected chemical constituents detected at eleven monitoring stations in subbasins that 

contribute water to Hobbs Brook and Stony Brook Reservoirs, eastern Massachusetts, September 1997–November 

1998—Continued.
80 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection

0110440501104410011044150110442001104430011044330110444001104390011044550110446001104475

K.

L.

U.S. GEOLOGICAL SURVEY STATION NUMBER

TRIHALOMETHANE FORMATION POTENTIAL

CONCENTRATION, IN MILLIGRAMS PER LITER

0

50

0.100

0.150

0.200

0.250

0.300

0.350

0.400

DISSOLVED ORGANIC CARBON CONCENTRATION,

IN MILLIGRAMS PER LITER

(12)

(9)

(8)

(8) (8) (9) (9)

(7)

(8)

(9)

(11)

(10)

(7)

(12)

(13)

(10)

(10)

(9)

(10)

(12) (12)

(8)

0

2

4

6

8

10

12

14

16

18

20 EXPLANATION

(12) NUMBER OF

MEASUREMENTS

90th percentile

75th percentile

Median

25th percentile

10th percentile

Figure 7. Concentrations of selected chemical constituents detected at eleven monitoring stations in subbasins that 

contribute water to Hobbs Brook and Stony Brook Reservoirs, eastern Massachusetts, September 1997–November 

1998—Continued.
APPENDIX: Cambridge, Mass., Drinking Water

Source-Area Water Quality Monitoring Program
Appendix 83

This appendix contains a brief description of the 

Cambridge, Mass., drinking water source-area waterquality monitoring program. The program was 

designed by the U.S. Geological Survey (USGS), in 

cooperation with the Cambridge Water Department 

(CWD), and is based in part on the results of a recent 

(1998) assessment of reservoir and tributary-stream 

quality. The assessment, which was conducted jointly 

by the USGS and the CWD, included a detailed analysis of the drainage basin and the identification of subbasins within the drainage basin that are exporting 

disproportionate amounts of nonpoint pollutants. This 

information then was used to help the design of the 

monitoring network (New England Interstate Water 

Pollution Control Commission, 1996).

Monitoring Objectives

The process of designing a water-quality monitoring program begins with a clear definition of program goals and objectives (Reinelt and others, 1988). 

The goals then guide the entire process of program 

design and implementation. Ideally, the data obtained 

through monitoring provide an objective source of 

information needed to support management decisions. 

Specifically, an effective water-quality monitoring program will provide quantitative answers to the following 

questions (Intergovernmental Task Force on Monitoring Water Quality, 1995):

• What is the condition of the source water?

• Where, how, and why are water-quality conditions 

changing over time?

• What problems are related to source-water quality? 

Where are the problems occurring and what is 

causing them?

• Are programs to prevent or remediate problems 

working effectively?

• Are water-quality goals and standards being met?

The primary goal of the Cambridge drinking 

water source-area monitoring program is to ensure that 

water withdrawn from Fresh Pond for treatment is as 

free as possible from contaminants, thereby minimizing the costs of treatment. Specific objectives of the 

program are to

• Monitor the condition of source waters in the 

Cambridge drinking water supply system;

• Determine where, when, and how water-quality 

conditions are changing over time;

• Identify actual and potential problems related to 

source-water quality;

• Evaluate effectiveness of programs to prevent or 

remediate problems;

• Ensure that all applicable water-quality goals, 

standards, and guidelines are being met; and

• Provide for rapid response to emerging problems.

Monitoring-Program Elements

The Cambridge source-area monitoring program 

consists of four major elements: (1) routine monitoring 

of reservoirs and tributary streams during dry weather, 

(2) event-based monitoring of streams, storm drains, 

and other outfalls during wet weather, (3) continuous 

recording of stage and selected water-quality characteristics at critical sites within the drainage basin, and (4) 

periodic monitoring of ground water in the vicinity of 

Fresh Pond. The program is guided by a Quality Assurance Program Plan (QAPP, U.S. Environmental Protection Agency, 1999) that includes detailed procedures 

for sample collection, preparation, and analysis; program quality objectives and measurement performance 

criteria; and procedures for data management and evaluation.

Routine (Dry Weather) 

Surface-Water Monitoring

Dry-weather sampling is conducted at 3 primary 

and 6 secondary reservoir-monitoring stations, and at 

10 primary and 6 secondary tributary-monitoring stations. The distinction between primary and secondary 

monitoring stations is based on the frequency of sampling and on the number of analyses performed on the 

samples. Locations of all monitoring stations are 

shown in figure A1.
84 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection

750,750

2,967,000 2,969,000

752,250

Weston

Lincoln

Lexington

Concord

Waltham

Hobbs Brook

Trapelo Road

128 Cherry Brook

Stony Brook

Hobbs Brook

2A

2

95

Hobbs Brook

Reservoir

Stony Brook

Reservoir

20

Sandy

Pond

117

0

0

1

1

2 MILES

2 KILOMETERS

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

Department, Harvard Design and Mapping, and Boston Edison. 

EXPLANATION

WETLAND 

BASIN BOUNDARY

SUBBASIN BOUNDARY

PRIMARY RESERVOIR-

 MONITORING STATION

PRIMARY STREAM-

 MONITORING STATION

SECONDARY RESERVOIR-

 MONITORING STATION

SECONDARY STREAM-

 MONITORING STATION

EVENT-MONITORING

 STATION

GROUND-WATER

 MONITORING SITE

CONTINUOUS-RECORD

 MONITORING STATION

71 ̊15'

42 ̊

22'

30"

71 ̊20'

42 ̊

27'

Treatment Plant

Intake

Winter Street

Gatehouse

Fresh Pond

0 1,000 FEET

0 500 METERS

Dam

Stony Brook

Intake

Stony Brook

Outfall

N

Figure A1. Cambridge, Mass., drinking-water source area water-quality monitoring network.
Appendix 85

At regular intervals (once each month from 

May through October and every other month from 

December through April), CWD staff measure Secchi 

disk transparency and depth profiles of specific conductance, pH, water temperature, turbidity, and dissolved oxygen concentration at both the primary and 

the secondary reservoir-monitoring stations. The sampling schedule (table A1) is based on the results of a 

USGS study which determined that monthly sampling 

was sufficient to characterize changes in reservoir 

water quality during the spring, summer, and early 

autumn months and that sampling every other month 

was sufficient during winter.

Secchi disk transparency is a measure of the 

depth of penetration of sunlight in a reservoir. It is measured by lowering a small horizontal disk on a calibrated line and noting the depth at which it is no longer 

visible from the surface (Lind, 1974). In the Cambridge 

drinking-water source area, the Secchi disk transparency is related mainly to the abundance of phytoplankton algae in the upper mixed layers of the reservoirs. 

Thus, it provides a quick and inexpensive indicator of 

eutrophication problems. Water temperature, specific 

conductance, pH, turbidity, and dissolved oxygen concentration are measured in situ with an electronic multiparameter water-quality monitoring system lowered 

on a cable. Depth profiles of these characteristics provide essential information on physical, chemical, and 

biological conditions in the reservoirs.

At the three primary reservoir-monitoring 

stations only (fig. A1), water samples are pumped with 

a peristaltic pump through pre-cleaned Tygon tubing 

from a depth of 6 ft when the water column is isothermal, or from three depths—6 ft below the surface, the 

depth of the thermocline (the point of maximum rate of 

change in water temperature with depth), and 2 to 6 ft 

above the bottom—when the water column is thermally 

stratified. Water from each sampling depth is collected 

in accordance with clean-sampling protocols (Wilde 

and others, 1999) into Teflon bottles. The samples are 

returned to the CWD laboratory and analyzed for color, 

alkalinity, and concentrations of major ions (sodium, 

calcium, chloride, and sulfate), nutrients (ammonia 

nitrogen, total Kjeldahl nitrogen, nitrate nitrogen, total 

phosphorus, and orthophosphate phosphorus), selected 

metals (aluminum, iron, and manganese), and phytoplankton chlorophyll a, using standard methods 

(American Public Health Association and others, 

1995). The USGS 1998 assessment of reservoir and 

tributary-stream quality determined that under most 

conditions, water-quality data collected in depth profiles at these stations are indicative of conditions 

throughout the reservoirs.

Color is measured spectrophotometrically and 

is primarily an indicator of the concentration of dissolved organic matter, which is abundant in sourcearea streams and reservoirs, and must be removed 

during treatment to prevent formation of organochlorine by-products. Alkalinity is a measure of the acidneutralizing capacity of a water sample and is mainly 

dependent on the quantities of carbonate and bicarbonate ions. The most accurate indicator of the abundance 

of phytoplankton algae is the amount of particulate 

chlorophyll a in the upper mixed layer of the reservoir. 

Changes in chlorophyll concentrations are indicative of 

changes in reservoir trophic state.

Nitrogen and phosphorus are plant nutrients that 

can, in sufficient quantities, cause algal blooms in the 

reservoirs and excessive growth of algae and higher 

plants in the streams. Ecologically significant forms of 

nitrogen include ammonia and nitrate nitrogen in 

runoff from areas that receive fertilizer applications 

and in wastewater discharges, and organic nitrogen 

produced by microbial processes. The concentration of 

organic nitrogen is determined by subtracting the concentration of ammonia nitrogen from that of total 

Kjeldahl nitrogen.

During each round of reservoir sampling, concentrations of fecal coliform bacteria are measured at 

the withdrawal points in all three reservoirs (fig. A1). 

The presence of fecal coliform bacteria in a water 

sample indicates that the water may have been contaminated with feces from humans or other warm-blooded 

animals. Such contamination can introduce diseasecausing viruses and other potential pathogens.

Water entering the reservoirs is monitored at 10 

primary and 5 secondary tributary-stream-monitoring 

stations (fig. A1). These stations represent streams that 

contribute water directly to the reservoirs and major 

tributaries, or integrate large areas of the drainage 

basin. Thus, the stations are important primary indicators of the condition of water likely to enter the reservoirs. At intervals of 2 months, the CWD measures 

stage and discharge and assesses water quality at each 

primary stream-monitoring station using USGS methods (Rantz and others, 1982; Wilde and others, 1999). 

The sampling frequency (table A1), in conjunction 

with the continuous monitors in each of the three 

reservoirs, is sufficient to capture changes in water 

quality in time to prevent contamination problems at 

the water-treatment plant intake.
86 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection

Table A1. Water sources, sampling frequencies, and monitored water-quality properties and constituents, for water-quality 

monitoring stations in the drinking-water source area for Cambridge, Massachusetts—Continued

Water source(s) Sampling frequency Monitored properties and constituents

Routine (Dry-Weather) Surface-Water Monitoring

Primary reservoir-monitoring stations (3) at the deepest 

points in Hobbs Brook Reservoir, Stony Brook 

Reservoir, and Fresh Pond

9 depth profiles per year Specific conductance

pH

Water temperature

Color

Turbidity

Secchi disk transparency

Dissolved oxygen concentration

Major ions1

Alkalinity

Nutrients2

Selected metals3

Phytoplankton chlorophyll a

Secondary reservoir-monitoring stations (6) in the 

upper and middle basins of Hobbs Brook Reservoir, 

at the Winter Street Gatehouse, the intakes at Stony 

Brook Reservoir and Fresh Pond, and the outfall 

from Stony Brook Reservoir in Fresh Pond

9 depth profiles per year Specific conductance

pH

Water temperature

Color

Turbidity

Secchi disk transparency

Dissolved oxygen concentration

Fecal coliform bacteria4

 Primary tributary-stream-monitoring stations (10) 6 times per year Stream stage and discharge

Specific conductance

pH

Water temperature

Color

Turbidity

Dissolved oxygen concentration

Fecal coliform bacteria

Major ions1

Alkalinity

Total suspended solids

Nutrients2

Selected metals3

Secondary tributary-stream-monitoring stations (6) 2 times per year Stream stage and discharge

Specific conductance

pH

Water temperature

Color

Turbidity

Dissolved oxygen concentration

Fecal coliform bacteria

Major ions1

Alkalinity

Total suspended solids

Nutrients2

Selected metals3

Table A1. Water sources, sampling frequencies, and monitored water-quality properties and constituents, for water-quality 

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

[Monitored properties and constituents: Italicized properties and constituents are measured in the field, non-italicized properties and constituents are 

measured on water samples returned to the laboratory. BTX, benzene, ethylbenzene, toluene, xylenes, TPH, total petroleum hydrocarbons]
Appendix 87

1Calcium, sodium, sulfate, chloride.

2Ammonia nitrogen, total Kjeldahl nitrogen, nitrate nitrogen, total phosphorus, orthophosphate phosphorus.

3Aluminum, iron, manganese.

4Measured only at the Winter Street Gatehouse and at the intakes in Stony Brook Reservoir and Fresh Pond.

5Measured at two continuous-record monitoring stations only.

6Arsenic, barium, cadmium, chromium, lead, mercury, nickel, selenium, silver .

Event-Based (Wet-Weather) Surface-Water Monitoring

Tributary-monitoring stations (9), storm drains (2), and 

other outfalls (4) in the drainage basin

3–5 storms per year Specific conductance

pH

Water temperature

Color

Turbidity

Dissolved oxygen concentration

Fecal coliform bacteria

Major ions1

Alkalinity

Total suspended solids 

Nutrients2

Selected metals3

Continuous-Record Surface-Water Monitoring

Two primary tributary-monitoring stations and three 

secondary reservoir-monitoring stations

every 15 minutes Stage

Water temperature

Specific conductance

Precipitation5

Ground-Water Monitoring

Ground-water-monitoring wells in the vicinity of Fresh 

Pond

2 times per year Specific conductance

pH

Water temperature

Color

Turbidity

Dissolved oxygen concentration

Fecal coliform bacteria

Major Ions1

Alkalinity

Total Suspended Solids

Nutrients2

Selected metals3

Trace metals6

TPH

BTX

Table A1. Water sources, sampling frequencies, and monitored water-quality properties and constituents, for water-quality 

monitoring stations in the drinking-water source area for Cambridge, Massachusetts—Continued

Water source(s) Sampling frequency Monitored properties and constituents
88 Reservoir and Stream Quality in the Cambridge, Mass., Drinking-Water Source Area and Implications for Source-Water Protection

Specific conductance, pH, water temperature, 

turbidity, and dissolved oxygen concentration are measured on site and water samples are collected using 

clean-sampling protocols (Wilde and others, 1999) into 

1-liter Teflon isokinetic samplers. Discharge-weighted, 

representative samples are collected from multiple vertical profiles distributed at equal distances along stream 

cross sections (Edwards and Glysson, 1999). The samples are then returned to the CWD laboratory for analysis of color, fecal coliform bacteria, alkalinity, total 

suspended solids, and concentrations of major ions, 

nutrients, and selected metals (table A1).

The five secondary stream-monitoring stations 

are monitored twice a year, usually during base flow 

and high flow. These stations are located higher up in 

the drainage basin on smaller tributaries or at points 

that discharge to the reservoirs predominantly during 

wet weather (fig. A1). The secondary stations are sampled biannually for the same constituents as the primary stations to provide indicators of potential changes 

in water quality or of base-flow conditions. Each round 

of periodic sampling includes quality-assurance 

samples (field and instrument blanks, duplicates, and 

sample splits) representing about 10 percent of the total 

number of samples analyzed.

Event-Based (Wet Weather) 

Surface-Water Monitoring

Storm-event sampling is conducted three to five 

times each year at a number of event-monitoring sites, 

some of which are primary and secondary streammonitoring stations and some of which are pipes and 

culverts that discharge to the reservoirs (fig. A1). The 

goal of the storm-event sampling is to collect samples 

of the first flush of runoff from storms producing 

0.5 inches or more of rain following a period of at least 

3 days of dry weather. This goal is accomplished by 

deploying passive samplers consisting of acid-washed 

polypropylene bottles with caps that fill automatically 

when the stream stage reaches a predetermined level or 

by manually collecting the first flush from pipes or 

culverts. The water samples are retrieved within 6 

hours after the storm ends and are analyzed for color, 

fecal coliform bacteria, alkalinity, total suspended solids, and concentrations of major ions, nutrients, and 

selected metals.

Continuous-Record

Surface-Water Monitoring

Continuous (15 minute interval) monitoring is 

contucted at two primary tributary-monitoring stations 

and three secondary reservoir-monitoring stations 

(fig. A1). These stations are operated and maintained 

by the USGS and the CWD for continuous measurement of stream and reservoir stage and temperaturecorrected specific conductance. Specific conductance, a 

measure of the ability of the water to conduct an electrical current, is an indicator of the concentrations of 

dissolved electrolytes in the water. The stations at 

Hobbs Brook Reservoir and Stony Brook Reservoir 

also monitor stage and specific conductance of the discharges from the reservoirs. The continuous streamstage data are converted to discharge by the use of 

stage-discharge relations (Rantz and others, 1982) and 

the specific conductance records are converted to concentrations of sodium, calcium, and chloride in a similar fashion (Granato and Smith, 1999). 

Ground-Water Monitoring

The quality of ground water in the area around 

Fresh Pond is assessed twice each year at monitoring 

wells (fig. A1). Water samples are collected using 

a stainless-steel submersible pumping system with 

Tygon tubing. The water is pumped slowly through a 

chamber for measurement of specific conductance, pH, 

water temperature, turbidity, and dissolved oxygen 

concentration. When conditions stabilize, the data are 

recorded, and water samples are collected for analysis 

of fecal coliform bacteria, alkalinity, total suspended 

solids, and concentrations of major ions, nutrients, 

major ions, trace metals (aluminum, iron, manganese, 

arsenic, barium, cadmium, chromium, lead, mercury, 
Appendix 89

selenium, silver, nickel), total petroleum hydrocarbons, 

and BTX (benzene, ethylbenzene, toluene, total 

xylenes).

Data Management, Interpretation,

Reporting, and Review

The monitoring and quality-assurance data are 

entered into a data base, maintained by the CWD as 

part of a Watershed Management Tool, that enables the 

CWD to efficiently map, plan, track, and report drainage-basin management and water-quality monitoring 

activities. Monitoring is conducted by a hydrologic 

technician and an assistant (Reservoir Caretaker). 

USGS methods and protocols are used in the program 

so that results may be compared to baseline data collected by the USGS during water year 1998. The entire 

program is reviewed periodically by a Technical Advisory Committee that includes members from the Cambridge academic community, and a Watershed 

Advisory Committee comprised of representatives 

from Cambridge, Waltham, Weston, Lexington, and 

Lincoln.

The CWD also conducts special investigations of 

water-quality related problems and situations within 

the source area. Such investigations may include intensive monitoring at present water-quality monitoring 

stations indicating increasing trends in contaminant 

loading, monitoring at locations where known disturbance is taking place, and monitoring to assess the 

effectiveness of new management practices or infrastructure. These investigations frequently require analysis of a variety of constituents and water-quality 

related properties.

References Cited

American Public Health Association, American Water Works 

Association, and Water Pollution Control Commission, 

1995, Standard methods for the examination of water 

and wastewater (19th ed.): Washington, D.C., APHA, 

variously paginated.

Edwards, T.K., and Glysson, G.D., 1999, Field methods for 

measurement of fluvial sediment: U.S. Geological 

Survey Techniques of Water-Resources Investigations, 

chap. C2, book 3, 89 p.

Granato, G.E., and Smith, K.P., 1999. Estimating 

concentrations of road-salt constituents in highway 

runoff from measurements of specific conductance: 

U.S. Geological Survey, Water-Resources 

Investigations Report 99-4077, 22 p.

Intergovernmental Task Force on Monitoring Water Quality, 

1995, The strategy for improving water-quality 

monitoring in the United States—Final report of the 

Intergovernmental Task Force on Monitoring Water 

Quality: Reston, Va., U.S. Geological Survey Office of 

Water Data Coordination, 25 p. plus Appendixes.

Lind, O.T., 1974, Handbook of common methods in 

limnology: St. Louis, The C.V. Mosby Co., 145 p.

New England Interstate Water Pollution Control 

Commission, 1996, Source protection. A guidance 

manual for small surface water supplies in New 

England: New England Interstate Water Pollution 

Control Commission, 77 p. plus Appendices.

Rantz, S.E., and others, 1982, Measurement and 

computation of streamflow—Volume 1. Measurement 

of stage and discharge—Volume 2. Computation of 

discharge: U.S. Geological Survey Water-Supply Paper 

2175, 631 p.

U.S. Environmental Protection Agency, 1999, Region I, 

EPA-New England Compendium of Quality Assurance 

Project Plan Requirements and Guidance: U.S. EPANew England, Region I, Office of Environmental 

Measurement and Evaluation, variously paginated.

Wilde, F.D., Radtke, D.B., Gibs, Jacob, and Iwatsubo, R.T., 

1999, Collection of water samples, National field 

manual for the collection of water-quality data: U.S. 

Geological Survey Techniques of Water-Resources 

Investigations, book 9, chap. A4, 103 p.