Water-Resources Investigations Report 03-4012

U.S. Department of the Interior

U.S. Geological Survey

NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

Water-Quality Trends in New England 

Rivers During the 20th Century
Cover photograph shows the Blackstone River and Slater Mill in Pawtucket, R.I. (Photograph 

reprinted from John H. Chafee Blackstone River National Heritage Corridor, National Park Service, 

and published with permission)
Water-Quality Trends in New England Rivers 

During the 20th Century

By Keith W. Robinson, Jean P. Campbell, and Norbert A. Jaworski

Pembroke, New Hampshire

2003

Water-Resources Investigations Report 03-4012

U.S. GEOLOGICAL SURVEY

NATIONAL WATER-QUALITY ASSESSMENT PROGRAM
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does 

not imply endorsement by the U.S. Government.

For additional information write to:

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New Hampshire/Vermont District

361 Commerce Way

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http://nh.water.usgs.gov

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FOREWORD

The U.S. Geological Survey (USGS) is 

committed to serve the Nation with accurate and 

timely scientific information that helps enhance 

and protect the overall quality of life, and 

facilitates effective management of water, 

biological, energy, and mineral resources. 

(http://www.usgs.gov/). Information on the quality 

of the Nation’s water resources is of critical interest 

to the USGS because it is so integrally linked to the 

long-term availability of water that is clean and 

safe for drinking and recreation and that is suitable 

for industry, irrigation, and habitat for fish and 

wildlife. Escalating population growth and 

increasing demands for the multiple water uses 

make water availability, now measured in terms of 

quantity and quality, even more critical to the longterm sustainability of our communities and 

ecosystems.

The USGS implemented the National WaterQuality Assessment (NAWQA) Program to support 

national, regional, and local information needs and 

decisions related to water-quality management and 

policy. (http://water.usgs.gov/nawqa/). Shaped by 

and coordinated with ongoing efforts of other 

Federal, State, and local agencies, the NAWQA 

Program is designed to answer: What is the 

condition of our Nation’s streams and ground 

water? How are the conditions changing over time? 

How do natural features and human activities affect 

the quality of streams and ground water, and where 

are those effects most pronounced? By combining 

information on water chemistry, physical 

characteristics, stream habitat, and aquatic life, the 

NAWQA Program aims to provide science-based 

insights for current and emerging water issues and 

priorities. NAWQA results can contribute to 

informed decisions that result in practical and 

effective water-resource management and 

strategies that protect and restore water quality.

Since 1991, the NAWQA Program has 

implemented interdisciplinary assessments in more 

than 50 of the Nation’s most important river basins 

and aquifers, referred to as Study Units. 

(http://water.usgs.gov/nawqa/nawqamap.html). 

Collectively, these Study Units account for more 

than 60 percent of the overall water use and 

population served by public water supply, and are 

representative of the Nation’s major hydrologic 

landscapes, priority ecological resources, and 

agricultural, urban, and natural sources of 

contamination. 

Each assessment is guided by a nationally 

consistent study design and methods of sampling 

and analysis. The assessments thereby build local 

knowledge about water-quality issues and trends in 

a particular stream or aquifer while providing an 

understanding of how and why water quality varies 

regionally and nationally. The consistent, multiscale approach helps to determine if certain types 

of water-quality issues are isolated or pervasive, 

and allows direct comparisons of how human 

activities and natural processes affect water quality 

and ecological health in the Nation’s diverse 

geographic and environmental settings. 

Comprehensive assessments on pesticides, 

nutrients, volatile organic compounds, trace 

metals, and aquatic ecology are developed at the 

national scale through comparative analysis of the 

Study-Unit findings. 

(http://water.usgs.gov/nawqa/natsyn.html). 

The USGS places high value on the 

communication and dissemination of credible, 

timely, and relevant science so that the most recent 

and available knowledge about water resources can 

be applied in management and policy decisions. 

We hope this NAWQA publication will provide 

you the needed insights and information to meet 

your needs, and thereby foster increased awareness 

and involvement in the protection and restoration 

of our Nation’s waters. 

The NAWQA Program recognizes that a 

national assessment by a single program cannot 

address all water-resource issues of interest. 

External coordination at all levels is critical for a 

fully integrated understanding of watersheds and 

for cost-effective management, regulation, and 

conservation of our Nation’s water resources. The 

Program, therefore, depends extensively on the 

advice, cooperation, and information from other 

Federal, State, interstate, Tribal, and local 

agencies, non-government organizations, industry, 

academia, and other stakeholder groups. The 

assistance and suggestions of all are greatly 

appreciated.

Robert M. Hirsch

Associate Director for Water
iv Contents

CONTENTS

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

Introduction ......................................................................................................................................................... 1

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

Acknowledgments ...................................................................................................................................... 3

Data Sources and Methods of Analysis ............................................................................................................... 3

Description of the Merrimack, Blackstone, and Connecticut Rivers .................................................................. 9

Changes in the Water Quality of the Merrimack, Blackstone, and Connecticut Rivers During the 

20th Century......................................................................................................................................................... 10

Sulfate ........................................................................................................................................................ 10

Chloride ...................................................................................................................................................... 11

Residues ..................................................................................................................................................... 13

Nitrate ......................................................................................................................................................... 14

Total Phosphorus ........................................................................................................................................ 16

Summary ............................................................................................................................................................. 17

References Cited ................................................................................................................................................. 18
Figures v

FIGURES

Figure 1. Photograph showing the “Mile of Mills” along the Merrimack River in Lowell, Mass., in 1893 .... 2

Figure 2. Photograph showing the confluence of the Concord and Merrimack Rivers in Lowell, Mass.,

in 2001 ............................................................................................................................................... 2

Figure 3. Map showing location of the Merrimack, Blackstone, and Connecticut Rivers in New England, and 

streamflow and water-quality data sites used to assess changes in the 20th century ........................ 4

Figure 4. Photograph showing the Connecticut River at Thompsonville, Conn., 2002.................................... 7

Figure 5. Photograph showing historic mills in the early years of the 20th century along the Blackstone River, 

Woonsocket, R.I. ............................................................................................................................... 9

Figure 6. Graphs showing annual mean streamflow and streamflow trends in the (A) Merrimack River at

Lowell, Mass., during 1935-2000; (B) Blackstone River at Millville, Mass., during 1929-2000;

and (C) Connecticut River at Thompsonville, Conn., during 1928-2000.......................................... 11

Figure 7. Graph showing trends in annual sulfate concentrations in the Merrimack, Blackstone, and

Connecticut Rivers from 1952-2000, and annual sulfur-dioxide emissions in the United States from 

1950-98 .............................................................................................................................................. 12

Figure 8. Graph showing trends in annual chloride concentrations in the Merrimack, Blackstone, and

Connecticut Rivers from 1900-2000, and annual amount of salt sold for highway de-icing in the

United States from 1940-2000........................................................................................................... 13

Figure 9. Graph showing trends in annual residue concentrations in the Blackstone and Connecticut Rivers from 

1900-2000, and population of New England from 1900-98.............................................................. 14

Figure 10. Graph showing trends in annual nitrate concentrations in the Merrimack, Blackstone, and

Connecticut Rivers from 1900-2000, annual nitrogen oxide omissions in the United States from

1940-98, and annual agricultural usage of nitrogen fertilizer in New England from 1945-91.......... 15

Figure 11. Graph showing trends in annual total phosphorus concentrations in the Merrimack, Blackstone,

and Connecticut Rivers from 1966-2000, and annual agricultural usage of phosphorus fertilizer in

New England from 1965-91 .............................................................................................................. 17
vi Tables

TABLES

Table 1. Anthropogenic sources of sulfate, chloride, residues, nitrate and phosphorus in rivers ................... 5

Table 2. Sources of water-quality and streamflow data for the Merrimack, Blackstone, and

Connecticut Rivers ............................................................................................................................ 6

Table 3. Land-use characteristics, population estimates, and municipal wastewater discharge yields in the

Merrimack, Blackstone, and Connecticut Rivers Basins................................................................... 9

Table 4. Summary of Kendall’s tau analysis for trends in streamflow and annual concentrations of sulfate,

chloride, residues, nitrate, and total phosphorus in the Merrimack, Blackstone, and Connecticut Rivers 

from 1900-2000 ................................................................................................................................ 12
Conversion Factors vii

CONVERSION FACTORS

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

Multiply By To obtain

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

square miles (mi2) 12.590 square kilometers (km2)

gallons (gal) 3.785 liter (L)

ton 0.9072 megagram (Mg)

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)
Introduction 1

Water-Quality Trends in New England Rivers During the 

20th Century

By Keith W. Robinson, Jean P. Campbell, and Norbert A. Jaworski

ABSTRACT

Water-quality data from the Merrimack, 

Blackstone, and Connecticut Rivers in New 

England during parts of the 20th century were 

examined for trends in concentrations of sulfate, 

chloride, residue upon evaporation, nitrate, and 

total phosphorus. The concentrations of all five of 

these constituents show statistically significant 

trends during the century. Annual concentrations 

of sulfate and total phosphorus decreased during 

the second half of the century, whereas annual 

concentrations of nitrate, chloride, and residues 

increased throughout the century. In the 

Merrimack River, annual chloride concentrations 

increased by an order of magnitude. Annual nitrate 

concentrations also increased by an order of 

magnitude in the Merrimack and Connecticut 

Rivers. These changes in the water quality 

probably are related to changing human activities. 

Most notable is the relation between increasing 

use of road de-icing salts and chloride 

concentrations in rivers. In addition, changes in 

concentrations of nitrate and phosphorus probably 

are related to agricultural use of nitrogen and 

phosphorus fertilizers. For all the water-quality 

constituents assessed, concentrations were greatest 

in the Blackstone River. The Blackstone River 

Basin is smaller and more highly urbanized than 

the other basins studied. Data-collection programs 

that span multiple decades can provide valuable 

insight on the effects of changing human 

population and societal activities on the water 

quality of rivers. This study was done as part of the 

U.S. Geological Survey’s National Water-Quality 

Assessment Program.

INTRODUCTION

The 20th century has been marked by major 

landscape and lifestyle changes in New England. The 

transition from an agricultural to an urban-based 

society began in the mid-19th century and continued 

through the 20th century. During this transition, 

automobile use increasingly dominated the way of life 

and settlement patterns. Older industries, such as paper 

and textile mills and agriculture, were replaced by 

service-oriented and high-technology industries. 

Human and industrial wastes that were routinely 

discharged untreated into waterways at the beginning 

of the century were eventually handled by wastewatertreatment facilities. These landscape and societal 

changes are likely to have affected the quality of New 

England rivers.

Forest clearing and agricultural expansion in 

New England began with European settlement and 

slowed in the mid-1800s (Foster, 1992). Reforestation 

then occurred in the region and has continued in many 

areas to the present (2003). The decline of agriculture 

was replaced by increases in mills and manufacturing 

industries in the major cities and along rivers. Rivers 

such as the Merrimack and Blackstone provided 

hydropower for the mills and other manufacturing 

industries (figs. 1 and 2). The construction of dams to 

provide mill power became so extensive that the 

Blackstone River, for example, had a dam present for 

almost every mile along its entire length (Shanahan, 

1994). The change to an industrial society resulted in 

large population increases in urban areas. For example, 

the population of New Bedford, Mass., increased from 

about 27,000 in 1880, when there were 2 textile mills, 

to about 121,000 in 1920 when there were 31 mills 

(U.S. Environmental Protection Agency, 2001). During 

the 40 years from 1880 to 1920, New England’s 

population almost doubled from 4.01 to 7.40 million 

people (U.S. Census Bureau, 2001). 
2 Water-Quality Trends in New England Rivers During the 20th Century

Throughout the 20th century, New England’s 

population continued to increase. As of year 2000, 

13.92 million residents lived in the six New England 

states; an increase of 149 percent from the 1900 

population of 5.59 million people (U.S. Census 

Bureau, 2001). As in other areas of the United States, 

the introduction of the automobile and the subsequent 

building of roads and interstate highways in the 20th

century led to the expansion of urban areas throughout 

New England. The expanding suburbs centered on the 

cities of Hartford, Conn., Providence R.I., and

Boston, Mass. The growth of suburban areas also has 

been fueled by the building of new commercial and 

industrial facilities along major roadways. The number 

of vehicle miles traveled in the region almost doubled 

during the 25 years from 1970 to 1995, from

60 million miles (mi) to 110 million mi yearly

(U.S. Environmental Protection Agency, 1996).

During the early to mid 20th century, rivers in 

New England were among the most polluted rivers in 

the United States because large amounts of untreated 

municipal and industrial sewage were released directly 

into surface waters. At the turn of the 20th century, 

outbreaks of typhoid fever and other infectious diseases 

were common in urban areas that used polluted rivers 

as a source of drinking water. As late as the mid-1960s, 

more than 120 million gallons per day (Mgal/d) of 

untreated or minimally treated wastewater were 

discharged into the Merrimack River (U.S. Department 

of the Interior, 1968). In the early 1970s, the 

Connecticut River was so polluted it was referred to as 

a “landscaped sewer” (U.S. Environmental Protection 

Agency, 2000a). The passage of the Federal Water 

Pollution Control Act in 1972 resulted in significant 

improvements in wastewater treatment throughout New 

England. At the beginning of the 21st century, most 

routinely discharged wastewaters in New England were 

being treated by facilities that removed 80-95 percent 

of the organic loads (Peter Kudarauskas, U.S. 

Environmental Protection Agency, oral commun., 

2002). 

The resulting effects of societal changes during 

the 20th century on the quality of New England’s major 

rivers has been minimally explored. Roman and others 

(2000) described total nitrogen and total phosphorus 

loadings from riverine sources to coastal estuaries from 

1900 to 1994 in the Northeastern United States. They 

reported that total nitrogen loadings increased steadily 

as a result of increasing use of nitrogen-based 

fertilizers and increased atmospheric deposition of 

nitrogen. Total phosphorus loadings to surface waters 

increased until the 1960s because of the use of 

phosphate-containing detergents, then decreased 

because of bans in the use of these detergents. Jaworski 

and Hetling (1996) presented water-quality trends for 

chloride, nitrate, and total residue concentrations in 12 

rivers in the eastern United States during the 20th

century. They found that concentrations of chloride, 

nitrate, and total residue increased during the century 

by a factor of 5.3, 4.7 and 1.6, respectively. Jaworski 

Figure 1. The “Mile of Mills” along the Merrimack River in Lowell, 

Mass., in 1893 (Photograph reprinted from the Lowell National 

Historic Park and published with permission).

Figure 2. The confluence of the Concord and Merrimack Rivers in 

Lowell, Mass., in 2001 (Photograph by Britt Stock, U.S. Geological 

Survey).
Data Sources and Methods of Analysis 3

and Hetling (1996) attributed the increasing total 

residue concentrations in the Blackstone River to 

municipal and industrial wastewater discharges. The 

small increases in total residue concentrations in the 

Connecticut River were probably related to 

reforestation of much of the basin (Jaworski and 

Hetling, 1996).

A number of studies have assessed water-quality 

changes in New England rivers over the last three 

decades of the 20th century. These studies were 

possible because of an increase in water-quality 

monitoring that began in the late 1960s and early 

1970s. Several studies reported upward trends in 

specific conductance and dissolved chloride 

concentrations in rivers in Connecticut, Massachusetts, 

Rhode Island, and New Hampshire (Trench, 1996; 

Strause, 1993; Kulp and Bohr, 1993; Bell, 1993; and 

Toppin, 1993). Zimmerman (1997) and Trench (1996) 

found increasing nitrate and decreasing total 

phosphorus concentrations in many Connecticut rivers. 

No trends in nitrate were reported in Massachusetts, 

New Hampshire, Rhode Island, and Maine streams 

between 1980-89 (Strause, 1993; Bell, 1993; Olsen and 

Cowing, 1993; and Toppin, 1993). Total-dissolvedsolids concentrations in these States during 1980-89 

also showed little change. Trench (1996) reported 

increasing total-dissolved solids in many Connecticut 

Rivers from 1969-88. Smith and others (1987 and 

1993) described water-quality trends Nationally for 

1974-81 and 1980-89, respectively. They reported 

generally increasing dissolved-solid concentrations in 

streams throughout the country primarily as a result of 

increased use of de-icing salts on roads, increasing and 

decreasing nitrate concentrations as a result of 

changing agricultural fertilizer use patterns, and 

decreasing total phosphorus concentrations in streams 

from reductions in phosphorus fertilizer use and 

phosphate-detergent bans.

There have been no studies that describe waterquality changes in New England rivers from the 

beginning to the end of the 20th century. The U.S. 

Geological Survey (USGS) National Water-Quality 

Assessment Program has used a combination of 

historical and current data to characterize changing 

water-quality conditions in New England rivers during 

the 20th century. This information will be useful in 

assessing how modifications in society and our 

lifestyles are affecting the quality of the region’s rivers. 

Purpose and Scope

This report examines trends in water quality in 

three large New England rivers—the Merrimack, 

Blackstone, and Connecticut Rivers—during the

20th century (fig. 3). Analyses of changes in the 

concentrations of sulfate, chloride, residues, nitrate, 

and total phosphorus in the three rivers for parts of the 

20th century are on the basis of the availability of 

historical and current water-quality data. Possible 

causes of the trends in concentrations of the five 

constituents (table 1) also are presented by examining 

information on changing human activities and 

environmental pollution-control measures during the 

century.

Acknowledgments

The authors acknowledge reviews of this report 

by Sarah Flanagan, Alisa Mast, Kenneth Toppin, and 

Debra Foster of the U.S. Geological Survey.

Ann Marie Squillacci and Anita Cotton of the

U.S. Geological Survey prepared the manuscript for 

publication.

DATA SOURCES AND METHODS OF 

ANALYSIS

The water-quality data used in this report for the 

Merrimack, Blackstone, and Connecticut Rivers are 

from 1900 to 1920 and from 1955 to 2000, and were 

collected in monitoring programs conducted by the 

State Department of Health of Massachusetts (DOH), 

the Massachusetts Department of Environmental 

Protection, formerly called the Massachusetts 

Department of Environmental Quality Engineering 

(DEQE), and the USGS (table 2). Few data are 

available for the period 1920 to 1955. Streamflow data 

for most of the 20th century from the USGS also were 

available for the analyses. Because the water-quality 

data are from nearly 100 years of monitoring efforts by 

several agencies, the data reflect changes in methods of 

sample collection and analysis and sampling location. 

Despite these changes, these data are comparable for 

the purpose of determining the relative change in 

concentration over time for the five constituents 

described in this report.
4 Water-Quality Trends in New England Rivers During the 20th Century

2 3

4

5

6

7 Millville

0

0

25 50 MILES

25 50 KILOMETERS

Connecticut River

Merrimack River

Blackstone River

71 ̊

72 ̊ 73 ̊ 45 ̊

44 ̊

43 ̊

42 ̊

Lake Champlain

Atlantic Ocean

New Bedford

Providence

Woonsocket

Worcester Boston

Manchester

Concord

Montpelier

Tyngsboro Lowell

Springfield

Thompsonville

Hartford

Connecticut

Rhode

Island

Massachusetts

Vermont

New Hampshire

New

York

Maine

CANADA

1

Basin boundaries from U.S. Geological Survey hydrologic unit boundaries, 1:2,000,000-scale, 1992

Rivers from U.S. Geological Survey digital line graphs, 1:2,000,000-scale, 1991

EXPLANATION

 Blackstone River Basin

 Connecticut River Basin

 Merrimack River Basin

 State capital

 Town or city

Sampling site and map number (see table 2)

 Streamflow

 Water quality

 Streamflow and water quality

2

6

1

Concord River

Figure 3. Location of the Merrimack, Blackstone, and Connecticut Rivers in New England, and streamflow and water-quality data sites used to 

assess changes in the 20th century.
Data Sources and Methods of Analysis 5

Water-quality data were collected at more than 

one location on the three rivers during the 20th century 

(table 2). For the Merrimack River, the data represent 

samples collected from locations in and near

Lowell, Mass., and at the confluence of the Concord 

River. Samples from the Blackstone River were 

collected in the area of Millville, Mass. Most data for 

the Connecticut River were from samples collected at 

Thompsonville, Conn. (fig. 4), although data from 

1900-17 were from samples collected about 7 mi 

upstream from Thompsonville at a site downstream 

from Springfield, Mass. Collectively, the data for each 

river represent the most similar sampling locations over 

time and reduce location as a source of variability in 

these data.

The mean of all available concentration data for 

each year was determined for sulfate, chloride, 

residues, nitrate, and total phosphorus and are referred 

to as annual concentrations in this report. Typically, 

sampling frequencies were from 4 to 6 samples per 

year for DOH and DEQE data, and from 4 to 12 

samples per year for USGS data. Samples generally 

were collected throughout the year and represent a 

variety of streamflow conditions that range from low to 

high flows. Water-quality data from 1900-17 were 

based on annual mean concentrations reported in the 

Third Annual Report of the DOH (State Department of 

Health of Massachusetts, 1918). 

Concentration values less than the reporting 

limit, or censored data, were set equal to the reporting 

limit for purposes of computing a mean concentration. 

The resulting annual concentrations of the constituents 

determined by this method will be slightly greater than 

if the mean concentration were determined using 

censored data set equal to half the reporting limit. Data 

were screened to eliminate concentration values that 

were either three times lower or higher than other 

concentrations from a similar time period.

The water-quality data analyzed for this report 

reflect a variety of sample-collection procedures and 

laboratory-analytical methods that were used 

throughout the 20th century. The accuracy and 

precision of the different methods were not compared, 

but the constituents being reported were verified to be 

consistent throughout the length of record. For 

example, the DOH reported chlorine data from 1900-

17, whereas other agencies reported chloride. Chlorine 

and chloride are different terms for the same 

constituent. A comparison of analytical methods used 

by the USGS are discussed in Trench and Vecchia 

(2002), who summarized the effect of changing USGS 

analytical methods during 1968-98 on values of sulfate, 

chloride, and total phosphorus. Sulfate concentrations 

could be biased high before 1990, and total phosphorus 

could be biased low before 1991.

For some constituents, different analytes were 

used to generate a common constituent measure. Either 

total- or dissolved-nitrate values were used to calculate 

annual concentrations, depending on which of the two 

analyses had the largest number of samples in a given 

year. A comparison of USGS total and dissolved nitrate 

data from the same sample was performed using the 

Wilcoxon signed rank statistical method with the 

statistical software StatView (SAS Institute, Inc., 

1999). This test indicated no difference between total 

and dissolved nitrate concentrations (p = 0.59);

Table 1. Anthropogenic sources of sulfate, chloride, residues, nitrate and phosphorus in rivers

[Data from Hem (1989)]

Sulfate Chloride Residues Nitrate Phosphorus

Combustion of fossil 

fuels, coal, and oil

Municipal and industrial 

wastewater

Municipal and industrial 

wastewater

Combustion of fossil fuels, 

coal, and oil

Detergents

Industrial processes 

(smelting)

Road salt Soil erosion (construction, 

forestry, agricultural)

Failing septic systems Failing septic systems

Municipal and industrial 

wastewater

Rock weathering Urban runoff Fertilizer (lawns and cropland) Fertilizer (lawns and 

cropland)

Rock weathering Rock weathering Livestock waste Livestock waste

Municipal and industrial 

wastewater

Municipal and industrial 

wastewater

Synthetic organic 

chemicals (insecticides)

Rock weathering
6 Water-Quality Trends in New England Rivers During the 20th Century

Table 2. Sources of water-quality and streamflow data for the Merrimack, Blackstone, and Connecticut Rivers

[USGS, U.S. Geological Survey; NWIS, National Water Inventory Systems, No., number. Site locations and map numbers are shown in figure 1.]

Constituent Date Collecting agency Data source

Data collection Map 

No. Site location Station No.

Merrimack River at Lowell, Massachusetts

Streamflow 1937-99 USGS NWIS Manchester, N.H. 01092000 1

Sulfate 1967-69 USGS NWIS Lowell, Mass. 01096570 2

1970-95 USGS NWIS above Lowell, Mass. 01096550 3

Chloride 1900-17 State Department of Health of 

Massachusetts

Third Annual Report1 above Lowell, Mass. None 2

1937 Massachusetts Department of 

Public Health

Report on Sources of 

Pollution Merrimack 

River Valley, 

Massachusetts2

Tyngsboro, Mass. Station #2 3

1967-69 USGS NWIS Lowell, Mass. 01096570 2

1970-95 USGS NWIS above Lowell, Mass. 01096550 3

Nitrate 1900-14 State Department of Health of 

Massachusetts

Third Annual Report1 above Lowell, Mass. None 2

1967-69 USGS NWIS Lowell, Mass. 01096570 2

1970-80, 

1985-95

USGS NWIS above Lowell, Mass. 01096550 3

1981 Massachusetts Department of 

Environmental Quality 

Engineering

The Merrimack River 19813 Rte. 113 bridge, 

Tyngsboro, Mass.

MR01 3

Phosphorus, total 1969-95 USGS NWIS above Lowell, Mass. 01096550 3

Blackstone River at Millville, Massachusetts

Streamflow 1930-99 USGS NWIS Woonsocket, R.I. 01112500 4

Sulfate 1973, 

1978-2000

USGS NWIS Millville, Mass. 01111230 5

Chloride 1900-17 State Department of Health of 

Massachusetts

Third Annual Report1 Millville, Mass. None 5

1971, 1973, 

1978-2000

USGS NWIS Millville, Mass. 01111230 5

1977 Massachusetts Department of 

Environmental Quality 

Engineering

Blackstone River 19774 Millville, Mass. BS18 5

Residues, total 1900-17 State Department of Health of 

Massachusetts

Third Annual Report1 Millville, Mass. None 5

1970 Massachusetts Department of 

Environmental Quality 

Engineering

Blackstone River Study 

19705

Railroad Bridge, 

Millville, Mass.

BS20 5

1977 Massachusetts Department of 

Environmental Quality 

Engineering

Blackstone River 19774 Millville, Mass. BS18 5

1978-2000 USGS NWIS Millville, Mass. 01111230 5

Nitrate 1900-14 State Department of Health of 

Massachusetts

Third Annual Report1 Millville, Mass. None 5

1971, 1973, 

1978-2000

USGS NWIS Millville, Mass. 01111230 5
Data Sources and Methods of Analysis 7

Blackstone River at Millville, Massachusetts--Continued

Phosphorus, total 1973,

1978-2000

USGS NWIS Millville, Mass. 01111230 5

1977 Massachusetts Department of 

Environmental Quality 

Engineering

Blackstone River 19774 Millville, Mass. BS18 5

Connecticut River at Thompsonville, Connecticut

Streamflow 1929-99 USGS NWIS Thompsonville, Conn. 01184000 6

Sulfate 1952-56,

1966-2000

USGS NWIS Thompsonville, Conn. 01184000 6

Chloride 1900-17 State Department of Health of 

Massachusetts

Third Annual Report1 below Springfield, Mass. None 7

1952-56,

1966-2000

USGS NWIS Thompsonville, Conn. 01184000 6

Residues, total 1900-04,

1906-17

State Department of Health of 

Massachusetts

Third Annual Report1 below Springfield, Mass. None 7

1969,

1974-2000

USGS NWIS Thompsonville, Conn. 01184000 6

Nitrate 1900-04,

1906-14

State Department of Health of 

Massachusetts

Third Annual Report1 below Springfield, Mass. None 7

1952-56,

1966-74,

1980-2000

USGS NWIS Thompsonville, Conn. 01184000 6

Phosphorus, total 1966-2000 USGS NWIS Thompsonville, Conn. 01184000 6

1State Department of Health of Massachusetts, 1918.

2Massachusetts Department of Public Health, 1938.

3Massachusetts Department of Environmental Quality Engineering, 1982. 4Massachusetts Department of Environmental Quality Engineering, 1978. 5Massachusetts Department of Environmental Quality Engineering, 1971.

Table 2. Sources of water-quality and streamflow data for the Merrimack, Blackstone, and Connecticut Rivers--Continued

[USGS, U.S. Geological Survey; NWIS, National Water Inventory Systems, No., number. Site locations and map numbers are shown in figure 1.]

Constituent Date Collecting agency Data source

Data collection Map 

No. Site location Station No.

Figure 4. The Connecticut River at Thompsonville, Conn., 2002 (Photograph by Karen Beaulieu, U.S. Geological Survey).
8 Water-Quality Trends in New England Rivers During the 20th Century

therefore, total and dissolved nitrate concentrations 

were used together to evaluate trends. For several 

samples, the USGS reported dissolved-nitrate 

concentrations “as nitrate.” These concentrations were 

converted to dissolved nitrate as “nitrogen,” by 

multiplying the “as nitrate” values by a correction 

factor (0.226) based on the atomic weight of nitrogen 

and oxygen. Total-phosphate concentrations analyzed 

by the USGS were converted to total phosphorus by 

multiplying the total-phosphate values by a correction 

factor (0.326) based on the atomic weight of 

phosphorus and oxygen.

The DOH reported total residues on evaporation, 

and the DEQE reported either total residues or total 

solids; all of these analyses measure the weight of 

material left in a vessel after evaporation. For the 

Blackstone and Connecticut Rivers, the total residues 

concentrations from 1900-17 were used with totalsolids concentrations collected by the USGS from 1970 

to 2000. For the Merrimack River, the early totalresidues concentrations were available along with 

dissolved-solids, and to a lesser amount, total-solids 

concentrations from the USGS in the later years of the 

20th century. Because the dissolved concentrations are 

statistically less than total concentrations based on the 

results of the Wilcoxon signed rank test (p < 0.001), the 

two data sets could not be combined to create a 

complete data set for 1970-2000. As a result, residues 

or solids data were not analyzed for trends at the 

Merrimack River. Both total residues and total solids 

are referred to as residues. 

Annual-mean streamflow data were examined 

for temporal trends to assess the potential effects of 

discharge on chemical constituent concentrations. 

Annual-mean streamflow is presented in cubic foot per 

second (ft3/s) in this report. The USGS stream-gaging 

stations used to determine annual streamflow for each 

water-quality site are listed in table 2. Estimates of 

streamflow were made for the Merrimack River at 

Lowell, Mass., and the Blackstone River at

Millville, Mass., because continuous streamflow data 

were not available directly at the water-qualitysampling sites. For the Merrimack River water-quality 

site, the nearest continuous-record stream-gaging 

station with long-term streamflow data is the 

Merrimack River near Goffs Falls, below

Manchester, N.H. (fig. 3). A correction factor based on 

the difference in drainage areas between the waterquality sampling site and stream-gaging station was 

applied to the annual-mean streamflow from the Goffs 

Falls station. Estimated streamflow at the Lowell 

sampling site, about 17 mi downstream from the

Goffs Falls station, is about 128 percent greater than 

the streamflow at Goffs Falls. For the Blackstone River 

at Millville, Mass., the nearest stream-gaging station 

with long-term streamflow data is the Blackstone River 

at Woonsocket, R.I. (fig. 3). A correction factor based 

on the difference in drainage areas between the watersampling site and the stream-gaging station was 

applied to the annual-mean streamflow from the 

Woonsocket station. Estimated streamflow at the 

Millville site, about 6.0 mi upstream from the 

Woonsocket station, is about 67 percent less than the 

streamflow at Woonsocket. 

Changes in annual concentration over time at the 

Merrimack, Blackstone, and Connecticut Rivers were 

plotted on scatter plots. The relation of annual 

concentration to time was demonstrated with a locally 

weighted scatter plot (LOWESS) smoothed line that 

shows a general trend. The LOWESS smoothed line 

was generated using the statistical computer software 

S-PLUS (MathSoft, 1999). The relation of streamflow 

to time was determined by use of linear regression. 

Trends in discharge and water-quality constituents 

were assessed with Kendall’s tau using S-PLUS 

software (MathSoft, 1999). Kendall’s tau computes the 

significance level of the trend based on ranks of the 

data and not the values themselves. A significance level 

(p-value) greater than 0.05 indicates that the trend 

observed is not statistically significant. Tau ranges 

from -1 to +1 and shows the strength of an upward 

trend (positive value) or a downward trend (negative 

value).

The magnitude of change in annual 

concentrations of the constituents was determined by 

calculating the difference (as a percentage) in the 

mean-annual concentration for one time period to the 

mean-annual concentration for a second time period. 

Data for each constituent was divided into two time 

periods. For chloride, residues, and nitrate, the two 

time periods were the first two decades of the

20th century and last four to five decades of the century. 

For sulfate and total phosphorus, the period of waterquality records was split into roughly two equal time 

periods. 

Finally, annual concentrations of chloride, 

nitrate, and total phosphorus were compared to ambient 

water-quality criteria for the protection of aquatic life 

or human health. No such criteria have been 

established for sulfate and residues.
Description of the Merrimack, Blackstone, and Connecticut Rivers 9

DESCRIPTION OF THE MERRIMACK, 

BLACKSTONE, AND CONNECTICUT RIVERS

The Merrimack, Blackstone, and Connecticut 

Rivers have drainage areas at their mouths of 5,010, 

480, and 11,260 mi2, respectively. The Connecticut 

River is the largest river in New England. The 

Merrimack and Connecticut Rivers originate in 

forested and mountainous lands in north-central

New England and flow southward through heavily 

populated urban centers before discharging into coastal 

waters. The Blackstone River originates in central 

Massachusetts and flows south through a primarily 

urban watershed to the Narragansett Bay in Rhode 

Island. 

The drainage areas for the Merrimack River at 

Lowell, Mass., Blackstone River at Millville, Mass., 

and the Connecticut River at Thompsonville, Conn., 

where water-quality data were collected, have drainage 

areas of 3,950, 277, and 9,660 mi2, respectively. The 

percentage of the drainage area classified as urban land 

in the Blackstone River Basin upstream of the waterquality-monitoring site is three to five times greater 

than that found in the Merrimack and Connecticut 

River Basins (table 3).

The Merrimack, Blackstone, and Connecticut 

Rivers were a focal point for human settlement and 

industrial development and played an important role in 

the development and expansion of the industrial 

revolution in the early and mid-1800s by serving as a 

source of waterpower for many mills and factories 

present along their waterways. The Blackstone River 

valley has been described as the birthplace of the 

American Industrial Revolution and was home to the 

first textile mill built in the United States. The mills, 

factories, and cities released large amounts of untreated 

wastewater to the rivers (fig. 5). Shanahan (1994) 

describes the history of industrial development and 

population growth in the Blackstone River Basin and 

its resultant effects on water quality in the river. 

Untreated wastes from pulp and paper and textile mills 

had detrimental effects on the quality of the Merrimack 

River from the 19th century to the late-1960s (U.S. 

Department of Interior, 1968). Because of disease 

outbreaks and water-quality problems in the 

Connecticut River and other waterbodies, the 

Connecticut State Supreme Court in 1900 ruled that the 

release of noxious substances to the State’s rivers were 

not permitted if the substances were harmful to 

downstream users (Symons, 2001).

Figure 5. Historic mills in the early years of the 20th century along the 

Blackstone River, Woonsocket, R.I. (Photograph reprinted from the

John H. Chafee Blackstone River National Heritage Corridor, National 

Park Service and published with permission).

Table 3. Land-use characteristics, population estimates, and municipal wastewater discharge yields in the Merrimack, Blackstone, and Connecticut 

Rivers Basins (Sources: Land-use data from Vogelmann and others, 2001; Population data from U.S. Census decennial files; Municipal wastewater 

discharge data from Medalie, 1996)

[Mgal/d, million gallons per day; mi2, square mile]

River basin

Data for

Urban 

(percent of 

basin)

Forests 

(percent of 

basin)

Agriculture 

(percent of 

basin)

1990 population

1990 population 

density

(people/mi2)

Municipal 

wastewater 

discharge yields 

(Mgal/d/mi2)

Merrimack at Lowell, Mass. (above 

Concord River)

7 76 7 813,000 197 0.08

Blackstone at Millville, Mass. 21 58 6 268,000 1,020 .35

Connecticut at Thompsonville, Conn. 4 80 8 1,093,000 113 .18
10 Water-Quality Trends in New England Rivers During the 20th Century

Flows in all three rivers are regulated by 

numerous dams that were built for power generation, 

flood control, and recreational purposes. Streamflow 

data during the length of record for the three rivers

(fig. 6) show much year-to-year variation in the meanannual flows, but only the Blackstone River had a 

significant increase in annual-mean flows during 1930-

99. The cause(s) of this trend is not known, but may be 

a result of reduced water consumption by industries, 

changes in flow regulation, and increased impervious 

surface area in the basin. The increased flows could 

also be contributing greater amounts of dissolved and 

suspended materials to the river or resulting in the 

dilution of some constituents, and may be contributing 

to changes in water quality during the 20th century. In 

the Merrimack and Connecticut Rivers, the lack of 

changes in annual-mean river flows indicates that 

water-quality changes are more likely a result of factors 

other than changing river flow. 

CHANGES IN THE WATER QUALITY OF THE 

MERRIMACK, BLACKSTONE, AND 

CONNECTICUT RIVERS DURING THE 20TH

CENTURY

Statistical analysis of annual concentrations of 

chloride, residues, and nitrate indicated upward trends 

in all three rivers (α ≤ 0.05) (table 4). Total phosphorus 

decreased in the three rivers, whereas sulfate decreased 

in the Connecticut and Merrimack Rivers. Residues 

increased in the Blackstone and Connecticut Rivers. 

For all the constituents analyzed in this report, annual 

concentrations in the Blackstone River are typically 

twice the annual concentrations in the Merrimack and 

Connecticut Rivers. This is likely the result of greater 

densities of population, wastewater discharges, and 

urban land use in the Blackstone River Basin compared 

to the Merrimack and Connecticut Rivers Basins

(table 3). 

The following sections describe the magnitude of 

change in annual concentrations of sulfate, chloride, 

residues, nitrate, and total phosphorus in the 

Merrimack, Blackstone, and Connecticut Rivers in the 

20th century. Ancillary data and literature information 

that characterize changes in the potential sources of 

these constituents in water are used to help identify 

potential causes for the observed water-quality 

changes. 

Sulfate

Sulfate data were available for only the second 

half of the 20th century for the three rivers studied. The 

Connecticut River has the longest length of record, 

from 1952 to 1956, and from 1966 to 2000 (table 2). 

Although all three rivers show a decrease in annual 

sulfate concentrations between 1950 and 2000 (fig. 7), 

the decrease in the Blackstone River is not statistically 

significant (p = 0.26) (table 4). The largest decreases 

occurred in the Connecticut River. The mean-annual 

sulfate concentrations (1) in the Connecticut River 

decreased by 31 percent from 14 mg/L (1952-56) to

9.6 mg/L (1984-2000), (2) in the Merrimack River 

decreased by 11 percent from 10.4 mg/L (1967-83) to 

9.3 mg/L (1984-2000), and in the Blackstone River 

decreased by 10 percent from 21.1 mg/L (1973-83) to 

18.1 mg/L (1984-2000).

The positive bias on sulfate concentrations 

reported by the USGS before 1990, could be 

contributing to the magnitude of change in sulfate 

concentrations in the three rivers. Annual sulfate 

concentrations, however, continued to decrease in the 

1990s, indicating that the changes during the decade 

were not solely the result of analytical methods.

Atmospheric deposition and weathering of 

sulfur-containing minerals are important sources of 

sulfate in water (Hem, 1989; and Mast and Turk, 1999). 

Mast and Turk (1999) concluded that wet and dry 

atmospheric deposition accounted for most of the 

sulfate in the Wild River in Maine, a generally 

undisturbed watershed. They also concluded that the 

decreases in annual sulfate concentrations in the

Wild River from 1964 to 1995 may be the result of 

decreasing sulfur-dioxide emissions in the 

Northeastern United States. Clow and Mast (1999) also 

found decreasing sulfate concentrations in stream water 

and precipitation at a number of sites in the 

Northeastern United States during 1984-96; concluding 

that a reduction in sulfur-dioxide emissions was the 

likely cause. Sulfur-dioxide emissions increased in the 

United States from 1955 to 1970, but decreased from 

1970 to 1998 (U.S. Environmental Protection Agency, 

2000b) (fig. 7). These emission reductions are the 

result of the conversion of the use of coal to natural gas, 

oil, and nuclear energy for electric power production, 

economic recessions, use of low-sulfur coals, and 

emission controls mandated by the Clean Air Act of 

1970 (U.S. Environmental Protection Agency, 2000b; 

Nizich and others, 1995). Although minimal sulfate 

data for streams (only for the Connecticut and
Changes in the Water Quality of the Merrimack, Blackstone, and Connecticut Rivers During the 20th Century 11

Merrimack Rivers) for 1955-70 is available that 

corresponds to the period of increasing sulfur-dioxide 

emissions (fig. 7), annual sulfate concentrations in the 

three New England Rivers do appear to decrease 

corresponding to sulfur-dioxide-emission reductions 

post 1970. This pattern indicates that sulfur-dioxide 

emissions likely affected the concentrations of sulfate 

in the Merrimack, Blackstone, and Connecticut Rivers. 

The significance of other factors affecting sulfate 

concentrations in the Blackstone River, such as 

wastewater discharges and industrial operations, is not 

known.

Chloride

Annual chloride concentrations in the 

Merrimack, Blackstone, and Connecticut Rivers 

increased during the 20th century (fig. 8 and table 4). 

The magnitude of these increases is greatest in the 

Merrimack River where the mean-annual concentration 

increased 760 percent from 2.9 mg/L (1900-17) to

24.9 mg/L (1976-95). Mean-annual concentrations of 

chloride in the Blackstone and Connecticut Rivers 

increased 186 percent and 344 percent, respectively, 

from 1900-17 to 1966-2000. In the 1990s, annual 

chloride concentrations approached 75 mg/L in the 

Blackstone River, whereas those in the Merrimack and

1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

WATER YEAR

2,000

3,250

4,500

5,750

7,000

8,250

9,500

10,750

12,000

ANNUAL MEAN STREAMFLOW, IN CUBIC FEET PER SECOND

A. Merrimack River

1929 1934 1939 1944 1949 1954 1959 1964 1969 1974 1979 1984 1989 1994 1999

WATER YEAR

100

200

300

400

500

600

700

800

900

ANNUAL MEAN STREAMFLOW, IN CUBIC FEET PER SECOND

B. Blackstone River

1928 1933 1938 1943 1948 1953 1958 1963 1968 1973 1978 1983 1988 1993 1998

WATER YEAR

7,500

10,000

12,500

15,000

17,500

20,000

22,500

25,000

27,500

C. Connecticut River

ANNUAL MEAN STREAMFLOW, IN CUBIC FEET PER SECOND

p=0.49 p=0.04

p=0.09

EXPLANATION

 Regression line

 Annual mean streamflow

 p Probability value of trend

 in streamflow over time;

 a trend is significant when p 

 is less than or equal to 0.05

Figure 6. Annual mean streamflow and streamflow trends in the (A) Merrimack River at Lowell, Mass., during 1935-2000; (B) Blackstone River at 

Millville, Mass., during 1929-2000; and (C) Connecticut River at Thompsonville, Conn., during 1928-2000.
12 Water-Quality Trends in New England Rivers During the 20th Century

Table 4. Summary of Kendall’s tau analysis for trends in streamflow and annual concentrations of sulfate, chloride, residues, nitrate, and total 

phosphorus in the Merrimack, Blackstone, and Connecticut Rivers from 1900-2000

[p-values less than 0.05, shown in bold type, indicate that a significant trend was found; --, no data]

Constituent

Merrimack River Blackstone River Connecticut River

Kendall’s Tau Number of 

samples in 

comparison

Kendall’s Tau Number of 

samples in 

comparison

Kendall’s Tau Number of 

samples in 

p-value tau p-value tau p-value tau comparison

Streamflow 0.4877 0.0599 63 0.0441 0.1644 70 0.0868 0.1388 71

Sulfate .0146 -.3202 29 .2582 -.160 24 .0000 -.6885 40

Chloride .0000 .7270 48 .0000 .7252 44 .0000 .7005 57

Residues -- -- -- .0000 .5903 43 .0010 .3394 45

Nitrate .0157 .2622 41 .0000 .5436 40 .0003 .3597 49

Phosphorus, total .0002 -.5100 27 .0054 -.3967 25 .0001 -.4756 35

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

WATER YEAR

5

10

15

20

25

CONCENTRATION OF SULFATE, IN MILLIGRAMS PER LITER

18,000,000

22,500,000

27,000,000

31,500,000

36,000,000

SULFUR DIOXIDE EMISSIONS IN THE UNITED STATES, IN SHORT TONS

EXPLANATION

 Sulfur dioxide emissions

LOWESS (trend) line and annual

 concentration data

 Merrimack River

 Blackstone River

 Connecticut River

Figure 7. Trends in annual sulfate concentrations in the Merrimack, Blackstone, and Connecticut Rivers from 1952-2000, and annual sulfurdioxide emissions in the United States from 1950-98 (Annual sulfur dioxide emissions data from the U.S. Environmental Protection Agency, 

2000b).
Changes in the Water Quality of the Merrimack, Blackstone, and Connecticut Rivers During the 20th Century 13

Connecticut Rivers approached 25 and 15 mg/L, 

respectively. These concentrations are well below the 

recommended ambient chloride water-quality criteria 

of 230 mg/L (U.S. Environmental Protection Agency, 

1988).

Increasing chloride concentrations of streams 

have been reported by other studies in New England, 

the Northeast, and throughout the United States 

(Trench, 1996; Strause, 1993; Toppin, 1993; Robinson 

and others, 1996; and Smith and others, 1987). The 

upward chloride trends reported in some of these 

studies are likely the result of road-salt application for 

de-icing purposes (Robinson and others, 1996; and 

Smith and others, 1987). From 1940 to 2000, the 

annual amount of salt sold for highway de-icing in

the United States increased from 164,000 to

18,101,000 tons; an increase of over 1,000 percent

(fig. 8) (Salt Institute, 2001). The increases in chloride 

concentrations in the three New England rivers does 

generally correspond to road-salt sales for the United 

States in the second half of the 20th century. Measures 

of other water-quality indicators, such as specificconductance values and sodium concentrations also are 

thought to mirror the increasing concentrations of 

chloride; this pattern was seen in other areas (Robinson 

and others 1996, and Trench, 1996). 

Residues

Residues data from 1900 to 1917 are compared 

to total solids data from 1969 to 2000 for the 

Blackstone and Connecticut Rivers (fig. 9). Residues 

increased in both rivers during the 20th century 

(table 4). Mean-annual concentrations of residues 

increased in the Blackstone River by 47 percent from 

116 mg/L (1900-17) to 170 mg/L (1970-2000), and in 

the Connecticut River by 17 percent from 71.5 mg/LSALT SALES IN THE UNITED STATES, IN TONS

1895 1905 1915 1925 1935 1945 1955 1965 1975 1995 2000

WATER YEAR

0

20

40

60

80

CONCENTRATION OF CHLORIDE, IN MILLIGRAMS PER LITER

0

6,250,000

12,500,000

18,750,000

25,000,000

1985

EXPLANATION

 Salt sales

LOWESS (trend) line and annual

 concentration data

 Merrimack River

 Blackstone River

 Connecticut River

Figure 8. Trends in annual chloride concentrations in the Merrimack, Blackstone, and Connecticut Rivers from 1900-2000, and annual amount

of salt sold for highway de-icing in the United States from 1940-2000 (Annual salt sales data from the Salt Institute, 2001).
14 Water-Quality Trends in New England Rivers During the 20th Century

(1900-17) to 83.7 mg/L (1969-2000). In the last decade 

of the 20th century, annual residue concentrations 

continue to increase in the Blackstone River, but 

changed little in the Connecticut River (fig. 9). 

Residues data can be used to reflect a variety of 

anthropogenic activities in a watershed—including 

wastewater discharges, soil erosion, and road salting. 

Annual residues concentration increased during 1900-

17, which generally corresponds to the growth of New 

England’s population during the same period (fig. 9). 

The relation of residues to population is less obvious 

from 1969 to 2000. This may be the result of improved 

wastewater treatment, and reforestation of the New 

England landscape that has resulted in reductions in 

soil erosion during the later part of the 20th century.

There were notable increases in residues during 

1910-17 in all three rivers. Changes in laboratory 

methods during 1910-17 may have contributed to the 

increasing concentrations. A review of standard 

laboratory methods from 1905 to 1917 identified 

changes in the suggested method of evaporating liquids 

and volatile substances (heating at 103oC for one-half 

hour changed to heating at 180oC for 1 hour) to 

determine total residues (American Public Health 

Association, 1905 and 1917). 

Nitrate

Annual nitrate concentrations in all three New 

England rivers increased during the 20th century

(table 4 and fig. 10). In the Merrimack and Connecticut 

Rivers, mean-annual concentrations increased by an 

order of magnitude. During 1900 to 1914, the meanannual concentration in both rivers was 0.04 mg/L. 

This compares to a mean-annual concentration of

0.33 mg/L in the Merrimack River from 1967 to 1996, 

and 0.35 mg/L in the Connecticut River from 1966 to 

2000. In the Blackstone River, mean-annual nitratePOPULATION

1895 1905 1915 1925 1935 1945 1955 1965 1975 1995 2005

WATER YEAR

25

75

125

175

225

CONCENTRATION OF TOTAL RESIDUES, IN MILLIGRAMS PER LITER

5,000,000

7,000,000

9,000,000

11,000,000

15,000,000

1985

13,000,000

EXPLANATION

 Population

LOWESS (trend) line and annual

 concentration data

 Blackstone River

 Connecticut River

Figure 9. Trends in annual residue concentrations in the Blackstone and Connecticut Rivers from 1900-2000, and population of New England 

from 1900-98 (Population data from the U.S. Census Bureau, 2001).
Changes in the Water Quality of the Merrimack, Blackstone, and Connecticut Rivers During the 20th Century 15

concentrations increased nearly 400 percent in the

20th century from 0.27 mg/L (1900-14) to 1.31 mg/L 

(1971-2000). During the last 10-20 years of the 

century, the increases in annual nitrate concentrations 

appear to have slowed in the Blackstone and 

Connecticut Rivers or been reversed in the Merrimack 

River. All annual nitrate concentrations are well below 

the U.S. Environmental Protection Agency (USEPA) 

criteria of 10 mg/L for the protection of human health 

(U.S. Environmental Protection Agency, 1986).

Large increases in nitrogen concentrations in 

rivers have been previously noted by a number of 

investigators (National Research Council, 2001). In 

addition, because nitrogen is often a limiting nutrient in 

estuarine and ocean waters, a number of studies have 

quantified and analyzed nitrogen loadings to coastal 

waters of the northeastern United States. Jaworski and 

Hetling (1996), using some of the data presented in this 

report, found nitrate concentrations in 12 rivers in the 

eastern United States increased by an average of

470 percent in the 20th century. Roman and others 

(2000) and Jaworski and Hetling (1996) report total 

nitrogen concentrations in rivers in the northeastern 

United States increased by 300 percent during the

20th century. These same studies show that the upward 

trends of nitrogen leveled off from 1970 to 1980. 

Howarth and others (1996) estimated the amount of 

total nitrogen from rivers to estuaries in northeastern 

United States increased 5-20 times in the 20th century.

Sources of the increasing nitrate concentrations 

in the Merrimack, Blackstone, and Connecticut Rivers 

during the 20th century likely include atmospheric 

deposition, fertilizer applications and other agricultural 

practices, and the presence of an increasing population. NITROGEN OXIDE EMISSIONS IN THE UNITED STATES, IN SHORT TONS

1895 1905 1915 1925 1935 1945 1955 1965 1975 1995 2000

WATER YEAR

0

0.5

1.0

1.5

2.0

CONCENTRATION OF NITRATE, IN MILLIGRAMS PER LITER

5,000,000

16,250,000

27,500,000

38,750,000

50,000,000

1985

NITROGEN FERTILIZER USAGE IN NEW ENGLAND, IN KILOGRAMS

EXPLANATION

 Nitrogen oxide emissions

 Nitrogen fertilizer usage

LOWESS (trend) line and annual

 concentration data

 Merrimack River

 Blackstone River

 Connecticut River

Figure 10. Trends in annual nitrate concentrations in the Merrimack, Blackstone, and Connecticut Rivers from 1900-2000, annual nitrogen oxide 

omissions in the United States from 1940-98, and annual agricultural usage of nitrogen fertilizer in New England from 1945-91 (Annual nitrogen 

oxide emissions data from the U.S. Environmental Agency, 2000b; annual nitrogen fertilizer usage data from Alexander and Smith, 1990, and 

Alexander, R.B., U.S. Geological Survey, written commun., 2001).
16 Water-Quality Trends in New England Rivers During the 20th Century

Jaworski and others (1997) show a relation between 

increasing total nitrogen loads in rivers in the 

northeastern United States with increasing nitrogenoxide emissions from fossil-fuel combustion and 

nitrogen-fertilizer application rates on agricultural 

lands. They also report a slight decrease in total 

nitrogen in rivers as decreases in both atmospheric 

deposition of nitrogen and agricultural fertilizer use 

occurred in the 1980s. Data on nitrogen-oxide 

emissions in the United States from the U.S. 

Environmental Protection Agency (2000b) show about 

a three-fold increase in emissions from 1940 to 1998 

(fig. 10). 

Agricultural use of nitrogen fertilizers in New 

England increased from the 1950s to early 1960s, 

changed little from the mid-1960s to mid 1980s, and 

then decreased from 1985 to 1990 (Alexander and 

Smith, 1990; and Richard Alexander, U.S. Geological 

Survey, written commun., 2001) (fig. 10). The lack of 

change or downward change in agricultural nitrogen 

fertilizer use from 1965 to 1991 matches the overall 

lack of nitrate concentration trends in the Blackstone 

and Connecticut Rivers for the later years of the

20th century. Increased human population density was 

related to increased nitrate fluxes in major rivers 

worldwide (Peierls and others, 1991). The increasing 

New England population (fig. 9) may be a contributing 

factor in the overall nitrate concentration increases in 

the three rivers. The cause(s) of decreasing annual 

nitrate concentrations in the Merrimack River during 

1967-95 are not known.

Total Phosphorus

Total phosphorus data from the three rivers 

studied were limited during 1965-2000 (table 2). Total 

phosphorus generally was not measured in stream 

water until the 1960s, when eutrophication of waters 

from phosphate-containing detergents became a 

concern. Annual concentrations of total phosphorus 

decreased in all three rivers during the last third of the 

20th century (table 4 and fig. 11). Comparisons of the 

annual concentration from each river from 1967 to 

1984, and from 1985 to 2000 were used to assess the 

amount of change. Mean-annual concentrations of total 

phosphorus in the Merrimack River decreased by

38 percent, from 0.13 (1967-84) to 0.08 mg/L (1985-

2000). The mean-annual concentrations decreased in 

the Blackstone River by 19 percent, from 0.31 mg/L 

(1967-84) to 0.25 mg/L (1985-2000), and in the 

Connecticut River by 40 percent, from 0.10 mg/L 

(1967-84) to 0.06 mg/L (1985-2000). Annual 

concentrations of total phosphorus at the end of the 

20th century continue to decline in the Merrimack and 

Blackstone Rivers, but appear to have stabilized in the 

Connecticut River (fig. 11). Nearly all annual 

concentrations of total phosphorus in these rivers are 

greater than the USEPA draft total phosphorus criteria 

of 0.031 mg/L for coastal areas of New England

(U.S. Environmental Protection Agency, 2000c).

The changes in total phosphorus concentrations 

detected in the three rivers are similar to the declines 

reported by Roman and others (2000) and Litke (1999). 

Both of these studies report that concentrations of total 

phosphorus in streams in the United States likely 

peaked in the 1960s and early 1970s as a result of 

phosphorus fertilizer use and discharged wastewater 

containing high phosphate levels from detergents and 

soaps. Litke (1999) also noted that loadings of total 

phosphorus in wastewater decreased by as much as

50 percent during the last quarter of the 20th century as 

phosphate detergent bans took effect. At the same time, 

phosphorus fertilizer use on agricultural lands 

nationally peaked in the early 1980s, and has since 

remained generally the same (Litke, 1999). In New 

England, agricultural use of phosphorus fertilizers 

decreased by 45 percent between 1965-91 (fig. 11), 

indicating that national phosphorus-fertilizer-use 

patterns are different than those found in New England 

(Alexander and Smith, 1990; and Richard Alexander, 

written commun., 2001). 

The trend in agricultural phosphorus fertilizer 

use in New England has a similar pattern to the 

decrease in total phosphorus concentrations seen in the 

Merrimack, Blackstone, and Connecticut Rivers. 

Although changes in total phosphorus releases by 

wastewater-treatment facilities during 1965-2000 have 

not been documented in New England, the changes 

seen in the total phosphorus concentrations in the three 

rivers are likely a result of reductions in phosphorus 

loads from wastewater-treatment facilities and in 

agricultural phosphorus fertilizer use. 
Summary 17

SUMMARY

A combination of historical and recent data for 

sulfate, chloride, residues, nitrate, and total phosphorus 

are used to determine how water-quality conditions 

have changed in the Merrimack, Blackstone, and 

Connecticut Rivers during the 20th century in New 

England. These data were collected by the U.S. 

Geological Survey and by the State of Massachusetts 

for different purposes and using different analytical 

methods. These data, however, are considered 

sufficiently comparable for identifying changes in 

water quality.

All of the water-quality constituents analyzed for 

this report show statistically significant trends in one or 

more of the three rivers during the 20th century. Annual 

concentrations of chloride and nitrate increased in all 

three rivers, whereas annual concentrations of residues 

increased only in the Blackstone and Connecticut 

Rivers. Annual chloride concentrations increased by an 

order of magnitude for the Merrimack, Blackstone, and 

Connecticut Rivers. Annual nitrate concentrations 

increased by an order of magnitude in the Merrimack 

and Connecticut Rivers. Annual sulfate concentrations 

in the three rivers decreased during the second half of 

the 20th century, although the decrease in the 

Blackstone River was not statistically significant. 

Annual total phosphorus concentrations also decreased 

in all three rivers during the last third of the century. 

The lack of statistical trends in annual-mean flow in the 

Merrimack and Connecticut Rivers during the periodof-flow record indicates that water-quality trends in 

these rivers are likely the result of factors other than 

1965 1970 1975 1980 1985 1990 2000

WATER YEAR

0

0.1

0.3

0.4

0.5

CONCENTRATION OF TOTAL PHOSPHORUS, IN MILLIGRAMS PER LITER

8,000,000

10,000,000

12,000,000

16,000,000

18,000,000

1995

PHOSPHORUS FERTILIZER USAGE IN NEW ENGLAND, IN KILOGRAMS

14,000,000

0.2

EXPLANATION

 Phosphorus fertilizer usage

LOWESS (trend) line and annual

 concentration data

 Merrimack River

 Blackstone River

 Connecticut River

Figure 11. Trends in annual total phosphorus concentrations in the Merrimack, Blackstone, and Connecticut Rivers from 1966-2000, and 

annual agricultural usage of phosphorus fertilizer in New England from 1965-91 (Annual phosphorus fertilizer usage data from Alexander and 

Smith, 1990, and Alexander, R.B., U.S. Geological Survey, written commun., 2001).
18 Water-Quality Trends in New England Rivers During the 20th Century

streamflow. The Blackstone River had an increase in 

annual-mean flows that could be resulting in changing 

water-quality conditions in this river.

The changes in river-water quality detected in the 

Merrimack, Blackstone, and Connecticut Rivers are 

similar to those reported in the literature. Decreases in 

sulfate concentrations follow a similar pattern of 

decreasing sulfur-dioxide emissions in the United 

States. The increased use of road de-icing salts and 

increasing chloride concentrations appear related. 

Increasing nitrate concentrations in rivers follow the 

general patterns of upward and stabilizing trends in the 

agricultural use of nitrogen fertilizers in New England, 

and, to a lesser degree, nitrogen-oxide emissions in the 

United States. Finally, decreases in annual total 

phosphorus concentrations are similar to decreases in 

the agricultural use of phosphorus fertilizer. In 

addition, bans on the use of phosphate-containing 

detergents by the New England States has likely 

contributed to the decrease in concentrations of 

phosphorus.

The Blackstone River consistently had greater 

concentrations than the Merrimack and Connecticut 

Rivers of all five constituents analyzed. Greater 

densities of population, wastewater discharges, and 

urban land uses are likely the primary reasons for these 

large concentrations.

The ability to synthesize water-quality data that 

represent a century of major societal changes in New 

England provides insight on how those changes affect 

the quality of rivers. As the human population and its 

associated development activities continue to increase 

in the 21st century, further changes in water quality are 

expected. The ability to predict and document these 

effects cannot be made by assuming that the rate of 

change observed in the 20th century will continue. 

These predictions will be dependent on additional 

water-quality data-collection programs that provide 

new information in the future.

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