U.S. Department of the Interior WRIR 02-4149

U.S. Geological Survey 2002

In cooperation with the

PROVIDENCE WATER SUPPLY BOARD

Sources of Sodium and Chloride in the Scituate Reservoir 

Drainage Basin, Rhode Island

By Mark T. Nimiroski and Marcus C. Waldron 

Findings−

• Deicing of State and local roads was the largest source of 

sodium and chloride in the Scituate Reservoir drainage 

basin during Water Year (WY) 2000. Of approximately 

1,000 tons of sodium and 2,300 tons of chloride introduced into the basin, 67 percent of the sodium and more 

than 90 percent of the chloride came from deicing of 

State highways, local roads, and other surfaces. Precipitation, individual sewage-disposal systems, and geologic 

weathering accounted for the remaining 33 percent of 

the sodium and 10 percent of the chloride inputs.

• Salting of State-maintained roads contributed more 

sodium and chloride during WY 2000 than did salting of 

locally maintained roads, although there were about 

45 percent more total lane-miles of locally maintained 

roads than State-maintained roads in the basin. Annual 

basin-wide application rates were estimated to be 

2.02 tons per lane mile for sodium and 8.03 tons per lane 

mile for chloride applied to State-maintained roads. For 

locally maintained roads, the average application rates 

were 1.06 tons per lane mile for sodium and 2.12 tons 

per lane mile for chloride. This application-rate 

difference may reflect the greater volume of traffic and 

higher speed limits on State-maintained roads.

• The median concentration of sodium measured periodically from 1983 through 2000 (calendar years) in 

streams that supply water to the Scituate Reservoir 

was positively correlated with the density of Statemaintained roads in the stream subbasins. About 62 

percent of the variation in median stream-sodium concentration was accounted for by variation in the density 

of State-maintained roads. In contrast, no correlation 

could be identified between median stream-sodium 

concentration and the density of locally maintained 

roads in the subbasins. This difference may result from a 

combination of higher road-salt application rates, greater 

proximity to tributary streams, and more efficient drainage systems associated with State-maintained roads as 

compared to locally maintained roads.

• The effectiveness of a State-mandated use of a 60:40 

mixture of calcium chloride and sodium chloride on 

State-maintained roads in the drainage basin beginning 

in 1990 was determined by comparing relations between 

subbasin-road density and median stream-sodium 

concentrations for the periods 1983 through 1989 and 

1990 through 2000. There was no change in the slope of 

the regression line for these relations following the introduction of the reduced-sodium deicing agent, indicating 

that there was no measurable change in the contribution 

of State-maintained roads to the sodium loads to the 

streams as a result of this action. It is not known if this 

apparent lack of effect is due to increased use of sodium 

chloride for deicing by municipalities and private individuals in the basin, or to mobilization of sodium 

previously applied and stored in the drainage basin.

The Scituate Reservoir system is the principal drinking-water supply for more than 60 percent of the 

population of Rhode Island. Sodium concentrations in the reservoir have been increasing slowly in recent years 

despite the long-term (12 years) use of a reduced-sodium deicing agent on State-maintained highways in the 

reservoir drainage basin. This report identifies the major sources of sodium and chloride in the drainage basin, 

and assesses the relative contribution of each source to the total amounts of these constituents that entered the 

basin from October 1999 through September 2000 (Water Year 2000).
2 

Introduction

The Scituate Reservoir system is the principal 

drinking-water supply for more than 60 percent of 

the population of Rhode Island. The system includes 

Scituate Reservoir and six tributary reservoirs—

the Ponaganset, Barden, Kimble, Westconnaug, Moswansicut, and Regulating Reservoirs—with a total surface 

area of 7.2 square miles (mi2) and a maximum storage 

capacity of 40 billion gallons (fig. 1). The drainage basin 

contributing water to the system covers a 94 mi2 area and 

is primarily undeveloped forest land. The proximity of the 

basin to Providence, Rhode Island, and other major New 

England urban centers has led to recent increases in 

transportation and suburban development in many parts of 

the basin. This development has increased the potential 

for transport of contaminants to the water supply.

One potential source of contamination is the 

increased use of sodium chloride as a deicing agent on 

highways, local roads, sidewalks, and paved parking lots 

in the drainage basin. From 1983 to 1987, the mean 

sodium concentration in tributaries to the Scituate 

Reservoir increased from 7.9 to 19.8 milligrams per liter 

(mg/L), and the mean chloride concentration increased 

from 16.9 to 26.4 mg/L (Rhode Island Department of 

Administration, 1988). During the same period, sodium 

concentrations in the reservoir at the treatment-plant 

intake increased from 7.7 mg/L to a high of 9.3 mg/L 

in 1985, whereas chloride concentrations at the intake 

increased from 10.7 to 13.2 mg/L (Rhode Island 

Department of Administration, 1988). Because of the 

association of dietary sodium with hypertension, the U.S. 

Environmental Protection Agency recommends that 

sodium concentrations in drinking water not exceed 

20 mg/L (U.S. Environmental Protection Agency, 1998). 

The National secondary drinking-water standard for 

chloride is 250 mg/L (U.S. Environmental Protection 

Agency, 2001). Both constituents are difficult and 

expensive to remove by conventional drinking-water 

treatment methods (Manahan, 1999).

In 1990, the Rhode Island Department of 

Transportation (RIDOT), in cooperation with the 

Providence Water Supply Board (PWSB), adopted the 

use of a 60:40 mixture of calcium chloride and sodium 

chloride, mixed with sand, in place of sodium chloride 

and sand, on State-maintained highways in the drainage 

basin. This practice was expected to reduce the sodiumapplication rate to these highways by approximately 

40 percent, and thereby lower the sodium concentration 

in the water supply (Rhode Island Department of 

Administration, 1988). However, semiannual analyses 

of raw water from the treatment-plant intake indicate 

that the mean concentration of sodium in the reservoir 

has not decreased appreciably following implementation 

of this policy. 

The U.S. Geological Survey (USGS) is cooperating 

with the PWSB, the agency responsible for managing the 

drainage basin and protecting the water supply, to develop 

a more complete understanding of the sources, transport 

mechanisms, and fate of sodium and chloride in the drainage basin. This report identifies the major sources of 

sodium and chloride in the Scituate Reservoir drainage 

basin and presents estimates of the amounts of these 

constituents that entered the basin during WY 2000.

Because road deicing was expected to be one of the 

most important sources, the effects of road density on 

sodium and chloride concentrations in tributary streams 

were examined using data collected from 1983 to 2000 by 

the PWSB at 31 monitoring stations located throughout 

the drainage basin. Road density was used as an indicator 

of the rate of application of road salt. Median sodium and 

chloride concentrations for individual streams during 

these 17 years were compared with 1995 road densities in 

the subbasins supplying the streams. Comparisons were 

made separately for data collected before and after introduction of the use of the reduced-sodium deicing agent on 

State-maintained roads in the basin.

U.S. Geological Survey hydrologist collecting water samples 

from a tributary to the Scituate Reservoir, Rhode Island.
 3

Ponaganset Riv er

Wesconnaug

Shippee Brook

Wilbur Hollow Brook

Wind or Brook

Brook

01115817

01115098 Peeptoad Brook

01115098

GLOCESTER

SCITUATE

FOSTER

JOHNSTON

CRANSTON

Base from USGS Digital Data

State Plane Feet Zone 5176

1:24,000 Scale

INDEX MAP OF RHODE ISLAND

RHODE ISLAND

SOUND

0 10 MILES

0 10 KILOMETERS

Westconnaug Reservoir

Ponaganset 

Reservoir 

Regulating 

Reservoir

Moswansicut 

 Reservoir

Kimball

Reservoir

Barden

Reservoir

Scituate Reservoir

0

0

1 2 3 4 5 MILES

1 2 5 KILOMETERS 3 4

EXPLANATION

WATER BODY

WETLAND

BASIN BOUNDARY

TOWN BOUNDARY

PRECIPITATION MONITORING SITE

U.S. GEOLOGICAL SURVEY STATION AND NUMBER

Figure 1. Location and hydrography of the Scituate Reservoir drainage basin, Rhode Island.
4 

Description of the Scituate Reservoir 

Drainage Basin

The Scituate Reservoir drainage basin is in northcentral Rhode Island, approximately 10 miles (mi) west 

of Providence. The basin includes most of the town of 

Scituate, and parts of Foster, Glocester, Johnston, and 

Cranston, Rhode Island (fig. 1). A large part (65.3 percent) of the drainage basin consists of undeveloped forest 

land. About 15 percent of the basin is developed, with 

residential (9.9 percent), commercial (0.5 percent), 

industrial (0.1 percent), institutional (0.2 percent), and 

agricultural (4.1 percent) land uses. The remaining land 

cover consists mainly of open water (7.5 percent) and 

wetlands (10.5 percent).

The topography of the drainage basin is characterized by steep slopes, rolling hills, and large rock outcrops. 

Slopes greater than 10 percent are found in about 20 percent of the basin (Breault and others, 2000). Soils 

generally are well-drained; poorly drained soils are found 

in only about 17 percent of the basin. Bedrock in the 

basin is composed primarily of Devonian and late 

Proterozoic igneous and metamorphic rocks and is 

overlain by glacial deposits consisting of till and 

meltwater deposits. Till covers most of the drainage basin, 

whereas meltwater deposits such as sand and gravel are 

limited to stream valleys and other low-lying areas 

(Breault and others, 2000).

The Ponaganset River, the principal stream in the 

basin, flows from its source at the Ponaganset Reservoir 

to its outlet at Barden Reservoir. The stream drains about 

14 mi2 (15 percent) of the basin. Other major tributaries 

include Peeptoad, Windsor, Shippee, Westconnaug, and 

Wilbur Hollow Brooks, which drain about 21 mi2 (20 

percent) of the basin (fig. 1). The USGS operates two continuous-record streamflow-monitoring stations in the 

drainage basin. During WY 2000, the annual mean 

discharges at the USGS continuous-record streamflowmonitoring stations on the Ponaganset River at South 

Foster, Rhode Island (01115187), and on Peeptoad Brook 

at Elmdale Road near North Scituate, Rhode Island 

(01115098), were 26.2 and 9.46 cubic feet per second 

(ft3

/s), respectively (Socolow and others, 2001).

Quantifying Sources of 

Sodium and Chloride

Information on the total amounts 

of sodium and chloride that entered the 

Scituate Reservoir drainage basin 

during WY 2000 were obtained from a 

variety of sources, including State and 

municipal public works departments, 

local precipitation-monitoring 

programs, State planning programs, 

and the scientific literature. This information was used to estimate sodium 

and chloride loads from each major 

source. The following sections present 

the methods and assumptions used in 

each case to develop the load estimates.

Precipitation

Total annual amounts of sodium 

and chloride that entered the drainage 

basin in precipitation were estimated 

from concentrations of sodium and 

chloride measured in precipitation 

samples collected weekly at the 

National Atmospheric Deposition 

Program/ National Trends Network 

(NADP/NTN) monitoring site CT15 in 

Abington, Conn., 15 miles west of the 

Scituate Reservoir drainage basin, and 

from monthly measurements of precipitation at five monitoring sites operated 

by the PWSB (fig. 1). The Thiessen 

method (Hammer and MacKichan, 

1981) was used to weight monthly precipitation totals for each station relative 

to the area of influence, and these 

values were summed to obtain the total 

annual precipitation falling on the 

drainage basin. This value was then 

multiplied by the area of the drainage 

basin to obtain the total volume of precipitation falling on the basin during 

WY 2000. Precipitation-weighted 

annual mean sodium and chloride concentrations during WY 2000 at the 

NADP/NTN site were determined from 

weekly samples by multiplying sample 

concentrations by their corresponding 

sample volumes, summing the values 

for WY 2000, and dividing by the total 

sample volume for the year. Total 

amounts of sodium and chloride were 

then estimated by multiplying the 

annual mean concentrations at the 

NADP/NTN site by the total volume of 

precipitation that fell on the drainage 

basin during WY 2000.

Road-Salt Applications

The amounts of sodium and 

chloride that entered the drainage basin 

from road deicing were determined 

from records maintained by the Rhode 

Island Department of Transportation 

(RIDOT), and by each of the four 

towns with roads in the basin, of the 

amounts of road salt applied or purchased during WY 2000. The RIDOT 

applies a 60:40 mixture of calcium 

chloride and sodium chloride, together 

with sand, to State-maintained roads. 

Before the sand is added, this mixture 

contains about 16 percent sodium, 62 

percent chloride, and 22 percent calcium by weight (Granato and others, 

1995). The town of Foster, Rhode 

Island, used the same reduced-sodium 

mixture during WY 2000. Amounts of 

sodium and chloride applied by the 

RIDOT and the town of Foster during 

FY 2000 were determined by taking 

appropriate percentages of the total 

amounts of salt mixtures purchased 

during the period. Other towns in the 

drainage basin used sodium chloride 

and sand. Records of amounts 

purchased (Johnston and Glocester) or 
 5

applied (Scituate), were used to calculate sodium and chloride applications 

based on the amounts of each constituent in sodium chloride.

Because the total amounts 

reported by the towns included salting 

of municipal roads outside of the drainage basin, these values were adjusted 

by multiplying by the fractions of the 

total municipal road miles actually 

present in the drainage basin. Amounts 

reported by the RIDOT were only for 

roads within the drainage basin.

Other Deicing Activities

Amounts of sodium and chloride 

deposited on sidewalks and driveways 

of private residences were estimated by 

assuming that 50 percent of the households in the drainage basin used an 

average of 20 pounds of salt (7.9 

pounds of sodium and 12.1 pounds of 

chloride) during WY 2000. The 

number of households (2,254) was estimated by dividing the projected 2000 

drainage basin population (see below) 

by 3.25 persons per average household 

(Runge and others, 1989). The high utilization rate (50 percent) was chosen to 

account for additional applications on 

commercial and industrial parking 

areas and sidewalks.

Septic-System Leachate

Amounts of sodium and chloride 

that entered the Scituate Reservoir 

drainage basin in septic-system 

leachate during WY 2000 were estimated from the number of people 

living in the basin and published measurements of per capita ISDS loading 

rates. Projected 2000 town populations 

were obtained from the Rhode Island 

Department of Administration Statewide Planning Program (Rhode Island 

Department of Administration, 2001) 

and were used to calculate an estimated 

percent change in the total population 

for each town. This percent change was 

applied to the populations in the 1990 

census blocks that fell wholly or partially within the drainage basin. These 

estimates were then adjusted according 

to the fractions of residential-land area 

(RIGIS) in the census blocks that fell 

within the drainage basin. The total 

estimated WY 2000 population living 

within the drainage basin (14,650) 

was then multiplied by per capita ISDS 

loads of sodium (21 pounds per year) 

and chloride (12 pounds per year) 

reported by Runge and others (1989) 

for the Scituate Reservoir drainage 

basin.

Geologic Weathering

Because no measurements of 

weathering rates for sodium and chloride in the Scituate Reservoir drainage 

basin were available, estimates were 

obtained from published values for 

other sites in New England (Likens and 

Borman, 1995) and the southeastern 

United States (Coleman and Diether, 

1986). These values range from 0.85 to 

1.65 tons per square mile per year. An 

average reported value of 1.25 tons per 

square mile per year was applied to the 

land-surface area of the drainage basin 

to obtain an estimate of the amount of 

sodium produced by weathering. It was 

assumed that the amount of chloride 

produced by weathering was negligible, because the amounts of chloride in 

rock types common in the drainage 

basin are extremely small.

Sodium and Chloride Enter the 

Scituate Reservoir Drainage 

Basin from Multiple Sources

Sodium and chloride may enter the Scituate 

Reservoir drainage basin in precipitation, including wet 

and dry deposition, in salt applied during winter deicing 

of roads, highways, sidewalks, and parking lots, in 

leachate from unprotected road-salt supplies, and in 

leachate from individual sewage-disposal systems (ISDS) 

(fig. 2). In addition, sodium may be added by weathering 

of sodium feldspars in metamorphic and igneous rocks in 

the basin. Of these sources, wet deposition (precipitation), 

deicing activities, ISDS leachate, and geologic weathering 

were thought to be the most important. Amounts of 

sodium and chloride in dry deposition in forested 

drainage basins in the Northeast usually are small relative 

to those in wet deposition (Likens and Bormann, 1995), 

and all salt-storage facilities in the basin have been 

covered since the late 1980s (Rhode Island Department of 

Administration, 1988), so amounts added from this source 

should be minimal. Accordingly, dry deposition and saltstorage facilities were not considered in this study.

When sodium and chloride are deposited in a 

drainage basin in precipitation or during road deicing, a 

portion of the salt enters the tributaries directly in runoff, 

whereas the remainder infiltrates into the soil and then to 

ground water. Sodium entering soil and ground water may 

undergo cation exchange reactions, displacing calcium, 

potassium, magnesium, and other constituents from soil 

clays and organic particles, or it may move slowly with 

the ground water to eventual discharge points in tributary 

streams and reservoirs. Chloride generally moves more 

readily than sodium through the hydrologic cycle because 

it is relatively unreactive in most environments (Hem, 

1985).
6 

EXPLANATION

POTENTIAL SOURCES OF SODIUM AND CHLORIDE AFFECTING

BASIN RUNOFF AND GROUND-WATER RECHARGE

Wet and dry deposition

Deicing of roads and highways

Deicing of sidewalks, parking lots, and other surfaces

Leachate from uncovered salt-storage piles

Leachate from individual sewage-disposal systems

Cation exchange in soils

Geologic weathering

1

2

3

4

5

6

7

2

4

2

5

6

3

6

7

HYDROLOGIC TRANSPORT PATHWAYS

ROADS

Precipitation

Infiltration

Runoff

Ground-water table

Ground-water flow direction

Surface-water flow direction

Basin boundary

Primary

Secondary

1

Figure 2. Potential sources and transport pathways of sodium and chloride in the Scituate Reservoir drainage basin, Rhode Island.

The rate of sodium transport in ground water 

depends, among other factors, on the cation-exchange 

capacity (CEC) of the soil and aquifer materials, and on 

the relative amounts of soil and rock in contact with the 

infiltrating water. Different subbasins in the Scituate 

Reservoir drainage basin may have different capacities 

for retaining sodium, depending on past road-salting 

practices, soil and rock CEC, and the proximity of 

roadways and other input sources to tributary streams.

The proportion of sodium that enters the ground 

water also depends on the presence or absence of 

impervious surfaces. Surfaces such as highways can 

convey salt-contaminated water directly to a stream, 

which readily transports the salt to the reservoirs. This 

is especially true of State-maintained roads, which 

frequently cross tributary streams.
 7

Deicing Activities Contributed More 

Sodium and Chloride to the Drainage 

Basin During Water Year 2000 Than Any 

Other Source

During WY 2000, about 1,000 tons of sodium and 

2,300 tons of chloride entered the basin or were released 

by weathering of soils and bedrock. Of these inputs, about 

67 percent of the sodium and more than 90 percent of the 

chloride came from deicing of State highways, local 

roads, and other surfaces. Precipitation accounted for only 

about 7 percent of the sodium and 6 percent of the 

chloride inputs. About 15 percent of the sodium and 

4 percent of the chloride came from ISDS leachate. 

Geologic weathering contributed about 11 percent of the 

sodium but was assumed not to be an important source of 

chloride. Because all ISDS leachate is discharged to 

ground water, the potential for interaction of dissolved 

constituents with soil and aquifer materials is high, and 

the time required for movement of mobile constituents 

may be very long (years to decades). Thus, the total 

estimated amounts of sodium and chloride released in 

ISDS leachate are not strictly comparable to those from 

inputs that are linked directly to surface water. The estimated total amounts of sodium and chloride that entered 

the drainage basin during WY 2000 from each major 

source are presented in table 1.

Salting of State-maintained roads contributed more 

sodium and chloride to the drainage basin during WY 

2000 than did salting of locally maintained roads, 

although there were about 45 percent more total lanemiles of locally maintained roads (277 lane miles) than 

State-maintained roads (191 lane miles). Annual basinwide application rates calculated from these road-saltapplication values were 2.02 tons per lane mile for 

sodium and 8.03 tons per lane mile for chloride applied 

to State-maintained roads. For locally maintained roads, 

the average application rates were 1.06 tons per lane 

mile for sodium and 2.12 tons per lane mile for chloride. 

This difference may reflect the greater volume of traffic 

and higher speed limits on State-maintained roads.

Tributary-Stream Concentrations of 

Sodium and Chloride Are Directly 

Related to the Densities of StateMaintained Roads in the Stream 

Drainage Areas

The importance of road-salting as a source of 

sodium and chloride in the Scituate Reservoir drainage 

basin prompted a more detailed examination of the relations between concentrations of sodium and chloride in 

tributary streams and deicing activities in the drainage 

basin. In 1995, there were 90.3 mi of State-maintained 

roads and 138.6 mi of locally maintained roads in the 

Scituate Reservoir drainage basin. Road locations relative 

to subbasins defined by 31 water-quality-monitoring 

stations used by the PWSB are shown in figure 3. Four 

additional stations in the the PWSB stream-water-qualitymonitoring network were considered unsuitable for this 

analysis, because they were excessively affected by highsodium ground water or by drainage from collecting 

basins for highway runoff.

The distinction between State-maintained and 

locally maintained roads was based on road-maintenance 

classifications in the RIGIS Statewide Road Inventory 

(Rhode Island Geographic Information System, 2001). 

State-maintained roads are State highways maintained by 

the RIDOT. Locally maintained roads include paved and 

unpaved public roads maintained by municipalities within 

the drainage basin. In addition, in 1995 there were about 

83 mi of privately maintained roads in the drainage basin, 

but more than 80 percent of these were fire lanes and logging roads on land owned by the PWSB and protected from 

road-salting. Road lengths in each category were obtained 

and totaled for each of the 31 subbasins from the RIGIS 

Table 1. Major sources of sodium and chloride in the Scituate Reservoir drainage 

basin, Rhode Island, October 1999 through September 2000

[--, contribution negligible]

Source Category

Water Year 2000 Load (tons)

Sodium Chloride

Precipitation .................................................. 75 140

Salting of State-maintained roads ................. 390 1,500

Salting of locally maintained roads............... 290 580

Other deicing activities ................................. 9 14

Individual sewage-disposal systems ............. 150 88

Geologic weathering ..................................... 110 --
8 

Central Pike

A. State and Local Roads 

3

29 28

8 9

12

5

4

6

33

1 2 32

31

13

22 21

34

19

16 20

14

15

18 17

23

27

25 26

24

10

11

7

30

EXPLANATION

ROADS

Primary, including

arterial, and

collector roads

Secondary, including

local roads

SCITUATE BASIN

BOUNDARY

Danielson Pike

116

6

14

101

0

0

1 2 3 4 5 MILES

1 2 5 KILOMETERS 3 4

Figure 3. Locations of Providence Water Supply Board water-quality-monitoring stations sampled from 1983 to 2000 in relation to (A)
 9

B. Subbasin Boundaries

3

5

6

11

30

29

28 8 9

12

4

33

1

2 32

31

13

22 21

34

19

20

16

14

15

17

18

23

27

25

26

24

10

7 EXPLANATION

RESERVOIRS, LAKES,

AND PONDS

BASIN BOUNDARY

WATER-QUALITY

MONITORING

SUBBASIN BOUNDARY

PROVIDENCE WATER

SUPPLY BOARD,

WATER-QUALITY

MONITORING STATION

AND NUMBER

1

State and local roads, and (B) subbasin boundaries in the Scituate Reservoir drainage basin, Rhode Island.
10 

0

5

10

15

20

25

30

0 2 4 6 8 10

y = 1.775x + 5.45

R2

 = 0.623

p < 0.001

0 2 4 6 810

y = 0.111x + 9.313

R2

 = 0.002

p = 0.811

MEDIAN SODIUM CONCENTRATION,

IN MILLIGRAMS PER LITER

ROAD DENSITY, IN LANE MILES PER SQUARE MILE

State-maintained roads Locally maintained roads

Statewide Road Inventory and converted to lane miles. 

Subbasin-road densities were then calculated by dividing 

the number of lane miles in each subbasin by the area of 

the subbasin in square miles. Of the 31 subbasins examined, 26 had higher densities (more lane-miles per mi2

) of 

locally maintained roads than State-maintained roads.

The effects of road density on sodium and chloride 

concentrations in streams that contribute water to the Scituate Reservoir were examined by the use of sodium- and 

chloride-concentration data collected at regular intervals 

from January 1983 through December 2000 at each of the 

31 the PWSB monitoring stations. Sodium data usually 

were obtained twice each year, in winter and summer, but 

some stations were sampled more frequently (up to six 

times per year). Water samples were collected by hand, 

usually in dry weather, when streamflow was essentially 

at baseflow. The total number of sodium samples ranged 

from 22 to 39 per station. Chloride data were obtained 

monthly at most stations. The total number of chloride 

samples ranged from 94 to 187 per station. In most cases, 

the roads cross the streams, and the monitoring stations 

are near the roads (fig. 3). Of the 31 stream-monitoring 

stations, 16 were associated with State-maintained roads, 

and 15 were associated with locally maintained roads.

Median stream-sodium concentration was 

positively correlated with the density of State-maintained 

roads in the subbasins (fig. 4). About 62 percent of the 

variation in median stream-sodium concentration was 

accounted for by the variation in density of Statemaintained roads. In contrast, no correlation could be 

identified between median stream-sodium concentration 

and the density of locally maintained roads in the subbasins. The relations between median chloride concentrations in the streams and the densities of State-maintained 

and locally maintained roads were similar to those for 

sodium. About 60 percent of the variation in streamchloride concentration was accounted for by the variation 

in density of State-maintained roads and no correlation 

(R2 = 0.002) could be identified between median streamchloride concentration and the density of locally 

maintained roads in the subbasins (fig. 5).

When the sodium- and chloride-concentration data 

were divided into those collected in December through 

May (winter-spring samples) and those collected in 

June through November (summer-autumn samples), the 

median concentrations for the two constituents showed 

little seasonal variation. Likewise, there were no seasonal 

differences in the relations between median constituent 

concentrations and the densities of State-maintained and 

Figure 4. Relations between median stream-sodium concentrations during 1983–2000, and the density of State-maintained 

and locally maintained roads in 31 subbasins of the Scituate Reservoir drainage basin, Rhode Island.
 11

0

10

20

30

40

MEDIAN CHLORIDE CONCENTRATION,

IN MILLIGRAMS PER LITER

State-maintained roads Locally maintained roads

0 2 4 6 8 10 0 2 4 6 8 10

ROAD DENSITY, IN LANE MILES PER SQUARE MILE

y = 2.906x + 7.971

R2

 = 0.596

p < 0.001

y = 0.186x +14.27

R2

 = 0.002

p = 0.810

locally maintained roads. Because most of the samples 

were collected during dry weather, the immediate effects 

of runoff from winter storms probably are not represented 

in the concentration data. Rather, the winter-spring 

concentrations reflect the addition of sodium and chloride 

from road salt in melting ice and snow and the diluting 

effects of higher average flows during the period. The 

summer-autumn concentrations are more indicative of 

sodium and chloride levels in ground water, some of 

which undoubtedly is affected by salting of locally maintained roads. Ice and snow that accumulates along the 

sides of roads following plowing contains high concentrations of road salt, and slow melting of this material may 

contribute sodium and chloride to riparian aquifers as 

well as directly to the streams. Thus, although the sources 

of sodium and chloride that appear in the streams may 

be different in winter and summer, the combination of 

higher road-salt-application rates and more efficient 

drainage systems of State-maintained roads as compared 

to locally maintained roads, apparently makes the density 

of State-maintained roads a better indicator of conditions 

in the streams.

Various investigators have reported correlations 

between subbasin-road densities and concentrations of 

sodium and chloride in streams draining the subbasins. 

Runge and others (1989) found strong positive correlations between total subbasin-road densities and baseflow 

concentrations of sodium and chloride in streams in the 

Scituate Reservoir drainage basin, and Heisig (2000) 

reported similar results for chloride in small streams in the 

Croton River drainage basin in southeastern New York. 

Mattson and Godfrey (1994), used a multiple-regression 

model to predict sodium concentrations in Massachusetts 

streams and concluded that salting of interstate highways 

and State roads contributed more sodium to the streams 

than did salting of town roads. Breault and others (2000) 

calculated instantaneous loads (measured concentration of 

a constituent divided by the mean daily discharge on the 

day the stream was sampled) and subbasin yields (instantaneous load divided by subbasin-drainage area) of dissolved chloride for 34 subbasins of the Scituate Reservoir 

drainage basin during WY 1995. They concluded that 

subbasin road density, together with subbasin slope and 

wetland acreage, was a major determinant of dissolved 

chloride yield.

Road-salt application rates in the Scituate Reservoir 

drainage basin are not high when compared with those in 

other parts of the northeastern United States. The combined sodium and chloride application rate for all types of 

roads in the Scituate Reservoir drainage basin during 

Figure 5. Relations between median stream-chloride concentrations during 1983–2000, and the density of State-maintained 

and locally maintained roads in 31 subbasins of the Scituate Reservoir drainage basin, Rhode Island.
12 

1999–2000 was 5.9 tons per lane-mile. Whereas this 

application rate is more than three times the rate estimated 

(1.7 tons per lane mile) by Runge and others (1989) for 

the same area during the early 1980s, it is lower than that 

reported by Heisig (2000) for the Croton River drainage 

basin (part of New York City’s water supply) during 

1996–97 (18.5 tons per lane-mile of State, county, and 

local roads), and lower than the average application rate 

reported by the Massachusetts Highway Department 

(1997) for State-maintained and locally maintained roads 

in the drainage basin that supplies the city of Cambridge, 

Massachusetts, during 1986–93 (29 tons per lane mile).

Sodium Concentrations in Tributary 

Streams Did Not Decrease Following a 

State-Mandated Reduction in the Rate of 

Sodium Applied to State-Maintained 

Roads in the Drainage Basin

To determine the effects of the State-mandated 

introduction in 1990 of the use of a reduced-sodium 

deicing agent on State-maintained roads in the Scituate 

Basin, the relation between median stream-sodium concentration and subbasin-road density was determined 

separately for data collected from January 1983 to 

December 1989 and from March 1990 to April 2000. If 

the alternative deicing procedure was effective at reducing sodium loading to tributary streams, then the slope of 

the relation between sodium concentration and the density 

of State-maintained roads would be expected to decrease 

after 1990, indicating that the effect on stream-sodium 

concentration per treated lane-mile was reduced. 

However, there was no change in the slope of the regression line relating stream-sodium concentration and the 

density of State-maintained roads following the introduction of reduced-sodium highway-deicing practices in 

1990 (fig. 6). These results are consistent with those of 

Breault and others (2000), who found significant decreasing trends in stream sodium concentrations in 4 of 34 Scituate Reservoir Subbasins examined from 1982 through 

1995, but found no trends in sodium concentrations in the 

remaining 30 tributary subbasins.

There are several possible reasons for the apparent 

ineffectiveness of the use of a lower-sodium deicing agent 

on State-maintained roads. It may be that the reduction in 

sodium applications to State-maintained roads was offset 

by concomitant increases in applications elsewhere in the 

drainage basin. Increased development since 1990 has 

resulted in more locally maintained roads, more ISDS 

installations, and more private use of road salt. Alternatively, winter storms in recent years may have required 

larger applications of conventional road salt. It also is 

likely that there are large amounts of sodium adsorbed to 

clays and other soil particles beneath heavily salted roads 

and previously unprotected salt-storage facilities. Sodium 

leached from these soils would continue to migrate to the 

streams for some time (possibly years) after any change in 

the road-salt application rate.

Regression Analysis

Regression analysis is a statistical 

technique that provides an equation 

describing the nature of the relation 

between two measured variables. In 

simple regression, the equation predicts 

the value of one of the variables (the 

dependent variable, or “y”) on the basis 

of the value of one other variable (the 

independent variable, or “x”). In this 

study, the dependent variables were the 

median concentrations of sodium and 

chloride in streams that supply water to 

the Scituate Reservoir. The independent variables were the densities of 

State-maintained and locally maintained roads in the subbasins supplying 

the streams. The simplest relation 

between the two variables, that of a 

straight line in which the value of the 

dependent variable changes in linear 

proportion to a change in the value of 

the independent variable, was assumed. 

The resulting equation describes the 

“best-fitting” straight line, in the sense 

that the distances between the line and 

each of the data points are minimized. 

Simple regression analysis can 

also be used to measure the accuracy 

with which the regression equation predicts values of the dependent variable 

on the basis of the value of the independent variable. This measure, known as 

R2

, is the proportion of the variation in 

the dependent variable that can be 

accounted for by variations in the independent variable. For example, an R2

 of 

0.62 for the equation relating median 

stream-sodium concentration to the 

density of State-maintained roads in the 

subbasins supplying the streams (fig. 4) 

indicates that 62 percent of the variation in sodium concentration is 

accounted for by the variation in road 

density. A second measure of the reliability of the regression equation is 

obtained by asking whether or not the 

observed relation could have appeared 

by chance alone. The p-values presented in figures 4-6 give the probabilities that a linear relation between the 

two variables could have arisen by 

chance. In all cases, the probabilities 

are small (less than 0.01 percent) that 

the strong positive relations between 

stream sodium and chloride concentrations and the densities of State-maintained roads are simply due to chance 

arrangements of the data, whereas the 

probabilities that the observed relations 

involving the two constituents and the 

densities of locally maintained roads 

are due to chance are about 81 percent.
 13

0

5

10

15

20

25

30

0 2 4 6 8 10

y = 1.761x + 6.340

R2

 = 0.5843

p < 0.0001

0 2 4 6 810

y = 1.767x + 4.803

R2

 = 0.6466

p < 0.0001

MEDIAN SODIUM CONCENTRATION,

IN MILLIGRAMS PER LITER

ROAD DENSITY, IN LANE MILES PER SQUARE MILE

1983–89 1990–2000

Summary and Conclusions

Major sources of sodium and chloride entering the 

Scituate Reservoir drainage basin in Rhode Island during 

WY 2000 were identified and quantified by the U.S. 

Geological Survey, in cooperation with the Providence 

Water Supply Board, by the use of data from State and 

municipal public works departments, local precipitationmonitoring programs, State planning programs, and scientific literature. About 1,000 tons of sodium and 2,300 tons 

of chloride entered the basin or were released by weathering of soils and bedrock during the water year. Of these 

inputs, 67 percent of the sodium and more than 90 percent 

of the chloride came from deicing of State highways, 

local roads, and other surfaces in the drainage basin. 

Precipitation accounted for only about 7 percent of the 

sodium and 6 percent of the chloride inputs. Leachate 

from ISDS contributed up to 15 percent of the sodium and 

4 percent of the chloride. Geologic weathering contributed about 11 percent of the sodium, but probably was not 

a major source of chloride.

Salting of State-maintained roads contributed about 

35 percent more sodium and 159 percent more chloride to 

the drainage basin during WY 2000 than did salting of 

locally maintained roads, although there were 45 percent 

more lane miles of locally maintained roads than there 

were of State-maintained roads. On average, State 

highway-maintenance crews applied twice the amount 

of sodium and four times the amount of chloride per 

lane mile during WY 2000 than did municipal roadmaintenance crews, although application rates varied 

greatly from town to town within the drainage basin.

The effects of road-salt applications on streams that 

contribute water to the Scituate Reservoir were examined 

by the use of sodium- and chloride-concentration data 

collected by the PWSB at regular intervals from January 

1983 through December 2000 at 31 tributary-streammonitoring stations in the Scituate Reservoir drainage 

basin. Median stream-sodium concentration was positively correlated with the number of lane miles of Statemaintained roads per mi2

 of subbasin area. About 63 percent of the variation in median stream-sodium concentration was accounted for by variation in the density of 

State-maintained roads, verifying that salting of Statemaintained roads is an important source of sodium in the 

streams. In contrast, no correlation could be identified 

between median stream-sodium concentration and the 

density of locally maintained roads in the subbasins. 

Similar results were obtained for the relation between 

median stream-chloride concentration and road density.

Figure 6. Relations between median stream-sodium concentrations and the density of State-maintained roads in 31 

subbasins of the Scituate Reservoir drainage basin, Rhode Island, during the periods 1983–89 and 1990–2000.
14 

The effectiveness of a State-mandated use of a 

60:40 mixture of calcium chloride and sodium chloride on 

State-maintained roads in the drainage basin beginning in 

1990 was determined by comparing relations between 

subbasin road density and median stream-sodium concentrations for the periods 1983 through 1989 and 1990 

through 2000. The slope of the regression line for these 

relations did not change following the introduction of the 

reduced-sodium deicing agent, indicating that there was 

no measurable change in the magnitude of the contribution of State-maintained roads to the sodium concentrations observed in the streams. The apparent lack of effect 

may be due to increased use of sodium chloride by municipalities and private individuals. The lack of effect may 

also indicate the mobilization of sodium previously 

applied and bound to soil particles.

References Cited

Breault, R.F., Waldron, M.C., and Striker, L.K., 2000, 

Relations between water-quality conditions and 

drainage basin characteristics in the Scituate 

Reservoir drainage basin, Rhode Island, 1982–95: 

U.S. Geological Survey Water-Resources 

Investigations Report 00-4086, 46 p.

Coleman, S.M., and Dethier, D.P. (eds.), 1986, Rates of 

chemical weathering of rocks and minerals: Orlando, 

FL, Academic Press, 790 p.

Granato, G.E., Church, P.E., and Stone, V.J., 1995, 

Mobilization of major and trace constituents of 

highway runoff in ground water potentially caused 

by chemical migration: Washington, D.C., 

Transportation Research Board National Research 

Council, Transportation Research Record 1483, 

p. 92–103.

Hammer, M.J., and MacKichan, K.A., 1981, Hydrology 

and quality of water resources: New York, John 

Wiley and Sons, 471 p.

Hem, J.D., 1985, Study and interpretation of the chemical 

characteristics of natural water (3d ed.): U.S. 

Geological Survey Water Supply Paper 2254, 263 p.

Heisig, P.M., 2000, Effects of residential and agricultural 

land uses on the chemical quality of baseflow of 

small streams inthe Croton Watershed, Southeastern 

New York: U.S. Geological Survey Water-Resources 

Investigations Report 99-4173, 15 p.

Likens, G.E., and Bormann, F.H., 1995, Biogeochemistry 

of a forested ecosystem (2d ed.): New York, 

Springer-Verlag, 159 p.

Manahan, S.E., 1999, Environmental chemistry (7th ed.): 

Boca Raton, FL, Lewis Publishers, 850 p.

U.S. Geological Survey hydrologist examines a highway 

storm drain during a winter storm in the Scituate Reservoir 

drainage basin.

Snow falling at an abandoned mill site off State Route 116 

between Moswansicut Pond and Regulating Reservoir.
 15

Mattson, M.D., and Godfrey, P. Jos., 1994, Identification 

of road salt contamination using multiple regression 

and GIS: Environmental Management, v. 18, no. 5, 

p 767–773.

Massachusetts Highway Department, 1997, Draft 

Environmental Impact Report—Volume 1. Hobbs 

Brook and Stony Brook Watersheds Highway 

Drainage Improvement Project: Boston, MA, EOEA 

Number 8263, variously paged.

Rhode Island Department of Administration, 1988, A 

road salt management strategy for the Scituate 

Reservoir Watershed: Providence, Rhode Island, 

Rhode Island Department of Administration, 

Division of Planning, Report Number 63, variously 

paged.

Rhode Island Department of Administration, 2001, 

Statewide Planning Program Web Site, 

accessed March 30, 2002, at URL http:// 

www.planning.state.ri.us/SPC/RIPOP.htm.

Rhode Island Geographic Information System, 2001, 

accessed March 30, 2002, at URL http:// 

www.edc.uri.edu/rigis-spf/statewide/state.html 

#transportation.

Runge, I., Wright, R.M., and Urish, D.W., 1989, 

Modeling sodium and chloride in surface streams 

during base flows: Journal of Environmental 

Engineering, v. 115, no. 3, p. 608–619.

Socolow, R.S., Whitley, J.S., Murino, D., Jr., and 

Ramsbey, L.R., 2001, Water resources data, 

Massachusetts and Rhode Island, Water Year 2000: 

U.S. Geological Survey Water-Data Report MA-RI00-1, 459 p.

U.S. Environmental Protection Agency, 1998, Safe 

drinking water—Sodium in drinking water, accessed 

March 30, 2002, at URL http://www.epa.gov/ 

safewater/ccl/sodium.html.

U.S. Environmental Protection Agency, 2001, Current 

drinking water standards—National secondary 

drinking-water regulations, accessed March 30, 2002, 

at URL http://www.epa.gov/safewater/mcl.html.

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

reservoir and stream-water quality in the Cambridge, 

Massachusetts, drinking-water source area and 

implications for source-water protection: U.S. 

Geological Survey Water-Resources Investigations 

Report 00-4262, 89 p.

Providence Water Supply Board water-quality monitoring 

station on Dolly Cole Brook, in South Foster, Rhode Island. 

The station is just downstream from State Route 6.

U.S. Geological Survey continuous streamflow and waterquality monitoring station on Peeptoad Brook near North 

Scituate, Rhode Island.
16 

U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary 

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director 

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 Discipline Box 25286

10 Bearfoot Road Denver, CO 80225-0286

Northborough, MA 01532

or visit our Web site at or through the Web at

http://ma.water.usgs.gov http://mapping.usgs.gov/esic/to_order.html

Layout and graphic design by Anne M. Weaver and Mark V. Bonito

View of the Scituate Reservoir, Rhode Island, shoreline in early autumn. More than 60 percent of the reservoir’s drainage basin is 

undeveloped forest land.