Prepared in cooperation with the Providence Water Supply Board

Water-Quality Trends in the Scituate Reservoir 

Drainage Area, Rhode Island, 1983–2012

Scientific Investigations Report 2015–5058

U.S. Department of the Interior

U.S. Geological Survey
Cover. Photograph of the Scituate Reservoir from Route 12, taken by Kimberly Campo.
Water-Quality Trends in the Scituate 

Reservoir Drainage Area, Rhode Island, 

1983–2012

By Kirk P. Smith

Prepared in cooperation with the Providence Water Supply Board

Scientific Investigations Report 2015–5058

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2015

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Suggested citation:

Smith, K.P., 2015, Water-quality trends in the Scituate reservoir drainage area, Rhode Island, 1983–2012: U.S. Geological Survey Scientific Investigations Report 2015–5058, 57 p., http://dx.doi.org/10.3133/sir20155058.

ISSN 2328-031X (print)

ISSN 2328-0328 (online)

ISBN 978–1–4113–3922–4
iii

Contents

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

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

Purpose and Scope ..............................................................................................................................2

Previous Investigations........................................................................................................................2

Description of the Study Area ............................................................................................................4

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

Climate...........................................................................................................................................4

Geology..........................................................................................................................................4

Hydrology ....................................................................................................................................11

Methods of Study.........................................................................................................................................11

Water-Quality Data.............................................................................................................................11

Compilation and Analysis of Streamflow and Continuous Water-Quality Data .......................12

Estimation of Loads and Yields .........................................................................................................12

Estimation of Daily Loads and Yields ......................................................................................12

Estimation of Continuous Loads and Yields of Chloride and Sodium................................13

Determination of Correlations and Trends......................................................................................15

Water-Quality Conditions and Constituent Loads, WYs 2003–12.........................................................16

Water-Quality Properties...................................................................................................................16

pH........ .........................................................................................................................................16

Color.... .........................................................................................................................................24

Turbidity .......................................................................................................................................24

Alkalinity......................................................................................................................................24

Constituent Concentrations ..............................................................................................................27

Chloride........................................................................................................................................27

Nutrients......................................................................................................................................27

Bacteria.......................................................................................................................................29

Correlations Between Water-Quality Properties and Constituent Concentrations.................30

Factors Affecting Water-Quality Properties and Constituent Concentrations.........................30

Subbasin Characteristics .........................................................................................................30

Streamflow..................................................................................................................................32

Comparison of Water-Quality Properties and Constituent Concentrations with 

Water-Quality Standards and Guidelines..........................................................................32

Water-Quality Properties..........................................................................................................34

Constituent Concentrations .....................................................................................................34

Loads and Yields of Selected Constituents, WYs 2003–12...........................................................34

Chloride........................................................................................................................................36

Nutrients......................................................................................................................................36

Bacteria.......................................................................................................................................36

Loads and Yields of Chloride and Sodium, WYs 2009–12.............................................................36

Trends in Water-Quality Properties and Constituent Concentrations.................................................38

Water-Quality Properties...................................................................................................................38

Chemical-Constituent Concentrations ............................................................................................40
iv

Summary........................................................................................................................................................51

Selected References...................................................................................................................................53

Appendix 1. Water-Quality Data Collected by the Providence Water Supply Board at 

37 Monitoring Stations in the Scituate Reservoir Drainage Area, Water Years 

1983–2012.........................................................................................................................................57

Figures

1. Map showing the Scituate Reservoir drainage area, Rhode Island, 2012..........................3

2. Map showing locations of tributary reservoir subbasins and stations in the 

Scituate Reservoir drainage area, Rhode Island, 2012 ..........................................................5

3. Map showing land use and locations of stations in the Scituate Reservoir 

drainage area, Rhode Island, 2012.............................................................................................8

4. Graph showing flow-duration curve and streamflow values on the dates when 

water-quality samples were collected for the U.S. Geological Survey station on 

Peeptoad Brook at North Scituate (station 01115098), Rhode Island, for water 

year 2012 ......................................................................................................................................12

5. Graph showing relations between specific conductance and concentrations of 

selected major ions in water-quality samples collected from tributaries at 14 

U.S. Geological Survey stations in the Scituate Reservoir drainage area, Rhode 

Island, water years 2009–12......................................................................................................16

6. Graphs showing relations between measured and estimated concentrations of 

A, chloride and B, sodium at 14 U.S. Geological Survey stations in the Scituate 

Reservoir drainage area, Rhode Island, water years 2009–12............................................17

7. Graphs showing distributions of measurements of A, pH, B, color, and C, turbidity 

and concentrations of D, alkalinity, E, chloride, F, nitrite, G, nitrate, H, nitrate plus 

nitrite, I, orthophosphate, J, total coliform bacteria, and K, Escherichia coli in 

samples collected at Providence Water Supply Board water-quality-monitoring 

stations in the Scituate Reservoir drainage area, Rhode Island, water years 

2003–12 .........................................................................................................................................20

8. Graphs showing relations between A, median values of turbidity and percent 

commercial and industrial land use; B, turbidity and percent residential land use; 

C, turbidity and percent impervious area; D, pH and forested land use; E, color and 

percent wetland area; F, alkalinity and percent agricultural land use; G, chloride 

and percent commercial and industrial land use; H, chloride and 20- to 49-percent 

canopy closure; I, chloride and percent impervious area; J, median concentrations 

of orthophosphate and percent water land cover; K, median numbers of Escherichia

coli colony-forming units per 100 milliliters and percent residential land use; and 

L, nitrate and percent residential land use at Providence Water Supply Board 

water-quality monitoring stations in the Scituate Reservoir drainage area, 

Rhode Island, water years 2003–12 .........................................................................................31

9. Graph showing relations between daily mean streamflow and daily mean 

concentrations of chloride estimated from continuous measurements of specific 

conductance for Quonapaug Brook in the Scituate Reservoir drainage area, 

Rhode Island, water years 2009–12 .........................................................................................32

10. Graphs showing relations between daily mean streamflow and daily mean values 

of specific conductance at A, Dolly Cole Brook in the Barden Reservoir Subbasin, 

B, Quonapaug Brook in the Direct Runoff Subbasin, and C, Rush Brook in the 

Regulating Reservoir Subbasin, Scituate Reservoir drainage area, Rhode Island, 

water year 2012...........................................................................................................................33
v

11. Graph showing daily maximum concentrations of chloride estimated from 

continuous records of specific conductance for Quonapaug Brook in the 

Scituate Reservoir drainage area, Rhode Island, water years 2009–12............................34

12. Map showing spatial distribution of sodium yields for 14 subbasins in the 

Scituate Reservoir drainage area, Rhode Island, water years 2009–12............................39

13. Graph showing average daily chloride loads estimated from continuous measurements of flow and estimated concentration data and average daily chloride 

loads estimated from water-quality samples and daily mean streamflow at 14 

Providence Water Supply Board stations in the Scituate Reservoir drainage area, 

Rhode Island, water years 2009–12 .........................................................................................40

14. Maps showing time trends of A, pH, B, color, C, turbidity, D, alkalinity, E, chloride, 

F, nitrate, G, nitrite, H, orthophosphate, I, total coliform bacteria, and J, Escherichia

coli at Providence Water Supply Board monitoring stations in the Scituate 

Reservoir drainage area, Rhode Island, water years 2003–12 and water years 

1983–2012 .....................................................................................................................................46

Tables

1. Providence Water Supply Board water-quality-monitoring stations, water-quality 

sample collection frequency, and available streamflow and specific conductance 

data by tributary-reservoir subbasins in the Scituate Reservoir drainage area, 

Rhode Island..................................................................................................................................6

2. Percentages of impervious area, land use, and canopy closure in the Scituate 

Reservoir drainage area, Rhode Island, 2012 ..........................................................................9

3. Stations in the Scituate Reservoir drainage area, Rhode Island, for which 

streamflow data were estimated, water years 2002–12, and U.S. Geological 

Survey index stations, period of streamflow measurements, and maintenance 

of variance extension type 1 method regression coefficients............................................14

4. Regression equation coefficients used to estimate concentrations of chloride 

and sodium from values of specific conductance at U.S. Geological Survey stations 

in the Scituate Reservoir drainage area, Rhode Island, water years 2009–12.................15

5. Median values for water-quality data collected at Providence Water Supply 

Board stations in the Scituate Reservoir drainage area, Rhode Island, water 

years 2003–12 ..............................................................................................................................18

6. Significance levels (p values) for rank correlations between selected physical 

properties and constituent concentrations at Providence Water Supply Board 

water-quality-monitoring stations in the Scituate Reservoir drainage area, 

Rhode Island, water years 2003–12 .........................................................................................24

7. Significance levels (p values) for rank correlations between subbasin 

characteristics and median values of selected physical properties and constituent 

concentrations at Providence Water Supply Board water-quality-monitoring 

stations in the Scituate Reservoir drainage area, Rhode Island, 2002–12........................25

8. Significance levels (p values) for rank correlations between mean daily 

streamflow and selected physical properties or constituent concentrations at 

Providence Water Supply Board monitoring stations in the Scituate Reservoir 

drainage area, Rhode Island, water years 1994–2012..........................................................26

9. Selected relevant Federal and Rhode Island State water-quality standards and 

guidelines.....................................................................................................................................27
vi

10. Percentages of samples, collected at Providence Water Supply Board monitoring 

stations in the Scituate Reservoir drainage area, Rhode Island, water years 

2003–12, with values or concentrations not meeting water-quality standards 

and guidelines .............................................................................................................................28

11. Median daily loads and yields of chloride, nitrite, nitrate, orthophosphate, and 

bacteria, in the Scituate Reservoir drainage area, Rhode Island, water years 

2003–12 .........................................................................................................................................35

12. Average chloride and sodium concentrations, loads, and yields estimated on the 

basis of streamflow and estimated concentration data collected at 14 Providence 

Water Supply Board stations in the Scituate Reservoir drainage area, Rhode Island, 

water years 2009–12...................................................................................................................37

13. Significance levels (p values) of seasonal Kendall tests for time trends in waterquality properties and constituent concentrations at Providence Water Supply 

Board monitoring stations in the Scituate Reservoir drainage area, Rhode Island, 

water years 2003–12...................................................................................................................42

14. Significance levels (p values) of seasonal Kendall tests for time trends in waterquality properties and constituent concentrations at Providence Water Supply 

Board water-quality monitoring stations in the Scituate Reservoir drainage area, 

Rhode Island, water years 1983–2012 .....................................................................................44
vii

Conversion Factors

International System of Units to Inch/Pound

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Area

square kilometer (km2

) 0.3861 square mile (mi2)

Volume

liter (L) 0.2642 gallon (gal)

cubic meter (m3

) 35.31 cubic foot (ft3)

Flow rate

cubic meter per second (m3

/s) 35.31 cubic foot per second (ft3/s)

cubic meter per second per square 

kilometer [(m3

/s)/km2]

91.49 cubic foot per second per square mile 

[(ft3/s)/mi2]

Mass

gram (g) 0.03527 ounce, avoirdupois (oz)

kilogram (kg) 2.205 pound avoirdupois (lb)

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

°C = (°F – 32) / 1.8.

Supplemental Information

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

25 °C).

Chemical concentration is given in units of milligrams per liter (mg/L) or milliequivalents per liter 

(meq/L). Milliequivalents per liter are units expressing the number of electron-moles of a solute 

per unit volume (liter). A mole is the mass in grams numerically equal to the atomic mass of a 

given element.
viii

Abbreviations

CFU colony-forming unit

DOC dissolved organic carbon

E. coli Escherichia coli

NWIS National Water Information System

NTU nephelometric turbidity unit

PCU platinum-cobalt unit

RIDEM Rhode Island Department of Environmental Management

RIGIS Rhode Island Geographic Information System

SDWR secondary drinking-water regulations

USGS U.S. Geological Survey

WY water year
Water-Quality Trends in the Scituate Reservoir Drainage 

Area, Rhode Island, 1983–2012

By Kirk P. Smith

Abstract

The Scituate Reservoir is the primary source of drinking 

water for more than 60 percent of the population of Rhode 

Island. Water-quality and streamflow data collected at 

37 surface-water monitoring stations in the Scituate Reservoir 

drainage area, Rhode Island, from October 2001 through 

September 2012, water years (WYs) 2002–12, were analyzed 

to determine water-quality conditions and constituent loads 

in the drainage area. Trends in water quality, including 

physical properties and concentrations of constituents, were 

investigated for the same period and for a longer period 

from October 1982 through September 2012 (WYs 1983–

2012). Water samples were collected and analyzed by the 

Providence Water Supply Board, the agency that manages 

the Scituate Reservoir. Streamflow data were collected by the 

U.S. Geological Survey. Median values and other summary 

statistics for pH, color, turbidity, alkalinity, chloride, nitrite, 

nitrate, total coliform bacteria, Escherichia coli (E. coli), 

and orthophosphate were calculated for WYs 2003–12 for 

all 37 monitoring stations. Instantaneous loads and yields 

(loads per unit area) of total coliform bacteria and E. coli, 

chloride, nitrite, nitrate, and orthophosphate were calculated 

for all sampling dates during WYs 2003–12 for 23 monitoring 

stations with streamflow data. Values of physical properties 

and concentrations of constituents were compared with State 

and Federal water-quality standards and guidelines and were 

related to streamflow, land-use characteristics, varying classes 

of timber operations, and impervious surface areas.

Tributaries in the Scituate Reservoir drainage area for 

WYs 2003–12 were slightly acidic (median pH of all stations 

equal to 6.1) and contained low median concentrations of 

chloride (22 milligrams per liter [mg/L]), nitrate (0.01 mg/L 

as nitrogen), nitrite (0.001 mg/L as nitrogen), and orthophosphate (0.02 milligrams per liter as phosphorus [mg/L as P]). 

Turbidity and alkalinity values also were low with medians 

of 0.57 nephelometric turbidity units and 5.1 mg/L as calcium 

carbonate, respectively. Total coliform bacteria and E. coli

were detected in most samples from all stations, but median 

concentrations were generally low—43 colony-forming units 

per 100 milliliters (mL) and 15 colony-forming units per 

100 milliliters, respectively.

Median values of several physical properties and median 

concentrations of several constituents correlated positively 

with the percentages of developed land and negatively with 

the percentages of forest cover in the drainage areas above 

the monitoring stations. Median concentrations of chloride 

correlated positively with the percentages of impervious land 

use in the subbasins of monitoring stations, likely reflecting 

the effects of deicing compounds applied to roadways during 

winter maintenance. Median concentrations of alkalinity also 

correlated positively with the percentage of impervious land 

use, which may be related to the deterioration of fabricated 

structures containing calcium carbonate. Median values of 

color correlated positively with the percentage of wetland area 

in the subbasins of monitoring stations, reflecting the natural 

sources of color in tributaries. Streamflows were negatively 

correlated with turbidity and concentrations of total coliform 

bacteria and E. coli, possibly reflecting seasonal patterns in 

which relatively high values of these properties and constituents occur during warmer low-flow conditions late in the water 

year. Similar seasonal patterns were observed for pH, alkalinity, and color. Negative correlations between concentrations of 

chloride and streamflow also were significant, indicating that 

deicing salts from roadways and other impervious surfaces 

that lack direct connection to the tributaries are likely infiltrating to the groundwater and discharging to some of the tributaries late in the water year. While salt-laden runoff directly 

enters some of the tributaries at roadway crossings, most of 

the roadway runoff infiltrates into the adjacent berms throughout the drainage area. Statistically significant correlations were 

not identified between various degrees of tree-canopy reduction caused by timber operations in the subbasins and median 

values or concentrations of water-quality properties.

Loads and yields of chloride, nitrate, nitrite, orthophosphate, and bacteria varied at monitoring stations in the 

Scituate Reservoir drainage area in WYs 2003–12. Loads 

generally were greater at stations in the Barden Reservoir and 

the Regulating Reservoir Subbasins that have larger drainage 

areas than in subbasins with smaller drainage areas. Subbasin 

yields of fecal-indicator bacteria and orthophosphate generally were largest in the Westconnaug Reservoir Subbasin, and 

subbasin yields for chloride, nitrate, and nitrite were largest in 

the Moswansicut Reservoir Subbasin in the northeastern part 

of the drainage area.
2 Water-Quality Trends in the Scituate Reservoir Drainage Area, Rhode Island, 1983–2012

Upward trends in pH were identified for nearly half of 

the monitoring stations for WYs 1983–2012 and may reflect 

regional reductions in acid precipitation. Many upward trends 

in alkalinity also were identified for both the WYs 1983–2012 

and for WYs 2003–12 periods and are likely related to the 

natural weathering of structures containing concrete or, in 

some cases, the application of lime or fertilizers on agriculture lands. Significant trends in chloride concentrations at 

most stations during WYs 1983–2012 were upward; however, 

results for WYs 2003–12 substantiate few significant upward 

trends and, in a few cases, downward trends were identified in 

several tributary drainage areas.

Introduction

Scituate Reservoir, the primary source of drinking water 

for more than 60 percent of the population of Rhode Island, 

covers about 94 square miles (mi2) in parts of the towns of 

Cranston, Foster, Glocester, Johnston, and Scituate, R.I. 

(fig. 1). The Providence Water Supply Board (Providence 

Water) manages the Scituate Reservoir and water-supply 

system from the Scituate Reservoir. The average demand for 

treated water is approximately 67 million gallons per day, of 

which 59 percent is for the cities of Cranston, Providence, 

and North Providence and for the town of Johnston R.I.; 

the remainder is sold to nine surrounding communities 

(Providence Water Supply Board, 2014).

Providence Water owns about one-third of the Scituate 

Reservoir drainage area and manages the area for waterquality protection and timber production (Richard Blodgett, 

Providence Water Supply Board, written commun., 2005); the 

remaining area is privately owned (Providence Water Supply 

Board, 2014). Human-induced changes in the drainage area, 

including changes in agricultural and industrial activities, 

urbanization, and other land-use changes, can cause degradation in the quality of the source water to the reservoir.

Providence Water has collected water-quality data in the 

area contributing water to the reservoir for more than 50 years 

as part of its efforts to maintain high-quality source water. 

Most of the data have been collected at 37 surface-water 

monitoring stations on tributaries to the Scituate Reservoir. 

Water-quality physical properties and constituent concentrations measured by Providence Water—pH, alkalinity, turbidity, 

color, chloride, nutrients, and indicator bacteria—are indicators of overall water-quality conditions.

Purpose and Scope

In 2008, the U.S. Geological Survey (USGS), in cooperation with Providence Water, began an investigation to 

evaluate water-quality conditions, including constituent loads, 

trends, and factors that affect water quality. Water-resources 

managers can use descriptions of water-quality conditions 

to develop management plans for the drainage area. Data on 

constituent loads and yields, which depend on both flow and 

constituent concentrations, contribute to an understanding of 

tributary water quality and the overall quality of the water 

in the reservoir. Knowledge of the time trends in the physical properties and constituents can enable water managers to 

predict future water-quality problems, assess the effectiveness 

of management actions, and identify improvements in water 

quality. Water-quality conditions and trends in the Scituate 

Reservoir drainage area may represent conditions in similar, 

relatively undeveloped or minimally developed drainage areas 

and reservoir source-water areas throughout the Northeastern 

United States.

This report presents analyses of streamflow and waterquality data collected in the Scituate Reservoir drainage area 

during water years1

 (WYs) 1983–2012 (October 1, 1982 

through September 30, 2012). Water-quality data were collected by Providence Water at 37 surface-water-monitoring 

stations distributed throughout the drainage area, streamflow 

data were collected by the USGS at 23 of the 37 stations, and 

specific conductance data were collected by USGS personnel 

at 14 of the 37 stations as part of the present study. Analyses performed on the data included (1) determination of 

median values and concentrations and other summary statistics for water-quality physical properties and constituents 

at all monitoring stations; (2) comparison of median values 

and concentrations with factors potentially affecting water 

quality, including streamflow, land use, and impervious area; 

(3) determination of the frequencies at which water-quality 

standards or guidelines were exceeded at tributary monitoring 

stations during the period; (4) estimation of loads and yields 

of selected water-quality constituents at the 23 stations for 

which streamflow data were available; (5) estimation of loads 

and yields of chloride and sodium at 14 stations for which 

streamflow and specific conductance data were available; and 

(6) identification of trends in values and concentrations of 

selected water-quality properties and constituents at all stations. Trends in water-quality properties and constituents were 

investigated for the 10-year study period of WYs 2003–12, 

representing a new period of analysis, and also were investigated for a longer 29-year period of WYs 1983–2012 which 

was inclusive of all available data (Nimiroski and others 

2008). Trends during the shorter period may be considered 

current trends in water-quality properties and constituent 

concentrations, whereas trends during the longer period may 

reflect ongoing or past changes.

Previous Investigations

Providence Water has collected water-quality data in 

the Scituate Reservoir drainage area since 1945. Breault and 

others (2000) and Nimiroski and others (2008) summarized 

water-quality data collected by Providence Water before the 

present study and include median values of physical properties 

1

A water year is a continuous period from October 1 through September 30 

of the following year and is designated by the year in which it ends.
Introduction 3

Narragansett

Bay

Block Island

Sound

41°45'

71°45' 71°30'

41°30'

42°00'

0 10 MILES

0 10 KILOMETERS

RHODE ISLAND

Base from U.S. Geological Survey digital data, 1:24,000 scale, 2014

Rhode Island State plane coordinate system, zone 5176

Westconnaug 

Reservoir

Ponaganset 

Reservoir

Regulating 

Reservoir

Barden

Reservoir

Moswansicut

Reservoir

Scituate 

Reservoir

0

0

1 2 3 4 5 MILES

1 2 3 4 5 KILOMETERS

DANIELSON PIKE

116

6

14

101

12

EXPLANATION

Town boundary

Primary roads, including

arterial and collector roads

Basin boundary

GLOCESTER

SCITUATE

FOSTER

JOHNSTON

CRANSTON

71°45'

71°37'30"

41°52'30"

41°45'

Wetlands

Figure 1. The Scituate Reservoir drainage area, Rhode Island, 2012.
4 Water-Quality Trends in the Scituate Reservoir Drainage Area, Rhode Island, 1983–2012

and constituent concentrations, trends in selected constituent 

concentrations, and relations between constituent concentrations and drainage-area characteristics. Since 2002, interim 

reports on water-quality data (WYs 2002–12) were published 

by USGS.

Since 1993, the USGS has cooperated with Providence 

Water and the Rhode Island Department of Environmental 

Management (RIDEM) to measure streamflow and specific 

conductance in selected tributaries to the Scituate Reservoir. 

Specific conductance serves as a surrogate for chloride and 

sodium, which are important concerns to local water-resources 

managers because they are difficult to remove from source 

water and can affect the quality of finished drinking water. 

Nimiroski and Waldron (2002) described potential sources of 

chloride and sodium in the Scituate Reservoir drainage area. 

Since 2009, streamflow and specific conductance have been 

continuously measured at 14 stations in the drainage area, and 

measurements of streamflow have been made periodically 

at 9 additional stations in the drainage area (fig. 2; table 1). 

These data are published in USGS annual reports (for example, Socolow and others, 2003) and are available through the 

National Water Information System (NWIS; U.S. Geological 

Survey, 2001). As part of a study of low-flow characteristics 

in northern Rhode Island, the USGS has collected streamflow 

data at several additional stations in the Scituate Reservoir 

drainage area (Kliever, 1995; Bent and others, 2014).

Description of the Study Area

The study area is in north-central Rhode Island and 

encompasses the Scituate Reservoir, five tributary reservoirs 

(Barden, Moswansicut, Ponaganset, Regulating, and 

Westconnaug) and numerous millponds (fig. 1). This series 

of reservoirs has a combined capacity of more than 40 billion 

gallons and covers a surface area of about 7.2 mi2. The 

drainage area surrounding the Scituate Reservoir represents 

about 9 percent of the total land area of Rhode Island.

Land Use

Land use in the Scituate Reservoir drainage area is 

primarily undeveloped with an impervious area of about 

3.8 percent (Rhode Island Geographic Information, 2013). 

Forest (66 percent), wetlands (10 percent), and water 

(8 percent) accounted for about 84 percent of the drainage 

area’s land use in 1995 (fig. 3; table 2). Developed areas 

consisted of residential (10 percent), agricultural (4 percent), 

commercial/industrial (1.1 percent), and other urban 

(0.6 percent) land uses boundaries (Rhode Island Geographic 

Information, 2005). The eastern part of the drainage area 

is more developed than the rest of the drainage area (fig. 3) 

and is close to Providence. The Moswansicut Reservoir 

and Regulating Reservoir Subbasins, small subbasins in 

the northeastern part of the drainage area, have the smallest 

percentages of undeveloped area (60 and 78 percent, 

respectively, including forest, wetland, and water) and largest 

percentages of residential land use (27 and 14 percent, 

respectively) relative to the other four subbasins (table 2). 

The Moswansicut Reservoir Subbasin also has the largest 

percentages (3.7 percent) of commercial/industrial land 

use and impervious area (10.5 percent) of the five major 

subbasins in the Scituate Reservoir drainage area. In contrast, 

the Barden Reservoir and Westconnaug Reservoir Subbasins 

in the western part of the drainage area has the greatest 

percentages (86 and 85 percent, respectively) of undeveloped 

land, relatively low percentages of residential land use (about 

8 and 9 percent, respectively), and of impervious area (about 

3 percent).

Providence Water manages its land within the Scituate 

Reservoir drainage area by implementing a strategic foreststewardship plan (Providence Water Supply Board, 2011). 

Timber harvesting is an important part of the plan because 

the age and species of trees in the forests can affect both 

the hydrology and quality of water in the tributary-reservoir 

subbasins (Binkley and others, 2004; Dawson, 1996). Timbermanagement activity in this report is expressed as percentage 

of area timbered and by the relative amount of open tree 

canopy remaining after the completion of logging activities 

(table 2). These data reflect all available reports of timbering 

from 1993 through 2013. The direct runoff subbasin, the area 

adjacent to the Scituate Reservoir, comprised the greatest 

amount of overall timbering activity (about 30 percent) among 

the reservoir subbasins. Overall timbering activity in the 

other five subbasins ranged from about 1 percent (Ponaganset 

Reservoir Subbasin) to about 15 percent (Moswansicut 

Reservoir and Westconnaug Reservoir Subbasins).

Climate

Climate in the Scituate Reservoir drainage area is 

temperate, with a mean annual temperature of 38.9 degrees 

Fahrenheit (WYs 1975–2012) at a National Oceanic and 

Atmospheric Administration climatological station in the 

Barden Reservoir Subbasin in Foster, R.I. (National Oceanic 

and Atmospheric Administration, 2014). The long-term 

mean annual precipitation and mean annual snowfall at 

the Foster station for WYs 1975–2012 were 53.69 inches 

(in.) and 57.1 in., respectively. During WYs 2003–12, 

mean annual precipitation and snowfall at this station were 

greater than the long-term mean annual values at 57.16 and 

63.0 in., respectively.

Geology

The bedrock in the drainage area is mostly granite and 

granitic gneiss; some metasedimentary and mafic igneous 

rocks also are present (Hermes and others, 1994). Glacial 

deposits of Pleistocene age overlie the bedrock (Richmond and 

Allen, 1951; Allen, 1953). Glacial materials consist of ice-laid 

deposits (till or ground moraine) and meltwater deposits (sand 
Introduction 5

71°45'

71°37'30"

41°52'30"

41°45'

Base from U.S. Geological Survey digital data, 1:24,000 scale, 2014

Rhode Island State plane coordinate system, zone 5176 0

0

1 2 3 4 5 MILES

1 2 3 4 5 KILOMETERS

DANIELSON PIKE

116

6

14

101

12

U.S. Geological Survey streamgage

Pawtuxet River at Cranston

3

29

28

8

9

12

5

4

6

33

1

2

32

31 13 22

21 34 19

20 16

14

15 17

18

23

27

25 26

24

10

11

7

30

35

36

37

Ponaganset Reservoir Subbasin

EXPLANATION

Barden Reservoir Subbasin Basin boundary

Subbasins

Unsampled area draining 

directly to Scituate Reservoir

Streamflow or water-quality monitoring station and identifier

Providence Water Supply Board 

water-quality sampling station and number

Primary roads, including

arterial and collector roads

Providence Water Supply Board water-quality 

sampling station and U.S. Geological Survey 

continuous-record streamgage station

Providence Water Supply Board water-quality 

sampling station and U.S. Geological Survey 

partial record streamgage station

U.S. Geological Survey downstream order numbers 

shown on table 1

Direct Runoff Subbasin

Moswansicut Reservoir Subbasin

Westconnaug Reservoir Subbasin

Regulating Reservoir Subbasin

Water-quality sampling

subbasin boundary

Subbasin boundary

Westconnaug 

Reservoir

Ponaganset 

Reservoir

Regulating 

Reservoir

Barden

Reservoir

Moswansicut

Reservoir

Scituate 

Reservoir

Figure 2. Locations of tributary reservoir subbasins and stations in the Scituate Reservoir drainage area, Rhode Island, 2012.
6 Water-Quality Trends in the Scituate Reservoir Drainage Area, Rhode Island, 1983–2012

Table 1. Providence Water Supply Board water-quality-monitoring stations, water-quality sample collection frequency, and available 

streamflow and specific conductance data by tributary-reservoir subbasins in the Scituate Reservoir drainage area, Rhode Island.

[Alternate station names given in parenthesis for stations where different historical names were used for the same sampling location by the Providence Water 

Supply Board (PWSB). Sums of drainage areas do not equal subbasin totals because some drainage areas are nested. USGS, U.S. Geological Survey; mi2, square 

miles; M, monthly; Q, quarterly]

PWSB

USGS 

station 

number

Station name

Drainage 

area 

(mi2

)

Water-quality samples 

for water years 

2003–12

Available USGS 

streamflow data 

2003–12

(water years)

Available 

USGS specific 

conductance 

data 2009–12

(water years)

Frequency 

of 

collection

Number1

Barden Reservoir Subbasin

24 01115190 Dolly Cole Brook 4.90 M 109 Periodic, continuous2 Continuous

25 01115200 Shippee Brook 2.35 Q 33 Periodic None

26 01115185 Windsor Brook 4.32 Q 36 Periodic None

27 011151845 Unnamed Tributary to Ponaganset 

River (Unnamed Brook B, Unnamed 

Brook West of Windsor Brook)

0.10 Q 23 None None

28 01115265 Barden Reservoir (Hemlock Brook) 8.72 M 116 Periodic, continuous2 Continuous

29 01115271 Ponaganset River (Barden Stream) 33.0 M 95 None None

35 01115187 Ponaganset River 14.0 M 107 Continuous Continuous

Direct Runoff Subbasin

1 01115180 Brandy Brook 1.57 M 116 Periodic None

2 01115181 Unnamed Tributary 2 to Scituate 

Reservoir (Unnamed Brook North of 

Bullhead Brook)

0.29 Q 19 None None

3 01115280 Cork Brook 1.79 M 104 Periodic, continuous2 Continuous

4 01115400 Kent Brook (Betty Pond Stream) 0.85 M 114 Periodic None

5 01115184 Spruce Brook 1.22 Q 34 Periodic, continuous2 Continuous

6 01115183 Quonapaug Brook 1.96 M 101 Periodic, continuous2 Continuous

7 01115297 Wilbur Hollow Brook 4.32 M 116 Periodic, continuous2 Continuous

8 01115276 Westconnaug Brook (Westconnaug 

Reservoir)

5.18 M 79 Periodic, continuous2 Continuous

9 01115275 Bear Tree Brook 0.62 Q 38 Periodic, continuous2 Continuous

30 01115350 Unnamed Tributary 4 to Scituate 

Reservoir (Coventry Brook, Knight 

Brook)

0.78 Q 36 None None

31 01115177 Toad Pond 0.04 Q 12 None None

32 01115178 Unnamed Tributary 1 to Scituate Reservoir (Pine Swamp Brook)0.45 Q 34 Periodic None

33 01115182 Unnamed Tributary 3 to Scituate Reservoir (Hall’s Estate Brook)0.28 Q 32 Periodic None

36 -- Outflow from King Pond 0.77 Q 32 None None

37 -- Fire Tower Stream 0.05 Q 32 None None
Introduction 7

Table 1. Providence Water Supply Board water-quality-monitoring stations, water-quality sample collection frequency, and available 

streamflow and specific conductance data by tributary-reservoir subbasins in the Scituate Reservoir drainage area, Rhode Island.

—Continued

[Alternate station names given in parenthesis for stations where different historical names were used for the same sampling location by the Providence Water 

Supply Board (PWSB). Sums of drainage areas do not equal subbasin totals because some drainage areas are nested. USGS, U.S. Geological Survey; mi2, square 

miles; M, monthly; Q, quarterly]

PWSB

USGS 

station 

number

Station name

Drainage 

area 

(mi2

)

Water-quality samples 

for water years 

2003–12

Available USGS 

streamflow data 

2003–12

(water years)

Available 

USGS specific 

conductance 

data 2009–12

(water years)

Frequency 

of 

collection

Number1

Moswansicut Reservoir Subbasin

19 01115170 Moswansicut Reservoir (Moswansicut 

Stream North, Moswansicut Pond)

3.25 M 96 Periodic, continuous2 Continuous

20 01115160 Unnamed Tributary 1 to Moswansicut 

Reservoir (Blanchard Brook)

1.18 M 97 None None

21 01115165 Unnamed Tributary 2 to Moswansicut 

Reservoir (Brook from Kimball 

Reservoir)

0.29 Q 28 Periodic None

22 01115167 Moswansicut Reservoir (Moswansicut 

Stream South)

0.30 M 99 None None

34 01115164 Kimball Stream 0.27 Q 24 None None

Ponaganset Reservoir Subbasin

23 011151843 Ponaganset Reservoir 1.92 M 103 None None

Regulating Reservoir Subbasin

13 01115176 Regulating Reservoir 22.1 M 112 None None

14 01115110 Huntinghouse Brook 6.23 M 105 Periodic, continuous2 Continuous

15 01115114 Rush Brook 4.70 M 109 Periodic, continuous2 Continuous

16 01115098 Peeptoad Brook (Harrisdale Brook) 4.96 M 115 Continuous Continuous

17 01115119 Dexter Pond (Paine Pond) 0.22 Q 25 None None

18 01115120 Unnamed Tributary to Regulating 

Reservoir (Unnamed Brook A)

0.28 Q 19 Periodic, continuous2 Continuous

Westconnaug Reservoir Subbasin

10 01115274 Westconnaug Brook 1.48 M 102 Periodic None

11 01115273 Unnamed Tributary to Westconnaug 

Reservoir (Unnamed Brook south of 

Westconnaug Reservoir)

0.72 Q 28 Periodic None

12 011152745 Unnamed Tributary to Westconnaug 

Brook (Unnamed Brook north of 

Westconnaug reservoir)

0.16 Q 22 None None

1

Not all samples were analyzed for all water-quality properties or constituents.

2

Continuous streamflow data available for water year 2009–12.
8 Water-Quality Trends in the Scituate Reservoir Drainage Area, Rhode Island, 1983–2012

Base from Rhode Island Geographic Information System digital data, 1:24000, 1995

Rhode Island state plane coordinate system, North American Datum of 1983

EXPLANATION

Land use catagory

Other urban

Forest

Commercial/Industrial 

Residential

Agriculture

Water

Wetland

0 2 4 MILES

0 2 4 KILOMETERS

Subbasin boundary

Basin boundary

Scituate Reservoir

Moswansicut Reservoir

Regulating Reservoir

Ponangaset Reservoir

Barden Reservoir

Westconnaug Reservoir

Sampling station

71°37'30"

71°45'

41°52'30" 41°52'30" 41°52'30"

41°45'

71°45'

Land use data from Rhode Island

Geographic Information System, 2013

Figure 3. Land use and locations of stations in the Scituate Reservoir drainage area, Rhode Island, 2012.
Introduction 

9

Table 2. Percentages of impervious area, land use, and canopy closure in the Scituate Reservoir drainage area, Rhode Island, 2012.[Land-use data from Rhode Island Geographic Information System (2005); impervious area data from Rhode Island Geographic Information System (2013). Timber operation calculated from data collected during water years 1992–2012. PWSB, Providence Water Supply Board; USGS, U.S. Geological Survey; <, less than; --, information not available]

PWSB 

station 

number

USGS 

station 

number

Impervious 

area 

(percent)

Land use (percent) Percentage of area timbered

Forest Water Wetland Agricultural Residential Commercial/ industrial Other urban

0–100 80–100 50–79 20–49 <20

Remaining canopy closure (percent) after logging

Barden Reservior Subbasin

24 01115190 3.5 75.5 1.4 9.6 1.8 10.5 0.3 0.8 3.6 3.6 0.0 0.0 0.0

25 01115200 2.8 75.8 0.7 14.0 1.4 7.2 0.9 0.0 0.0 0.0 0.0 0.0 0.0

26 01115185 3.5 72.2 0.2 12.1 5.6 7.6 1.4 0.8 0.0 0.0 0.0 0.0 0.0

27 011151845 4.9 68.0 0.0 11.7 3.4 15.7 1.2 0.0 0.3 0.3 0.0 0.0 0.028 01115265 3.1 69.9 0.5 14.1 4.2 8.9 2.1 0.3 0.0 0.0 0.0 0.0 0.0

29 01115271 3.2 73.4 2.9 10.6 3.1 8.5 1.1 0.4 6.5 5.0 0.9 0.4 0.2

35 01115187 3.5 72.3 3.1 10.9 3.3 8.6 1.2 0.6 1.5 1.5 0.0 0.0 0.0

Subbasin -- 3.3 72.5 1.4 12.0 3.5 8.8 1.3 0.5 1.7 1.5 0.1 0.0 0.0

Direct Runoff Subbasin

1 01115180 6.1 44.2 4.3 18.5 9.9 20.7 2.3 0.1 24.4 18.8 4.5 1.0 0.1

2 01115181 8.0 56.1 3.6 8.7 3.0 28.5 0.0 0.0 44.6 28.7 15.9 0.0 0.0

3 01115280 6.0 59.9 0.1 15.6 6.3 16.5 1.3 0.3 3.0 3.0 0.0 0.0 0.0

4 01115400 2.5 55.5 4.7 13.7 6.5 3.1 1.3 15.2 36.3 19.4 16.3 0.6 0.0

5 01115184 5.3 66.8 0.1 10.9 4.2 17.4 0.0 0.6 9.2 8.9 0.2 0.0 0.0

6 01115183 6.2 53.6 0.1 20.4 7.0 13.8 1.2 4.0 68.3 41.9 18.9 7.4 0.0

7 01115297 3.2 69.0 1.2 15.5 5.0 8.8 0.1 0.4 16.9 14.7 1.7 0.1 0.4

8 01115276 2.9 73.7 5.5 7.9 4.5 6.0 1.9 0.5 30.1 23.3 5.8 1.1 0.0

9 01115275 5.1 57.7 0.3 12.4 9.3 8.0 12.2 0.1 2.9 2.9 0.0 0.0 0.0

30 01115350 2.7 85.2 0.2 7.8 0.0 5.9 0.0 1.0 97.4 24.0 58.4 14.1 0.9

31 01115177 18.7 36.9 10.9 0.0 0.0 18.0 18.3 16.0 43.3 22.8 15.3 0.0 5.2

32 01115178 5.6 56.6 0.0 16.1 1.9 20.7 4.7 0.0 46.3 7.0 33.6 4.5 1.1

33 01115182 5.5 60.0 0.0 17.1 1.9 21.0 0.0 0.0 30.6 28.7 0.0 1.8 0.0

36 -- 1.9 74.0 4.2 12.6 4.5 4.1 0.7 0.0 38.0 33.2 3.1 0.6 1.1

37 -- 1.9 99.6 0.0 0.4 0.0 0.0 0.0 0.0 53.5 23.8 21.1 4.6 4.0

Subbasin -- 4.4 63.7 1.9 14.2 5.5 11.7 1.4 1.5 29.7 19.0 8.5 1.9 0.3
10 Water-Quality Trends in the Scituate Reservoir Drainage Area, Rhode Island, 1983–2012 Table 2. Percentages of impervious area, land use, and canopy closure in the Scituate Reservoir drainage area, Rhode Island, 2012.—Continued [Land-use data from Rhode Island Geographic Information System (2005); impervious area data from Rhode Island Geographic Information System (2013). Timber operation calculated from data collected dur- ing water years 1992–2012. PWSB, Providence Water Supply Board; USGS, U.S. Geological Survey; <, less than; --, information not available] PWSB station number USGS station number Impervious area (percent) Land use (percent) Percentage of area timbered Forest Water Wetland Agricultural Residential Commercial/ industrial Other urban 0–100 80–100 50–79 20–49 <20 Remaining canopy closure (percent) after logging Moswansicut Reservoir Subbasin 19 01115170 9.2 36.0 14.4 13.4 8.2 25.2 2.9 0.1 21.1 18.0 2.9 0.2 0.0 20 01115160 10.3 30.3 0.0 22.2 12.4 33.1 1.9 0.2 0.9 0.9 0.0 0.0 0.0 21 01115165 8.9 51.9 11.4 17.2 1.4 14.0 4.1 0.0 55.8 50.1 5.6 0.0 0.0 22 01115167 19.1 44.6 0.0 4.9 5.6 30.9 13.9 0.0 0.4 0.2 0.2 0.0 0.0 34 01115164 8.7 52.7 12.3 16.6 1.6 12.4 4.4 0.0 59.4 53.3 6.0 0.0 0.0 Subbasin -- 10.5 36.2 7.6 16.1 8.9 27.4 3.7 0.1 15.3 13.3 1.9 0.1 0.0 Ponaganset Reservoir Subbasin 23 011151843 5.1 59.1 19.7 7.0 0.0 13.7 0.3 0.2 0.9 0.9 0.0 0.0 0.0 Regulating Reservoir Subbasin 13 01115176 7.2 59.7 4.4 11.9 6.5 14.9 1.8 0.8 10.9 6.8 2.9 1.0 0.3 14 01115110 3.4 70.1 0.5 10.1 6.8 12.2 0.1 0.2 1.7 1.0 0.4 0.2 0.1 15 01115114 6.6 60.1 0.3 15.9 6.8 15.0 1.1 0.8 2.6 1.7 0.1 0.1 0.7 16 01115098 4.9 64.4 1.8 9.6 7.1 13.2 2.4 1.5 0.5 0.4 0.1 0.0 0.0 17 01115119 5.4 56.8 2.2 19.4 8.2 10.4 2.3 0.7 0.0 0.0 0.0 0.0 0.0 18 01115120 12.5 47.2 0.0 19.0 4.5 14.9 6.3 8.1 0.1 0.1 0.0 0.0 0.0 Subbasin -- 5.2 64.3 1.3 11.9 6.8 13.6 1.3 0.9 2.8 1.8 0.6 0.2 0.2

Westconnaug Reservoir Subbasin

10 01115274 2.9 76.0 0.0 8.2 5.9 9.0 0.4 0.6 19.2 19.1 0.0 0.1 0.0

11 01115273 1.4 73.2 0.3 16.2 5.6 3.4 1.3 0.0 2.3 0.2 2.1 0.0 0.0

12 011152745 10.4 61.4 0.7 9.8 4.7 19.1 1.8 2.3 28.0 16.2 8.9 2.8 0.0

Subbasin -- 2.9 74.3 0.1 10.6 5.7 7.9 0.8 0.5 14.7 13.4 1.1 0.2 0.0

Scituate Reservoir Drainage Area

-- -- 3.8 65.8 8.0 10.5 4.1 9.9 1.1 0.6 -- -- -- -- --
Methods of Study 11

and gravel). Till or ground moraine, locally called hardpan, 

is a mixture of nonsorted sand, silt, clay, and boulders that is 

generally compacted and blankets the bedrock surface. Typically, till was not carried far and thus reflects the character 

of the underlying bedrock (Robinson, 1961). The meltwater 

deposits, which are primarily in stream valleys, consist of 

poorly sorted to well sorted sand and gravel. As a result, meltwater deposits typically underlie the reservoir water bodies.

Hydrology

The Scituate Reservoir drainage area encompasses 

the Scituate Reservoir and five smaller tributary reservoirs 

that receive water from numerous small rivers and streams 

(fig. 1). The Ponaganset River, Peeptoad Brook, and Hemlock 

Brook are the largest tributaries to the Scituate Reservoir 

(fig. 2; table 1). The Pawtuxet River begins at the outlet of 

the Scituate Reservoir and flows eastward to Narragansett 

Bay (not shown on figs. 2 and 3). The mean annual flow and 

water yield in the Pawtuxet River at Cranston, R.I., (USGS 

station 01116500) were 356 cubic feet per second (ft3

/s) and 

1.78 cubic feet per second per square mile (ft3/s/mi2), respectively, for the 72-year period of record from 1940 through 

2012 (http://waterdata.usgs.gov/nwis/). During WYs 2003–12, 

mean annual streamflow and water yield at the Pawtuxet River 

at Cranston, R.I., station, were 410 ft3/s and 2.05 ft3/s/mi2, 

respectively, about 15 percent greater than the long-term mean 

annual streamflow. Mean annual streamflows and water yields 

at the USGS stations on the Ponaganset River (Providence 

Water station 35, USGS station 0111578) and Peeptoad Brook 

(Providence Water station 16, USGS station 01115098; fig. 2) 

in the Scituate Reservoir drainage area averaged about 32 ft3/s 

and 2.20 ft3/s/mi2 and 12 ft3/s and 2.37 ft3/s/mi2, respectively, 

for WYs 2003–12. The latter two stations have drainage areas 

of 14.4 and 4.92 mi2, respectively (table 1), whereas the drainage area above the USGS station on the Pawtuxet River at 

Cranston is 200 mi2.

Surface water and groundwater are closely connected 

in the Scituate River drainage area, as is typical of valley-fill 

hydrogeologic settings in the Northeastern United States. 

Groundwater is recharged from precipitation and generally 

flows from topographic highs in the uplands toward stream 

channels and meltwater glacial deposits in the stream valleys 

(Hahn and Hansen, 1961; Hansen, 1962; Pollock, 1960). 

Streams, along with wetlands, typically are discharge areas 

for groundwater.

Methods of Study

Water-quality data were obtained from Providence Water. 

The data are based on samples collected and analyzed by 

Providence Water personnel during WYs 2003–12 and previous years. The USGS, in cooperation with Providence Water 

and the RIDEM, has collected streamflow data at many of the 

Providence Water monitoring stations since 1993.

Water-Quality Data

Water-quality data from WYs 2003–12 were obtained 

annually from a water-quality database maintained by 

Providence Water (appendix 1; Richard Blodgett, Providence 

Water Supply Board, written commun., 2005–12). Waterquality datasets for trend analysis for years before 2003 were 

obtained periodically from Providence Water. During WYs 

2003–12, the water-quality samples were collected monthly 

at 19 stations and quarterly at 18 stations in the Scituate 

Reservoir drainage area (table 1). Sampling frequencies for 

data collected before WY 1996 varied (Breault and others, 

2000). Typically, samples were collected at approximately the 

same day of each month. Occasionally, samples could not be 

collected because streams at the sampling stations were dry 

or frozen.

Water-quality samples were collected by dipping the 

sample bottle into the stream at the center of flow (Richard 

Blodgett, Providence Water Supply Board, written commun., 

2005). Samples were transported on ice to the water-quality 

laboratory of Providence Water at the P.J. Holton Water 

Purification Plant in Scituate, R.I., for analysis. Samples were 

not filtered prior to analysis. Complete sample-collection, 

analytical, and laboratory quality-control procedures are 

described in a Providence Water quality assurance program 

manual (Providence Water Supply Board Water Quality 

Laboratory, 2005, 2012). Field quality-control samples were 

not collected.

Physical properties and constituent concentrations measured in the water samples included pH, color, turbidity, total 

coliform bacteria, Escherichia coli (E. coli), alkalinity, chloride, nitrite, nitrate, and orthophosphate. Analytical methods 

used for the determination of values or concentrations of pH, 

color, turbidity, bacteria, alkalinity, chloride, and nitrite are 

those documented by the American Public Health Association 

and others (2012). Concentrations of nitrate were determined 

by the Hach Nitrate LR Method (Hach Method 8192; Hach 

Company, 2000). Concentrations of orthophosphate were 

determined by the Hach PhosVer Method (Hach Method 8048; 

Hach Company, 2000). In July 2012, the analytical method for 

bacteria was changed to the Standard Methods 9222 method, 

which has a higher upper reporting limit than the previously 

used Standard Methods 9221 (multiple tube fermentation 

method that limited values to 2,400 colony-forming units 

per 100 milliliters (CFU/100 mL; American Public Health 

Association and others, 2012). Not all the analyses were done 

throughout the entire 29-year period, (WYs 1983–2012), and 

some of the data are missing. Monitoring of nitrate began 

in WY 1988, monitoring of nitrite began in WY 1985, and 

monitoring of orthophosphate began in WY 1997. Monitoring 

of E. coli began in 1992, but data for E. coli collected before 

WY 1995 were not available.

The interpretations of water-quality analyses made for 

this study were limited in some cases by a lack of available 

information about the historical and current water-quality data. 

For example, it was not possible to evaluate the accuracy, 
12 Water-Quality Trends in the Scituate Reservoir Drainage Area, Rhode Island, 1983–2012

precision, or bias of laboratory analyses because laboratory 

quality-control data were not available. Similarly, it was not 

possible to evaluate the in situ variability of values of physical 

properties and constituent concentrations or the potential for 

sample contamination because field quality-control data were 

not available. Where possible, statistical approaches were used 

to minimize the limitations that may have resulted from the 

use of existing data or to assure that interpretations were based 

on a well-represented dataset. For example, a lack of ancillary 

information or sample documentation precluded the identification of data values that may have been suspect. The use 

of nonparametric, rank-based analytical methods reduced the 

potential effects of these outliers.

The samples analyzed for physical properties and 

constituents generally were collected during a wide range of 

flow conditions. As an example, the daily mean flow-duration 

curve for the Peeptoad Brook at North Scituate station 

(USGS station number 01115098) for WY 2012 represents the 

percentage of time that each flow was exceeded at this station 

(fig. 4). The flows at this station are represented by the points 

plotted on the curve for the days when water-quality samples 

were collected. In this example, samples were collected 

at flow durations ranging from the 20th percentile to the 

98th percentile.

Changes in analytical methods or lab personnel could 

be unidentified sources of variability and trends in the waterquality data. The potential effects of changing analytical 

detection limits and the upper method performance levels 

0.1 1 10 40 70 95 99.5

0.1

1

10

100

1,000

Percentage of time streamflow was equaled or exceeded

Streamflow, in cubic feet per second

Water-quality sample 

collected at Peeptoad Brook

Figure 4. Flow-duration curve and streamflow values on the 

dates (represented by points) when water-quality samples were 

collected for the U.S. Geological Survey station on Peeptoad 

Brook at North Scituate (station 01115098), Rhode Island, for water 

year 2012.

over time were addressed by applying uniform assessment 

levels, which represent the reported detection limits in the 

dataset, to selected constituents. These assessment levels 

were 0.001 milligrams per liter as nitrogen (mg/L as N) 

for nitrite, 0.01 mg/L as N for nitrate, 0.003 milligrams per 

liter as phosphorus (mg/L as P) for orthophosphate, and 

3 CFU/100 mL (lower) and 2,400 CFU/100 mL (upper) for 

total coliform bacteria and E. coli, respectively.

Compilation and Analysis of Streamflow and 

Continuous Water-Quality Data

Streamflow data have been collected at 23 of the 

37 Providence Water monitoring stations (table 1) since 

1994. Streamflow has been monitored continuously at 2 stations (Providence Water stations 16 and 35) since 1994 and 

at 12 additional stations since WY 2009 (table 1). Streamflow 

was measured intermittently at 9 other stations (partial-record 

stations) during WYs 1994–2012. Specific conductance and 

water temperature has been monitored concurrently with 

streamflow at 14 stations since WY 2009 (table 1). Streamflow, specific conductance, and water temperature data for 

continuous- and partial-record stations are available through 

NWIS (U.S. Geological Survey, 2001).

Estimation of Loads and Yields

Loads are the quantity of a constituent transported by 

a stream during a specific time interval and are determined 

by multiplying the concentration of a constituent by the 

streamflow. Yields are loads normalized by dividing the 

load by the drainage area of each subbasin. Daily loads and 

yields of total coliform bacteria, E. coli, alkalinity, chloride, 

nitrite, nitrate, and orthophosphate were estimated from 

analyte concentrations in samples collected by Providence 

Water. Daily loads and yields were only estimated for stations 

which streamflow data were available. Loads and yields of 

chloride and sodium were estimated from continuous records 

of streamflow and specific conductance, and estimated 

dissolved concentrations of chloride and sodium derived from 

correlations with specific conductance for all monitoring 

stations with these data records.

Estimation of Daily Loads and Yields

Daily loads and yields of water-quality constituents 

were calculated for all dates during the study period when 

Providence Water collected water-quality samples at the 

23 stations for which periodic or continuous streamflow data 

were available (table 1). These loads were calculated by 

multiplying the constituent concentrations in single samples 

by the mean daily flow for the date when the sample was 

collected. These flows, which in most cases were estimated 

(before WY 2008, in particular), were assumed to reasonably 
Methods of Study 13

represent the flow at the time of the sample collection. Daily 

loads and yields were calculated for total coliform bacteria, 

E. coli, chloride, nitrate, nitrite, and orthophosphate. Median 

values of loads and yields are reported for all water-quality 

sampling dates during WYs 2003–12.

For constituents with censored data (concentrations 

reported as less than the minimum method detection level or 

greater than the upper method performance level), minimum 

loads were calculated by multiplying the mean daily flow 

by half of the method detection level as the concentration of 

each constituent and where values were greater than the upper 

method performance level, maximum loads were estimated by 

multiplying the mean daily flow by the upper assessment level 

(2,400 CFU for bacteria). The calculation of yields allows for 

direct comparison of loads at stations with different drainage 

areas. Yields were calculated by dividing load by the drainage 

area for each station. Loads are given in kilograms per day 

(kg/d) or millions of colony-forming units per day (MCFU/d) 

for bacteria, and yields are given in kilograms per day per 

square mile or millions of colony-forming units per day per 

square mile for bacteria.

Mean daily flows at the partial-record stations on 

the water-quality sampling dates were estimated with the 

Maintenance of Variance Extension type 1 method (MOVE.1), 

as described by Ries and Friesz (2000); data needed to 

estimate streamflows at partial-record sites were retrieved 

from NWIS (http://waterdata.usgs.gov/nwis/). In the MOVE.1 

method, an equation (equation 1) is developed to relate 

measured instantaneous flow at a partial-record station and 

flow at one or more continuous-record streamgaging stations. 

When this relation is developed, streamflow at the ungaged 

partial-record station can be estimated for any date on which 

the continuous streamflow records are available. It is assumed 

that the relation between flow at the partial-record station and 

flow at the continuous-record station remains constant through 

time and is consistent over the range of flows in the time 

period of interest.

( ) m Qq b = × , (1)

where

Q is the mean daily flow at the station of 

interest, in cubic feet per second;

q is the mean daily flow at the reference station, 

in cubic feet per second;

m is the slope from the MOVE.1 analysis 

(table 3); and

b is the intercept from the MOVE.1 analysis 

(table 3).

Data from the station on the Ponaganset River 

(Providence Water station 35; USGS station 01115187) were 

used primarily to estimate flow at the partial-record stations 

(table 3). Although flow data from other stations outside the 

Scituate Reservoir drainage area were available for analyses, 

inclusion of these data did not reduce the difference between 

predicted and observed streamflow values as indicated by the 

resultant average standard error of the estimate2

 of the relation 

with data from Ponagaset River alone. For several stations, 

flows after WY 2008 were estimated with separate equations 

developed from the continuous-record streamflow-gaging 

station on Quonapaug Brook (Providence Water station 6; 

USGS station 01115183). In general, data from Quonapaug 

Brook are more hydrologically similar to many of the partial 

record stations than they are to Ponaganset River. As a result, 

the average standard error of the estimate for regression 

equations developed from continuous records of flow from 

Quonapaug Brook after 2008 are smaller than the average 

standard error of the estimate for regression equations used 

before WY 2008 (table 3).

Estimation of Continuous Loads and Yields of 

Chloride and Sodium

Specific conductance measurements are commonly used 

to estimate concentrations of dissolved major ions (Hem, 

1982, 1985; Miller and others, 1988; Church and others, 1996; 

Granato and Smith, 1999; Smith and Granato, 2010; Smith, 

2013). In this study, the measurements were used to estimate 

concentrations and loads of dissolved chloride and sodium in 

14 subbasins in the Scituate Reservoir drainage area in WYs 

2009–12. Concentrations of dissolved chloride and sodium 

were not only measured in water-quality samples primarily 

collected during WYs 2009–12 at the 14 continuous-record 

stations (table 1; http://waterdata.usgs.gov/nwis/), but also 

were estimated from the more frequent, in situ measurements 

of specific conductance. These continuous specific conductance data were available for nearly every day of the study and 

represent a variety of hydrologic conditions (base flow, rain, 

mixed precipitation, and snowmelt runoff events).

Instantaneous concentrations of chloride and sodium 

were estimated from continuous measurements of specific 

conductance by using equations developed to relate specific 

conductance to concentrations of the respective ions 

(equation 2). These regression equations were developed with 

the MOVE.1 technique based on concurrent measurements 

of log-transformed values of specific conductance and 

concentrations of chloride and sodium in tributary water 

samples. The data set includes 185 concentrations for these 

constituents, where specific conductance values ranged from 

44 to 351 microsiemens per centimeter at 25 degrees Celsius 

(U.S. Geological Survey, 2001). The MOVE.1 technique was 

chosen for regression analysis because it minimizes errors in 

both the x and y directions, producing a unique equation that 

can be used to predict either variable from the other (Helsel 

and Hirsch, 2002).

2

Average standard error of the estimate is a measure of the average variation 

between regression estimates and observed values. About two-thirds of the 

regression estimates have errors less than the average standard error of estimate and about one-third of the estimates have errors larger than the average 

standard error of estimate.
14 Water-Quality Trends in the Scituate Reservoir Drainage Area, Rhode Island, 1983–2012 Table 3. Stations in the Scituate Reservoir drainage area, Rhode Island, for which streamflow data were estimated, water years 2002–12, and U.S. Geological Survey index stations, period of streamflow measurements, and maintenance of variance extension type 1 method regression coefficients. [Values in parentheses are applicable for estimated streamflow data during water years 2009–12. PWSB, Providence Water Supply Board; USGS, U.S. Geological Survey] PWSB station number USGS station number Number of USGS station used in analysis Number of streamflow measurements used in analysis Years of streamflow measurements Slope Intercept Standard error of regressions (percent) Period of estimated streamflow (water years) Barden Reservoir Subbasin 24 01115190 01115187 114 1994–2007, 2009–13 1.211 0.155 81.7 2003–08 25 01115200 01115187, (01115183) 31 (11) 1994–95, 2003, 2009–13 1.560 (1.372) 0.031 (0.0758) 97.3 (89.6) 2003–12 26 01115185 01115187, (01115183) 79 (11) 1994–2007, 2009–13 1.358 (1.392) 0.0692 (1.034) 116 (103) 2003–12 28 01115265 01115187 94 1996–2007, 2009–13 1.091 0.467 45.1 2003–08 Direct Runoff Subbasin

1 01115180 01115187, (01115183) 89 (12) 1994–2007, 2009–13 0.7177 (0.7263) 0.3094 (1.004) 49.1 (37.1) 2003–12

3 01115280 01115187 111 1994–2007, 2009–13 1.1512 0.0650 67.3 2003–08

4 01115400 01115187 20 1994–2007, 2009–13 1.3674 0.0139 123 2003–12

5 01115184 01115187 112 1994–2007, 2009–13 1.0189 0.0973 70.4 2003–08

6 01115183 01115187 105 1994–2007, 2009–13 1.1194 0.1055 75.1 2003–08

7 01115297 01115187 112 1994–2007, 2009–13 1.0057 0.3288 59.1 2003–08

8 01115276 01115187 48 1994–95, 2003, 2009–13 0.4352 2.3202 42.3 2003–08

9 01115275 01115187 93 1994–95, 2000–07, 2009–13 0.5334 0.3024 48.1 2003–08

32 01115178 01115187, (01115183) 22 (11) 1994–95, 2003, 2009–13 0.8375 (0.8206) 0.0472 (0.2165) 52.5 (44.3) 2003–12

33 01115182 01115187 23 1994–95, 2003, 2009–13 1.0596 0.0125 69.5 2003–12

Moswansicut Reservoir Subbasin

19 01115170 01115187 91 1994–95, 2000–07, 2009–13 0.9341 0.3034 85.9 2003–08

21 01115165 01115187, (01115183) 28 (11) 1994–95, 2003, 2009–13 0.7346 (0.8708) 0.0643 (0.2134) 95.3 (51.4) 2003–12

Regulating Reservoir Subbasin

14 01115110 01115187 106 1994–2007, 2009–13 1.3161 0.1276 57.2 2003–08

15 01115114 01115187 81 1997–2007, 2009–13 1.3014 0.0948 62.6 2003–08

18 01115120 01115187 73 1994–2007, 2009–13 1.6816 0.0009 109 2003–08

Westconnaug Reservoir Subbasin

10 01115274 01115187, (01115183) 28 (11) 1994–95, 2003, 2009–13 1.793 (1.064) 0.0130 (0.5206) 136 (36.3) 2003–12

11 01115273 01115187, (01115183) 28 (7) 1994–95, 2003, 2009–13 0.9838 (0.8601) 0.0740 (0.4255) 49.4 (33.1) 2003–12
Methods of Study 15

( ) m C Spc b = × , (2)

where

C is the concentration of the ion of interest, in 

milligrams per liter

Spc is the specific conductance, in microsiemens 

per centimeter at 25 degrees Celsius;

m is the slope from the MOVE.1 analysis 

(table 4); and

b is the intercept from the MOVE.1 analysis 

(table 4).

With the exception of chloride and sodium, concentrations of other major ions did not vary substantially as specific 

conductance increased from sample to sample and from station to station (fig. 5). Only minimal increases in the concentrations of calcium and sulfate, trace constituents of road 

salt (Smith and Granato, 2010), occurred over the range of 

specific conductance. As a result, concentrations of chloride 

and sodium, the primary constituents of road salt, governed 

the relation with specific conductance and in many cases, the 

same equation coefficients produced satisfactory estimates for 

concentrations of chloride and sodium at multiple stations. At 

some stations, however, separate coefficients were developed 

to improve the average standard error of the estimate of the 

relations between measurements of specific conductance to 

concentrations of chloride and sodium. The average standard 

error of the estimate ranged from about 5 to 9 percent for 

chloride and from about 7 to 11 percent for sodium for all 

14 stations. These equations provide reasonable estimates for 

dissolved concentrations of chloride and sodium on the basis 

of specific conductance measurements (fig. 6).

Loads were calculated by multiplying estimated flowweighted concentrations of chloride and sodium, in milligrams 

per liter, for each day by the daily-mean flow, in liters per 

day, and summed for each water year. The average daily load, 

yield, and concentration of chloride and sodium were estimated for WYs 2009–12 at each of the 14 stations.

Determination of Correlations and Trends

To examine factors affecting water quality in the Scituate 

Reservoir drainage area, correlation analyses were performed 

relating streamflow and subbasin characteristics, such as land 

use, and impervious area, to median values or concentrations 

of pH, color, turbidity, total coliform bacteria and E. coli, 

chloride, nitrite, nitrate, and orthophosphate from WYs 

2003–12. Correlation analyses also were performed relating 

Table 4. Regression equation coefficients used to estimate concentrations of chloride and sodium from values of specific 

conductance at U.S. Geological Survey stations in the Scituate Reservoir drainage area, Rhode Island, water years 

2009–12.

[PWSB, Providence Water Supply Board; USGS, U.S. Geological Survey]

PWSB 

station 

number

USGS 

station 

number

Chloride Sodium

Slope Intercept

Standard 

error of 

regressions 

(percent)

Slope Intercept

Standard 

error of 

regressions 

(percent)

24 01115190 1.0995 0.1368 5.2 1.0522 0.1043 7.1

28 01115265 1.0995 0.1368 5.2 1.0522 0.1043 7.1

35 01115187 1.0995 0.1368 5.2 1.0522 0.1043 7.1

3 01115280 1.0995 0.1368 5.2 1.0522 0.1043 7.1

5 01115184 1.1901 0.0772 8.5 1.0808 0.0760 10.6

6 01115183 1.1561 0.0942 5.1 0.9534 0.1577 7.0

7 01115297 1.1901 0.0772 8.5 1.0808 0.0760 10.6

8 01115276 1.0995 0.1368 5.2 1.0522 0.1043 7.1

9 01115275 1.0995 0.1368 5.2 1.0522 0.1043 7.1

19 01115170 1.0995 0.1368 5.2 1.0522 0.1043 7.1

14 01115110 1.1901 0.0772 8.5 1.0808 0.0760 10.6

15 01115114 1.0995 0.1368 5.2 1.0522 0.1043 7.1

16 01115098 1.1561 0.0942 5.1 0.9534 0.1577 7.0

18 01115120 1.0995 0.1368 5.2 1.0522 0.1043 7.1
16 Water-Quality Trends in the Scituate Reservoir Drainage Area, Rhode Island, 1983–2012

EXPLANATION

50 100 150 200 250 300 350

0

20

40

60

80

100

 Chloride

 Sulfate

 Calcium

 Magnesium

 Potassium

 Sodium

Concentration, in milligrams per liter

Specific conductance, in microsiemens per centimeter

0

Figure 5. Relations between specific conductance and 

concentrations of selected major ions in water-quality samples 

collected from tributaries at 14 U.S. Geological Survey stations in 

the Scituate Reservoir drainage area, Rhode Island, water years 

2009–12.

the density of timber activity expressed as varying degrees of 

tree-canopy reduction in each subbasin to median values or 

concentrations of pH, color, turbidity, total coliform bacteria 

and E. coli, chloride, nitrite, nitrate, and orthophosphate from 

WYs 1992–2012. The nonparametric Spearman correlation 

test, which is performed on the ranks of the data, was used 

(Helsel and Hirsch, 2002). Censored data values were set 

equal to a value less than the uniform assessment level for 

each constituent; this approach is appropriate for correlation 

analysis based on rank-based, nonparametric methods (Helsel, 

2005). A significant correlation was defined as one with a 

probability of results from chance associations of the data 

equal to less than 5 percent (p value less than 0.05).

Time trends in concentrations of water-quality 

constituents can indicate long-term changes in streamwater quality. Statistical tests to identify time trends were 

performed for pH, color, turbidity, total coliform bacteria, 

E. coli, alkalinity, chloride, nitrite, nitrate, and orthophosphate 

at all monitoring stations. Trend tests were performed 

separately on water-quality physical properties and constituent 

concentrations for WYs 2003–12 and for WYs 1983–2012.

Trends for each multiyear period were analyzed by the 

seasonal Kendall test (Helsel and Hirsch, 2002). This test 

accounts for the natural seasonal variation in values of waterquality properties or concentrations of constituents caused 

by changes in streamflow, temperature, biological activity, 

or other factors. A trend was considered significant if the 

calculated p value of the test was less than 0.05, as described 

previously. Significant trends were identified as either upward 

or downward.

Water-Quality Conditions and 

Constituent Loads, WYs 2003–12

Water-quality conditions in the Scituate Reservoir 

drainage area for WYs 2003–12 were described in terms of 

median values (table 5) and boxplots (fig. 7) of water-quality 

properties and constituent concentrations in samples from 

the 37 monitoring stations in the drainage area. Correlations 

among water-quality properties and constituent concentrations 

(table 6), and correlations between water-quality properties 

and subbasin characteristics and streamflow and between 

constituent concentrations and subbasin characteristics and 

streamflow also were used to evaluate water-quality patterns 

in the drainage area (tables 7 and 8). Values of properties and 

concentrations of constituents were compared to available 

State and Federal guidelines and standards (tables 9 and 10) to 

describe water-quality conditions in terms of potential effects 

on human health and aquatic life.

Water-Quality Properties

Providence Water measured four water-quality properties—pH, color, turbidity, and alkalinity as general indicators 

of water-quality conditions. The measurements were used to 

identify site-specific concerns and were compared with State 

and Federal guidelines.

pH

Measurements of pH represent the negative base-10 

logarithm of the hydrogen-ion concentration or activity. pH 

values near 7 are considered neutral, below 7 are acidic, and 

above 7 are alkaline or basic. The pH of river water generally 

ranges from about 6.5 to 8.5 (Hem, 1985). The pH of water 

affects the solubility and biological availability of chemical 

constituents such as nutrients and metals. The pH of the 

stream and reservoir waters also affects water-treatment 

procedures. Median pH in the tributaries in the Scituate 

Reservoir drainage area ranged from 5.4 to 6.7 with a median 

for all stations of 6.1. Thus, tributaries were slightly acidic, 

reflecting the low pH of precipitation in the Northeastern 

United States (mean annual precipitation-weighted pH was 

4.7 for WYs 2003–12; National Atmospheric Deposition 

Program, 2014) and the relatively nonreactive character of 

rock types in the drainage area. Water samples collected at two 

monitoring stations in the Westconnaug Reservoir Subbasin, 

Providence Water stations 10 and 11, had the lowest median 

pH values (5.4 and 5.6, respectively; table 5); water samples 

collected in several tributary subbasins had median pH 

values less than 6.0. The highest median pH value, 6.7, was 

determined for Moswansicut Stream (Providence Water station 

19) and Brandy Brook (Providence Water station 1)—in areas 

that are less forested and more developed than those in the 

Westconnaug Reservoir Subbasin (table 2).
Water-Quality Conditions and Constituent Loads, WYs 2003–12 17

U.S. Geological Survey

station number

EXPLANATION

A

0 10 20 30 40 50 60

0

10

20

30

40

50

60

Estimated concentrations of sodium, in milligrams per liter

Measured concentrations of sodium, in milligrams per liter

0 10 20 30 40 50 60 70 80

0

10

20

30

40

50

60

70

80

 01115114

 01115120

 01115170

 01115187

 01115190

 01115265

 01115275

 01115276

 01115280

 01115297

 01115184

 01115114

 01115098

 01115183

Estimated concentrations of chloride, in milligrams per liter

Measured concentrations of chloride, in milligrams per liter

1:1 line

Plus and minus 5 percent about the 1:1 line

B

Figure 6. Relations between measured and estimated concentrations of A, chloride and B, sodium at 14 U.S. Geological 

Survey stations in the Scituate Reservoir drainage area, Rhode Island, water years 2009–12.
18 Water-Quality Trends in the Scituate Reservoir Drainage Area, Rhode Island, 1983–2012 Table 5. Median values for water-quality data collected at Providence Water Supply Board stations in the Scituate Reservoir drainage area, Rhode Island, water years 2003–12. [Water-quality data are from samples collected and analyzed by Providence Water Supply Board (PWSB). Not all samples were analyzed for all water-quality properties or constituents. USGS, U.S. Geological Survey; PCU, platinum cobalt units; NTU, nephelometric turbidity units; CFU/100mL, colony-forming units per 100 milliliters; E. coli, Escherichia coli; mg/L, milligrams per liter; CaCO3, calcium carbonate; N, nitrogen; P, phosphorus; <, less than; --, no USGS station number assigned] PWSB station number USGS station number Properties Constituents pH (units) Color (PCU) Turbidity (NTU) Total coliform bacteria (CFU/100mL) E. coli (CFU/100mL) Alkalinity (mg/L as CaCO3

)

Chloride

(mg/L)

Nitrite

(mg/L as N)

Nitrate

(mg/L as N)

Orthophosphate

(mg/L as P)

Barden Reservoir Subbasin

24 01115190 5.9 54 0.59 43 10 3.8 25 0.002 0.01 0.02

25 01115200 5.9 36 0.37 23 9 3.5 11 0.001 0.01 0.02

26 01115185 6.2 35 0.33 23 8 3.9 20 0.001 0.01 0.03

27 011151845 5.7 25 0.26 23 9 3.9 9.9 0.001 0.02 0.03

28 01115265 5.8 80 0.51 43 23 3.5 22 0.002 0.01 0.03

29 01115271 6.0 45 0.60 9 4 3.6 19 0.001 0.01 0.02

35 01115187 6.1 49 0.60 23 12 3.5 18 0.001 0.01 0.02

Direct Runoff Subbasin

1 01115180 6.7 72 1.4 75 23 9.0 11 0.002 0.01 0.03

2 01115181 6.1 18 0.28 93 23 3.7 53 0.001 0.03 0.02

3 01115280 6.3 34 0.32 43 9 4.7 31 0.001 0.02 0.03

4 01115400 6.3 30 0.57 23 4 6.4 4.5 0.001 0.01 0.02

5 01115184 6.1 52 0.42 43 9 4.1 17 0.001 0.03 0.03

6 01115183 6.3 98 1.0 150 43 8.2 34 0.002 0.01 0.03

7 01115297 6.1 82 0.78 43 23 5.5 10 0.002 0.01 0.02

8 01115276 5.9 20 0.42 4 <3 2.9 12 0.001 0.01 0.02

9 01115275 6.3 36 0.32 43 9 6.5 60 0.001 0.11 0.02

30 01115350 6.1 48 0.38 43 23 3.9 20 0.001 0.02 0.02

31 01115177 6.4 35 0.93 350 59 14 87 0.003 0.13 0.04

32 01115178 6.4 63 0.70 59 23 5.9 12 0.002 0.02 0.03

33 01115182 6.1 32 0.36 23 20 4.7 13 0.001 0.02 0.02

36 -- 6.4 25 0.31 23 4 3.9 3.8 0.001 0.01 0.02

37 -- 5.8 24 0.26 23 4 3.1 4.2 0.001 0.01 0.02
Water-Quality Conditions and Constituent Loads, WYs 2003–12 19

Table 5. Median values for water-quality data collected at Providence Water Supply Board stations in the Scituate Reservoir drainage area, Rhode Island, water years 2003–12.—Continued[Water-quality data are from samples collected and analyzed by Providence Water Supply Board (PWSB). Not all samples were analyzed for all water-quality properties or constituents. USGS, U.S. Geological Survey; PCU, platinum cobalt units; NTU, nephelometric turbidity units; CFU/100mL, colony-forming units per 100 milliliters; E. coli, Escherichia coli; mg/L, milligrams per liter; CaCO3, calcium carbonate; N, nitrogen; P, phosphorus; <, less than; --, no USGS station number assigned]

PWSB 

station 

number

USGS 

station 

number

Properties Constituents

pH 

(units)

Color

(PCU) 

Turbidity

(NTU) 

Total 

coliform bacteria

(CFU/100mL) 

E. coli

(CFU/100mL) 

Alkalinity

(mg/L as CaCO3)

Chloride

(mg/L)

Nitrite

(mg/L as N)

Nitrate

(mg/L as N)

Orthophosphate

(mg/L as P)

Moswansicut Reservoir Subbasin

19 01115170 6.7 25 0.93 23 5 8.4 36 0.002 0.01 0.01

20 01115160 6.0 135 0.64 240 43 6.1 45 0.003 0.01 0.03

21 01115165 6.6 55 0.89 43 11 10 32 0.002 0.03 0.02

22 01115167 6.5 35 1.5 460 75 14 39 0.005 0.14 0.03

34 01115164 6.3 50 0.76 23 4 10 33 0.002 0.01 0.03

Ponaganset Reservoir Subbasin

23 011151843 5.7 12 0.43 9 2 2.3 12 0.001 0.01 0.01

Regulating Reservoir Subbasin

13 01115176 6.6 32 0.71 9 4 7.9 31 0.001 0.01 0.02

14 01115110 6.4 36 0.56 84 41 6.6 10 0.001 0.01 0.03

15 01115115 6.6 51 0.76 150 23 8.4 42 0.002 0.01 0.03

16 01115098 6.5 36 0.76 75 23 9.6 34 0.002 0.01 0.02

17 01115119 5.8 56 0.53 23 4 5.2 31 0.002 0.01 0.02

18 01115120 6.4 66 0.75 75 23 8.9 45 0.001 0.02 0.03

Westconnaug Reservoir Subbasin

10 01115274 5.4 26 0.22 43 17.5 2.3 24 0.001 0.01 0.03

11 01115273 5.6 120 0.56 210 23 3.9 6.1 0.003 0.01 0.02

12 011152745 5.9 72 0.94 43 23 5.1 43 0.002 0.02 0.02

Scituate Reservoir Drainage Area

Minimum 5.4 12 0.22 4 <3 2.3 3.8 0.001 0.01 0.01

Median 6.1 36 0.57 43 15 5.1 22.0 0.001 0.01 0.02

Maximum 6.7 135 1.5 460 75 14.0 87.0 0.005 0.14 0.04
20 Water-Quality Trends in the Scituate Reservoir Drainage Area, Rhode Island, 1983–2012

 Providence Water station number

24252627282935 1 2 3 4 5 6 7 8 930313233

3637192021223423131415161718101112

0.01

0.1

1

10

100

4

5

6

7

8

1

10

100

1,000

25th percentile

75th percentile

Median

Maximum

Minimum

Mean

Acceptable color range stated in the National 

Secondary Drinking Water Regulation

Acceptable range in turbidity stated in

the U.S. Environmental Protection Agency

proposed guideline for level 3 ecoregion 59

Subbasins

Barden Reservoir Subbasin 

Direct Runoff Subbasin

Moswansicut Reservoir Subbasin

Ponaganset Reservoir Subbasin

Regulating Reservoir Subbasin

Westconnaug Reservoir Subbasin

A

B

C

pH, in standard units Color, in platinum cobalt units Turbidity, in nephelometric turbidity units

EXPLANATION

Acceptable pH range stated in the U.S. Environmental

Protection Agency secondary drinking-water regulation

Figure 7. Distributions of measurements of A, pH, B, color, and C, turbidity and concentrations of D, alkalinity, E, chloride, 

F, nitrite, G, nitrate, H, nitrate plus nitrite, I, orthophosphate, J, total coliform bacteria, and K, Escherichia coli (E. coli) in 

samples collected at Providence Water Supply Board (Providence Water) water-quality-monitoring stations in the Scituate 

Reservoir drainage area, Rhode Island, water years 2003–12.
Water-Quality Conditions and Constituent Loads, WYs 2003–12 21

0.1

1

10

100

1

10

100

1,000

0.01

0.1

1

Providence Water station number

0.1

0.001

0.0001

Acceptable chloride range stated

in the U.S. Environmental Protection Agency

secondary drinking-water regulation

Lower assessment level = 0.001

Assessment level

D

E

F

25th percentile

75th percentile

Median

Maximum

Minimum

Mean

Subbasins

EXPLANATION

Barden Reservoir Subbasin 

Direct Runoff Subbasin

Moswansicut Reservoir Subbasin

Ponaganset Reservoir Subbasin

Regulating Reservoir Subbasin

Westconnaug Reservoir Subbasin

Alkalinity, in milligrams per liter Chloride, in milligrams per liter Nitrite as N, in milligrams per liter

24252627282935 1 2

3 4

56789

30313233

363719202122342313

1415161718101112

Figure 7. Distributions of measurements of A, pH, B, color, and C, turbidity and concentrations of D, alkalinity, 

E, chloride, F, nitrite, G, nitrate, H, nitrate plus nitrite, I, orthophosphate, J, total coliform bacteria, and K, Escherichia coli 

(E. coli) in samples collected at Providence Water Supply Board (Providence Water) water-quality-monitoring stations in 

the Scituate Reservoir drainage area, Rhode Island, water years 2003–12.—Continued
22 Water-Quality Trends in the Scituate Reservoir Drainage Area, Rhode Island, 1983–2012

Providence Water station number

0.01

0.1

1

10

Orthophosphate as phosphorus, in milligrams per liter

0.01

0.1

1

10

Nitrate as N, in milligrams per liter

0.001

Lower assessment level = 0.01

0.001

Acceptable range for the sum of nitrate and nitrite

stated in the U.S. Environmental Protection Agency

proposed guideline for level 3, ecoregion 59

Lower assessment level = 0.003

0.001

Acceptable range for total phosphorus stated in the

U.S. Environmental Protection Agency proposed guideline 

for level 3 ecoregion 59

Assessment level

0.01

0.1

1

10

G

H

I

25th percentile

75th percentile

Median

Maximum

Minimum

Mean

Subbasins

EXPLANATION

Barden Reservoir Subbasin 

Direct Runoff Subbasin

Moswansicut Reservoir Subbasin

Ponaganset Reservoir Subbasin

Regulating Reservoir Subbasin

Westconnaug Reservoir Subbasin

Nitrate plus nitrite as N, in milligrams per liter

24252627282935 1 2 3 4 5 6 7 8 9303132

333637192021223423131415161718101112

Figure 7. Distributions of measurements of A, pH, B, color, and C, turbidity and concentrations of D, alkalinity, E, chloride, F, nitrite, 

G, nitrate, H, nitrate plus nitrite, I, orthophosphate, J, total coliform bacteria, and K, Escherichia coli (E. coli) in samples collected 

at Providence Water Supply Board (Providence Water) water-quality-monitoring stations in the Scituate Reservoir drainage area, 

Rhode Island, water years 2003–12.—Continued
Water-Quality Conditions and Constituent Loads, WYs 2003–12 23

24252627282935 1

23456789

30313233

3637192021223423131415161718101112

1

10

100

1,000

10,000

Providence Water station number

Upper assessment level = 2,400

Lower assessment level = 3

1

10

100

1,000

10,000

100,000

Upper assessment level = 2,400

Lower assessment level = 3

J

Assessment level

25th percentile

75th percentile

Median

Maximum

Minimum

Mean

Subbasins

EXPLANATION

K

Barden Reservoir Subbasin 

Direct Runoff Subbasin

Moswansicut Reservoir Subbasin

Ponaganset Reservoir Subbasin

Regulating Reservoir Subbasin

Westconnaug Reservoir Subbasin

Total coliform bacteria, in colony-forming

units per 100 milliliters E. coli bacteria, in colony-forming units per 100 milliliters

Figure 7. Distributions of measurements of A, pH, B, color, and C, turbidity and concentrations of D, alkalinity, 

E, chloride, F, nitrite, G, nitrate, H, nitrate plus nitrite, I, orthophosphate, J, total coliform bacteria, and K, Escherichia 

coli (E. coli) in samples collected at Providence Water Supply Board (Providence Water) water-quality-monitoring 

stations in the Scituate Reservoir drainage area, Rhode Island, water years 2003–12.—Continued
24 Water-Quality Trends in the Scituate Reservoir Drainage Area, Rhode Island, 1983–2012

Table 6. Significance levels (p values) for rank correlations between selected physical properties and constituent concentrations at 

Providence Water Supply Board water-quality-monitoring stations in the Scituate Reservoir drainage area, Rhode Island, water years 

2003–12.

[All significant correlations are positive; p values for correlations considered significant are shown in bold. E. coli, Escherichia coli; <, less than value shown]

Property or constituent pH Color Turbidity Alkalinity

Total 

coliform 

bacteria

E. coli 

bacteria Chloride Nitrite Nitrate

Color 0.958

Turbidity 0.001 0.001

Alkalinity <0.001 0.068 <0.001

Total coliform bacteria 0.072 0.002 0.025 0.002

E. coli bacteria 0.193 0.001 0.008 0.010 <0.001

Chloride 0.059 0.294 0.019 0.005 0.009 0.033

Nitrite 0.237 <0.001 <0.001 <0.001 <0.001 0.001 0.021

Nitrate 0.219 0.656 0.963 0.085 0.031 0.054 0.012 1.000

Orthophosphate 0.390 0.027 0.718 0.044 0.002 0.004 0.504 0.236 0.352

Color

Color in stream water, typically pale yellow to dark 

brown, is usually caused by dissolved organic material, such 

as humic and fulvic acids, derived from natural sources 

(Hem, 1985). Inorganic materials, such as metals, also can be 

common sources of color. Dark-colored water, indicating a 

substantial concentration of dissolved organic carbon (DOC), 

may be of concern in source waters because some forms of 

DOC can be transformed into chlorinated organic compounds, 

such as trihalomethanes, during water chlorination (Miller, 

1993; Breault and others, 2000). Color intensity is measured 

in standard units (platinum-cobalt units), based on dilutions of 

a chemical solution that closely matches the color of natural stream water. Median values of color ranged from 12 to 

135 platinum-cobalt units (PCU), with the highest value determined at the monitoring station in the Moswansicut Reservoir 

Subbasin (Providence Water station 20; table 5). The majority 

of discrete measurements of color at most monitoring stations 

were less than 100 PCU (fig. 7B).

Turbidity

Turbidity in stream water is caused by the presence of 

suspended particles, such as silt, clay, organic matter, and 

microorganisms, and dissolved colored material. Turbidity 

is measured in standard units that quantify the ability of 

the water to transmit light and is considered an indicator of 

environmental health and early stages of ecosystem change 

caused by excessive productivity (U.S. Environmental 

Protection Agency, 2000). High levels of turbidity also can 

interfere with water-disinfection processes (World Health 

Organization, 2004). The turbidity of tributaries in the 

drainage area generally was low; no median values exceeded 

1.5 nephelometric turbidity unit (NTU) (fig. 7C; table 5) 

and all were less than the proposed U.S. Environmental 

Protection Agency (EPA) reference value of 1.68 NTU for 

what the agency describes as “minimally impacted conditions” 

for streams in the Northeastern Coastal Zone ecoregion 

(U.S. Environmental Protection Agency, 2000). The median 

turbidity was 0.57 NTU (table 5) among all stations in the 

drainage area. About 25 percent or more of the discrete 

turbidity measurements at Providence Water stations 1, 6, 

and 31 in the Direct Runoff Subbasin and station 22 in the 

Moswansicut Reservoir Subbasin were greater than 1.5 NTU 

(fig. 7C).

Alkalinity

Alkalinity is a measure of the ability of water to 

neutralize acid primarily as a result of the concentrations 

of dissolved carbon dioxide species in most natural waters 

(Hem, 1985). Similar in effect to pH, alkalinity can affect 

the corrosivity of water and is an important factor in water 

treatment. Alkalinity of stream water can result from 

weathering of carbonate rocks, natural organic activity, 

and human activities, such as waste disposal or fertilizer 

applications that add organic or inorganic carbon to the 

environment. Median alkalinity in samples from tributaries 

was low, ranging from 2.3 to 14 milligrams per liter as 

calcium carbonate (mg/L as CaCO3). Among all stations in 

the drainage area, median alkalinity was 5.1 mg/L as CaCO3

(table 5). Alkalinity was consistently low at monitoring 

stations in the western part of the drainage area—in the 

Barden Reservoir, Ponaganset Reservoir, and Westconnaug 

Reservoir Subbasin—and was highest at monitoring stations 

in the Direct Runoff, Moswansicut Reservoir, and Regulating 

Reservoir Subbasin (fig. 7D).
Water-Quality Conditions and Constituent Loads, WYs 2003–12 25

Table 7. Significance levels (p values) for rank correlations between subbasin characteristics and median values of selected physical properties and constituent 

concentrations at Providence Water Supply Board water-quality-monitoring stations in the Scituate Reservoir drainage area, Rhode Island, 2002–12.

[p values for trends considered significant are shown in bold. Median values of physical properties and constituent concentrations for timber operation calculated from data collected during water years 1992–2012. E. coli, Escherichia coli; (+), variables are positively correlated; (-), variables are negatively correlated]

Subbasin 

characteristic pH Color Turbidity Total coliform bacteria E. coli bacteria Alkalinity Chloride Nitrite Nitrate Orthophos- phate

Agricultural 0.105 0.118 0.138 0.187 0.552 0.029 (+) 0.227 0.309 0.734 0.303

Commercial and 

industrial

0.016 (+) 0.138 <0.001 (+) 0.141 0.327 <0.001 (+) 0.001 (+) 0.045 (+) 0.121 0.486

Forest <0.001 (-) 0.173 <0.001 (-) 0.012 (-) 0.091 <0.001 (-) 0.001 (-) 0.007 (-) 0.014 (-) 0.131

Other urban 0.841 0.432 0.094 0.877 1.000 0.595 0.099 0.465 0.308 0.978

Residential 0.032 (+) 0.564 0.006 (+) 0.023 (+) 0.008 (+) 0.002 (+) 0.001 (+) 0.078 0.001 (+) 0.054

Water 0.058 0.246 0.056 0.125 0.045 (-) 0.350 0.673 0.809 0.107 <0.001 (-)

Wetlands 0.269 <0.001 (+) 0.069 0.074 0.328 0.043 (+) 0.618 0.027 (+) 0.694 0.061

Imperviousness 0.003 (+) 0.260 <0.001 (+) 0.021 (+) 0.026 (+) <0.001 (+) <0.001 (+) 0.039 (+) 0.004 (+) 0.095

Timber operations

Sum of all areas 0.971 0.520 0.708 0.486 0.986 0.803 0.301 0.468 0.756 0.503

80–100 percent 

canopy closure

0.830 0.224 0.605 0.377 0.601 0.775 0.475 0.382 0.412 0.550

50–79 percent 

canopy closure

0.639 0.885 0.321 0.985 0.500 0.691 0.264 0.895 0.740 0.194

20–49 percent 

canopy closure

0.601 0.794 0.538 0.467 0.547 0.135 0.005 (-) 0.074 0.142 0.405

Less than 

20 percent canopy closure

0.119 0.903 0.571 0.509 0.639 0.716 0.189 0.355 0.126 0.984
26 Water-Quality Trends in the Scituate Reservoir Drainage Area, Rhode Island, 1983–2012

Table 8. Significance levels (p values) for rank correlations between mean daily streamflow and selected physical properties or 

constituent concentrations at Providence Water Supply Board monitoring stations in the Scituate Reservoir drainage area, Rhode 

Island, water years 1994–2012.

[Station locations shown on figure 2. p values for correlations considered significant are shown in bold. PWSB, Providence Water Supply Board; USGS, U.S. 

Geological Survey; E. coli, Escherichia coli; (-), negative correlation; (+), positive correlation; <, less than value shown]

PWSB 

station 

number

USGS 

station 

number

Turbidity Total coliform 

bacteria

E. coli 

bacteria Chloride Nitrate Nitrite Orthophosphate

Barden Reservoir Subbasin

24 01115190 <0.001 (-) 0.001 (-) <0.001 (-) 0.749 0.711 <0.001 (-) 0.740

25 01115200 0.002 (-) 0.003 (-) 0.001 (-) 0.181 0.262 0.276 0.546

26 01115185 0.802 0.109 0.026 (-) 0.026 (-) 0.872 0.121 0.864

28 01115265 <0.001 (-) 0.004 (-) 0.002 (-) <0.001 (-) 0.447 0.004 (-) 0.686

35 01115187 <0.001 (-) <0.001 (-) <0.001 (-) 0.031 (-) 0.806 0.050 0.259

Direct Runoff Subbasin

1 01115180 <0.002 (-) 0.001 (-) 0.045 (-) 0.012 (-) 0.096 0.067 0.067

3 01115280 0.013 (+) 0.268 0.325 0.255 0.300 0.256 0.253

4 01115400 0.015 (-) <0.001 (-) <0.001 (-) 0.131 0.064 0.374 0.295

5 01115184 0.003 (-) 0.008 (+) 0.010 (+) 0.015 (-) 0.085 0.989 0.444

6 01115183 <0.001 (-) <0.001 (-) <0.001 (-) <0.001 (-) 0.501 <0.001 (-) 0.224

7 01115297 <0.001 (-) 0.008 (-) 0.162 0.003 (-) 0.265 0.001 (-) 0.923

8 01115276 0.217 <0.001 (-) 0.095 0.984 0.820 0.647 0.211

9 01115275 0.083 <0.001 (-) 0.010 (-) <0.001 (-) 0.165 0.025 (+) 0.427

32 01115178 0.001 (-) 0.015 (-) 0.024 (-) 0.619 0.011 (+) 0.870 0.563

33 01115182 <0.001 (-) <0.001 (-) 0.001 (-) 0.086 0.159 0.431 0.285

Moswansicut Reservoir Subbasin

19 01115170 0.003 (-) 0.024 (-) 0.205 0.428 0.248 0.022 (+) 0.225

21 01115165 0.076 0.017 (-) 0.129 0.136 0.001 (+) 0.668 0.658

Regulating Reservoir Subbasin

14 01115110 0.002 (-) 0.001 (-) 0.134 0.733 0.844 0.523 0.761

15 01115114 0.040 (-) 0.068 0.608 <0.001 (-) 0.148 0.010 (+) 0.531

16 01115098 0.817 0.308 0.036 (+) 0.387 <0.001 (+) 0.035 (+) 0.076

18 01115120 0.965 0.689 0.519 0.188 0.643 0.865 0.423

Westconnaug Reservoir Subbasin

10 01115274 0.980 <0.001 (-) <0.001 (-) 0.234 <0.001 (+) 0.663 0.663

11 01115273 <0.001 (-) 0.174 0.262 0.683 0.078 0.003 (-) 0.287
Water-Quality Conditions and Constituent Loads, WYs 2003–12 27

Table 9. Selected relevant Federal and Rhode Island State water-quality standards and guidelines.

[Sources of standards and guidelines include Rhode Island Department of Environmental Management (RIDEM; 2010) and U.S. 

Environmental Protection Agency (EPA; 2009). Class AA waters are waters used for drinking-water supply or tributary stream water 

to drinking-water sources in Rhode Island. NTU, nephelometric turbidity units; CFU/100 mL, colony-forming units per 100 milliliters; mg/L, milligrams per liter; <, less than; >, greater than; --, none; P, phosphorus]

Parameter or constituent RIDEM water-quality standards 

for Class AA waters

USEPA National Secondary 

Drinking Water Regulations

pH, in standard units 6.5–9.01 6.5–8.5

Color, platinum cobalt units -- 15

Turbidity, in NTU <5 above background --

Total coliform bacteria, in CFU/100 mL Geometric mean 20; 

<10 percent of samples >200

--

Chloride, in mg/L -- 250

Total phosphorus, mg/L as P 0.025 --

1

Or as occurs naturally.

Constituent Concentrations

Providence Water also monitors concentrations of three 

categories of constituents—chloride, nutrients, and bacteria. 

Similar to values of physical properties, constituent concentrations are used as an indicator of overall water quality and of 

compliance with Federal and State guidelines.

Chloride

Chloride, a nonreactive ion commonly found in stream 

water, is found in precipitation, minerals and soils in the 

environment (Hem, 1985), septic effluent, industrial wastes, 

and wastewater (Mullaney and others, 2009). The mean annual 

precipitation-weighted chloride was less than 0.5 milligrams 

per liter (mg/L) during WYs 2002–12 west of the study 

area near Abington, Connecticut (National Atmospheric 

Deposition Program, 2014). Chloride is a major constituent of 

road salt and other deicing compounds. The median chloride 

concentration for discrete samples collected by Providence 

Water for all stations in the drainage area was low, 22 mg/L 

(table 5), which is typical of natural stream water in coastal 

New England (Robinson and others, 2003). Median values 

for individual stations ranged from 3.8 to 87 mg/L. Discrete 

chloride concentrations were greatest at a few stations in 

the Direct Runoff Subbasin (fig. 7E). The greatest median 

chloride concentrations, 87 and 60 mg/L, were measured 

at Toad Pond (Providence Water station 31) and Bear Tree 

Brook (Providence Water station 9), respectively. A formerly 

uncovered salt-storage facility in the Bear Tree Brook 

Subbasin likely contributed to the chloride concentrations at 

that monitoring station (Nimiroski and Waldron, 2002). Toad 

Pond receives flows from a developed area with relatively 

large percentage of impervious area (about 19 percent) in 

comparison with the other monitoring stations in the Scituate 

Reservoir drainage area.

Nutrients

Nutrients, including chemical species3 of nitrogen and 

phosphorus, are essential elements for plant and animal 

life. Nutrient enrichment, however, can lead to excessive 

productivity and plant growth with consequent low 

concentrations of dissolved oxygen, and to degradation of 

aquatic life (U.S. Environmental Protection Agency, 2000). 

Sources of nitrogen in the tributaries include atmospheric 

deposition, leaching of naturally occurring organic material, 

discharge of groundwater that is enriched in nitrate from 

septic-system leachate, and runoff contaminated with fertilizer 

or animal waste. In tributaries in the Scituate Reservoir 

drainage area, concentrations of nutrient species nitrite, nitrate, 

and orthophosphate were low (table 5) and, in many cases, 

were not detected at the reporting limits; (figs. 7F, G, and I). 

Other forms of nitrogen, such as dissolved and particulate 

ammonia, were not measured as part of this study.

Median concentrations of nitrate were less than 0.1 mg/L 

as N at most monitoring stations. High median concentrations 

of nitrate at some monitoring stations, such as Moswansicut 

Reservoir (Providence Water station 22; 0.14 mg/L as N) 

in the Moswansicut Reservoir Subbasin and Toad Pond 

(Providence Water station 31; 0.13 mg/L as N) in the Direct 

Runoff Subbasin, may be caused by nitrogen-enriched 

runoff or groundwater. Median concentrations of nitrite at all 

monitoring stations were less than 0.006 mg/L as N (table 5). 

Median concentrations of nitrite at all monitoring stations in 

the Moswansicut Reservoir Subbasin were greater than the 

median Scituate Reservoir drainage area value of 0.001 mg/L 

as N. The median values for the sum of nitrite and nitrate 

concentrations at all stations were less than the proposed 

EPA reference concentration for the sum of nitrite and nitrate 

concentrations (0.31 mg/L as N) for minimally impacted 

3

A chemical species of an element may be an atom, molecule, or ion containing that element.
28 Water-Quality Trends in the Scituate Reservoir Drainage Area, Rhode Island, 1983–2012

Table 10. Percentages of samples, collected at Providence Water Supply Board monitoring stations in the Scituate Reservoir drainage 

area, Rhode Island, water years 2003–12, with values or concentrations not meeting water-quality standards and guidelines.

[PWSB, Providence Water Supply Board; USGS, U.S. Geological Survey; PCU, platinum-cobalt units; NTU, nephelometric turbidity units; mg/L, milligrams 

per liter; <, less than; >, greater than; --, none; P, phosphorus]

PWSB 

station 

number

USGS 

station 

number

Properties Constituents

pH Color Turbidity Chloride Orthophosphate

<6.5 >15 PCU >5 NTU >250 mg/L >0.025 mg/L as P

Barden Reservoir Subbasin

24 01115190 93 100 0 0 32

25 01115200 97 100 0 0 27

26 01115185 92 97 0 0 56

27 011151845 100 86 0 0 52

28 01115265 97 100 0 0 53

29 01115271 89 100 0 0 37

35 01115187 92 100 0 0 40

Average 94 98 0 0 42

Direct Runoff Subbasin

1 01115180 17 100 1 0 55

2 01115181 95 65 0 0 26

3 01115280 78 97 0 0 53

4 01115400 89 97 0 0 37

5 01115184 91 97 0 0 53

6 01115183 82 99 5 0 65

7 01115297 91 100 0 0 49

8 01115276 91 72 0 0 33

9 01115275 76 97 0 0 45

30 01115350 97 100 0 0 29

31 01115177 58 100 0 0 85

32 01115178 74 100 0 0 53

33 01115182 100 93 0 0 44

36 -- 63 97 0 0 47

37 -- 100 90 0 0 26

Average 80 94 0 0 47

Moswansicut Reservoir Subbasin

19 01115170 18 96 0 0 32

20 01115160 97 99 0 0 56

21 01115165 29 100 4 0 29

22 01115167 38 85 8 1 54

34 01115164 71 100 0 0 50

Average 50 96 2 0 44

Ponaganset Reservoir Subbasin

23 011151843 99 25 0 0 24
Water-Quality Conditions and Constituent Loads, WYs 2003–12 29

Table 10. Percentages of samples, collected at Providence Water Supply Board monitoring stations in the Scituate Reservoir drainage 

area, Rhode Island, water years 2003–12, with values or concentrations not meeting water-quality standards and guidelines.—Continued

[PWSB, Providence Water Supply Board; USGS, U.S. Geological Survey; PCU, platinum-cobalt units; NTU, nephelometric turbidity units; mg/L, milligrams per 

liter; <, less than; >, greater than; --, none; P, phosphorus]

PWSB 

station 

number

USGS 

station 

number

Properties Constituents

pH Color Turbidity Chloride Orthophosphate

<6.5 >15 PCU >5 NTU >250 mg/L >0.025 mg/L as P

Regulating Reservoir Subbasin

13 01115176 36 100 0 0 40

14 01115110 66 99 0 0 57

15 01115115 32 99 2 0 56

16 01115098 43 100 0 0 35

17 01115119 100 100 0 0 48

18 01115120 84 100 0 0 58

Average 60 100 0 0 49

Westconnaug Reservoir Subbasin

10 01115274 98 97 2 0 50

11 01115273 100 100 0 0 46

12 011152745 100 100 0 0 41

Average 99 99 1 0 46

Scituate Reservoir Drainage Area

Minimum 17 25 0 0 24

Median 91 99 0 0 47

Maximum 100 100 8 1 85

conditions in streams in the Northeastern Coastal Zone 

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

Orthophosphate is generally considered the most biologically available form of phosphorus (Reddy and others, 1999), 

which is considered the limiting nutrient in inland aquatic 

systems; therefore, orthophosphate often is used as an indicator of potential water-quality problems, such as algae blooms 

or excessive aquatic plant growth (Schlesinger, 1991). The 

median of the median orthophosphate concentrations for the 

37 monitoring stations was 0.02 mg/L as P and median values 

ranged from 0.01 to 0.04 mg/L as P. Median orthophosphate 

concentrations, as well as much of the discrete sample data 

(fig. 7I) at many of the monitoring stations, were greater than 

the EPA-proposed total phosphorus reference concentration of 

0.024 mg/L for minimally impacted conditions for streams in 

the Northeastern Coastal Zone ecoregion (U.S. Environmental 

Protection Agency, 2000).

Bacteria

Streams and reservoirs commonly contain a variety 

of microorganisms, some of which can cause disease in 

humans. Some microorganisms are introduced into water 

supplies by sewage disposal, either directly to surface waters 

or through septic-system failure. Septic or other individual 

sewage-disposal systems are predominantly used throughout 

the Scituate Reservoir drainage area (Richard Blodgett, 

Providence Water Supply Board, written commun., 2014). 

Other sources of bacteria to surface waters include waterfowl 

and runoff from impervious areas that are affected by animal 

waste (Weiskel and others, 1996). Total coliform bacteria and 

E. coli are indicators used to identify bacterial growth and the 

presence of sewage contamination. Although these bacteria 

groups generally do not include disease-causing organisms, 

they may indicate the presence of human pathogens, including 

bacteria, protozoans (Cryptosporidium and Giardia), and 

enteric viruses. Total coliform bacteria include types that 

can grow in the environment (for example, in soils and on 

vegetation) and in human and animal intestines, whereas 

E. coli are specifically intestinal bacteria and thus are a more 

definitive indicator of fecal contamination (Francy and others, 

2000; World Health Organization, 2004).

Median concentrations of total coliform bacteria and 

E. coli were greater than the detection limit (3 CFU/100 mL) 

at nearly all stations (table 5; figs. 7J and K). Median concentrations of E. coli were typically less than the associated 

median concentrations of total coliform bacteria. The median 

concentrations of total coliform and E. coli bacteria for all 
30 Water-Quality Trends in the Scituate Reservoir Drainage Area, Rhode Island, 1983–2012

stations in the drainage area were 43 and 15 CFU/100 mL, 

respectively. A substantial number of discrete concentrations 

of total coliform bacteria and E. coli were greater at specific 

stations in the Direct Runoff (Providence Water stations 2, 6, 

and 31) and Moswansicut Reservoir Subbasins (Providence 

Water station 22; figs. 7J and K) than at other stations in the 

Scituate Reservoir drainage area. Drainage conveyance structures often direct stormwater runoff, one source of bacteria, 

to streams and ponds; thus, the relatively high percentage of 

impervious surfaces (table 2) in the drainage areas of stations 31 (Toad Pond) and 22 (Moswansicut Stream South) 

may contribute to the bacteria concentrations in water-quality 

samples. Some low concentrations of indicator bacteria were 

determined from samples collected at monitoring stations 

immediately downstream from subbasin reservoirs, such as 

Providence Water station 23 (at the outlet of the Ponaganset 

Reservoir), station 13 (at the outlet of Regulating Reservoir), 

and station 29 (at the outlet of the Barden Reservoir). Low 

concentrations at such stations may be the result of settling 

and dieoff of indicator bacteria during the passage of water 

through the impoundments.

Correlations Between Water-Quality Properties 

and Constituent Concentrations

Relations between water-quality properties and 

constituent concentrations may be useful to identify potential 

sources of constituents or natural processes occurring in 

the tributary drainage areas. Correlations between discrete 

measurements of turbidity and pH, color, and alkalinity were 

significant (table 6); as a result, tributary subbasins with 

high values of pH, color, and alkalinity tended to have high 

turbidity values. Correlations also were significant between 

discrete measurements of turbidity and concentrations of 

bacteria, chloride, and nitrite constituents that are often 

associated with stormwater runoff from impervious area. 

No significant correlations were found between nitrite, 

nitrate, and orthophosphate; however, positive relations 

between orthophosphate and color, alkalinity, and bacteria 

were significant (table 6). Concentrations of chloride were 

significantly correlated with concentrations of nitrite, nitrate, 

and most physical properties in the drainage area.

Factors Affecting Water-Quality Properties and 

Constituent Concentrations

Values of water-quality properties and constituents are 

affected by many factors, including land use, impervious area, 

and streamflow. Comparison of median values of water-quality 

properties and constituent concentrations with percentages 

of land-use area, impervious area, and timber activity can 

assist water-resources managers with drainage-area-protection 

strategies, such as purchasing land and restricting certain land 

uses. Many water-quality properties and concentrations vary 

with streamflow, either directly through the association of high 

flows that entrain additional constituents with surface runoff or 

through the dilution or addition of contaminants derived from 

base flow dominated by groundwater.

Subbasin Characteristics

Land use, impervious surface areas, and timber activity 

in the subbasins are factors that can affect constituent concentrations in the tributaries. Percentages of residential land 

use and of impervious area in tributary subbasins correlated 

significantly with median values or concentrations of several 

water-quality properties that can indicate human activities, 

including pH, turbidity, alkalinity, and concentrations of total 

coliform bacteria, E. coli, chloride, nitrite, and nitrate (fig. 8; 

table 7). Correlations with commercial and industrial land 

use also were significant for many of the same properties and 

constituents, except for correlations with concentrations of 

total coliform bacteria, E. coli, and nitrate. Median values of 

pH, turbidity, alkalinity, and concentrations of total coliform 

bacteria, chloride, nitrite, and nitrate correlated negatively 

with the percentage of forest in subbasins of the monitoring 

stations. The correlations between chloride concentrations and 

various land uses related to impervious area are likely a direct 

result of the application of deicing compounds during winter 

maintenance activities throughout the drainage area. Road salt 

was previously determined to be the largest source of chloride 

in the drainage area (Nimiroski and Waldron, 2002). Median 

values of color correlated positively with the percentage of 

wetlands in the tributary subbasins; this correlation suggests 

that naturally occurring organic material and possibly dissolved iron and manganese were likely sources of color in 

tributary stream water (Nimroski and others, 2008).

Median values of alkalinity correlated positively with 

the percentage of agriculture in the tributary subbasins; 

positive correlations with pH were only significant at a lower 

probability level (p value of about 10-percent). This relation 

may be associated with the application of lime to the poorly 

buffered croplands in the subbasins (Shearin and others, 

1943) or as a result of nitrification following applications of 

fertilizer that promotes soil acidification and consequently 

accelerates chemical weathering of the soil materials (Kaushal 

and others, 2013). The application or remnants of animal 

manure on agricultural land may also increase the alkalinity 

in runoff entering the tributaries (Hansen and others, 1976). 

Median values of alkalinity also correlated positively with 

the percentage of imperviousness and other land use related 

to development. The relation between the level of alkalinity 

in the tributaries and these subbasin characteristics may 

be related to the weathering of anthropogenic structures 

containing concrete (sidewalks, buildings, drainage 

convenience structures, and so forth) through contact with 

relatively low-pH precipitation common to this region of the 

country (Prestegaard and others, 2005).

Providence Water manages the forests around the 

reservoirs through a selective timber management program 
Water-Quality Conditions and Constituent Loads, WYs 2003–12 31

Orthophosphate, in milligrams

per liter as phosphorus

Water

Escherichia coli bacteria, in colony 

 forming units per 100 milliliters

Residential

Nitrate, in milligrams

per liter as nitrogen

Residential

0.0

0.4

0.8

1.2

1.6

2.0

Turbidity,

in nephelometric turbidity units

Commercial/Industrial

0.0

0.4

0.8

1.2

1.6

2.0

Impervious area (percent)

pH, in standard units

0

30

60

90

120

150

Color, in platinum-colbalt units 

Wetland

0

4

8

12

16

20

Alkalinity, in milligrams

per liter calcium carbonate

Agricultural

Turbidity,

in nephelometric turbidity units

Turbidity,

in nephelometric turbidity units

0 4 8 12 16 20 0 4 8 12 16 20

0 4 8 12 16 20 24 0 2 4 6 8 10 12

0 4 8 12 16 20

D E F

G H I

J K L

0 10 20 30 40 0.0

0.4

0.8

1.2

1.6

2.0

Residential

0 10 20 30 40 50 60 70 80 90 100 5.0

5.2

5.4

5.6

5.8

6.0

6.2

6.4

6.6

6.8

7.0

Forest

0.00

0.04

0

20

40

60

80

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.02

0.01

0.03

0 10 20 30 40 0 10 20 30 40

0 4 8 12 16 20 0

20

40

60

80

Chloride, in milligrams per liter

Commercial/Industrial

0

20

40

60

80

Crown closure (20–49 percent)

0

20

40

60

80

Impervious area (percent)

Chloride, in milligrams per literChloride, in milligrams per liter

0 4 8 12 16 20 0 4 8 12 16 20

A B C

Figure 8. Relations between A, median values of turbidity and percent commercial and industrial land use; B, turbidity and percent 

residential land use; C, turbidity and percent impervious area; D, pH and forested land use; E, color and percent wetland area; 

F, alkalinity and percent agricultural land use; G, chloride and percent commercial and industrial land use; H, chloride and 20- to 

49-percent canopy closure; I, chloride and percent impervious area; J, median concentrations of orthophosphate and percent water 

land cover; K, median numbers of Escherichia coli colony-forming units per 100 milliliters and percent residential land use; and L, nitrate 

and percent residential land use at Providence Water Supply Board water-quality monitoring stations in the Scituate Reservoir drainage 

area, Rhode Island, water years 2003–12.
32 Water-Quality Trends in the Scituate Reservoir Drainage Area, Rhode Island, 1983–2012

0.01 0.1 1 10 100

10

20

30

40

50

60

70

Daily mean chloride concentration, in milligrams per liter

Daily mean discharge, in cubic feet per second

Chloride concentration

Best-fit line

EXPLANATION

R2

=0.88

(Providence Water Supply Board, 2011). Some amount of 

timbering was performed in nearly all tributary drainage areas 

during WYs 1992–2012; however, most of the operations 

were focused in the Direct Runoff Subbasin. With only 

one exception (concentrations of chloride), no statistically 

significant correlations were determined among percentages 

of various degrees of tree-canopy reduction in drainage area 

of monitoring stations and median values or concentrations 

of water-quality properties (table 7). While the reduction of 

a forest canopy can affect water quality, increases in chloride 

concentrations are not normally documented as a result 

(Martin and others, 1986).

Streamflow

Values of water-quality properties and constituent 

concentrations may vary with flow because of seasonal 

changes in precipitation or stormwater runoff. For example, 

concentrations of some constituents vary during warm lowflow periods of the year because of biological activity. Several 

water-quality properties and constituent concentrations are 

negatively correlated with streamflow (table 8). In general, 

turbidity, concentrations of total coliform bacteria, and E. coli

often are negatively correlated with flow at most stations. 

Concentrations of chloride also correlated negatively with 

streamflow. The strong relation between streamflow and 

chloride, estimated from continuous measurements of specific 

conductance, is illustrated for Quonapaug Brook (Providence 

Water station 6; fig. 9). At some stations in the Barden 

Figure 9. Relations between daily mean streamflow and daily 

mean concentrations of chloride estimated from continuous 

measurements of specific conductance for Quonapaug Brook in 

the Scituate Reservoir drainage area, Rhode Island, water years 

2009–12.

Reservoir, Direct Runoff, and Regulating Reservoir Subbasins, 

concentrations of chloride tends to increase during the summer 

when streamflow is low and decrease during higher base-flow 

conditions and stormflows. The seasonal relation between 

daily mean streamflow and daily mean specific conductance, 

a surrogate for chloride, is illustrated for Dolly Cole Brook, 

Quonapaug Brook, and Rush Brook (Providence Water 

stations 24, 6, and 15) in figure 10. This observation indicates 

that chloride in runoff containing deicing compounds, which 

in many cases infiltrates along the roadways and parking lots, 

is entering the groundwater and discharging to the tributaries 

later in the year

Median concentrations of nitrate correlated positively 

with flow at only a few stations found in the Direct Runoff, 

Moswansicut Reservoir, Regulating Reservoir, and 

Westconnaug Reservoir Subbasins (table 8). Nitrogenenriched runoff in streamflow during storms could be a factor 

in these relations. Correlations between median concentrations 

of nitrite and flow were significant at about one-third of the 

stations; however, the direction of the trends was inconsistent. 

Correlations between median concentrations of orthophosphate and flow were only significant at p-values of about 0.07 

at Brandy Brook and Peeptoad Brook (Providence Water stations 1 and 16, respectively; table 8).

Comparison of Water-Quality Properties and 

Constituent Concentrations with Water-Quality 

Standards and Guidelines

Values of water-quality properties and concentrations of 

constituents can be compared to several types of standards 

and guidelines. RIDEM sets water-quality standards for 

surface water based on the intended uses of the water. Some 

inland surface-water bodies that are sources of drinking 

water and their tributaries are designated class AA (Rhode 

Island Department of Environmental Management, 2010). 

Class AA waters have standards for pH, turbidity, and 

maximum allowable concentrations of total phosphorus 

(table 9). The EPA has established secondary drinkingwater regulations (SDWRs) for measurements of pH and 

color and concentrations of chloride (table 9). SDWRs are 

nonmandatory water-quality guidelines established by the 

EPA and are designed to assist public water suppliers in 

managing aesthetic aspects of water, such as taste, odor, color, 

foaming, corrosivity, staining, scaling, and sedimentation; 

the regulations are not related to risk to human health 

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

these guidelines typically are applied to finished water, the 

presence of these constituents at concentrations exceeding 

SDWRs can result in damage to equipment and reduce 

the effectiveness of treatment for other constituents. Some 

constituents, such as chloride and nitrates, are not effectively 

removed by many water treatment systems (Smith, 2007).
Water-Quality Conditions and Constituent Loads, WYs 2003–12 33

0.1

1

10

100

200

Daily mean streamflow, in cubic feet per second

 Daily mean streamflow

 Daily mean specific conductance

50

60

70

80

90

100

110

120

130

140

150

Daily mean specific conductance,

 in microsiemens per centimeter

Daily mean streamflow, in cubic feet per second

0.01

0.1

1

10

100

50

75

100

125

150

175

200

225

250

275

300

Daily mean specific conductance,

 in microsiemens per centimeter

0.01

0.1

1

10

100

200

Daily mean streamflow, in cubic feet per second

50

75

100

125

150

175

200

225

250

275

300

Daily mean specific conductance,

 in microsiemens per centimeter

EXPLANATION

Oct. Dec. Feb. Apr. June Aug. Oct.

A

B

C

Figure 10. Relations between daily mean streamflow and daily mean values of specific conductance at A, Dolly 

Cole Brook in the Barden Reservoir Subbasin, B, Quonapaug Brook in the Direct Runoff Subbasin, and C, Rush 

Brook in the Regulating Reservoir Subbasin, Scituate Reservoir drainage area, Rhode Island, water year 2012.
34 Water-Quality Trends in the Scituate Reservoir Drainage Area, Rhode Island, 1983–2012

Water-Quality Properties

RIDEM water-quality standards for class AA waters 

establish an allowable range for pH of 6.5 to 9.0 or, as occurs 

naturally; the EPA’s recommended pH range of drinking water 

is 6.5 to 8.5 (table 9). The pH of water samples collected 

during WYs 2003–12 by Providence Water did not exceed 

8.5; however, the pH was less than 6.5 in 17 to 100 percent 

of samples at stations (table 10) in the Scituate Reservoir 

drainage area. Low pH values are common in New England 

streams and result primarily from the low pH of precipitation 

(National Atmospheric Deposition Program, 2014); the low 

buffering capacity of the soil material in the Scituate Reservoir 

drainage area contributes to the low pH values observed there.

Water-quality guidelines for color consist only of the EPA 

SDWR for public drinking-water supplies (table 9). Color 

in source water, however, is an important indicator of DOC, 

which can contribute to the formation of trihalomethanes 

when chlorine is added to finished drinking water; thus, DOC 

is a factor in water treatment. Most water samples at most 

stations exceeded the SDWR for color of 15 PCU during WYs 

2003–2012 (fig. 7B).

Turbidity values in tributaries in the drainage area rarely 

exceeded the background level by 5 NTU or more (table 10), 

the maximum allowable for RIDEM class AA waters (table 9). 

Similarly, the turbidity value of 1.68 NTU, the EPA proposed 

reference concentration for minimally affected conditions in 

the region (U.S. Environmental Protection Agency, 2000), was 

rarely exceeded at most of the monitoring stations (fig. 7C). 

These results suggest that the water quality of tributaries in 

the Scituate Reservoir drainage area is minimally affected 

by suspended particles relative to stream water in the 

Northeastern Coastal Zone ecoregion of the United States, 

as noted previously.

Constituent Concentrations

RIDEM water-quality standards for class AA waters 

establish maximum concentrations for total phosphorus. 

Orthophosphate (a component and variable fraction of the total 

phosphorus) in stream water was the only phosphorus species 

measured in this study; concentrations of orthophosphate were 

greater than the RIDEM standard for total phosphorus in 24 to 

85 percent of samples collected from all stations (table 10). At 

16 of the 37 stations, the median orthophosphate concentration (table 5) was two or more times greater than the standard 

value of 0.025 mg/L. Phosphorus concentrations that exceed 

the standard could be of concern because phosphorus can 

contribute to excessive plant and algae growth.

The EPA SDWR for chloride (250 mg/L) was only 

exceeded once—in a single sample collected at Moswansicut 

Reservoir (Providence Water station 22)—during WYs 2003–

12; however, instantaneous chloride concentrations estimated 

from continuous records of specific conductance (representing all base flow and runoff conditions) from Quonapaug 

Brook for water years 2009–12 (Providence Water station 6) 

exceeded the SDWR in WYs 2011 and 2012 during winter 

runoff events (fig. 11). Although elevated concentrations of 

chloride commonly occur during winter runoff events in many 

of the tributaries in the drainage area, estimated concentrations 

of chloride did not exceed the SDWR at the other 13 stations 

(table 1) equipped to continuously monitor specific conductance during WYs 2009–12.

Loads and Yields of Selected Constituents, 

WYs 2003–12

The potential adverse effects of a constituent on reservoir 

water could depend on the total amount of the constituent 

added as well as on the concentrations of the constituent in 

tributary waters. This mass flux of a constituent, the constituent load, can be calculated on an annual, daily, or instantaneous basis from streamflow and concentration data. In this 

study, daily loads were calculated for selected constituents for 

all sampling dates during WYs 2003–12. Daily yields, or loads 

per unit of subbasin drainage area, also were calculated to 

allow for direct comparisons of loads among sampling stations 

with drainage areas of different sizes. The median daily loads 

and yields were calculated for the 23 monitoring stations for 

which sufficient data were available (table 11).

Discrete loads and yields represent point measurements of continuous mass flux that may vary seasonally (for 

example, in association with hydrologic conditions or roadsalt application) and daily (for example, in association with 

10-09 02-10 06-10 10-10 03-11 07-11 11-11 03-12 08-12

10

100

1,000

Daily maximum chloride concentration, in milligrams per liter

Month-Year

Chloride concentration

Chloride limit stated in

 the U.S. Environmental 

 Protection Agency

 secondary drinking-

 water regulation

2,000

EXPLANATION

Figure 11. Daily maximum concentrations of chloride estimated 

from continuous records of specific conductance for Quonapaug 

Brook in the Scituate Reservoir drainage area, Rhode Island, 

water years 2009–12.
Water-Quality Conditions and Constituent Loads, WYs 2003–12 35

Table 11. Median daily loads and yields of chloride, nitrite, nitrate, orthophosphate, and bacteria, in the Scituate Reservoir drainage area, Rhode Island, water years 2003–12.[Water-quality data are from samples collected and analyzed by the Providence Water Supply Board (PWSB). USGS, U.S. Geological Survey; CFU×106/d, millions of colony forming units per day; E. coli, 

Escherichia coli; N, nitrogen; P, phosphorus; kg/d, kilograms per day; kg/d/mi2, kilograms per day per square mile; g/d, grams per day; g/d/mi2, grams per day per square mile]

PWSB 

station 

number

USGS 

station 

number

Chloride Nitrite (as N) Nitrate (as N) Orthophosphate (as P) Total coliform bacteria E. coli

(kg/d) (kg/d/mi2) (g/d) (g/d/mi2) (g/d) (g/d/mi2) (g/d) (g/d/mi2) (CFU×106/d) (CFU×106/mi2) (CFU×106/d) (CFU×106/mi2)

Barden Reservoir Subbasin

24 01115190 450 92 27 5.5 170 34 300 60 6,900 1,400 1,600 330

25 01115200 90 38 10 4.3 100 44 130 54 1,400 600 460 200

26 01115185 160 37 10 2.2 160 37 170 39 1,200 280 530 120

28 01115265 530 61 50 5.7 190 21 510 58 14,000 1,600 5,700 650

35 01115187 1,100 79 83 5.9 560 40 1,200 86 17,000 1,200 5,300 380

Direct Runoff Subbasin

1 01115180 70 45 15 9.2 70 44 130 85 2,800 1,800 580 370

3 01115280 130 73 5.5 3.1 80 45 120 65 1,700 950 500 280

4 01115400 12 14 2.9 3.4 25 29 34 40 480 560 95 110

5 01115184 80 66 7.3 6.0 120 94 85 70 1,500 1,200 210 170

6 01115183 300 150 17 8.7 110 56 240 120 11,000 5,600 4,000 2,000

7 01115297 190 43 36 8.2 180 41 410 94 6,600 1,500 2,500 570

8 01115276 300 58 24 4.6 240 46 360 69 750 140 440 85

9 01115275 180 290 3.7 6.0 300 480 57 92 1,400 2,200 230 370

32 01115178 19 41 2.2 4.9 23 50 32 71 610 1,400 300 670

33 01115182 13 46 1.1 3.9 21 75 20 70 230 800 89 320

Moswansicut Reservoir Subbasin

19 01115170 420 130 27 8.2 200 62 210 65 2,600 780 570 170

21 01115165 57 190 3.6 12 44 150 25 87 520 1,800 100 340

Regulating Reservoir Subbasin

14 01115110 200 32 22 3.5 190 30 490 79 21,000 3,400 5,000 800

15 01115114 460 98 18 3.8 130 28 330 70 15,000 3,200 3,000 640

16 01115098 590 120 27 5.4 180 36 280 57 12,000 2,400 2,300 460

18 01115120 34 120 1.3 4.6 11 39 22 79 360 1,300 61 220

Westconnaug Reservoir Subbasin

10 01115274 130 88 5.1 3.4 57 38 130 85 2,100 1,400 520 350

11 01115273 29 40 9.6 13 37 51 95 130 6,200 8,500 1,100 1,500

Scituate Reservoir Drainage Area

Minimum 12 14 1.1 2.2 11 21 20 39 230 140 61 85

Median 160 66 10 5.4 120 44 130 70 2,100 1,400 530 350

Maximum 1,100 290 83 13 560 480 1,200 130 21,000 8,500 5,700 2,000
36 Water-Quality Trends in the Scituate Reservoir Drainage Area, Rhode Island, 1983–2012

biologic activity and, even more frequently in association 

with precipitation and runoff). Sampling dates also differed 

among stations. Thus, differences in median values of loads 

and yields among monitoring stations incorporate variability 

resulting from the above factors, as well as from differences in 

constituent sources and transport characteristics.

Chloride

Chloride loads and yields differed among monitoring 

stations in the drainage area (table 11). Median daily loads 

of chloride estimated on the basis of Providence Water 

sample data ranged from 12 to 1,100 kg/d. The largest 

median chloride loads were determined for Providence 

Water stations 16, 18, 19, 6, 21, and 9 in the Direct Runoff, 

Moswansicut Reservoir, and Regulating Reservoir Subbasins.

The relatively large median load (1,100 kg/d) at 

Providence Water station 35 (Ponaganset River) is a result of 

this station’s large drainage area (14 mi2; table 11); the yield 

was 79 kilograms per day per square mile (kg/d/mi2), which is 

comparable to the median drainage area yield (66 kg/d/mi2). 

Median daily yields ranged from 14 to 290 kg/d/mi2. The largest median chloride yield was determined for Bear Tree Brook 

(Providence Water station 9) and was caused in part by the 

location of a formerly uncovered salt-storage facility within 

the subbasin drainage area. Relatively high chloride yields 

ranging from 120 to 190 kg/d/mi2 in several subbasins likely 

reflect the greater percentages of developed land uses and 

imperviousness in their drainage areas (table 2).

Nutrients

Loads of nitrite, nitrate, and orthophosphate were greatest 

in the subbasins at several stations with large drainage areas 

in the Barden Reservoir Subbasin; at stations with moderately 

large drainage areas (5–6 mi2

) in the Regulating Reservoir 

Subbasin; and at stations in the eastern part of the Scituate 

Reservoir drainage area that are more developed than other 

parts of the drainage area (fig. 2; table 11). Yields ranged 

from 2.2 to 13 grams per day per square mile as nitrogen 

(g/d/mi2

 as N) for nitrite, 21 to 480 g/d/mi2 as N for nitrate, 

and 39 to 130 grams per day per square mile as phosphorus 

(g/d/mi2 as P) for orthophosphate. The median yields for 

the Scituate Reservoir drainage area were 5.4 g/d/mi2 as 

N for nitrite, 44 g/d/mi2 as N for nitrate, and 70 kilograms 

per day per square mile as phosphorus (kg/d/mi2 as P) for 

orthophosphate, respectively. Nutrient yields varied from 

station to station; however, subbasin yields estimated from 

available data (sum of subbasin loads divided by subbasin 

area) indicate that yields for nitrite and nitrate were greatest 

in the Moswansicut Reservoir Subbasin (8.6 and 69 g/d/mi2

as N, respectively) and subbasin yields for orthophosphate 

were greatest in the Westconnaug Reservoir Subbasin 

(100 kg/d/mi2 as P). Yields of nitrite and orthophosphate were 

greatest at Providence Water station 11 in the Westconnaug 

Reservoir Subbasin (fig. 2; table 11). The greatest median 

yield of nitrate (480 kg/d/mi2 as N) was determined for 

Providence Water station 9 (Bear Tree Brook) in the Direct 

Runoff Subbasin and was about 10 times greater than the 

median yield for the Scituate Reservoir drainage area.

Bacteria

Loads and yields of total coliform bacteria and E. coli

varied among stations by more than two orders of magnitude 

(table 11). Median loads of total coliform and E. coli bacteria 

were 2,100 and 530 MCFU/d, respectively. The greatest 

median load of total coliform bacteria (21,000 million CFU/d) 

and E. coli (5,700 million CFU/d) were found at the stations 

on Hunting House Brook (Providence Water station 14) and at 

Hemlock Brook (Providence Water station 14), respectively. 

Median yields of total coliform and E. coli bacteria were 

1,400 million colony-forming units per square mile 

(MCFU/d/mi2

) and 350 MCFU/d/mi2, respectively. The 

greatest median yields of total coliform bacteria (5,600–

8,500 MCFU/d/mi2) and E. coli (800–2,000 MCFU/d/mi2) 

were found at the station on Quonapaug Brook (Providence 

Water station 6), an unnamed tributary to the Westconnaug 

Reservoir (Providence Water station 11), and at Hunting 

House Brook (Providence Water station 14). As previous 

discussed, concentrations of bacteria tended to vary with 

concentrations of chloride and nutrients and, as a result, 

stations with high chloride and nutrient yields tend to have 

high yields for bacteria. High median yields of fecal indicator 

bacteria (greater than 2,000 MCFU/d/mi2

) for these stations 

could result from runoff from impervious areas, waterfowl 

defecation, and other animal waste. Although relatively high 

for monitoring stations in the Scituate Reservoir drainage area, 

median bacteria yields for these stations were low overall 

compared to yields of bacteria for sewage-contaminated 

stream water or stream water affected by stormwater 

runoff in an urban environment which could be more than 

1 trillion colony-forming units per square mile (Breault and 

others, 2002).

Loads and Yields of Chloride and Sodium, 

WYs 2009–12

Average daily loads and yields of chloride and sodium 

were estimated based on streamflow and estimated concentration data collected at 14 stations for WYs 2009–12 (table 12). 

Similar data were not available at these 14 stations before 

WYs 2009. Concentrations of chloride and sodium were 

estimated using the correlations between concentration data 

and values of specific conductance. The average daily load for 

each station was calculated by dividing the sum of the daily 

loads for WYs 2009–12 by the number of days in that period; 

the average daily yield for each station was calculated by 

dividing the average daily load by the drainage area of each 

station. Average daily loads (herein referred to as USGS daily 
Water-Quality Conditions and Constituent Loads, WYs 2003–12 37

Table 12. Average chloride and sodium concentrations, loads, and yields estimated on the basis of streamflow and estimated 

concentration data collected at 14 Providence Water Supply Board stations in the Scituate Reservoir drainage area, Rhode Island, 

water years 2009–12.

[Mean annual concentrations were calculated by dividing the annual load for each year by the total discharge for that year. PWSB, Providence Water Supply Board; USGS, U.S. Geological Survey; mg/L, milligrams per liter; kg/d, kilograms per day; kg/mi2

/d, kilograms per square mile per day; Cl, chloride; 

Na, sodium]

PWSB 

station 

number

USGS 

station 

number

Concentration Load Yield

Cl

(mg/L)

Na

(mg/L)

Cl

(kg/d)

Na

(kg/d)

Cl

(kg/mi2

/d)

Na

(kg/mi2/d)

Barden Reservoir Subbasin

24 01115190 21.2 13.0 550 340 110 69

28 01115265 15.6 9.68 800 490 91 57

35 01115187 15.8 9.81 1,300 780 90 56

Direct Runoff Subbasin

3 01115280 26.5 16.1 250 150 140 85

5 01115184 13.5 8.22 89 54 73 45

6 01115183 30.4 18.4 330 200 170 100

7 01115297 8.64 5.50 200 130 46 29

8 01115276 19.9 12.2 380 230 74 45

9 01115275 47.4 28.0 170 100 280 160

Moswansicut Reservoir Subbasin

19 01115170 35.8 21.5 640 380 200 120

Regulating Reservoir Subbasin

14 01115110 8.6 5.47 280 180 46 29

15 01115114 29.3 17.7 680 410 140 87

16 01115098 26.8 16.6 800 490 160 100

18 01115120 36.9 22.1 52 31 190 110

Scituate Reservoir Drainage Area

Average Total Average

24 15 6,500 4,000 100 64

loads) and yields, estimated using continuous records of specific conductance and flow, were expected to be more accurate 

than average daily loads estimated on the basis of intermittent 

sample data collected by Providence Water (herein referred 

to as Providence Water daily loads) because the continuousrecord-estimated loads and yields represented a broader range 

of hydrologic conditions than the daily mean streamflow data 

associated with individual samples and contained information 

from all runoff events, including snowmelts.

The USGS daily loads of chloride and sodium ranged 

from 52 to 1,300 kg/d and 31 to 780 kg/d (table 12), respectively, where the magnitude of the loads generally correlated 

with the size of the drainage area of each subbasin. Ponaganset 

River (Providence Water station 35) had the largest USGS 

daily loads for chloride and sodium loads (1,300 and 780 kg/d, 

respectively), which represented 20 percent of the total load 

for the 14 stations. Bear Tree Brook (Providence Water 

station 9) had the highest average chloride and sodium daily 

yields at 280 and 160 kg/d/mi2, respectively. The average daily 

yields of chloride and sodium for the drainage areas above the 

14 USGS stations, which represent nearly 66 percent of the 

Scituate Reservoir drainage area, were 100 and 64 kg/d/mi2,

respectively (table 12). These yields represent low to moderate 

values in comparison to other areas in the northeast region of 

the country. For example, yields of chloride in surface water in 

the glaciated Northern United States were estimated to range 

from 16 kg/d/mi2 in forested areas to 220 kg/d/mi2 in urbanized areas (Mullaney and others, 2009). Yields of chloride 

and sodium in the drinking-water supply area for Cambridge, 

Massachusetts, ranged from 170 and 96 kg/d/mi2, respectively, 

in a low-density residential area to 1,700 to 810 kg/d/mi2, 

respectively, in an area dominated by commercial land use and 

large percentages of imperviousness (Smith, 2013). With the 

exception of the drainage area for Bear Tree Brook, estimated 
38 Water-Quality Trends in the Scituate Reservoir Drainage Area, Rhode Island, 1983–2012

average daily yields for chloride and sodium at the 14 USGS 

stations in the Scituate Reservoir drainage area tended to 

be greatest in the northeastern part of the drainage area (for 

sodium, see fig. 12) that contains greater percentages of developed land uses and impervious surfaces.

The spatial distributions of yields for chloride were 

similar to the distributions of yields for sodium; however, the 

yields for chloride were about 1.6 times higher than the yields 

for sodium. The difference in magnitude between the yields is 

primarily explained by the difference in the molecular mass of 

the two elements since the relation for sodium to chloride in 

water samples was nearly 1:1.

The USGS daily loads of chloride for WYs 2009–12 

were compared to the Providence Water daily loads of chloride 

(arithmetic average of intermittent daily loads for WYs 2009–

12; fig. 13). The Providence Water daily loads of chloride 

differed from the USGS daily loads of chloride by 0.2 to 

83 percent. At five stations (Providence Water stations 24, 

28, 35, 3, and 6), the Providence Water daily chloride loads 

differed from the USGS daily chloride loads by less than 

5 percent and were within the 95 percent confidence interval 

about the USGS daily chloride loads. These data indicate 

that the collection of periodic (monthly at these stations) 

samples by Providence Water was sufficient to characterize 

the average daily chloride load at these five stations. At 9 of 

the 14 stations, the difference between the Providence Water 

daily chloride loads and the USGS daily chloride loads was 

10 percent or greater. The difference between the Providence 

Water daily chloride loads and the USGS daily chloride loads 

was greater than 45 percent for Wilbur Hollow Brook, Hunting 

House Brook, Peeptoad Brook, and Unnamed Tributary to 

Scituate Reservoir (Providence Water stations 7, 14, 16, and 

18). For these four stations in particular, the limited number of 

daily chloride load values estimated based on periodic samples 

collected by Providence Water and daily mean streamflow 

were not sufficient to characterize the average daily chloride 

load for WYs 2009–12. The Providence Water daily chloride 

loads at these stations likely were greater than the USGS daily 

chloride loads because periodic daily load estimates often 

were not available during the summer months when loads of 

chloride generally were small. With the exception of periods 

of stormflow, the tributaries at these four stations during the 

summer often were dry or streamflow was too low to collect 

samples during scheduled visits by Providence Water. In the 

absence of these low load estimates, the Providence Water 

daily chloride loads likely were overestimated at these four 

stations for WYs 2009–12. The dissimilarities between load 

estimates were not necessarily surprising considering the 

difference in the magnitude of data used to estimate the loads 

in both methods—the collection of tens of thousands of 

discrete measurements of specific conductance values at each 

station used to estimate concentrations of chloride compared 

with the collection and analysis of monthly or quarterly 

grab samples.

Trends in Water-Quality Properties and 

Constituent Concentrations

Trends in water-quality properties and constituents were 

investigated for the 10-year study period of WYs 2003–12 

(table 13) and for the 29-year period WYs 1983–2012 

(table 14). Trends during the WYs 2003–12 shorter period 

may be considered current trends in water-quality properties 

and constituent concentrations, whereas trends during the 

WYs 1983–2012 longer period may reflect ongoing or past 

changes. Differences between the two trends could result 

from the relative ease in detecting trends in larger and longer 

duration datasets than in smaller and shorter duration datasets. 

Other sources of differences in the trends could be caused by 

less variability in the shorter dataset than the longer one, as 

well as by changes in sampling and analytical procedures.

Water-Quality Properties

Significant trends of upward pH were identified for 

samples collected at 19 stations for the shorter period and 

at 11 stations during the longer period (tables 13 and 14). 

Upward trends in the pH of samples collected from tributaries 

may be associated with reductions in acid precipitation in 

North America since the 1980s (Lynch and others, 2000). 

Downward pH trends were statistically significant in samples 

collected from Providence Water stations 8, 31, 17 and 11 

during the longer period (table 14; fig. 14A); no significant 

downward trends in pH were identified for the shorter period 

(fig. 14A). The low p values for nine stations (p value less than 

0.001; table 14) indicate that the statistically significant trends 

in pH for the longer period are strong.

Significant upward trends in color were identified for 

samples from 25 of 37 stations during the longer period. 

Significant trends in color were identified in samples from 

only five stations during the shorter period. Because color 

is likely to have a natural source, such as dissolved organic 

material or suspended or colloidal iron or manganese, upward 

trends in color are difficult to interpret. Trends in color could 

be related to changes in the geochemical conditions that favor 

the mobility of dissolved organic material or other color 

sources. Samples collected at Providence Water station 20 in 

the Moswansicut Reservoir Subbasin had the only significant 

downward trend in color during the WYs 2003–12 trendanalysis period (fig. 14B).

Significant trends in turbidity were identified for samples 

from 10 of 37 stations for WYs 2003–12 and for samples from 

9 of 37 stations for WYs 1983–2012 (fig. 14C; tables 13 and 

14). Six significant trends identified during the longer period 

were not identified as significant during the shorter period. 

Upward trends in turbidity were identified at nine stations for 

the shorter period; seven of these trends were not significant 

for WYs 1983–2012. Westconnaug Brook (Providence Water 

station 10) and Ponaganset Reservoir (Providence Water 
Trends in Water-Quality Properties and Constituent Concentrations 39

0 1.5 3 MILES

0 1.5 3 KILOMETERS

Base from U.S. Geological Survey, StreamStats digital data, 1:24000, 2013

Rhode Island stateplane projection, North American Datum of 1983

Scituate Reservoir

01115265

01115187

01115098

01115190 01115114

01115297

01115110

01115276

01115183

01115280

01115170

01115184

01115275

01115120

U.S. Geological Survey streamgage

01115110 USGS station number

Scituate Reservoir drainage area boundary

Subbasin boundary

10,000

20,000

30,000

40,000

50,000

60,000

EXPLANATION

Mean annual sodium yield,

 in kilograms per square mile Data not available

Impervious area

71°45' 71°37'30"71°37'30"

41°52'30"41°52'30"

41°45'

Impervious area data from Rhode Island

Geographic Information System, 2013

Figure 12. Spatial distribution of sodium yields for 14 subbasins in the Scituate Reservoir drainage area, Rhode Island, water 

years 2009–12.
40 Water-Quality Trends in the Scituate Reservoir Drainage Area, Rhode Island, 1983–2012

EXPLANATION

Average daily chloride load estimated from

continuous flow and estimated chloride 

concentration data collected by the 

U.S. Geological Survey

Upper 95-percent confidence limit of the 

U.S. Geological Survey average daily chloride load 

Lower 95-percent confidence limit of the 

U.S. Geological Survey average daily chloride load

Average daily chloride load estimated from 

periodic water samples collected by the

Providence Water Supply Board and

daily mean streamflow

Providence Water Supply Board station number

24 18161514199876533528

100

200

300

400

500

600

700

800

900

1,000

1,100

1,200

1,300

1,400

1,500

Average daily chloride load, in kilograms per day

0

Figure 13. Average daily chloride loads estimated from continuous measurements of flow and estimated concentration data 

and average daily chloride loads estimated from water-quality samples and daily mean streamflow at 14 Providence Water 

Supply Board stations in the Scituate Reservoir drainage area, Rhode Island, water years 2009–12.

station 23) were the only stations with a significant upward 

trend in turbidity for the shorter period within the significant 

downward trend of their longer period. Although turbidity 

values were low throughout the Scituate Reservoir drainage 

area, data collected at these nine stations indicated that 

turbidity values were increasing during the shorter period. 

Toad Pond (Providence Water station 31) in the Direct Runoff 

Subbasin was the only tributary with a significant downward 

trend during both periods.

Significant trends in alkalinity, like trends in pH, were 

primarily upwards during WYs 2003–12 and WYs 1983–2012 

(fig. 14D) and may be related in part to the relatively low 

pH of precipitation in this region of the country. Upward 

trends in alkalinity are common in the Northeastern United 

States (Kaushal and others, 2013). As previously discussed, 

low pH precipitation can increase alkalinity in tributaries 

through chemical weathering of bedrock and overlying soils, 

and through chemical weathering of structures containing 

concrete. All significant trends in alkalinity identified during 

the shorter period were upward, including trends at six new 

stations (Providence Water stations 27, 28, 35, 20, 10, and 11; 

fig. 14D) differing significantly from the values determined 

for the longer period. Changes in the applications of lime 

and fertilizer in agricultural areas, which occupy as much 

as 12 percent of the subbasin area (table 2), may explain the 

differences between trends in alkalinity for some tributaries 

during the two periods.

Chemical-Constituent Concentrations

Twenty-seven significant trends in chloride 

concentrations were upward during WYs 1983–2012; two, 

at Providence Water stations 8 and 9 in the Direct Runoff 

Subbasin, were identified as downward trends (fig. 14E). Only 

nine trends were significant for WYs 2003–12. Trends for the 

shorter period indicate fewer statistically significant upward 

trends than during the longer period; several downward 

trends were statistically significant in the shorter period: at 

Providence Water station 28 in the Barden Reservoir Subbasin 

and at stations 13, 15, and 18 in the Regulating Reservoir 

Subbasin. Stations with significant upward chloride trends 

in the shorter period include stations 27, 1, 33, 20, 4, and 23 

(fig. 14E). The smaller number of statistical significant trends 

in chloride concentrations during the shorter period indicate 

that the chloride concentrations in many subbasins were 

neither increasing nor decreasing during WYs 2003–12. These 

results may reflect minimal development in the subbasins 

(U.S. Census Bureau, 2014) or a somewhat consistent use 

of road salt during the relatively short period. Conversely, 

significant downward trends identified at several stations, 

particularly in subbasins that are more developed and contain 

higher percentages of State-maintained roadways than 

others, may reflect in part the effects of changes in winter 

maintenance methods used by the Rhode Island Department of 
Trends in Water-Quality Properties and Constituent Concentrations 41

Transportation (RIDOT; 2014). Although the RIDOT still uses 

sodium chloride for winter maintenance activities throughout 

the State, the use of vehicles equipped with new spreader 

systems and real-time measurements of roadway temperature 

have resulted in reported decreases in the amount of material, 

including sodium chloride, applied to the roadways during 

2005–12 (Rhode Island Department of Administration, 2014). 

The amount of reduction in the amount of deicing materials 

applied to the State-maintained roads within the Scituate 

Reservoir drainage area is not known, and the new winter 

maintenance methods used by the RIDOT have been offered 

as one potential explanation for declining trends in chloride 

concentrations in some subbasins.

Trends in nitrate concentrations were downward and 

significant at nearly every station for WYs 1988–2012 

(fig. 14F). Significant trends were primarily downward for 

WYs 2003–12 as well; however, fewer significant trends 

were identified, and trends were upward at Providence Water 

station 33 in the Direct Runoff Subbasin and at station 18 in 

the Regulating Reservoir Subbasin. Significant downward 

trends for nitrite concentrations were identified at nine of the 

stations during the longer period (fig. 14G). Fewer significant 

trends in concentrations of nitrite were identified during 

WYs 2003–12 than during WYs 1985–2012; however, four 

significant upward nitrite concentration trends were identified 

at Providence Water stations 25, 7, 18, and 12. Trends in 

nitrate and nitrite concentrations in tributary water could be 

affected by changes in atmospheric deposition of nitrogen, but 

these changes have been variable in the Eastern United States 

since the 1980s (Lynch and others, 2000). Trends in nitrate 

and nitrite concentrations also could be affected by changes 

in land use, drainage area processes, or their concentrations in 

base flow. Nitrate and nitrite concentrations at many stations, 

however, are low and sometimes less than reporting limits. 

Concentration data that are near the method detection limit 

also tend to be variable, and results often can be improperly 

interpreted (for example, false-positive detections). As a result, 

factors other than environmental sources could affect the 

identification of statistically significant trends.

Orthophosphate concentration data were not available 

until WYs 1997. Eleven upward trends were identified as 

significant during WYs 1997–2012 (fig. 14H; table 14); however, neither upward nor downward trends were identified as 

significant during WYs 2003–12 (fig. 14H; table 13). Trends 

identified as upward during the WYs 1997–2012 period might 

reflect environmental changes or be affected by the relatively 

small difference in the duration of the datasets or by changes 

in analytical methods.

Nine significant downward and three significant upward 

trends in concentrations of total coliform bacteria were 

identified for WYs 1983–2012 in the Scituate Reservoir 

drainage area (fig. 14I; table 14). For the WYs 2003–12, a 

single significant upward trend was identified at Providence 

Water station 10 in the Westconnaug Reservoir Subbasin 

(fig. 14I). For WYs 1995–2012, downward trends in E. coli

concentrations at Providence Water stations 35, 8, and 19 

also were significant (table 14). Significant upward E. coli

concentration trends were identified for Providence Water 

station 6 in the Direct Runoff Subbasin and for station 14 in 

the Regulating Reservoir Subbasin for the longer period. Only 

a single significant trend in E. coli concentrations (upward) 

was identified for the Scituate Reservoir drainage area during 

WYs 2003–12 (Providence Water station 26). Bacteria 

concentrations in the tributaries were extremely variable 

and were strongly affected by flow and land use (table 7), 

particularly subbasin areas containing high percentages of 

residential and impervious areas (table 2). Thus, trends in 

bacteria concentrations can be difficult to interpret. Changes 

in sampling or analytical methods or procedures for handling 

censored data also could affect the results of tests for trends in 

concentrations of bacteria. Downward trends in concentrations 

of total coliform and E. coli bacteria may have resulted from 

changes in the routing or disposal of stormwater runoff, 

changes in waterfowl abundance, and improvements to onsite 

sewage-disposal systems.
42 Water-Quality Trends in the Scituate Reservoir Drainage Area, Rhode Island, 1983–2012 Table 13. Significance levels (

p values) of seasonal Kendall tests for time trends in water-quality properties and constituent concentrations at Providence Water 

Supply Board monitoring stations in the Scituate Reservoir drainage area, Rhode Island, water years 2003–12.

[Stations locations shown on figure 2. p values for trends considered significant are shown in bold. PWSB, Providence Water Supply Board; USGS, U.S. Geological Survey; E. coli, Escherichia coli; 

(+), variables are positively correlated; (-), variables are negatively correlated; --, not available]

PWSB 

station 

number

USGS 

station 

number

Properties Constituents

pH Color Turbidity

Total 

coliform bacteria

E. coli Alkalinity Chloride Nitrate Nitrite Orthophosphate

Barden Reservoir Subbasin

24 01115190 0.018 0.032 0.119 0.355 0.303 0.006 (+) 0.794 0.109 0.385 0.216

25 01115200 0.472 0.777 0.052 0.318 0.644 0.347 1.000 0.457 0.048 (+) 0.900

26 01115185 0.107 0.331 0.062 0.157 0.013 (+) 0.297 0.274 0.119 0.714 0.353

27 011151845 0.082 0.636 0.177 0.114 0.328 0.024 (+) 0.005 (+) 0.027 (-) 1.000 1.000

28 01115265 0.047 (+) 0.325 0.032 (+) 0.083 0.106 0.033 (+) 0.041 (-) 0.029 (-) 1.000 0.591

29 01115271 0.675 0.473 0.416 0.063 0.232 0.135 1.000 0.032 (-) 0.915 0.277

35 01115187 0.046 (+) 0.068 0.042 (+) 0.659 0.16 0.040 (+) 0.365 0.285 0.301 0.154

Direct Runoff Subbasin

1 01115180 0.583 0.596 0.533 0.933 0.919 0.015 (+) 0.010 (+) 0.022 (-) 0.308 0.269

2 01115181 0.075 0.826 0.461 0.840 0.387 0.141 0.089 0.034 (-) 0.518 0.687

3 01115280 0.134 0.043 (+) 0.243 0.440 0.254 0.023 (+) 0.076 0.026 (-) 0.210 0.059

4 01115400 0.572 0.210 0.034 (+) 0.669 0.418 0.191 0.008 (+) 0.113 0.123 0.466

5 01115184 0.486 0.325 0.594 0.249 0.340 0.179 0.911 0.071 0.687 0.378

6 01115183 0.003 (+) 0.521 0.139 0.295 0.238 0.008 (+) 0.176 0.030 (-) 0.679 0.473

7 01115297 0.018 (+) 0.263 0.29 0.081 0.218 1.000 0.181 0.297 0.006 (+) 0.524

8 01115276 0.065 0.786 0.116 0.964 0.467 0.124 0.771 0.005 (-) 0.309 0.678

9 01115275 0.088 0.225 0.095 0.610 0.453 0.352 0.260 0.074 0.713 0.467

30 01115350 0.227 0.867 0.065 0.256 0.695 0.066 0.060 0.053 0.840 0.158

31 01115177 0.135 0.875 0.046 (-) 0.900 0.213 0.902 0.108 0.764 0.379 0.060

32 01115178 0.189 0.803 0.068 0.324 0.571 0.832 0.154 0.052 0.771 0.349

33 01115182 0.007 (+) 0.324 0.120 0.884 0.284 0.685 0.043 (+) 0.039 (+) 0.808 0.949

36 -- 0.497 0.237 0.096 0.065 0.379 0.466 0.149 0.040 (-) 0.454 0.935

37 -- 0.042 (+) 0.073 0.022 (+) 0.161 0.345 0.125 0.054 0.015 (-) 0.836 0.248
Trends in Water-Quality Properties and Constituent Concentrations 43

Table 13. Significance levels (p values) of seasonal Kendall tests for time trends in water-quality properties and constituent concentrations at Providence Water 

Supply Board monitoring stations in the Scituate Reservoir drainage area, Rhode Island, water years 2003–12.—Continued

[Stations locations shown on figure 2. p values for trends considered significant are shown in bold. PWSB, Providence Water Supply Board; USGS, U.S. Geological Survey; E. coli, Escherichia coli; 

(+), variables are positively correlated; (-), variables are negatively correlated; --, not available]

PWSB 

station 

number

USGS 

station 

number

Properties Constituents

pH Color Turbidity

Total 

coliform bacteria

E. coli Alkalinity Chloride Nitrate Nitrite Orthophosphate

Moswansicut Reservoir Subbasin

19 01115170 0.297 0.293 0.303 0.666 0.295 0.485 0.071 0.013 (-) 0.039 (-) 0.811

20 01115160 0.004 (+) 0.046 (-) 0.031 (+) 0.826 0.886 0.015 (+) 0.973 0.005 (-) 0.974 0.656

21 01115165 0.333 0.57 0.017 (+) 0.162 0.066 0.746 0.244 0.048 (-) 0.790 0.44

22 01115167 0.051 0.620 0.869 0.262 0.402 0.889 0.061 0.003 (-) 0.014 (-) 0.822

34 01115164 0.335 0.908 0.256 1.000 0.350 0.697 0.301 0.677 1.000 0.221

Ponaganset Reservoir Subbasin

23 011151843 0.008 (+) 0.041 (+) 0.020 (+) 0.159 0.408 0.020 (+) 0.022 (+) 0.008 (-) 0.300 0.179

Regulating Reservoir Subbasin

13 01115176 0.121 0.044 (+) 0.026 (+) 0.613 0.388 0.044 (+) 0.043 (-) 0.009 (-) 0.548 0.716

14 01115110 0.413 0.218 0.072 0.796 0.483 0.057 0.109 0.006 (-) 0.412 0.305

15 01115114 0.095 0.171 0.180 0.496 0.683 0.027 (+) 0.004 (-) 0.018 (-) 0.648 0.768

16 01115098 0.860 0.187 0.152 0.812 0.830 0.513 0.459 0.008 (-) 0.403 0.199

17 01115119 0.451 0.625 0.636 0.065 0.245 0.268 1.000 0.259 0.315 0.345

18 01115120 0.481 0.073 0.505 0.306 0.720 0.848 0.013 (-) 0.036 (+) 0.039 (+) 0.705

Westconnaug Reservoir Subbasin

10 01115274 0.002 (+) 0.004 (+) 0.047 (+) 0.021 (+) 0.641 0.002 (+) 0.062 0.006 (-) 0.038 0.616

11 01115273 0.010 (+) 1.000 0.550 0.245 0.323 0.038 (+) 0.028 0.603 0.080 0.417

12 011152745 0.002 (+) 0.124 0.496 0.235 0.177 0.810 0.222 0.940 0.006 (+) 0.913
44 Water-Quality Trends in the Scituate Reservoir Drainage Area, Rhode Island, 1983–2012 Table 14. Significance levels (

p values) of seasonal Kendall tests for time trends in water-quality properties and constituent concentrations at Providence Water Supply Board 

water-quality monitoring stations in the Scituate Reservoir drainage area, Rhode Island, water years 1983–2012.

[Station locations shown on figure 2. p values for trends considered significant are shown in bold. Escherichia coli (E. coli) data not available until after 1995; nitrate data not available until after 1987; nitrite 

data not available until 1985; and orthophosphate data not available until 1997. PWSB, Providence Water Supply Board; USGS, U.S. Geological Survey; (+), variables are positively correlated; (-), variables are negatively correlated; --, not available]

PWSB 

station 

number

USGS 

station 

number

Properties Constituents

pH Color Turbidity

Total 

coliform bacteria

E. coli Alkalinity Chloride Nitrate Nitrite Orthophosphate

Barden Reservoir Subbasin

24 01115190 <0.001 (+) 0.001 (+) 0.308 0.256 0.385 0.006 (+) <0.001 (+) 0.001 (-) 0.901 0.265

25 01115200 0.008 (+) 0.053 0.102 0.094 0.968 0.968 0.028 (+) 0.001 (-) 0.267 0.612

26 01115185 <0.001 (+) 0.029 (+) 0.092 0.019 (-) 0.427 1.000 <0.001 (+) <0.001 (-) 0.448 0.032 (+)

27 011151845 <0.001 (+) 0.003 (+) 0.184 0.417 0.363 0.157 <0.001 (+) 0.056 0.314 0.456

28 01115265 0.663 <0.001 (+) 0.733 0.004 (+) 0.296 0.324 <0.001 (+) <0.001 (-) 0.874 0.030 (+)

29 01115271 0.013 (+) <0.001 (+) 0.292 0.019 (-) 0.869 0.428 <0.001 (+) <0.001 (-) 0.991 0.032 (+)

35 01115187 0.276 0.002 (+) 0.188 0.419 0.043 (-) 0.723 <0.001 (+) 0.001 (-) <0.001 (-) 0.081

Direct Runoff Subbasin

1 01115180 0.261 <0.001 (+) 0.002 (+) 0.328 0.322 0.005 (+) <0.001 (+) <0.001 (-) 0.490 0.261

2 01115181 0.020 (+) 0.220 0.051 0.588 0.157 0.858 0.089 0.004 (-) 0.477 0.957

3 01115280 0.001 (+) <0.001 (+) 0.142 0.102 0.255 0.001 (+) 0.001 (+) <0.001 (-) 0.713 0.2204 01115400 0.006 (+) 0.001 (+) 0.204 0.782 0.684 <0.001 (+) 0.877 <0.001 (-) 0.436 0.0475 01115184 0.059 0.016 (+) 0.822 0.140 0.258 0.149 0.016 (+) 0.074 0.249 0.137

6 01115183 <0.001 (+) 0.015 (+) 0.194 0.883 0.006 (+) <0.001 (+) 0.001 (+) <0.001 (-) 0.968 0.022 (+)7 01115297 <0.001 (+) <0.001 (+) 0.571 0.001 (-) 0.669 0.045 (+) <0.001 (+) <0.001 (-) 0.554 0.079

8 01115276 0.002 (-) 0.419 0.001 (-) <0.001 (-) 0.004 (-) <0.001 (-) 0.001 (-) <0.001 (-) 0.610 0.032 (+)

9 01115275 0.026 (+) 0.392 0.674 0.562 0.363 0.034 (+) 0.001 (-) 0.002 (-) 0.418 0.274

30 01115350 <0.001 (+) 0.139 0.345 0.546 0.403 0.406 0.012 (+) 0.006 (-) 0.444 0.292

31 01115177 0.003 (-) 0.610 0.023 (-) 0.516 0.853 0.426 0.598 0.298 0.546 0.780

32 01115178 0.877 0.002 (+) 0.503 0.529 0.772 0.255 0.002 (+) <0.001 (-) 0.016 (-) 0.12333 01115182 0.825 0.069 0.162 0.783 0.527 0.182 0.002 (+) <0.001 (-) 0.014 (-) 0.391

36 -- 0.521 0.008 (+) 0.302 0.024 (+) 0.290 0.844 0.008 (+) 0.009 (-) 0.396 0.056

37 -- 0.125 0.011 (+) 0.262 0.009 (+) 0.181 0.974 0.014 (+) 0.021 (-) 0.689 0.302
Trends in Water-Quality Properties and Constituent Concentrations 45

Table 14. Significance levels (p values) of seasonal Kendall tests for time trends in water-quality properties and constituent concentrations at Providence Water Supply Board 

water-quality monitoring stations in the Scituate Reservoir drainage area, Rhode Island, water years 1983–2012.—Continued

[Station locations shown on figure 2. p values for trends considered significant are shown in bold. Escherichia coli (E. coli) data not available until after 1995; nitrate data not available until after 1987; nitrite 

data not available until 1985; and orthophosphate data not available until 1997. PWSB, Providence Water Supply Board; USGS, U.S. Geological Survey; (+), variables are positively correlated; (-), variables are negatively correlated; --, not available]

PWSB 

station 

number

USGS 

station 

number

Properties Constituents

pH Color Turbidity

Total 

coliform bacteria

E. coli Alkalinity Chloride Nitrate Nitrite Orthophosphate

Moswansicut Reservoir Subbasin

19 01115170 0.002 (+) 0.001 (+) <0.001 (+) <0.001 (-) 0.025 (-) 0.001 (+) <0.001 (+) <0.001 (-) 0.003 (-) 0.019 (+)

20 01115160 0.377 0.003 (+) 0.272 0.777 0.506 0.208 <0.001 (+) <0.001 (-) 0.724 0.072

21 01115165 0.106 0.007 (+) 0.918 0.050 0.759 0.001 (+) 0.182 0.007 (-) 0.277 0.221

22 01115167 0.016 (+) 0.638 <0.001 (-) 0.003 (-) 0.448 0.033 (+) 0.206 <0.001 (-) 0.001 (-) 0.26834 01115164 0.574 0.726 0.307 0.746 0.331 0.131 0.273 0.189 0.798 0.009 (+)

Ponaganset Reservoir Subbasin

23 011151843 <0.001 (+) <0.001 (+) 0.017 (-) 0.249 0.303 0.001 (+) <0.001 (+) 0.001 (-) 0.017 (-) 0.268

Regulating Reservoir Subbasin

13 01115176 0.082 <0.001 (+) 0.858 0.013 (-) 0.276 <0.001 (+) 0.001 (+) <0.001 (-) 0.012 (-) 0.040 (+)

14 01115110 0.036 (+) <0.001 (+) 0.919 0.378 0.032 (+) 0.199 <0.001 (+) <0.001 (-) 0.647 0.008 (+)

15 01115115 <0.001 (+) <0.001 (+) 0.228 0.417 0.208 0.001 (+) 0.024 (+) <0.001 (-) 0.022 (-) 0.07316 01115098 0.450 <0.001 (+) 0.080 0.345 0.165 0.032 (+) <0.001 (+) <0.001 (-) 0.910 0.019 (+)

17 01115119 0.017 (-) 0.663 0.014 (-) 0.001 (-) 1.000 0.021 (-) 0.007 (+) 0.001 (-) 0.006 (-) 0.058

18 01115120 0.205 0.199 0.864 0.004 (-) 0.839 0.005 (-) 0.143 0.001 (-) 0.336 0.026 (+)

Westconnaug Reservoir Subbasin

10 01115274 0.002 (+) 0.004 (+) <0.002 (-) 0.757 0.683 0.306 <0.001 (+) <0.001 (-) 0.809 0.208

11 01115273 0.015 (-) <0.001 (+) 0.284 0.409 0.473 0.298 0.056 0.020 (-) 0.283 0.393

12 011152745 <0.001 (+) 0.390 0.007 (+) 0.140 0.194 0.001 (+) <0.001 (+) 0.441 0.139 0.418
46 Water-Quality Trends in the Scituate Reservoir Drainage Area, Rhode Island, 1983–2012

1

2

3

4

5

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8 9

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Ponaganset Reservoir Subbasin

Barden Reservoir Subbasin Basin boundary

Subbasins and reservoir areas

30

0

0

1 2 3 4 5 MILES

1 3 42 5 KILOMETERS

A. pH

2003−12 1983−2012

B. Color

EXPLANATION

Westconnaug Reservoir Subbasin

Regulating Reservoir Subbasin

Moswansicut Reservoir Subbasin

Direct Runoff Subbasin

Providence Water Supply Board station number

and approximate location

Downward trend

Upward trend

Water-quality sampling

subbasin boundary

2003−12 1983−2012

Figure 14. Time trends of A, pH, B, color, C, turbidity, D, alkalinity, E, chloride, F, nitrate, G, nitrite, H, orthophosphate, I, total coliform 

bacteria, and J, Escherichia coli at Providence Water Supply Board monitoring stations in the Scituate Reservoir drainage area, 

Rhode Island, water years 2003–12 and water years 1983–2012.
Trends in Water-Quality Properties and Constituent Concentrations 47

2 1

3

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6

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37

2 1

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C. Turbidity

2003−12 1983−2012

D. Alkalinity

2003−12 1983−2012

Ponaganset Reservoir Subbasin

Barden Reservoir Subbasin Basin boundary

Subbasins and reservoir areas

30

0

0

1 2 3 4 5 MILES

1 3 42 5 KILOMETERS

EXPLANATION

Westconnaug Reservoir Subbasin

Regulating Reservoir Subbasin

Moswansicut Reservoir Subbasin

Direct Runoff Subbasin

Providence Water Supply Board station number

and approximate location

Downward trend

Upward trend

Water-quality sampling

subbasin boundary

Figure 14. Time trends of A, pH, B, color, C, turbidity, D, alkalinity, E, chloride, F, nitrate, G, nitrite, H, orthophosphate, I, total coliform 

bacteria, and J, Escherichia coli at Providence Water Supply Board monitoring stations in the Scituate Reservoir drainage area, 

Rhode Island, water years 2003–12 and water years 1983–2012.—Continued
48 Water-Quality Trends in the Scituate Reservoir Drainage Area, Rhode Island, 1983–2012

2 1

3

4

5

6

7

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F . Nitrate

E. Chloride

2003−12 1983−2012

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Barden Reservoir Subbasin Basin boundary

Subbasins and reservoir areas

30

0

0

1 2 3 4 5 MILES

1 3 42 5 KILOMETERS

EXPLANATION

Westconnaug Reservoir Subbasin

Regulating Reservoir Subbasin

Moswansicut Reservoir Subbasin

Direct Runoff Subbasin

Providence Water Supply Board station number

and approximate location

Downward trend

Upward trend

Water-quality sampling

subbasin boundary

Figure 14. Time trends of A, pH, B, color, C, turbidity, D, alkalinity, E, chloride, F, nitrate, G, nitrite, H, orthophosphate, I, total coliform 

bacteria, and J, Escherichia coli at Providence Water Supply Board monitoring stations in the Scituate Reservoir drainage area, 

Rhode Island, water years 2003–12 and water years 1983–2012.—Continued
Trends in Water-Quality Properties and Constituent Concentrations 49

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H. Orthophosphate

G. Nitrite

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Barden Reservoir Subbasin Basin boundary

Subbasins and reservoir areas

30

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0

1 2 3 4 5 MILES

1 3 42 5 KILOMETERS

EXPLANATION

Westconnaug Reservoir Subbasin

Regulating Reservoir Subbasin

Moswansicut Reservoir Subbasin

Direct Runoff Subbasin

Providence Water Supply Board station number

and approximate location

Downward trend

Upward trend

Water-quality sampling

subbasin boundary

Figure 14. Time trends of A, pH, B, color, C, turbidity, D, alkalinity, E, chloride, F, nitrate, G, nitrite, H, orthophosphate, I, total coliform 

bacteria, and J, Escherichia coli at Providence Water Supply Board monitoring stations in the Scituate Reservoir drainage area, 

Rhode Island, water years 2003–12 and water years 1983–2012.—Continued
50 Water-Quality Trends in the Scituate Reservoir Drainage Area, Rhode Island, 1983–2012

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J. Escherichia coli bacteria

I . Total coliform bacteria

2003−12 1983−2012

2003−12 1995−2012

Ponaganset Reservoir Subbasin

Barden Reservoir Subbasin Basin boundary

Subbasins and reservoir areas

30

Westconnaug Reservoir Subbasin

Regulating Reservoir Subbasin

Moswansicut Reservoir Subbasin

Direct Runoff Subbasin

Providence Water Supply Board station number

and approximate location

Downward trend

Upward trend

Water-quality sampling

subbasin boundary

EXPLANATION

0

0

1 2 3 4 5 MILES

1 3 42 5 KILOMETERS

Figure 14. Time trends of A, pH, B, color, C, turbidity, D, alkalinity, E, chloride, F, nitrate, G, nitrite, H, orthophosphate, I, total coliform 

bacteria, and J, Escherichia coli at Providence Water Supply Board monitoring stations in the Scituate Reservoir drainage area, 

Rhode Island, water years 2003–12 and water years 1983–2012.—Continued
Summary 51

Summary

Water-quality and streamflow data were collected at 

37 surface-water monitoring stations in the Scituate Reservoir 

drainage area, Rhode Island, from October 1, 2002, through 

September 30, 2012 (water years [WYs] 2003–12). These 

data were analyzed to determine water-quality conditions 

and constituent loads and yields in the drainage area. 

Trends in selected water-quality properties and constituent 

concentrations also were investigated for WYs 2003–12 

and for WYs 1983–2012, by using historical data. Waterquality samples were collected and analyzed by personnel 

of Providence Water Supply Board (Providence Water), the 

agency that manages the reservoir. Streamflow and specific 

conductance data were collected by U.S. Geological Survey 

(USGS) personnel. Median values and other summary 

statistics were calculated for all water samples collected at 

37 monitoring stations during WYs 2003–12 for pH, color, 

turbidity, alkalinity, chloride, nitrate, nitrite, orthophosphate, 

and total coliform bacteria and E. coli. Daily loads and 

subbasin yields of chloride, nitrate, nitrite, orthophosphate, 

and total coliform bacteria and E. coli were calculated for all 

sampling dates during WYs 2003–12 for the 23 stations for 

which sufficient flow data were available. Values of physical 

properties and concentrations of constituents were compared 

with State and Federal water-quality standards and guidelines 

and were related to streamflow, land-use characteristics, 

timber operations, and imperviousness.

The interpretations of the results of water-quality 

analyses presented in this report were limited because of 

the absence of quality-control data. Without laboratory and 

field quality-control data, it was not possible to evaluate the 

accuracy, precision, or bias of laboratory analyses, the in situ 

variability of water-quality properties and constituents, or the 

potential for sample contamination. Wherever possible, however, statistical approaches were used to minimize the limitations that may have resulted from the use of existing data.

Tributary water in the Scituate Reservoir drainage area 

during WYs 2003–12 was slightly acidic, with a median pH 

for all stations of 6.1. The pH was less than 6.5, the lower 

pH limit established by the Rhode Island Department of 

Environmental Management (RIDEM) for class AA waters, 

in 17 to 100 percent of samples collected at stations in the 

Scituate Reservoir drainage area. The low pH values in 

tributaries likely reflect the low pH of precipitation and the 

relatively nonreactive rock types in the drainage area.

Median values of color ranged from 12 to 135 platinumcobalt units (PCU) and as a result, most median values and the 

majority of discrete measurements of color at most monitoring 

stations exceeded the U.S. Environmental Protection Agency 

(EPA) secondary drinking-water regulation (SDWR) value of 

15 PCU during WYs 2003–2012.

Turbidity values in tributaries in the drainage area were 

typically low and rarely exceeded 5 NTU, the maximum 

allowable value above background for RIDEM class AA 

waters. The median turbidity values at all 37 stations were 

less than 1.68 nephelometric turbidity units (NTU), the 

proposed EPA reference value for streams in the Northeastern 

Coastal Zone ecoregion. These results indicate that the water 

quality of tributaries in the Scituate Reservoir drainage area 

was minimally affected by suspended particles relative to 

stream water in the Northeastern Coastal Zone ecoregion. 

Correlations between turbidity and pH, color, and alkalinity 

were significant.

Median alkalinity values measured in samples from 

tributaries ranged from 2.3 to 14 milligrams per liter calcium 

carbonate (mg/L as CaCO3), and the median value for all 

sampling stations for the entire Scituate Reservoir drainage 

area was 5.1 mg/L as CaCO3. Alkalinity was consistently 

low in the Barden Reservoir, Ponaganset Reservoir, and 

Westconnaug Reservoir Subbasins in the western part of the 

study area; and was highest in the Direct Runoff, Moswansicut 

Reservoir, and Regulating Reservoir Subbasins in the eastern 

part of the study area.

The median chloride concentration for the entire study 

area was 22 milligrams per liter (mg/L), a low value typical 

of natural stream water in coastal New England. Median 

values for individual stations ranged from 3.8 to 87 mg/L. 

Median chloride concentrations were highest in samples 

collected from Toad Pond and Bear Tree Brook. A formerly 

uncovered salt-storage facility in the Bear Tree Brook 

Subbasin likely contributed to the high chloride concentrations 

at that monitoring station. With the exception of one sample 

collected from Moswansicut Stream South (Providence Water 

station 22), concentrations of chloride in water samples did 

not exceed the 250-mg/L EPA SDWR for chloride during 

WYs 2003–12. Concentrations of chloride estimated from 

continuous records of specific conductance, which represent 

a broad concentration range during base-flow, runoff, and 

snowmelt conditions, also did not exceed the EPA SDWR for 

chloride during WYs 2009–12 except at Quonapaug Brook 

(Providence Water station 6) in WY 2011 and WY 2012 during 

winter runoff events.

Concentrations of the nutrient species nitrate, nitrite, and 

orthophosphate were low. Median concentrations of nitrate 

did not exceed 0.14 milligrams per liter as nitrogen (mg/L 

as N) throughout the drainage area, although concentration 

at most stations were about 0.01 mg/L as N. Median nitrite 

concentrations at all 37 stations were less than or equal to 

0.005 mg/L as N. The median values for the sum of nitrite 

and nitrate concentrations at all stations were less than the 

proposed EPA reference concentration for the sum of nitrite 

and nitrate concentrations that represent minimally affected 

conditions for streams in the Northeastern Coastal Zone 

ecoregion. Median concentrations of orthophosphate ranged 

from 0.01 to 0.04 milligrams per liter as phosphorus (mg/L as 

P). Concentrations of orthophosphate in water samples from 

the tributaries often were greater than the RIDEM standard 

of 0.025-mg/L as P for total phosphorus, and at 16 of the 

37 stations, the median orthophosphate concentration was two 

or more times greater than the RIDEM standard. Phosphorus 

concentrations that exceed the standard could be of concern 
52 Water-Quality Trends in the Scituate Reservoir Drainage Area, Rhode Island, 1983–2012

because phosphorus can contribute to excessive plant and 

algae growth.

The median concentrations of total coliform bacteria and E. coli for the 37 stations in the drainage area were 

43 colony-forming units per 100 milliliters (CFU/100 mL) 

and 15 CFU/100 mL, respectively. Concentrations of total 

coliform bacteria and E. coli in many water samples were high 

at stations on the Unnamed Brook North of Bullhead Brook, 

Quonapaug Brook, Toad Pond, and Moswansicut Stream 

South in the Moswansicut Reservoir Subbasin. The relatively 

high percentage of impervious area in the drainage areas of 

stations on Toad Pond and Moswansicut Stream South, which 

can convey surface-water runoff to these tributaries, may 

contribute to the bacteria concentrations measured in waterquality samples.

Median values of some water-quality properties and 

constituent concentrations that are affected by human activities correlated positively with the percentages of developed 

land and negatively with the percentage of forest cover. In 

general, statistically significant correlations (p value less than 

0.05) were not identified between median values or concentrations of water-quality properties and constituents and various degrees of tree-canopy reduction resulting from timber 

operations in the subbasins. Significant positive correlations 

were identified between median concentrations of chloride and 

impervious area and developed area. These relations are likely 

associated with the application of road salt and other deicing 

compounds during winter maintenance activities. The significant positive correlation between median values of color and 

the percentages of wetlands in the drainage areas indicates that 

some sources of color in tributary waters are likely natural. 

Negative correlations of turbidity and indicator bacteria with 

streamflow at many stations likely reflect seasonal patterns 

in which these values are higher during low-flow conditions 

in summer and fall at the end of the water year. The negative 

correlation between chloride concentrations and streamflow at 

stations primarily in the Barden Reservoir and Direct Runoff 

Subbasins indicates that road salt is likely infiltrating to the 

groundwater and discharging as base flow. Median concentrations of nitrate correlated positively with streamflow at four 

stations. Correlations between concentrations of nitrite and 

streamflow were numerous.

The highest median chloride load (1,100 kilograms/day 

[kg/d]) was measured at the station on the Ponaganset River 

above the Barden Reservoir; this high value may be related 

in part to the large area of this subbasin. The largest median 

chloride yield (290 kilograms per day per square mile 

[kg/d/mi2

]) was determined for Bear Tree Brook that has historically been affected by the location of a salt-storage facility 

within the subbasin drainage area. Relatively high chloride 

yields ranging from 120 to 190 kg/d/mi2 likely reflect the 

greater percentages of developed land uses and imperviousness in several subbasins. Average daily loads and yields of 

chloride and sodium also were calculated for 14 continuousmonitoring stations in the Scituate Reservoir drainage area 

for WYs 2009–12. These USGS data were estimated based on 

continuous records of flow and specific conductance and relations between specific conductance values and concentrations 

of chloride and sodium. The USGS daily loads of chloride 

and sodium ranged from 52 to 1,300 kg/d and 31 to 780 kg/d, 

respectively. The largest USGS daily loads for chloride and 

sodium (1,300 and 780 kg/d, respectively) were calculated for 

Ponaganset River and represent 20 percent of the total load for 

the 14 stations. The USGS daily yields of chloride and sodium 

for the drainage areas above the 14 stations, which represent 

nearly 66 percent of the Scituate Reservoir drainage area, were 

100 and 64 kg/d/mi2,respectively.

Median nutrient loads (nitrate, nitrite, and orthophosphate) tended to be larger in tributary subbasins with large 

drainage areas and in the eastern part of the Scituate Reservoir 

drainage area, which is more developed than the other parts. 

The median nutrient yields for the Scituate Reservoir drainage 

area were 5.4 grams per day per square mile as nitrogen 

(g/d/mi2

 as N) for nitrite, 44 g/d/mi2 as N for nitrate, and 

70 kilograms per day per square mile as phosphorus 

(kg/d/mi2

 as P) for orthophosphate. Mean subbasin yields for 

nitrite and nitrate were largest in the Moswansicut Reservoir 

Subbasin (8.6 and 69 g/d/mi2 as N, respectively), and the mean 

yield for orthophosphate was largest in the Westconnaug 

Reservoir Subbasin (100 kg/d/mi2 as P). The largest median 

yield of nitrate (480 kilograms per day per square mile as 

nitrogen) was determined for Bear Tree Brook in the Direct 

Runoff Subbasin and was about 10 times larger than the 

median yield for the Scituate Reservoir drainage area.

Loads and yields of total coliform bacteria and E. coli

varied by nearly two orders of magnitude among the stations 

in the Scituate Reservoir drainage area. Median loads of total 

coliform and E. coli bacteria were 2,100 million colonyforming units per day (MCFU/d) and 530 MCFU/d, respectively. The greatest median load of total coliform bacteria 

(21,000 MCFU/d) and E. coli (5,700 MCFU/d) were found 

at the stations on Hunting House Brook (Providence Water 

station 14) and at Hemlock Brook (Providence Water station 

14), respectively. Median yields of total coliform and E. coli

bacteria were 1,400 million colony-forming units per day per 

square mile (MCFU/d/mi2) and 350 MCFU/d/mi2, respectively. The largest median yields of total coliform and E. coli

bacteria were at Quonapaug Brook and several other monitoring stations in the Westconnaug Reservoir and Regulating 

Reservoir Subbasins.

Significant upward and downward time trends (p values 

less than 0.05) were identified in water-quality properties and 

concentrations for several constituents for WYs 2003–12 and 

WYs 1983–2012. Significant upward trends in pH were identified at nearly half of the monitoring stations for WYs 1983–

2012 and might reflect regional reductions in acid precipitation 

in the Northeast. All significant trends in color were upward 

in samples from 25 of the 37 stations for WYs 1983–2012, 

but few significant trends were observed for WYs 2003–12. 

Significant upward trends in turbidity were identified at about 

one-quarter of the stations for both trend periods. Trends for 

WYs 2003–12 indicated that turbidity values for nine statins, 
Selected References 53

including Westconnaug Brook and Ponaganset Reservoir, were 

increasing. A consistent downward trend in turbidity was identified at a single monitoring station during both test periods—

Toad Pond in the Direct Runoff Subbasin.

Significant trends in alkalinity were upward during both 

trend periods and were likely related either to the natural 

weathering of structures containing concrete or to the application of lime or fertilizers that can increase alkalinity in the 

poorly buffered soils in the drainage area. Thirteen significant 

upward trends in alkalinity were identified during WYs 2003–

12, many of which were not identified for WYs 1983–2012.

Trends in chloride concentrations (primarily upward) 

for WYs 1983–2002 were identified at 29 of the 37 stations 

in the Scituate Reservoir drainage area; however, only 

nine significant trends were identified for WYs 2003–12. 

Significant downward trends for WYs 2003–12 were 

identified for monitoring stations in the Barden Reservoir 

and Regulating Reservoir Subbasins. The reduction in the 

number of significant trends for the recent period indicates that 

chloride concentrations are neither increasing nor decreasing. 

Decreasing trends at some stations, particular those with large 

percentages of roadways, may indicate the effects of either 

changes in the quantity of the deicing compounds applied 

during the study period or changes in winter maintenance 

procedures by the Rhode Island Department of Transportation 

and other local agencies responsible for road maintenance.

Significant trends in nitrate concentrations were downward at 32 of the 37 stations for WYs 1988–2012 and at 

20 stations for WYs 2003–12. Nine significant downward 

trends for nitrite were identified for WYs 1985–2012; only 

two significant upward trends were identified for the shorter 

period. Four upward trends in nitrite concentrations also were 

identified for WYs 2003–12. Concentrations of both nitrate 

and nitrite typically were low and close to reporting limits; 

as a result, trends could be affected by factors like changes in 

environmental sources and changes in analytical procedures 

or sampling irregularities not identified because of the absence 

of field quality-control data. Eleven significant increasing 

trends were identified for concentrations of orthophosphate 

during WYs 1997–2002; no trends were significant during 

WYs 2003–12.

Nine significant downward and three significant upward 

time trends in concentrations of total coliform bacteria were 

identified for WYs 1983–2012 in the Scituate Reservoir drainage area; a single significant upward trend was identified in the 

Westconnaug Reservoir Subbasin for the shorter period. Three 

downward and two upward trends in E. coli concentrations for 

WYs 1995–2012 were significant, but only a single significant 

upward trend in E. coli concentrations was identified for a station in the Barden Reservoir Subbasin during WYs 2003–12. 

Bacteria concentrations in the tributaries were extremely variable with discrete concentrations often ranging by nearly two 

orders of magnitude at a single monitoring station.

In conclusion, water-quality conditions for WYs 2003–12 

continue to reflect the generally high-quality, drinking-watersource tributaries in the Scituate Reservoir drainage area. 

Monitored water-quality properties and constituent concentrations for the current study were similar to historical values at 

most stations. Fewer significant trends for most parameters 

and constituents were identified for WYs 2003–12 than were 

identified for WYs 1983–2012. The differences in trends for 

WYs 2003–12, in general, did not contradict the trend results 

for WYs 1983–2012, with the exception of trends in chloride 

concentrations, which indicated that concentrations were 

decreasing at several monitoring stations. The relations of 

concentrations and values of several water-quality constituents 

and properties to land use and impervious area in the drainage 

areas of the monitoring stations point to the effects of human 

activities on tributary water quality.

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Bent, G.C., Steeves, P.A., and Waite, A.M., 2014, Equations 

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Binkley Dan, Ice, G.G., Kaye, Jason, and Williams, C.A., 

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Breault, R.F., 2010, Streamflow, water quality, and constituent loads and yields, Scituate Reservoir drainage area, 

Rhode Island, water year 2002: U.S. Geological Survey 

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of/2009/1041/.

Breault, R.F., and Campbell, J.P., 2010a, Streamflow, water 

quality, and constituent loads and yields, Scituate Reservoir 

drainage area, Rhode Island, water year 2003: U.S. 

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Breault, R.F., and Campbell, J.P., 2010b, Streamflow, water 

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drainage area, Rhode Island, water year 2004: U.S. 

Geological Survey Open-File Report 2010–1044, 24 p., 

http://pubs.usgs.gov/of/2010/1044/.
54 Water-Quality Trends in the Scituate Reservoir Drainage Area, Rhode Island, 1983–2012

Breault, R.F., and Campbell, J.P., 2010c, Streamflow, water 

quality, and constituent loads and yields, Scituate Reservoir 

drainage area, Rhode Island, water year 2005: U.S. 

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Breault, R.F., and Campbell, J.P., 2010d, Streamflow, water 

quality, and constituent loads and yields, Scituate Reservoir 

drainage area, Rhode Island, water year 2006: U.S. 

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Breault, R.F., and Smith, K.P., 2010, Streamflow, water 

quality, and constituent loads and yields, Scituate Reservoir 

drainage area, Rhode Island, water year 2009: U.S. 

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Breault, R.F., Sorensen, J.R., and Weiskel, P.K., 2002, 

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Breault, R.F., Waldron, M.C., Barlow, L.K., and Dickerman, 

D.C., 2000, Water-quality conditions in relation to drainagebasin characteristics in the Scituate Reservoir basin, Rhode 

Island, 1982–95: U.S. Geological Survey Water-Resources 

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Church, P.E., Armstrong, D.S., Granato, G.E., Stone, V.J., 

Smith, K.P., and Provencher, P.L., 1996, Effectiveness of 

highway-drainage systems in preventing contamination 

of ground water by road salt, Route 25, southeastern 

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programs, and methodology: U.S. Geological Survey 

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Dawson, T.E., 1996, Determining water use by trees and 

forests from isotopic, energy balance and transpiration 

analyses—The roles of tree size and hydraulic lift: Tree 

Physiology, v. 16, p. 263–272.

Francy, D.S., Myers, D.N., and Helsel, D.R., 2000, Microbiological monitoring for the U.S. Geological Survey National 

Water-Quality Assessment Program: U.S. Geological 

Survey Water-Resources Investigations Report 00–4018, 

31 p. [Also available at http://oh.water.usgs.gov/reports/

Abstracts/wrir00-4018.html.]

Granato, G.E., and Smith, K.P., 1999, Estimating concentrations of road-salt constituents in highway-runoff from measurements of specific conductance: U.S. Geological Survey 

Water-Resources Investigations Report 99–4077, 22 p. 

[Also available at http://pubs.usgs.gov/wri/wri99-4077/.]

Hach Company, 2000, Hach 2010 spectrophotometer procedures manual, rev. 2: Loveland, Colo., Hach Company, 

657 p.

Hahn, G.W., and Hansen, A.J., 1961, Ground water map of 

the Chepachet quadrangle, Rhode Island: Providence, R.I., 

Rhode Island Water Resources Coordinating Board Ground 

Water Map 15, 1 pl., scale 1:24,000.

Hansen, A.J., 1962, Ground water map of the Clayville quadrangle, Rhode Island: Providence, R.I., Rhode Island Water 

Resources Coordinating Board Ground Water Map 17, 1 pl., 

scale 1:24,000.

Hansen, R.W., Harper, J.M., Stone, M.L., Ward, G.M., and 

Kidd, R.A., 1976, Manure harvesting practices—Effects 

on waste characteristics and runoff: U.S. Environmental 

Protection Agency EPA/600/2–76–292, 104 p.

Helsel, D.R., 2005, Nondetects and data analysis—Statistics 

for censored environmental data: Hoboken, N.J., John 

Wiley and Sons, 250 p.

Helsel, D.R., and Hirsch, R.M., 2002, Statistical methods in 

water resources: U.S. Geological Survey Techniques of 

Water-Resources Investigations, book 4, chap. A3, 522 p. 

[Also available at http://pubs.usgs.gov/twri/twri4a3/pdf/

twri4a3-new.pdf.]

Hem, J.D., 1982, Conductance—A collective measure of 

dissolved ions, in Minear, R.A., and Keith, L.A., eds., 

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Appendix 1. Water-Quality Data Collected by the 

Providence Water Supply Board at 37 Monitoring 

Stations in the Scituate Reservoir Drainage Area, 

Water Years 1983–2012

[Available separately at http://pubs.usgs.gov/sir/2015/5058/]

For more information concerning this report, contact:

Director, New England Water Science Center

U.S. Geological Survey 

10 Bearfoot Road 

Northborough, MA 01532

dc_nweng@usgs.gov

or visit our Web site at:

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http://dx.doi.org/10.3133/sir20155058

Smith—Water-Quality Trends in the Scituate Reservoir Drainage Area, Rhode Island, 1983–2012—Scientific Investigations Report 2015–5058

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