Streamflow, Water Quality, and Constituent Loads and Yields, Scituate Reservoir Drainage Area, Rhode Island, Water Year 2022

Data Report 1205
Prepared in cooperation with Providence Water
By:  and 

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Plain Language Summary

The U.S. Geological Survey, in cooperation with Providence Water (formerly Providence Water Supply Board), conducted a long-term program to monitor water quality in the Scituate Reservoir drainage area in Rhode Island to collect streamflow and water-quality data from 16 tributaries to the reservoir during the water year 2022. These data were used to estimate loads of chloride and sodium. Additionally, water-quality samples were collected at 37 sampling stations on the tributaries, and the data were summarized using central tendency values.

  • Annual mean streamflows for monitoring stations ranged from 0.31 to 28.0 cubic feet per second.
  • Tributaries transported about 2,600 metric tons of chloride and 1,600 metric tons of sodium to the reservoir.
  • Annual yields ranged from 15 to 100 metric tons per square mile for chloride and 10 to 59 metric tons per square mile for sodium.
  • The medians of the median daily loads were 55,000 million colony forming units per day for coliform bacteria, 1,300 million colony forming units per day for Escherichia coli, 230 kilograms per day for chloride, 11 grams per day as nitrogen for nitrite, 620 grams per day as nitrogen for nitrate, and 440 grams per day as orthophosphate for phosphate.
  • The medians of the median daily yields were 25,000 million colony forming units per day per square mile for coliform bacteria, 810 million colony forming units per day per square mile for Escherichia coli, 110 kilograms per day per square mile for chloride, 5.1 grams per day per square mile as nitrogen for nitrite, less than 300 grams per day per square mile as nitrogen for nitrate, and 230 grams per day per square mile as orthophosphate for phosphate.

Abstract

As part of a long-term cooperative program to monitor water quality within the Scituate Reservoir drainage area, the U.S. Geological Survey, in cooperation with Providence Water (formerly the Providence Water Supply Board), collected streamflow and water-quality data in tributaries to the Scituate Reservoir, Rhode Island. Streamflow and concentrations of chloride and sodium estimated from records of specific conductance for 16 tributaries were used to calculate loads of chloride and sodium during water year 2022 (October 1, 2021, through September 30, 2022). Water-quality samples were collected by Providence Water at 37 sampling stations on tributaries to the Scituate Reservoir during water year 2022. These water-quality data are summarized by using values of central tendency and are used, in combination with measured (or estimated) streamflows, to calculate loads and yields of selected water-quality constituents for water year 2022.

Annual mean streamflows for monitoring stations in this study ranged from about 0.31 to 28.0 cubic feet per second during water year 2022. At the 16 continuous-record streamgages, tributaries transported about 2,600 metric tons of chloride and 1,600 metric tons of sodium to the Scituate Reservoir; annual chloride yields for the tributaries ranged from 15 to 100 metric tons per square mile, and annual sodium yields ranged from 10 to 59 metric tons per square mile. At the stations where water-quality samples were collected by Providence Water, the medians of the median daily loads were 55,000 million colony forming units per day for coliform bacteria, 1,300 million colony forming units per day for Escherichia coli, 230 kilograms per day for chloride, 11 grams per day as nitrogen for nitrite, 620 grams per day as nitrogen for nitrate, and 440 grams per day as orthophosphate for phosphate, The medians of the median yields were 25,000 million colony forming units per day per square mile for coliform bacteria, 810 million colony forming units per day per square mile for Escherichia coli, 110 kilograms per day per square mile for chloride, 5.1 grams per day per square mile as nitrogen for nitrite, less than 300 grams per day per square mile as nitrogen for nitrate, and 230 grams per day per square mile as orthophosphate for phosphate.

Introduction

The Scituate Reservoir is the primary source of drinking water for more than 60 percent of the population of Rhode Island. The Scituate Reservoir drainage area consists of six subbasins and covers an area of about 94 square miles (mi2) in parts of the towns of Cranston, Foster, Glocester, Johnston, and Scituate, R.I. (fig. 1). The six subbasins are referred to in this report as the Barden Reservoir, “Direct runoff,” Moswansicut Pond reservoir, Ponaganset Reservoir, “Regulating reservoir,” and Westconnaug Reservoir subbasins (informal names are used for subbasins that do not have official names). Information about the water quality of the reservoir and its tributaries is important for management of the water supply and for the protection of human health. Providence Water (PW; formerly the Providence Water Supply Board) is the agency responsible for the management and distribution of the Scituate Reservoir water supply and has been monitoring and assessing water quality in the reservoir and reservoir drainage area for more than 60 years.

Six reservoir subbasins and an unsampled area draining directly to Scituate Reservoir;
                     northwestern Rhode Island.
Figure 1.

Map showing locations of tributary-reservoir subbasins and stations in the Scituate Reservoir drainage area, Rhode Island, October 1, 2021, through September 30, 2022. Modified from Breault (2010). Data are from Smith and Spaetzel (2021).

Since 1993, the U.S. Geological Survey (USGS) has been cooperating with PW and the Rhode Island Department of Environmental Management to measure streamflow in tributaries to the Scituate Reservoir. Streamflow has been continuously measured by the USGS at 10 streamgages in the drainage area (table 1) since 2009. Streamflow also was continuously measured at four streamgages from 2009 to 2014 and periodically measured at nine additional streamgages on tributaries in the drainage area. In October 2020, three streamgages began or resumed collection of continuous streamflow data (table 1). At 11 streamgages, daily mean streamflow was estimated for the period between October 1, 2021, and September 30, 2022, by using methods developed by the USGS (table 1; Hirsch, 1982). The USGS also has been continuously measuring specific conductance at 14 monitoring stations since 2009 and at two additional monitoring stations since 2020 (table 1). Equations that relate specific conductance to concentrations of chloride and sodium in stream water were developed as part of previous cooperative studies of the USGS and PW (Nimiroski and Waldron, 2002; Smith, 2015b, 2018a, 2022a; Spaetzel and Smith, 2022a). These equations, used together with measured (or estimated) streamflows, allow for nearly continuous estimation of chloride and sodium loads to the reservoir.

Table 1.    

Providence Water water-quality sampling stations and corresponding U.S. Geological Survey streamgages in the Scituate Reservoir drainage area, Rhode Island, and data collection and monitoring statistics from October 1, 2021, to September 30, 2022.

[Data are from Smith and Spaetzel (2021) and U.S. Geological Survey (2024). Alternate station names given for stations where different historical names were used for the same sampling location by Providence Water (PW). Locations of stations are shown on figure 1. USGS, U.S. Geological Survey; mi2, square mile; WQ, water quality; Na, sodium; Cl, chloride; M, monthly; Q, quarterly; Y, yes; N, no; —, not applicable]

PW station number USGS station number USGS station short name Alternate name Drainage area (mi2) Frequency of WQ sample collection by PW Number of samples collected by PW1 Daily estimated Na and Cl loads Streamflow data availability Specific conductance data availability
24 01115190 Dolly Cole Brook 4.9 M 11 Y Continuous Continuous
25 01115200 Shippee Brook 2.37 Q 2 N Estimated None
26 01115185 Winsor Brook 4.33 Q 4 Y Continuous2 Continuous2
27 011151845 Unnamed tributary to Ponaganset River Unnamed brook B, unnamed brook west of Winsor Brook 0.10 Q 3 N None None
28 01115265 Hemlock Brook 8.72 M 12 Y Continuous Continuous
29 01115271 Ponaganset River Barden Stream 33.0 M 11 N None None
35 01115187 Ponaganset River 14.0 M 11 Y Continuous Continuous
1 01115180 Brandy Brook 1.57 M 9 N Estimated None
2 01115181 Unnamed tributary 2 to Scituate Reservoir Unnamed brook north of Bullhead Brook 0.22 Q 2 N None None
3 01115280 Cork Brook 1.87 M 9 Y Continuous Continuous
4 01115400 Kent Brook Betty Pond Stream 0.85 M 12 N Estimated None
5 01115184 Spruce Brook 1.26 Q 3 Y Continuous2 Continuous
6 01115183 Quonopaug Brook 1.96 M 10 Y Continuous Continuous
7 01115297 Wilbur Hollow Brook 4.33 M 10 Y Estimated Continuous
8 01115276 Westconnaug Brook Westconnaug Reservoir 5.18 M 12 Y Continuous Continuous
9 01115275 Bear Tree Brook 0.62 Q 3 Y Estimated Continuous
30 01115350 Coventry Brook 0.79 Q 2 N None None
31 01115177 Toad Pond 0.03 Q 0 N None None
32 01115178 Pine Swamp Brook 0.45 Q 3 N Estimated None
33 01115182 Halls Estate Brook 0.28 Q 2 N Estimated None
36 Not available Outflow from King Pond 0.76 Q 3 N None None
37 Not available Fire tower stream 0.03 Q 3 N None None
38 01115278 Swamp Brook 1.92 Q 2 Y Continuous2 Continuous2
19 01115170 Moswansicut Stream 3.25 M 11 Y Continuous Continuous
20 01115160 Blanchard Brook 1.18 M 8 N None None
21 01115165 Unnamed tributary 2 to Moswansicut Pond reservoir Brook from Kimball Reservoir 0.30 Q 4 N Estimated None
22 01115167 Unnamed tributary 3 to Moswansicut Pond reservoir 0.10 M 7 N None None
34 01115164 Unnamed tributary from Kimball Reservoir Kimball Stream 0.27 Q 4 N None None
23 011151843 Ponaganset Reservoir 1.92 M 11 N None None
13 01115176 Regulating reservoir 22.1 M 10 N None None
14 01115110 Huntinghouse Brook 6.29 M 11 Y Continuous Continuous
15 01115114 Rush Brook 4.70 M 11 Y Continuous Continuous
16 01115098 Peeptoad Brook Harrisdale Brook 4.97 M 9 Y Continuous Continuous
17 01115119 Dexter Pond Paine Pond 0.22 Q 2 N None None
18 01115120 Unnamed tributary to Regulating reservoir Unnamed brook A 0.28 Q 1 Y Estimated Continuous
10 01115274 Westconnaug Brook 1.48 M 10 N Estimated None
11 01115273 Unnamed tributary to Westconnaug Reservoir Unnamed brook south of Westconnaug Reservoir 0.72 Q 3 N Estimated None
12 011152745 Unnamed tributary to Westconnaug Brook Unnamed brook north of Westconnaug Reservoir 0.16 Q 3 N None None
Table 1.    Providence Water water-quality sampling stations and corresponding U.S. Geological Survey streamgages in the Scituate Reservoir drainage area, Rhode Island, and data collection and monitoring statistics from October 1, 2021, to September 30, 2022.
1

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

2

Continuous monitoring began or resumed in October 2020.

In 2022, PW regularly, either monthly or quarterly, visited fixed sites on 38 tributaries within the Scituate Reservoir drainage area and collected water-quality samples; however, no samples were collected at the Toad Pond (PW station 31; USGS station 01115177) site because it was routinely dry. Therefore, a total of 37 sites were sampled by PW between October 1, 2021, and September 30, 2022, of which, eight were sampled fewer than three times. Compiled and tabulated streamflow (measured or estimated by the USGS) and water-quality data (collected by PW) have been published in Breault and others (2000), Nimiroski and others (2008), Breault (2010), Breault and Campbell (2010a–d), Breault and Smith (2010), Smith and Breault (2011), Smith (2013, 2014, 2015ab, 2016, 2018a–d, 2019ab, 2022ab, 2024), Smith and Spaetzel (2021, 2024), and Spaetzel and Smith (2022ab).

This report presents data on streamflow, water quality, and loads and yields of selected constituents for water year (WY) 2022 in the Scituate Reservoir drainage area. Data were collected in past studies by the USGS, in cooperation with PW and the Rhode Island Department of Environmental Management, and discrete sample data provided by PW (Smith and Spaetzel, 2021). This report summarizes measured and estimated streamflows presented for the 13 continuous-record and 11 partial-record streamgages (called estimated in table 1) in the drainage area. Estimated monthly and annual loads and yields of chloride and sodium are presented for the 16 streamgages at which specific conductance is continuously monitored by the USGS. Summary statistics for water-quality data collected by PW for 37 sampling stations (table 1) during WY 2022 also are presented. These data were used to calculate loads and yields of selected water-quality constituents where flow data were available. Water-quality data related to the Scituate Reservoir drainage area have been published serially by the USGS since 2000 (Breault and others, 2000). The presentation and content of this report has been replicated from Breault (2010), with annually updated methods, data, and interpretations (Breault and Campbell, 2010a–d; Breault and Smith, 2010; Smith and Breault, 2011; Smith 2013, 2014, 2015a, 2016, 2018ab, 2019a, 2022a; Smith and Spaetzel, 2024).

Streamflow Data Collection and Estimation

Streamflow was measured or estimated by the USGS at 24 streamgages (table 1). Measured and estimated streamflows are necessary to estimate water volume and water-quality constituent loads and yields from tributary basins. Stream gage height was measured every 10 minutes at most continuous-record streamgages. Streamflow was computed with a gage height to discharge relation (known as a rating), which was developed on the basis of periodic manual measurements of streamflow. Daily mean streamflow at a streamgage was calculated by dividing the total volume of water that passed the streamgage each day by 86,400 (the number of seconds in a day). Periodic manual streamflow measurements at partial-record streamgages were used concurrently with continuous-record measurements from streamgages in nearby hydrologically similar drainage areas to estimate a continuous daily record at the partial-record streamgages. Specifically, daily streamflow records for the 11 partial-record sites in the Scituate Reservoir drainage area (table 1) were estimated by using the Maintenance of Variance Extension type 1 (MOVE.1) method, as described by Ries and Friesz (2000), Smith (2015b), and Spaetzel and Smith (2022a); data needed to estimate streamflows at partial-record sites were retrieved from the USGS National Water Information System (NWIS; USGS, 2024). The upper and lower 90-percent confidence limits for the estimated mean annual streamflows, as described by Tasker and Driver (1988), are listed in table 2. These USGS data indicate that there is a 90-percent chance that the estimated mean annual streamflow is between the upper and lower 90-percent confidence limits.

Table 2.    

Measured or estimated annual mean streamflow for tributary streams in the Scituate Reservoir drainage area, Rhode Island, from October 1, 2021, through September 30, 2022.

[Data were collected by the U.S. Geological Survey (USGS; USGS, 2024). Locations of stations are shown on figure 1. PW, Providence Water; ft3/s, cubic foot per second; (ft3/s)/mi2, cubic foot per second per square mile]

PW station number USGS station number Annual mean streamflow (ft3/s) Upper 90-percent confidence interval (ft3/s) Lower 90-percent confidence interval (ft3/s) Annual mean streamflow yield (ft3/s/mi2)
24 01115190 10.7 11.9 9.50 2.18
25 01115200 7.39 24.1 2.27 3.12
26 01115185 7.85 8.72 6.97 1.81
28 01115265 16.5 18.4 14.5 1.89
35 01115187 28.0 31.1 25.2 2.01
1 01115180 2.94 6.23 1.39 1.87
3 01115280 3.42 3.86 2.98 1.83
4 01115400 1.54 6.94 0.34 1.81
5 01115184 2.10 2.30 1.89 1.66
6 01115183 3.55 3.95 3.16 1.81
7 01115297 7.39 15.2 3.59 1.71
8 01115276 8.06 8.58 7.53 1.56
9 01115275 1.42 2.43 0.83 2.29
32 01115178 0.61 1.14 0.33 1.36
33 01115182 0.31 0.71 0.13 1.10
38 01115278 3.27 3.65 2.88 1.70
19 01115170 5.32 5.74 4.90 1.64
21 01115165 0.56 1.19 0.26 1.86
14 01115110 10.1 11.4 8.88 1.61
15 01115114 8.23 9.40 7.06 1.75
16 01115098 7.92 8.73 7.10 1.59
18 01115120 0.49 2.14 0.11 1.75
10 01115274 3.42 8.19 1.43 2.31
11 01115273 2.00 5.29 0.75 2.77
Table 2.    Measured or estimated annual mean streamflow for tributary streams in the Scituate Reservoir drainage area, Rhode Island, from October 1, 2021, through September 30, 2022.

Continuous-record streamgages were operated and maintained by the USGS during WY 2022 (fig. 1; table 1). Streamflow data for these streamgages were collected at 10- or 15-minute intervals (near-real-time streamflow data), were updated at 1-hour intervals on the internet and are available through NWIS (USGS, 2024). Error associated with measured streamflows was generally within about 15 percent, as noted in the annual water year summary for each USGS streamgage (USGS, 2024).

Water-Quality Data Collection and Analysis

Water-quality data were collected by the USGS and PW. Concentrations of sodium and chloride were estimated by the USGS from continuous records of specific conductance from 16 of the 24 streamgages. Water-quality samples were collected monthly or quarterly at 37 sampling stations in the Scituate Reservoir drainage area by PW during WY 2022 as part of a long-term sampling program table 1).

Data Collected by the U.S. Geological Survey

Three or more water-quality samples were collected by the USGS at each of the 16 streamgages equipped with continuous specific conductance monitors in the Scituate Reservoir drainage area during WY 2022 (table 1), except from the unnamed tributary to Regulating reservoir (PW station 18; USGS station 01115120) and Cork Brook (PW station 3; USGS station 01115280), where samples could not be collected during the summer because the streambeds were dry. Samples were collected in the centroid of the streams during fall, winter, and summer. Water samples were processed in the USGS New England Water Science Center laboratory in Northborough, Massachusetts, at the conclusion of scheduled sampling. After processing, the samples were packed in ice and shipped overnight to the USGS National Water Quality Laboratory in Lakewood, Colorado. Analytical results for the USGS water-quality samples are available through NWIS (USGS, 2024); these include specific conductance and dissolved concentrations of sodium and chloride.

The USGS collected and analyzed continuous-record specific conductance data at 16 streamgages (fig. 1; table 1). Measurements of specific conductance were recorded automatically at 10- or 15-minute intervals at each streamgage. Measurements were made by using an instream probe and standard USGS methods for continuous water-quality monitoring at streams (Wagner and others, 2006). The specific conductance measurement data are available through the NWIS web interface (USGS, 2024).

Concentrations of chloride and sodium were estimated from continuous measurements of specific conductance by using equations that were developed by the USGS to relate specific conductance to concentrations of chloride and sodium, as follows:

CCl=SPCm×b
and
(1)
CNa=SPCm×b
,
(2)
where

CCl

is the chloride concentration, in milligrams per liter;

CNa

is the sodium concentration, in milligrams per liter;

SPC

is the specific conductance, in microsiemens per centimeter;

m

is the slope from the MOVE.1 analysis (table 3); and

b

is the intercept from the MOVE.1 analysis (table 3).

These regression equations were developed by using the MOVE.1 method (also known as the line of organic correlation; Helsel and others, 2020) on the basis of concurrent measurements of specific conductance (USGS observed property “Specific conductance, water, unfiltered, normalized to 25 degrees Celsius, laboratory,” formerly parameter code 90095) along with chloride (USGS observed property “Chloride, water filtered,” formerly parameter code 00940) and sodium (USGS observed property “Sodium, water filtered,” formerly parameter code 00930) concentrations measured in water-quality samples collected by the USGS from tributaries in the Scituate Reservoir drainage area during WY 2000, WY 2005, and WYs 2009–23 (table 3; USGS, 2024).

Table 3.    

Regression equation coefficients used to estimate concentrations of chloride and sodium from values of specific conductance for U.S. Geological Survey streamgage stations in the Scituate Reservoir drainage area, Rhode Island, from October 1, 2021, through September 30, 2022.

[Data were collected by the U.S. Geological Survey (USGS; USGS, 2024). Locations of stations are shown on figure 1. Constituent concentrations, continuous specific conductance, and parameter codes are available in National Water Information System (USGS, 2024). USGS parameter codes: specific conductance, 90095; chloride, 00940; sodium, 00930. USGS observed properties: “Chloride, water filtered” with units of mg/L; “Sodium, water filtered” with units of mg/L; “Specific conductance, water, unfiltered, normalized to 25 degrees Celsius, laboratory,” with units of µS/cm. PW, Providence Water; ≥, greater than or equal to; f3/s, cubic foot per second]

PW station number USGS station number Samples used in analyses Chloride Sodium Percentage of specific conductance record unavailable where streamflow is ≥0.01 ft3/s
Sample date range Sample count Slope Intercept Standard error of regressions (percent) Slope Intercept Standard error of regressions (percent)
24 01115190 03/08/2000; 03/29/2005; 01/22/2009–08/22/2023 44 1.1657 0.10659 3.2 1.1018 0.08995 5.0 9.0
26 01115185 01/22/2020–08/22/2023 12 1.1063 0.14054 6.3 0.9185 0.21066 2.9 0.3
28 01115265 03/28/2001; 03/30/2005; 01/22/2009–08/22/2023 44 1.1437 0.11781 3.8 1.0533 0.11041 5.3 1.3
35 01115187 03/28/2001; 03/29/2005; 01/22/2009–08/21/2023 44 1.1723 0.10101 4.0 1.0951 0.08976 5.2 2.8
3 01115280 03/08/2000; 03/30/2005; 01/22/2009–08/21/2023 42 1.2008 0.08650 3.1 1.0875 0.09106 4.8 1.9
5 01115184 03/05/2009–08/22/2023 41 1.2314 0.07031 3.8 1.0848 0.08551 4.8 2.6
6 01115183 03/08/2000; 03/30/2005; 01/22/2009–08/22/2023 55 1.1793 0.08437 4.6 1.1894 0.04825 6.2 3.2
7 01115297 03/28/2001; 03/30/2005; 01/22/2009–08/21/2023 43 1.0316 0.14569 4.4 0.8601 0.20013 5.3 19
8 01115276 01/22/2009–08/22/2023 41 1.0871 0.14598 2.6 1.0286 0.12185 3.5 0.27
9 01115275 03/08/2000; 03/30/2005; 01/22/2009–08/22/2023 43 1.0548 0.18169 2.2 1.0818 0.09317 3.0 1.4
38 01115278 01/22/2020–08/23/2023 12 1.4687 0.02243 11.2 1.2124 0.04999 7.6 0.00
19 01115170 03/08/2000; 03/29/2005; 01/22/2009–08/21/2023 50 1.2118 0.07599 2.3 1.2127 0.04507 2.6 2.1
14 01115110 03/28/2001; 03/29/2005; 01/22/2009–08/22/2023 51 0.9935 0.18278 7.2 0.9335 0.14700 7.6 3.3
15 01115114 01/22/2009–08/22/2023 58 1.1372 0.11558 3.7 1.0679 0.10135 5.2 20
16 01115098 03/28/2001; 03/29/2005; 01/22/2009–08/22/2023 44 1.2439 0.06412 4.0 1.06100 0.09808 6.0 1.1
18 01115120 01/22/2009–08/21/2023 34 1.1610 0.09934 2.6 1.1454 0.06242 3.2 13
Table 3.    Regression equation coefficients used to estimate concentrations of chloride and sodium from values of specific conductance for U.S. Geological Survey streamgage stations in the Scituate Reservoir drainage area, Rhode Island, from October 1, 2021, through September 30, 2022.

MOVE.1 was chosen for regression analysis to maintain variance (Hirsch and Gilroy, 1984). Under some circumstances, specific conductance records were unavailable, possibly because of the following reasons: a sensor malfunctioned; was affected by debris, fouling, or ice; or was not submerged during low flow. In these cases, values of specific conductance were estimated by proportional distribution between recorded values. In general, the period of specific conductance record when streamflow occurred that was unavailable for each USGS station represents a small fraction of the record period for WY 2022 (table 3).

Data Collected by Providence Water

Water-quality samples were collected by PW at 37 fixed stations on tributaries draining to the Scituate Reservoir during WY 2022. Samples were scheduled to be collected monthly at 19 stations and quarterly at another 18 stations (table 1). Water-quality samples were not collected according to specific weather conditions; instead, a periodic water-quality sampling schedule was followed so that water-quality samples would be representative of a variety of weather conditions. However, sometimes samples could not be collected because tributaries at the sampling stations were dry or frozen. When possible, water-quality samples were collected by dipping the sample bottle into the tributary at the center of flow (Richard Blodgett, PW, written commun., 2005). Samples were transported on ice to the PW water-quality laboratory at the P.J. Holton Water Purification Plant in Scituate, R.I. Water-quality properties and constituent concentrations were measured by using unfiltered water samples. These water-quality properties included pH, color, turbidity, and alkalinity; concentrations of chloride, nitrite, nitrate, orthophosphate; and Escherichia coli (E. coli) and total coliform bacteria. These data, collected by PW, are published in Smith and Spaetzel (2021). In this report, orthophosphate is the name for compounds with only one phosphate (PO4) unit, whereas phosphate is used to name any compound having one or more PO4 units. Analytical methods used to determine the values or concentrations of pH, color, turbidity, alkalinity, and chloride are documented by Baird and others (2018). Concentrations of nitrite were determined by U.S. Environmental Protection Agency method 353.2 (U.S. Environmental Protection Agency, 1993). Concentrations of nitrate were determined by Standard Method 4500–NO3 (Holm and others, 2018). Concentrations of orthophosphate were determined by the Hach PhosVer Method (Hach Method 8048; Hach Company, 2000). Standard Method 9223 was used for the determination of concentrations of bacteria (Best and others, 2018).

Water-quality samples were collected by PW during a wide range of flow conditions. During WY 2022, the measured or estimated daily mean flow-duration curves for the USGS streamgages at Dolly Cole Brook (PW station 24; USGS station 01115190) and the unnamed tributary 2 to Moswansicut Pond reservoir (PW station 21; USGS station 01115165) are shown in figure 2. The curves represent the percentage of time that each flow was equaled or exceeded at the respective stations; the flows at each station on days when water-quality samples were collected are represented by the plotted points superimposed on the curves. At Dolly Cole Brook, the 11 monthly samples were collected at flow rates that were exceeded between 9.7 and 91 percent of the time; this range indicates that the water-quality samples collected in WY 2022 represent a large range of the flow conditions representing flows between 0.38 and 28 ft3/s. Samples collected on a quarterly schedule at the unnamed tributary 2 to Moswansicut Reservoir were collected at flow rates that were exceeded between 15 and 74 percent of the time; this range of flow rates excludes the flow conditions for both the lower and upper flow range at that station during WY 2022 (fig. 2).

Eleven samples were collected from Dolly Cole Brook and four from the unnamed tributary
                        2 to Moswansicut Pond reservoir.
Figure 2.

Graph showing flow-duration curves and streamflow on the dates (represented by points) when water-quality samples were collected by Providence Water at the U.S. Geological Survey (USGS) streamgages Dolly Cole Brook (station 01115190) in South Foster, Rhode Island, and the unnamed tributary 2 to Moswansicut Pond reservoir (station 01115165) in North Scituate, Rhode Island, from October 1, 2021, through September 30, 2022. Station information is shown in table 1. Modified from Breault (2010). Data are from Smith and Spaetzel (2021) and USGS (2024).

Estimating Daily, Monthly, and Annual Loads and Yields

Daily, monthly, and annual chloride and sodium loads (in kilograms) were estimated for all streamgages for which continuous streamflow and specific-conductance data were available for WY 2022. Daily flow-weighted concentrations of chloride and sodium were calculated by multiplying instantaneous flows by concurrent concentrations of chloride and sodium (estimated from measurements of specific conductance) for each day and dividing the sum by the total flow for that day. At the three instrumented monitoring stations, where continuous flow was unavailable (table 1), daily mean concentrations of chloride and sodium were calculated from the daily mean value of specific conductance for each day. The latter method may result in less accurate concentrations because instantaneous measurements of specific conductance may change (decrease or increase) with surface-water runoff; however, the variability of instantaneous measurements of specific conductance at these streamgages was generally small and daily mean values did not differ substantially from daily flow-weighted values estimated during prior water years when instantaneous flow data were available. Daily loads of chloride and sodium were estimated by multiplying daily flow-weighted concentrations of chloride and sodium (in milligrams per liter) by daily discharge (in liters per day). Daily data were summed to estimate monthly or annual loads (converted to metric tons).

Daily loads of water-quality constituents (in samples collected by PW) were calculated for all sampling dates during WY 2022 (table 4) for which periodic- or continuous-streamflow data were available (table 1). These loads were calculated by multiplying constituent concentrations (in milligrams per liter or colony forming units per liter) in individual samples by the daily discharge (in liters per day) for the day on which each sample was collected. The flows, which in some cases were estimates, were assumed to be representative of the flow at the time of the sample collection. Loads (in million colony forming units per day, kilograms per day, or grams per day) and yields (in million colony forming units per day per square mile, kilograms per day per square mile, or grams per day per square mile) were calculated for bacteria, chloride, nitrite, nitrate, and orthophosphate. Censored data (concentrations reported as less than method detection limits) were replaced with concentrations equal to one-half the method detection limit.

Table 4.    

Daily loads of bacteria, chloride, nitrite, nitrate, and orthophosphate in the Scituate Reservoir drainage area, Rhode Island, from October 1, 2021, through September 30, 2022.

[Data from Smith and Spaetzel (2021). Water-quality data are from samples collected and analyzed by Providence Water (PW). Locations of stations shown on figure 1. USGS, U.S. Geological Survey; ft3/s, cubic foot per second; CFU×106/d; millions of colony forming units per day; E. coli, Escherichia coli; kg/d, kilogram per day; g/d as N, gram per day as nitrogen; g/d as PO4, gram per day as phosphate <, less than; >, greater than; NA, not available]

PW station number USGS station number Date Daily mean streamflow (ft3/s) Total coliform bacteria (CFUx106/d) E. coli (CFUx106/d) Chloride (kg/d) Nitrite (g/d as N) Nitrate (g/d as N) Orthophosphate (g/d as PO4)
24 01115190 10/15/21 3.48 1,200,000 13,000 190 17 560 850
11/23/21 12.1 160,000 <3,000 570 59 <1,500 6,500
01/03/22 11.0 130,000 2,700 600 27 1,500 3,200
02/17/22 28.2 330,000 <6,900 1,500 69 7,400 5,500
03/04/22 25.1 140,000 <6,100 1,500 61 4,600 4,300
04/12/22 13.4 850,000 <3,300 760 33 <1,600 1,300
05/26/22 1.32 38,000 3,900 88 6.5 230 32
06/03/22 1.19 100,000 3,700 79 5.8 260 87
07/26/22 0.38 36,000 2,000 25 1.9 140 93
08/25/22 0.62 100,000 5,700 42 1.5 290 46
09/23/22 3.21 410,000 26,000 180 7.9 500 240
25 01115200 10/25/21 1.59 65,000 1,600 51 7.8 230 430
04/28/22 6.01 110,000 2,900 170 15 <740 2,200
26 01115185 10/25/21 3.54 98,000 4,300 260 8.7 480 430
01/28/22 3.61 21,000 880 220 8.8 870 350
04/28/22 5.96 62,000 1,500 400 15 <730 1,600
08/30/22 0.07 6,600 660 5.6 0.17 29 8.6
28 01115265 10/05/21 42.1 7,500,000 840,000 2,800 410 5,800 7,200
11/04/21 19.3 260,000 4,700 910 140 2,500 4,700
12/03/21 8.95 160,000 20,000 470 44 1,200 2,600
01/18/22 33.6 1,000,000 99,000 1,400 160 <4,100 3,300
02/08/22 98.9 4,000,000 150,000 5,300 480 17,000 17,000
03/03/22 30.9 110,000 30,000 1,900 76 3,900 6,000
04/05/22 17.1 170,000 13,000 1,100 42 <2,100 1,300
05/12/22 7.32 180,000 1,800 470 36 <900 1,100
06/09/22 9.19 1,400,000 130,000 720 67 1,500 670
07/11/22 0.56 7,500 <140 46 4.1 230 55
08/02/22 0.06 1,600 29 5.1 0.15 43 8.8
09/08/22 21.5 3,400,000 440,000 1,300 160 <2,600 2,100
35 01115187 10/15/21 9.28 3,200,000 120,000 510 45 1,400 1,600
11/23/21 42.7 470,000 10,000 1,800 100 <5,200 11,000
01/03/22 37.7 350,000 <9,200 1,900 92 5,800 9,200
02/17/22 62.9 1,300,000 15,000 2,700 150 17,000 7,700
03/04/22 53.6 260,000 26,000 2,600 130 12,000 7,900
04/12/22 39.6 1,400,000 19,000 1,900 97 <4,800 2,900
05/26/22 5.01 300,000 16,000 280 25 960 370
06/03/22 3.19 250,000 11,000 170 23 1,300 310
07/26/22 0.53 50,000 1,100 34 2.6 170 65
08/25/22 0.33 50,000 3,300 14 1.6 100 32
09/23/22 38.5 9,900,000 1,000,000 2,000 190 6,200 940
1 01115180 10/01/21 1.14 9,200 <280 33 5.6 790 250
11/01/21 7.13 240,000 1,700 260 70 1,400 1,700
12/02/21 2.71 49,000 4,000 92 20 990 660
01/06/22 4.13 45,000 1,000 140 40 2,200 1,200
03/18/22 3.53 59,000 <860 130 17 1,000 350
06/10/22 2.73 310,000 9,400 96 20 510 200
07/15/22 0.64 NA 470 19 3.1 310 94
08/29/22 0.12 7,300 380 3.7 0.29 95 26
09/09/22 1.27 190,000 3,800 35 6.2 380 160
3 01115280 10/07/21 3.28 87,000 1,600 340 8.0 1,200 480
11/02/21 5.55 160,000 4,100 450 27 <680 810
12/07/21 4.78 86,000 1,200 400 23 1,200 1,100
01/04/22 3.07 27,000 <750 290 7.5 1,300 380
02/03/22 3.4 21,000 <830 290 8.3 2,300 830
03/28/22 2.58 23,000 1,300 270 6.3 750 440
04/15/22 3.37 48,000 3,300 320 8.2 830 490
05/16/22 1.49 28,000 360 140 3.6 730 290
06/07/22 0.52 28,000 <130 48 1.3 420 38
4 01115400 10/01/21 0.18 3,400 88 2.9 0.44 <22 92
11/01/21 3.93 150,000 960 93 9.6 540 1,700
12/02/21 0.54 4,200 <130 8.2 1.3 66 170
01/06/22 1.26 13,000 <310 22 3.1 <150 340
02/18/22 32.7 240,000 <8,000 420 80 <4,000 3,200
03/07/22 6.64 39,000 <1,600 83 16 <810 810
04/01/22 4.02 NA <980 57 9.8 <490 300
05/06/22 1.59 85,000 390 26 3.9 <190 120
06/10/22 0.47 79,000 230 11 1.1 <57 11
07/15/22 0.04 NA 88 0.53 0.20 <4.9 2.9
08/29/22 0.02 2,800 <4.9 0.27 0.049 2.8 2.0
09/09/22 0.16 5,900 <39 2.3 0.39 24 16
5 01115184 10/29/21 1.47 55,000 360 72 11 390 220
03/18/22 3.12 87,000 760 150 7.6 2,200 530
04/22/22 3.35 91,000 <820 150 16 1,400 330
6 01115183 10/01/21 0.59 18,000 1,000 15 4.3 180 250
11/01/21 9.97 250,000 4,900 720 120 <1,200 3,200
12/02/21 2.16 21,000 2,100 160 11 1,200 530
01/06/22 4.03 39,000 4,900 340 20 2,300 690
02/18/22 18.2 1,300,000 27,000 1,200 130 7,700 2,200
03/07/22 9.58 26,000 2,300 840 47 5,800 1,200
04/01/22 7.01 NA 27,000 560 34 3,200 510
05/06/22 3.38 79,000 6,600 260 17 1,000 330
06/10/22 4.50 510,000 45,000 180 66 1,400 770
09/09/22 3.35 860,000 67,000 250 16 1,200 250
7 01115297 10/07/21 5.99 430,000 16,000 170 44 1,100 880
11/02/21 11.7 440,000 42,000 250 110 <1,400 2,000
12/07/21 8.75 96,000 6,400 300 43 2,000 1,500
01/04/22 4.61 66,000 4,600 120 23 1,500 790
02/03/22 4.66 19,000 <1,100 160 11 2,500 1,100
03/28/22 7.52 140,000 1,800 180 37 1,200 1,100
04/15/22 11.8 410,000 5,800 290 58 <1,400 1,400
05/16/22 6.99 500,000 120,000 290 51 1,000 860
06/07/22 1.87 110,000 3,400 56 9.2 <230 140
07/01/22 1.07 75,000 3,400 25 7.9 180 180
8 01115276 10/19/21 4.05 76,000 <990 120 9.9 640 790
11/16/21 11.7 140,000 <2,900 350 29 <1,400 4,000
12/21/21 6.64 6,700 <1,600 300 32 1,600 320
01/21/22 11.2 8,200 <2,700 360 27 <1,400 1,400
02/22/22 20.7 32,000 <5,100 720 51 <2,500 1,500
03/08/22 17.3 13,000 <4,200 660 42 <2,100 2,500
04/18/22 10.5 25,000 <2,600 410 26 <1,300 510
05/20/22 5.78 34,000 <1,400 200 14 <710 710
06/30/22 2.58 270,000 <630 84 6.3 <320 440
07/28/22 1.74 83,000 <430 60 4.3 <210 85
08/10/22 1.41 33,000 <340 50 3.4 <170 140
09/30/22 2.69 120,000 660 93 6.6 <330 200
9 01115275 03/18/22 2.63 55,000 <640 320 6.4 2,500 320
04/22/22 2.99 83,000 <730 380 7.3 230 220
08/19/22 0.23 10,000 56 41 0.56 420 73
32 01115178 10/22/21 0.36 11,000 440 13 1.8 76 35
02/24/22 1.50 22,000 <370 47 7.3 1,400 37
06/24/22 0.125 7,600 230 4.3 0.92 160 40
33 01115182 11/29/21 0.14 990 <34 5.3 0.34 38 27
06/16/22 0.05 1,200 <12 2.5 0.24 30 15
38 01115278 02/24/22 8.06 40,000 <2,000 910 20 <990 590
09/30/22 0.52 7,200 <130 37 <1.3 <64 51
19 01115170 10/18/21 2.03 3,300,000 5,100 270 5 370 150
11/15/21 8.92 44,000 6,500 2,300 44 1,500 2,000
12/14/21 5.21 3,800 1,300 670 25 1,900 250
01/13/22 4.16 3,100 2,000 510 20 1,200 710
03/22/22 6.25 <1,500 <1,500 800 31 2,400 460
04/14/22 8.21 8,000 <2,000 1,100 40 3,000 1,200
05/23/22 2.48 70,000 <610 320 12 540 120
06/17/22 0.96 180,000 190 130 4.7 <120 190
07/25/22 0.16 11,000 <39 22 0.39 20 7.8
08/18/22 0.15 5,100 <37 21 0.37 <18 33
09/26/22 2.48 170,000 <610 320 6.1 410 180
21 01115165 10/21/21 0.35 15,000 340 35 1.7 170 34
03/11/22 0.94 9,700 230 81 6.9 1,100 160
05/27/22 0.26 15,000 380 33 3.8 370 38
09/15/22 0.35 37,000 1,500 37 1.7 240 94
14 01115110 10/04/21 12.2 >7,200,000 180,000 430 90 4,500 2,100
11/10/21 5.94 120,000 16,000 210 15 <730 1,000
12/17/21 8.85 130,000 6,500 340 22 1,800 1,900
01/10/22 17.1 200,000 8,400 780 42 4,300 4,200
02/11/22 32.3 760,000 16,000 1,100 79 8,200 7,100
03/29/22 10.4 79,000 <2,500 340 25 1,900 2,300
04/07/22 23.0 460,000 34,000 920 56 3,100 2,800
05/13/22 4.93 130,000 4,800 150 12 600 1,100
06/17/22 0.90 57,000 1,400 30 8.8 490 130
07/14/22 0.16 18,000 3,600 4.3 0.78 72 20
09/23/22 4.38 1,900,000 140,000 200 21 1,200 320
15 01115114 10/04/21 13.9 >8,200,000 200,000 1,600 240 5,100 3,400
11/10/21 3.83 54,000 5,600 370 9.4 <470 840
12/17/21 7.46 110,000 5,500 650 18 <910 1,100
01/10/22 14.5 200,000 11,000 1,700 35 3,200 2,500
02/11/22 20.9 190,000 15,000 1,800 51 6,500 4,600
03/29/22 6.82 25,000 <1,700 800 17 1,500 1,200
04/07/22 16.7 120,000 8,200 1,800 41 22,000 1,200
05/13/22 2.19 110,000 4,600 270 5.4 <270 210
06/17/22 0.50 1,600 98 59 2.4 240 110
07/14/22 0.14 25,000 830 29 0.69 24 31
09/23/22 5.94 1,700,000 210,000 630 29 1,600 440
16 01115098 10/04/21 11.3 480,000 5,500 1,000 55 2,100 1,700
11/10/21 15.8 220,000 12,000 1,500 39 3,100 1,500
12/17/21 7.19 58,000 1,800 920 35 2,300 350
01/10/22 7.66 36,000 1,900 710 37 3,200 1,300
02/11/22 8.42 110,000 8,200 790 21 4,200 1,900
03/29/22 11.6 88,000 <2,800 1,200 28 3,300 2,000
04/07/22 16.7 290,000 12,000 1,700 41 4,500 1,600
05/13/22 4.83 250,000 1,200 460 12 <590 350
06/17/22 1.00 320,000 49 100 4.9 <120 98
18 01115120 03/11/22 0.91 6,100 1,300 130 4.5 620 200
10 01115274 10/05/21 11.2 1,200,000 79,000 820 55 1,400 2,500
11/04/21 3.02 73,000 3,000 190 7.4 <370 440
12/03/21 1.99 48,000 <490 130 4.9 <240 390
01/18/22 11.9 370,000 71,000 650 29 <1,500 4,400
02/08/22 15.1 430,000 11,000 940 37 2,000 1,800
03/03/22 7.54 85,000 3,700 500 18 <920 1,300
04/05/22 3.47 180,000 <850 240 8.5 <420 510
05/12/22 1.39 110,000 <340 95 <3.4 <170 170
06/09/22 2.58 820,000 62,000 190 6.3 <320 320
09/08/22 1.76 300,000 14,000 110 4.3 <220 220
11 01115273 03/10/22 3.82 42,000 <930 56 9.3 <470 750
05/31/22 0.25 14,000 120 4.3 2.4 <31 49
07/29/22 0.02 2,500 25 0.48 0.24 3.3 6.4
Table 4.    Daily loads of bacteria, chloride, nitrite, nitrate, and orthophosphate in the Scituate Reservoir drainage area, Rhode Island, from October 1, 2021, through September 30, 2022.

Streamflow

Monitoring streamflow is a necessary step for computing the volume of water and estimating constituent loads to the Scituate Reservoir. The Ponaganset River is the largest monitored tributary to the Scituate Reservoir. Mean annual streamflow at the streamgage on the Ponaganset River (PW station 35; USGS station 01115187) for the entire period of its operation (mean of the annual mean streamflows for the period of record, WYs 1995–2021) before WY 2022 was 29.1 cubic feet per second (ft3/s; USGS, 2024). During WY 2022, the annual mean streamflow of 28.0 ft3/s was just below the median for the period of record (fig. 3). Daily mean streamflows were commonly within the 10th and 90th percentile of all mean daily streamflows for WYs 1995–2021; however, daily streamflows from the fall, early winter, and late summer exceeded the 90th percentile on many occasions. Daily mean streamflows in WY 2022 fell below the 10th percentile values for periods from May through August (fig. 3). The other long-term continuous-record streamgage in the Scituate Reservoir drainage area is the Peeptoad Brook streamgage (PW station 16; USGS station 01115098). The mean annual streamflow at the Peeptoad Brook streamgage for the period of record (WYs 1995–2021), before WY 2022, was 10.6 ft3/s (USGS, 2024). The annual mean streamflow at the Peeptoad Brook streamgage during WY 2022 (7.9 ft3/s) also was lower than the median annual streamflow for its period of record (11 ft3/s). The annual mean measured or estimated streamflows for the other monitoring stations in this study ranged from about 0.31 to 16.5 ft3/s (table 2).

Some daily streamflows were clearly above the 90th percentile in October, November,
                     February, and September and below the 10th percentile in May, June, and August. Annual
                     mean streamflow for 2022 was below the median.
Figure 3.

Hydrologic data taken at the Providence Water station 35 (U.S. Geological Survey [USGS] station 01115187 on the Ponaganset River in South Foster, Rhode Island); A, Graph showing measured daily mean streamflow for water year 2022 (October 1, 2021, through September 30, 2022), and the 10th percentile, median, and 90th percentile values of daily streamflow from October 1, 1994, through September 30, 2021; and B, Boxplot showing annual mean streamflow during water year 2022 and the distribution of mean annual streamflows for water years 1995–2021. Location of station is shown on figure 1. Modified from Breault (2010). Data are from USGS (2024).

Water Quality and Constituent Loads and Yields

Water-quality conditions in the Scituate Reservoir drainage area are described by summary statistics for water-quality properties, constituent concentrations, and estimated constituent loads and yields. Loads and yields measure the rates at which masses of constituents are transferred to the reservoir by tributaries. Tributaries with high flows tend to have high loads because the greater volume of water can carry more of the constituent to the reservoir per unit time than tributaries with low flows. Yield represents the constituent load per unit of drainage area and is calculated by dividing the load estimated for a streamgage by the drainage area for the monitoring station. Yields are useful for comparison among streamgages that have different drainage areas because each basin size and therefore total streamflow volume is normalized. Yields also are useful for examining potential differences among basin properties that may contribute to water quality in the reservoir.

Summary statistics include means and medians. For some purposes, median values are more appropriate because they are less likely to be affected by high or low concentrations (outliers). Medians are especially important to use for summarizing a relatively limited number of values. In contrast, datasets that include a large number of values, such as continuously monitored streamflow and loads of chloride and sodium (estimated from measurements of specific conductance), are better summarized in terms of means because large datasets are more resistant to the effects of outliers than small datasets. Mean values also are particularly appropriate for measuring loads because outlier values, which typically represent large flows, are important to include in estimates of constituent masses delivered to receiving waters.

Uncertainties associated with measuring streamflow and specific conductance and with chloride and sodium sample collection, preservation, and analysis produce uncertainties in load and yield estimates. The load and yield estimates presented in the text and tables are the most likely values for chloride and sodium inputs from tributaries or their drainage basins, based on the available data and analysis methods. It may be best to discuss loads and yields in terms of a range within which the true values lie; however, the most likely values of loads and yields are presented for ease of discussion and presentation. The range within which the true values lie also depends on the uncertainties in individual measurements of streamflow and concentration, which are difficult to quantify with the available information. The uncertainties associated with streamflow are commonly assumed to have a larger effect on load and yield calculations than the errors associated with measuring specific conductance or chemical analysis. The uncertainties associated with estimated streamflow are greater than those associated with measured streamflow. The most likely values of loads and yields presented in the tables and text are sufficient for planning-level analysis of water quality in tributaries and their drainage basins.

Chloride and Sodium Loads and Yields Estimated From Specific-Conductance Monitoring Data

Chloride and sodium are constituents of special concern in the Scituate Reservoir drainage area; chloride is difficult to remove from finished drinking water and can affect the taste; and sodium is a constituent of potential concern for human health; some people on restricted diets might need to limit their sodium intake. Chloride and sodium are major constituents of road salt used for deicing, and several major roadways cross the Scituate Reservoir drainage area. State Routes 12 and 14 cut across the main body of the reservoir, and Route 116 parallels the eastern limb of the reservoir (fig. 1). Nimiroski and Waldron (2002) indicated that tributaries in basins with State-maintained roads had substantially higher concentrations of chloride and sodium than tributaries in basins with low road density, presumably because of deicing activities. Smith (2015b) indicated relations between concentrations of chloride collected from the tributaries to the Scituate Reservoir and total impervious area of the respective subbasins were significant; and Spaetzel and Smith (2022a) found 32 significant upward trends in tributary chloride concentrations at the 37 stations during WYs 1983–2019.

Monthly mean concentrations were calculated by dividing the total monthly load by the total discharge for the month. Estimated monthly mean chloride concentrations in tributaries of the Scituate Reservoir drainage area ranged from 8.6 to 80 milligrams per liter (mg/L), and estimated monthly mean sodium concentrations ranged from 6.0 to 48 mg/L (table 5). Both highest monthly mean concentrations of chloride and sodium (80 and 48 mg/L, respectively) were recorded at Quonopaug Brook (PW station 6; USGS station 01115183) in August 2022. The estimated monthly mean concentrations of chloride and sodium were greatest in July, August, and September 2022 at most stations, compared with the estimated monthly mean concentrations during the winter months. Monthly estimated chloride and sodium mean concentrations were either highest or tied for highest in August for 11 of the 16 stations. Monthly estimated mean concentrations were highest in January for the unnamed tributary to Regulating reservoir (PW station 18; USGS station 01115120) and highest in March for Swamp Brook (PW station 38; USGS station 01115278). The estimated monthly mean concentrations of chloride and sodium in Dolly Cole Brook (PW station 24; USGS station 01115190), Cork Brook (PW station 3; USGS station 01115290), Wilbur Hollow Brook (PW station 7; USGS station 01115297) and Moswansicut Stream (PW station 19; USGS station 01115170) remained consistent throughout the water year, ranging only by less than 7.1 mg/L and less than 4.1 mg/L, respectively. The greatest variation of estimated monthly mean concentrations of chloride and sodium was for Quonopaug Brook (PW station 6; USGS station 01115183), Swamp Brook (PW station 38; USGS station 01115278), and the unnamed tributary to Regulating reservoir (PW station 18; USGS station 01115120).

Table 5.    

Monthly mean concentrations of chloride and sodium estimated from continuous measurements of specific conductance in the Scituate Reservoir drainage area, Rhode Island, October 1, 2021, through September 30, 2022.

[Data were collected by the U.S. Geological Survey (USGS; USGS, 2024). Locations of stations are shown on figure 1. Monthly mean concentrations were calculated by dividing the monthly load by the total discharge for the month. All units in milligrams per liter. PW, Providence Water; Cl, chloride; Na, sodium; —, not applicable]

PW station number USGS station number October 2021 November 2021 December 2021 January 2022 February 2022 March 2022 April 2022 May 2022 June 2022 July 2022 August 2022 September 2022
Cl Na Cl Na Cl Na Cl Na Cl Na Cl Na Cl Na Cl Na Cl Na Cl Na Cl Na Cl Na
24 01115190 21 13 19 12 21 14 23 15 23 14 24 15 23 14 24 15 26 16 24 15 26 16 26 16
26 01115185 22 14 19 12 22 14 22 14 19 12 23 14 21 14 27 16 32 19 33 20 28 17 32 19
28 01115265 23 14 18 11 21 13 22 14 18 11 21 13 20 12 26 16 31 19 32 19 33 20 33 20
35 01115187 18 11 15 9.9 17 11 17 11 15 9.7 17 11 17 11 20 12 22 14 23 14 24 15 23 14
3 01115280 31 19 28 17 34 20 34 20 30 18 34 20 34 20 33 20 31 18 34 21 34 21 32 19
5 01115184 23 14 19 12 21 13 19 12 17 11 17 11 16 10 19 12 20 12 27 16 32 18 16 10
6 01115183 32 20 28 17 31 19 31 19 28 17 29 18 27 16 32 19 35 21 50 30 80 48 37 22
7 01115297 10 6.9 8.6 6.0 12 7.9 12 7.9 9.1 6.3 9.1 6.3 8.9 6.2 11 7.2 11 7.2 11 7.2 16 9.9 14 9.1
8 01115276 23 15 20 13 22 14 21 14 19 12 20 13 20 13 23 15 26 17 26 17 25 16 27 17
9 01115275 53 31 52 31 53 31 55 32 42 25 45 27 47 28 54 32 56 34 65 39 66 39 47 28
38 01115278 27 17 21 14 26 17 35 22 44 26 53 30 38 23 28 18 32 20 22 15 12 8.9 40 24
19 01115170 49 29 48 29 49 29 51 31 50 30 50 30 50 30 51 31 52 31 53 32 55 33 53 32
14 01115110 12 7.6 11 7.2 13 8.0 13 8.2 10 6.4 11 7.2 11 7.1 13 8.1 14 8.8 15 9.1 18 11 16 9.6
15 01115114 29 18 26 16 27 17 47 29 52 31 27 17 32 20 45 27 50 30 52 31 48 29 43 26
16 01115098 35 21 33 20 32 20 36 22 33 20 33 20 36 22 38 23 41 24 42 25 44 26 41 24
18 01115120 41 24 49 29 61 35 68 39 51 30 53 30 48 28 61 35 60 34 52 30 33 19 36 21
Mean 28 17 26 16 29 18 32 19 29 17 29 18 28 17 31 19 34 20 35 21 36 22 32 19
Table 5.    Monthly mean concentrations of chloride and sodium estimated from continuous measurements of specific conductance in the Scituate Reservoir drainage area, Rhode Island, October 1, 2021, through September 30, 2022.

Annual mean concentrations were calculated by dividing the total annual load by the total discharge for the year. The highest annual mean concentrations of chloride and sodium were estimated to be 52 and 30 mg/L, respectively (table 6), in the unnamed tributary to Regulating reservoir (PW station 18; USGS station 01115120) and 50 and 30 mg/L, respectively, in Moswansicut Stream (PW station 19; USGS station 01115170). The stations on the Moswansicut Pond reservoir basin and the unnamed tributary to Regulating reservoir are in the more developed, northeastern part of the Scituate Reservoir drainage area (fig. 1). The similarly high annual mean concentrations of 49 mg/L of chloride and 29 mg/L of sodium in Bear Tree Brook (PW station 9; USGS station 01115275) are the result of residual chloride and sodium leaching to groundwater from a formerly uncovered salt storage pile (Nimiroski and Waldron, 2002).

Table 6.    

Estimated annual mean chloride and sodium concentrations, loads, and yields at streamgage stations in the Scituate Reservoir drainage area, Rhode Island, from October 1, 2021, through September 30, 2022.

[Data were collected by the U.S. Geological Survey (USGS; USGS, 2024). Locations of stations are shown on figure 1. Annual mean concentrations were calculated by dividing the annual load by the total discharge for the year; annual mean yields were calculated by dividing the sum of individual loads by the sum of the drainage area. PW, Providence Water; Cl, chloride; mg/L, milligram per liter; Na, sodium; t/yr, metric ton per year; (t/yr)/mi2, metric ton per year per square mile; —, not applicable]

PW station number USGS station number Concentration Load Yield
Cl (mg/L) Na (mg/L) Cl (t/yr) Na (t/yr) Cl ([t/yr]/mi2) Na ([t/yr]/mi2)
24 01115190 22 14 210 130 43 27
26 01115185 22 14 150 96 35 22
28 01115265 20 13 300 190 34 21
35 01115187 17 11 430 270 30 19
3 01115280 32 19 97 58 52 31
5 01115184 18 11 34 21 27 17
6 01115183 30 18 95 57 49 29
7 01115297 10 6.9 67 45 15 10
8 01115276 21 14 150 97 29 19
9 01115275 49 29 62 37 100 59
38 01115278 37 22 110 65 56 34
19 01115170 50 30 240 140 73 44
14 01115110 12 7.3 110 66 17 10
15 01115114 38 23 280 170 59 36
16 01115098 35 21 250 150 49 30
18 01115120 52 30 23 13 80 46
Mean concentration or yield 29 18 38 23
Total load 2,600 1,600
Table 6.    Estimated annual mean chloride and sodium concentrations, loads, and yields at streamgage stations in the Scituate Reservoir drainage area, Rhode Island, from October 1, 2021, through September 30, 2022.

During WY 2022, the Scituate Reservoir received about 2,600 metric tons (t) of chloride and 1,600 t of sodium from tributaries that are equipped with instrumentation capable of continuously monitoring specific conductance (table 6). The highest annual chloride and sodium loads in the Scituate Reservoir drainage area during WY 2022 were estimated to be 430 and 270 t, respectively, at the Ponaganset River station (PW station 35; USGS station 01115187; table 6). Monthly estimated chloride and sodium loads tended to be lowest in August at each station, except at Spruce Brook (PW station 5; USGS station 01115184) and Cork Brook (PW station 3; USGS station 01115280) where the loads were slightly lower in July (fig. 4). Monthly estimated chloride and sodium loads were highest in February at all stations; however, loads tended to remain relatively high through April at each station before diminishing through the summer months (fig. 4). From January through April, the sums of the monthly estimated loads of chloride and sodium for the respective drainage areas upstream from each streamgage accounted for about 53 to 76 percent of the annual loads of chloride and sodium.

Chloride and sodium loads were lowest in July and August at every station.
Figure 4.

Graphs showing monthly loads of chloride and sodium estimated from streamflow and specific conductance data and annual mean discharge for October 1, 2021, through September 30, 2022, at 16 Providence Water (PW) sampling stations with continuous-record U.S. Geological Survey (USGS) water-quality data in the Scituate Reservoir drainage area, Rhode Island. Locations of stations are shown on figure 1; station information is shown in table 1. Data are from USGS (2024).

During WY 2022, estimated annual loads of chloride and sodium at the continuous streamgage stations were greater than the median estimated annual loads for WYs 2009–21 for 8 of the 16 USGS stations (fig. 5; note that two stations do not have estimates for WYs 2009–20). The sums of annual loads of chloride and sodium during WY 2022 for 14 stations with continuous monitoring in WYs 2009–21 (2,300 and 1,400 t, respectively) were about 12 percent lower than the average sums of loads of chloride (2,600 t) and sodium (1,600 t) during WYs 2009–21 (fig. 6). Annual loads from Swamp Brook (PW station 38; USGS station 01115278) and Winsor Brook (PW station 26; USGS station 01115185), which were incorporated into the monitoring network in 2021, represented about 10 percent of the sum of annual loads of chloride and sodium (fig. 6) in WY 2022. The annual mean discharge yield in cubic feet per second per square mile for WY 2022 was lower (by 0.17 cubic foot per second per square mile) than the WY 2009–21 average (table 2).

Chloride and sodium loads varied by station, from less than the minimum to somewhat
                        greater than the median for 2009–21.
Figure 5.

Graph showing annual loads of chloride and sodium estimated from streamflow and specific conductance data for October 1, 2021, through September 30, 2022, and associated minimum, maximum, and median annual loads for October 1, 2008, through September 30, 2021, at 16 Providence Water sampling stations with continuous-record U.S. Geological Survey water quality data in the Scituate Reservoir drainage area, Rhode Island. Locations of stations are shown on figure 1; only 2 years of available data are available for PW stations 26 and 38. Station information is shown in table 1. Modified from Smith (2016).

Chloride loads, sodium loads, and annual mean discharge fluctuate in similar patterns.
Figure 6.

Bar chart showing all available estimated annual loads of chloride and sodium estimated from continuous measurements of streamflow and specific conductance from water years 2009–22 (October 1, 2008, through September 30, 2022), for the area upstream from 16 Providence Water sampling stations in the Scituate Reservoir drainage area, Rhode Island. Modified from Smith (2019a). Note: there were no data available to show chloride and sodium loads associated with Providence Water stations 26 and 38 prior to water year 2021.

Watershed yields ranged from 15 to 100 metric tons per year per square mile ([t/yr]/mi2) for estimated chloride and from 10 to 59 (t/yr)/mi2 for estimated sodium. The lowest yields for estimated chloride and sodium were 15 and 10 (t/yr)/mi2, respectively, for Wilbur Hollow Brook (PW station 7; USGS Station 01115297) and 17 and 10 (t/yr)/mi2, respectively, for Huntinghouse Brook (PW station 14; USGS station 01115110). The highest yields for chloride and sodium among the 16 USGS stations, 100 and 59 (t/yr)/mi2, respectively, were in Bear Tree Brook (PW station 9; USGS station 01115275), which is downstream from a formerly uncovered salt storage pile. The second highest chloride and sodium yields, 80 and 46 (t/yr)/mi2, respectively, were in the small watershed for the unnamed tributary to Regulating reservoir (PW station 18; USGS station 01115120; table 6). Chloride and sodium yields for Ponaganset River (PW station 35; USGS station 01115187), the largest subbasin in the Scituate Reservoir watershed, were more than three times lower at 30 and 19 (t/yr)/mi2, respectively, than the yields for Bear Tree Brook. The estimated annual mean yields of chloride and sodium for the drainage area upstream from the 16 USGS streamgage stations were 38 and 23 (t/yr)/mi2, (table 6), respectively. These estimated annual mean yields of chloride and sodium for WY 2022 were lower than estimated annual mean yields of chloride and sodium in the prior water year by about 27 percent (Smith, 2024).

Physical and Chemical Properties and Daily Loads and Yields Estimated From Data Collected by Providence Water

PW routinely measured four water-quality properties (pH, color, turbidity, and alkalinity), and concentrations of chloride, nitrite, nitrate, orthophosphate, total coliform bacteria, and Ecoli bacteria in monthly or quarterly samples of tributary water. These data are general indicators of water-quality conditions in the Scituate Reservoir drainage area.

Physical and Chemical Properties

Physical and chemical properties including pH, color, turbidity, and alkalinity were routinely measured to determine water quality in each of the six subbasins in the Scituate Reservoir drainage area (table 7) by PW. Specifically, pH is a measure of the effective hydrogen-ion concentration (USGS, 2021) representing the negative base-10 logarithm of hydrogen-ion activity of a solution, in moles per liter; color can be an indirect measure of the amount of organic carbon dissolved in the water column; turbidity is an indirect measure of suspended particles; and alkalinity is a measure of the buffer capacity of water.

Table 7.    

Median values for water-quality data collected at Providence Water stations in the Scituate Reservoir drainage area, Rhode Island, from October 1, 2021, through September 30, 2022.

[Data from Smith and Spaetzel (2021). Water-quality data are from samples collected and analyzed by Providence Water (PW). Locations of stations are shown on figure 1. If fewer than three samples were collected, the sample values are reported instead of a median. Minimum, median, and maximum values of station medians are based on stations with more than two samples. USGS, U.S. Geological Survey; PCU, platinum-cobalt units; NTU, nephelometric turbidity unit; CFU/100 mL, colony forming unit per 100 milliliters; E. coli., Escherichia coli; mg/L, milligram per liter ; CaCO3, calcium carbonate; N, nitrogen; PO4, phosphate; <, less than; —, no data; shaded rows, also indicated by footnotes, show actual values where less than three samples were collected]

PW station number USGS station number Properties Constituents
pH Color (PCU) Turbidity (NTU) Total coliform bacteria (CFU/100 mL) E. coli (CFU/100 mL) Alkalinity (mg/L as CaCO3) Chloride (mg/L) Nitrite (mg/L as N) Nitrate (mg/L as N) Orthophosphate (mg/L as PO4)
24 01115190 6.5 45 0.60 2,600 121 7.4 23.3 0.001 0.07 0.07
125 01115200 6.3; 6.5 70; 60 0.93; 0.64 1,660; 776 40; 20 7.9; 5.4 13; 11.7 0.002; 0.001 0.06; <0.05 0.11; 0.15
26 01115185 6.4 30 0.28 778 30 7.2 28.8 0.001 0.08 0.05
27 011151845 6.0 20 0.21 637 <10 4.8 11.8 0.001 <0.05 0.10
28 01115265 6.1 70 0.78 1,070 50 6.6 25.6 0.002 0.05 0.06
29 01115271 6.5 40 0.75 384 <10 5.6 21 0.001 0.05 0.04
135 01115187 6.4 35 0.83 2,480 83 6.1 20.4 0.002 0.08 0.05
1 01115180 7.0 125 2.0 1,060 30 16 13.9 0.002 0.15 0.09
12 01115181 6.5; 6.5 65; 8 1.1; 0.21 24,200; 310 <10; 2,600 9.2,7.7 54.9; 66.5 0.002; 0.001 0.06; 0.27 0.03; 0.07
3 01115280 6.5 37 0.34 738 <10 7.8 37.9 0.001 0.15 0.06
4 01115400 6.3 30 0.47 1,140 10 8.5 6.1 0.001 <0.05 0.04
5 01115184 6.2 85 0.34 1,140 10 5.3 19.1 0.002 0.18 0.06
6 01115183 6.4 125 1.0 1,020 65 13 30.8 0.002 0.16 0.06
7 01115297 6.4 110 0.81 1,470 58 8.6 11 0.002 0.07 0.07
8 01115276 6.5 18 0.48 374 <10 5.2 14.2 0.001 <0.05 0.05
9 01115275 6.6 35 0.38 1,140 <10 9.0 51.3 0.001 0.39 0.05
130 01115350 5.7; 5.9 40; 62 0.48; 0.43 464; 738 <10; 10 4.9; 5.9 32.8; 23.5 0.001; 0.002 0.08; 0.05 0.08; 0.05
32 01115178 6.6 85 0.93 1,290 50 12 14.1 0.002 0.38 0.04
133 01115182 6.2; 6.5 25; 40 0.22; 0.43 290; 988 <10; <10 9.8; 11.8 15.6; 20.2 0.001; 0.002 0.11; 0.25 0.08; 0.12
36 6.8 30 0.33 730 <10 6.4 4.9 0.001 <0.05 0.08
37 5.8 25 0.21 630 <10 5.1 6.6 0.001 <0.05 0.06
138 01115278 6.3; 6.6 25; 20 0.19; 0.13 201; 563 <10; <10 4; 7.4 46.3; 29.2 0.001; <0.001 <0.05; <0.05 0.03; 0.04
19 01115170 7.2 25 0.76 1,150 <10 13 53.5 0.002 0.07 0.03
20 01115160 6.3 135 0.98 516 10 8.9 66.4 0.003 0.07 0.08
21 01115165 6.5 45 2.1 2,000 50 17 42.2 0.003 0.39 0.07
22 01115167 6.7 35 1.7 4,610 130 23 50.2 0.010 0.98 0.07
34 01115164 6.4 83 0.82 690 25 18 37.4 0.003 <0.05 0.08
23 011151843 6.2 20 0.51 749 <10 4.2 19 0.001 0.06 0.03
13 01115176 7.0 35 1.0 670 <10 12 39.1 0.001 <0.05 0.05
14 01115110 6.6 35 0.81 960 60 11 14.3 0.001 0.10 0.07
15 01115114 6.9 50 0.83 573 30 12 45.7 0.001 0.09 0.07
16 01115098 6.7 35 0.98 576 10 14 38.9 0.001 0.11 0.04
117 01115119 6.4; 5.9 32; 250 0.54; 1.0 684; 8,160 <10; 70 8.6; 12.9 26.6; 55.4 0.001; 0.005 <0.05; 0.06 0.07; 0.13
18 01115120 6.5 42 0.90 275 60 9.6 58.6 0.002 0.28 0.09
10 01115274 5.7 30 0.27 1,710 35 4.0 26.6 0.001 <0.05 0.06
11 01115273 5.9 90 1.2 2,360 20 7.6 7.1 0.004 <0.05 0.08
12 011152745 6.2 25 0.87 4,880 30 7.0 13.6 0.001 <0.05 0.03
Minimum median 5.7 18 0.21 275 <10 4.0 4.9 0.001 <0.05 0.03
Median median 6.5 35 0.81 1,020 25 8.5 23.3 0.001 0.10 0.06
Maximum median 7.2 135 2.1 4,880 130 23 66.4 0.010 0.98 0.10
Table 7.    Median values for water-quality data collected at Providence Water stations in the Scituate Reservoir drainage area, Rhode Island, from October 1, 2021, through September 30, 2022.
1

Less than three samples were collected at this station; actual values rather than medians are listed.

The median pH in tributaries in the Scituate Reservoir drainage area ranged from 5.7 to 7.2; the median of the medians for all stations with more than two samples was 6.5. Median values of color ranged from 18 to 135 platinum-cobalt units; the median for all stations was 35 platinum-cobalt units. Median values of turbidity ranged from 0.21 to 2.1 nephelometric turbidity units; the median for all stations was 0.80 nephelometric turbidity unit. Median alkalinity values in tributaries were low, ranging from 4.0 to 23 mg/L as calcium carbonate; the median for all stations was 8.5 mg/L alkalinity as calcium carbonate (table 7).

Constituent Concentrations and Daily Loads and Yields

Fecal indicator bacteria, chloride, and nutrients such as nitrogen and phosphorus are commonly detected in natural water; at elevated concentrations, these constituents can cause or contribute to water-quality impairments. Fecal indicator bacteria, which are found in the intestines of warm-blooded animals, may indicate impairment from sewage contamination or from livestock or wildlife that defecate in or near the stream margin. Chloride originates in tributary stream water from precipitation, weathering, or human activities such as waste disposal, use of septic systems, and road deicing. Sources of nutrients in tributary stream water include atmospheric deposition, leaching of naturally occurring organic material, discharge of groundwater that is enriched with nutrients from septic-system leachate, and runoff contaminated with fertilizer or animal waste. The ultimate intended use of water in the tributaries is drinking water, which must meet specific water-quality standards. For this reason, PW closely monitors concentrations of these constituents in tributaries. Median concentrations, loads, and yields of water-quality constituents are listed in tables 78, and 9.

Table 8.    

Median daily loads of bacteria, chloride, nitrite, nitrate, and orthophosphate in the Scituate Reservoir drainage area, Rhode Island, from October 1, 2021, through September 30, 2022.

[Concentration data from Smith and Spaetzel (2021). Water-quality data are from samples collected and analyzed by Providence Water (PW). Locations of stations are shown on figure 1. USGS, U.S. Geological Survey; (CFU×106)/d; millions of colony forming units per day; E. coliEscherichia coli; kg/d, kilogram per day; g/d, gram per day; N, nitrogen; PO4, phosphate; —, not applicable]

PW station number USGS station number Total coliform bacteria ([CFUx106]/d) E. coli ([CFUx106]/d) Chloride (kg/d) Nitrite (as N) (g/d) Nitrate (as N) (g/d) Orthophosphate (as PO4) (g/d)
24 01115190 140,000 3,900 190 17 560 850
25 01115200 88,000 2,300 110 11 <490 1,300
26 01115185 42,000 1,200 240 8.8 <610 390
28 01115265 220,000 25,000 1,000 72 <2,300 2,400
35 01115187 350,000 15,000 1,800 92 <4,800 1,600
1 01115180 54,000 1,000 92 17 790 250
3 01115280 28,000 1,200 290 8.0 830 480
4 01115400 26,000 <270 17 2.2 <110 145
5 01115184 87,000 <760 150 11 1,400 330
6 01115183 79,000 5,800 300 27 1,300 610
7 01115297 130,000 5,200 180 40 <1,300 990
8 01115276 33,500 <1,500 250 20 <1,000 610
9 01115275 55,000 <640 320 6.4 420 220
32 01115178 11,000 <370 13 1.8 160 37
33 01115182 1,100 <23 3.9 0.29 34 21
38 01115278 24,000 <1,100 470 <11 <530 320
19 01115170 11,000 1,300 320 12 540 190
21 01115165 15,000 360 36 2.8 310 66
14 01115110 130,000 8,400 340 22 1,800 1,900
15 01115114 110,000 5,600 650 18 1,500 1,100
16 01115098 220,000 <2,800 920 35 3,100 1,500
18 01115120 6,100 1,300 130 4.5 620 200
10 01115274 240,000 7,400 220 8.0 <400 480
11 01115273 14,000 120 4.3 2.4 <31 49
Minimum median 1,100 <23 3.9 0.29 <31 21
Median median 55,000 1,300 230 11 620 440
Maximum median 350,000 25,000 1,800 92 <4,800 2,400
Table 8.    Median daily loads of bacteria, chloride, nitrite, nitrate, and orthophosphate in the Scituate Reservoir drainage area, Rhode Island, from October 1, 2021, through September 30, 2022.

Table 9.    

Median daily yields of bacteria, chloride, nitrite, nitrate, and orthophosphate in the Scituate Reservoir drainage area, Rhode Island, from October 1, 2021, through September 30, 2022.

[Concentration data from Smith and Spaetzel (2021). Water-quality data are from samples collected and analyzed by Providence Water (PW). Locations of stations shown on figure 1. USGS, U.S. Geological Survey; ([CFU×106]/d)/mi2; millions of colony forming units per day per square mile; E. coliEscherichia coli; N, nitrogen; PO4, phosphate; (kg/d)/mi2, kilogram per day per square mile; (g/d)/mi2, gram per day per square mile; —, none]

PW station number USGS station number Total coliform bacteria ([CFUx106]/mi2) E. coli ([CFUx106]/mi2) Chloride ([kg/d]/mi2) Nitrite (as N) ([g/d]/mi2) Nitrate (as N) ([g/d]/mi2) Orthophosphate (as PO4) ([g/d]/mi2)
24 01115190 29,000 800 39 3.5 110 170
25 01115200 37,000 950 47 4.8 <200 550
26 01115185 10,000 270 55 2.0 <140 90
28 01115265 25,000 2,900 120 8.2 <260 270
35 01115187 25,000 1,100 130 6.6 <340 110
1 01115180 34,000 640 59 11 500 160
3 01115280 15,000 640 160 4.3 440 260
4 01115400 31,000 <320 19 2.6 <130 170
5 01115184 69,000 <600 120 8.7 1,100 260
6 01115183 40,000 2,900 150 14 660 310
7 01115297 29,000 1,200 40 9.2 <300 230
8 01115276 6,500 <290 48 3.9 <190 120
9 01115275 89,000 <1,000 520 10 680 350
32 01115178 24,000 <820 29 4.0 360 82
33 01115182 3,900 <82 14 1.0 120 75
38 01115278 12,000 <550 250 <5.5 <270 170
19 01115170 3,400 400 100 3.7 170 58
21 01115165 50,000 1,200 120 9.2 1,000 220
14 01115110 21,000 1,300 54 3.5 290 300
15 01115114 23,000 1,200 140 3.8 320 230
16 01115098 44,000 <560 190 7.0 620 300
18 01115120 22,000 4,600 460 16 2,200 710
10 01115274 160,000 5,000 150 5.4 <270 320
11 01115273 19,000 170 6.0 3.3 <43 68
Minimum median 3,400 <82 6.0 1.0 <43 58
Median median 25,000 810 110 5.1 <300 230
Maximum median 160,000 5,000 520 16 2,200 710
Table 9.    Median daily yields of bacteria, chloride, nitrite, nitrate, and orthophosphate in the Scituate Reservoir drainage area, Rhode Island, from October 1, 2021, through September 30, 2022.

Bacteria

Median concentrations of total coliform bacteria were above the detection limit (1 colony forming unit per 100 milliliters [CFU/100 mL]) at all sites (table 7). Median concentrations of E. coli were censored below the detection limit of 10 CFU/100 mL at 10 of the 37 stations that had more than two samples collected in WY 2022. Median concentrations of E. coli were uncensored for 22 stations, and these concentrations ranged from 10 to 130 CFU/100 mL. For seven stations, medians were not determined because fewer than three samples were collected, and these concentrations ranged from less than 10 to 2,600 CFU/100 mL. Total coliform bacteria concentrations were greater than E. coli concentrations (as expected because total coliform is a more inclusive measure than E. coli); the medians of median concentrations for all sites in the drainage area were 1,020 CFU/100 mL for total coliform bacteria and 25 CFU/100 mL for E. coli (table 7). The highest median concentration of total coliform bacteria, 4,880 CFU/100 mL, was at the unnamed tributary to Westconnaug Brook (PW station 12; USGS station 011152745), which drains directly to the Scituate Reservoir (table 1). Median concentrations of total coliform bacteria were equal to or exceeded 2,000 CFU/100 mL at five other stations, including Dolly Cole Brook (PW station 24; USGS station 01115190), Ponaganset River (PW station 35; USGS station 01115187), unnamed tributary 2 to Moswansicut Pond reservoir (PW station 21; USGS station 011151650, unnamed tributary 3 to Moswansicut Pond reservoir (PW station 22; USGS station 01115167), and unnamed tributary to Westconnaug Reservoir (PW station 11; USGS station 01115273). Median concentrations of total coliform bacteria were lowest at the unnamed tributary to Regulating reservoir (PW station 18; USGS station 01115120). The highest median concentration of E. coli, 130 CFU/100 mL (table 7), was at the unnamed tributary 3 to Moswansicut Pond Reservoir (PW station 22; USGS station 1115167).

Median daily loads and yields of total coliform bacteria and E. coli varied by two orders of magnitude or more (tables 8 and 9). The median daily loads of total coliform bacteria for all subbasins in the Scituate Reservoir drainage area ranged from 1,100 to 350,000 million colony forming units per day ([CFU×106]/d), and yields ranged from 3,400 to 160,000 million colony forming units per day per square mile ([(CFU×106)/d]/mi2); E. coli loads ranged from less than 23 to 25,000 (CFU×106)/d, and yields ranged from less than 82 to 5,000 ([CFU×106]/d)/mi2 (tables 8 and 9). The highest median daily yield of total coliform bacteria at 160,000 ([CFU×106]/d)/mi2 was at Westconnaug Brook (PW station 10; USGS station 01115274), and the E. coli median yield at this station was 5,000 ([CFU×106]/d)/mi2 (table 9). Although relatively high for sampling stations in the Scituate Reservoir subbasin, median daily bacteria yields at these stations were low to moderate for yields of indicator bacteria in sewage-contaminated stream water or stream water affected by stormwater runoff in an urban environment (Breault and others, 2002).

Chloride

Median chloride concentrations among the PW stations ranged from 4.9 to 66.4 mg/L. The highest median concentration was collected at the unnamed tributary 1 to Moswansicut Pond reservoir (PW station 20; USGS station 01115160; table 7). The median of median concentrations for all sites in the drainage area was 23.3 mg/L (table 7), which is similar to the median of median concentrations in WY 2020 and 2021 (24.8 and 23.7 mg/L, respectively; Smith, 2024, Smith and Spaetzel 2024). Median daily chloride loads and yields estimated from samples collected by PW varied among monitoring stations in the drainage area (tables 8 and 9). Ponaganset River (PW station 35; USGS station 01115187) had the largest median daily chloride load at 1,800 kilograms per day (kg/d), followed by Hemlock Brook (PW station 28; USGS station 01115265) and Peeptoad Brook (PW station 16; USGS station 01115098; table 8). Bear Tree Brook (PW station 9; USGS station 01115275) had the largest median yield at 520 kilograms per day per square mile ([kg/d]/mi2). Median chloride yields ranged from 6 to 250 (kg/d)/mi2at the other streamgages, except for the small drainage area above the unnamed tributary to Regulating reservoir (PW station 18; USGS station 01115120), which was 460 (kg/d)/mi2 (table 9). The median daily chloride yield for monitored areas within the drainage area was 110 (kg/d)/mi2.

Nutrients

Median concentrations of nitrite and nitrate (table 7) were 0.001 and 0.10 mg/L as nitrogen, respectively. The highest median concentrations of nitrite (0.010 mg/L as nitrogen) and nitrate (0.98 mg/L as nitrogen) were at the unnamed tributary 3 to Moswansicut Pond reservoir (PW station 22; USGS station 01115167). The median concentration of orthophosphate for the entire study area (table 7) was 0.06 mg/L as phosphate. The maximum median concentration of orthophosphate was 0.10 mg/L as phosphate, measured in a sample collected at the unnamed tributary to Ponaganset River (PW station 27; USGS station 011151845).

Median daily nitrite and nitrate loads were largest at Ponaganset River (PW station 35; USGS station 01115187) at 92 grams per day (g/d) and less than 4,800 g/d (table 8). The largest uncensored median daily nitrate load was at Peeptoad Brook (PW station 16; USGS station 01115098) at 3,100 g/d as nitrogen. Median daily orthophosphate loads were largest (2,400 g/d The medians of median daily loads were 11 g/d for nitrite as nitrogen, 620 g/d for nitrate as nitrogen, and 440 g/d for orthophosphate as phosphate as phosphate) at Hemlock Brook (PW station 28; USGS station 01115265; table 8).

The maximum median daily yields for nitrite and nitrate were 16 grams per day per square mile ([g/d]/mi2) as nitrogen and 2,200 (g/d)/mi2 as nitrogen, respectively, at the unnamed tributary to Regulating reservoir (PW station 18; USGS station 01115120; table 9). These maximum daily yields were more than three and seven times greater, respectively, than the median yields among all stations (5.1 and less than 300 (g/d)/mi2, respectively). For orthophosphate, the maximum median daily yield was 710 (g/d)/mi2 as phosphate at the unnamed tributary to Regulating reservoir. This maximum median daily yield of orthophosphate was about three times greater than the median among all station medians (230 [g/d]/mi2 as phosphate).

Summary

Since 1993, the U.S. Geological Survey (USGS), in cooperation with Providence Water (PW; formerly the Providence Water Supply Board), has maintained a long-term cooperative water-quality monitoring program within the Scituate Reservoir drainage area. PW also has been independently monitoring and assessing water quality in the reservoir and reservoir drainage area for more than 60 years. Together, the data collected by the USGS and PW are used to calculate concentrations, loads, and yields of chloride, sodium, nutrients, and bacteria for tributaries within Scituate Reservoir drainage area on an annual basis.

During water year (WY) 2022, the USGS measured or estimated streamflow at 24 streamgages; 16 of these streamgages are equipped with instrumentation capable of continuously monitoring water level, specific conductance, and water temperature. Prior to WY 2021, 14 streamgages were equipped with continuous-monitoring instrumentation; therefore, for annual comparisons of total chloride and sodium loads measured in the drainage area over the WY 2009–21 period, loads from the two streamgages added in WY 2022 were omitted. Water-quality samples were periodically collected by the USGS at each of the 16 streamgages and analyzed for dissolved concentrations chloride and sodium. Concentrations of chloride and sodium collected during WY 2022 were combined with data collected in previous WYs to support and refine relations between each ion and specific conductance. Monthly and annual concentrations, loads, and yields were estimated for the 16 streamgages by using equations to relate specific conductance to concentrations of chloride and sodium and measured or estimated streamflow data.

At 16 of the 24 USGS streamgages, where both streamflow and continuous specific conductance data were available, estimated monthly mean chloride concentrations ranged from 8.6 to 80 milligrams per liter (mg/L) and estimated monthly mean sodium concentrations ranged from 6.0 to 48 mg/L in tributaries of the Scituate Reservoir drainage area. The highest annual mean concentrations of chloride and sodium in the more developed, northeastern part of the Scituate Reservoir drainage area were estimated to be 52 and 30 mg/L, respectively, in the unnamed tributary to Regulating reservoir (PW station 18; USGS station 01115120) and 50 and 30 mg/L, respectively, in Moswansicut Stream (PW station 19; USGS station 01115170). Estimated loads of 2,600 metric tons of chloride and 1,600 metric tons of sodium were transported to the Scituate Reservoir during WY 2022 from the 16 tributaries equipped with instrumentation. Annual chloride yields for tributaries in the drainage area ranged from 15 to 100 metric tons per square mile, and annual sodium yields ranged from 10 to 59 metric tons per square mile. The sum of estimated chloride and sodium annual loads during WY 2022 for 14 stations with continuous monitoring in WYs 2009–21 was about 12 percent lower than the sum of annual loads estimated during the previous water year and less than the average annual load for WYs 2009–21 for the same stations.

PW collected at least one water-quality sample at 37 of 38 sampling stations in WY 2022, including at the 16 USGS continuous-record streamgages, as part of their long-term sampling program in the Scituate Reservoir drainage area. In WY 2022, only Toad Pond (PW station 31; USGS station 01115177) was not sampled. Water-quality samples are analyzed by PW for pH, color, turbidity, alkalinity, and concentrations of chloride, nutrients, and bacteria. Water-quality data collected by PW are summarized by using values of central tendency and are used in combination with periodic- or continuous-streamflow data available at 24 of the 37 stations sampled in WY 2022 to calculate loads and yields of chloride, nutrients, and bacteria.

For water samples collected by PW, the median of the median pH values for samples from all stations on tributaries in the Scituate Reservoir drainage area was 6.5; the median value for color was 35 platinum-cobalt units; the median value for turbidity was 0.80 nephelometric turbidity unit; and the median concentration for alkalinity was 8.5 mg/L as calcium carbonate. The medians of the median concentrations for water samples from all stations were 1,020 colony forming units per 100 milliliters for total coliform bacteria, 25 colony forming units per 100 milliliters for Escherichia coli, 23.3 milligrams per liter for chloride, 0.001 milligram per liter as nitrogen for nitrite, 0.07 milligram per liter as nitrogen for nitrate, and 0.06 milligram per liter as phosphate for orthophosphate. The medians of the median daily loads were 55,000 million colony forming units per day for coliform bacteria, 1,300 million colony forming units per day for Escherichia coli, 230 kilograms per day for chloride, 11 grams per day as nitrogen for nitrite, 620 grams per day as nitrogen for nitrate, and 440 grams per day as orthophosphate for phosphate. The medians of the median yields were 110 kilograms per day per square mile for chloride, 5.1 grams as nitrogen per day per square mile for nitrite, less than 300 grams as nitrogen per day per square mile for nitrate, 230 grams as orthophosphate per day per square mile for phosphate, 25,000 million colony forming units per day per square mile for coliform bacteria, and 810 million colony forming units per day per square mile for Escherichia coli.

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Conversion Factors

U.S. customary units to International System of Units

Multiply By To obtain
mile (mi) 1.609 kilometer (km)
square mile (mi2) 2.590 square kilometer (km2)
liter (L) 0.03531 cubic foot (ft3)
cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)
cubic foot per second per square mile ([ft3/s]/mi2) 0.01093 cubic meter per second per square kilometer ([m3/s]/km2)
liter per day (L/d) 0.03531 cubic foot per day (ft3/d)
kilogram (kg) 2.205 pound avoirdupois (lb)
metric ton (t) 1.102 ton, short [2,000 lb]
gram per day (g/d) 0.0022 pound per day (lb/d)
kilogram per day (kg/d) 2.205 pound per day (lb/d)
metric ton per year (t/yr) 2205 pound per year (lb/yr)
gram per day per square mile ([g/d]/mi2) 0.0022 pound per day per square mile ([lb/d]/m2)
kilogram per day per square mile ([kg/d]/mi2) 2.590 kilogram per day per square kilometer ([kg/d]/km2)
kilogram per day per square mile ([kg/d]/mi2) 2.205 pound per day per square mile ([lb/d]/mi2)
metric ton per year per square mile ([t/yr]/mi2) 2.590 metric ton per year per square kilometer ([t/yr]/mi2)
metric ton per year per square mile ([t/yr]/mi2) 2205 pound per year per square mile ([lb/yr]/mi2)

Datum

Vertical coordinate information is referenced to North American Vertical Datum of 1988 (NAVD 88).

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

Supplemental Information

Concentrations of constituents in water are given in either milligrams per liter (mg/L) or colony forming units per 100 milliliters (CFU/100 mL).

Loads of bacteria in water are given in million colony forming units per day ([CFU×106]/d).

Yields of bacteria are given in million colony forming units per day per square mile ([(CFU×106)/d]/mi2).

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

Color is given in platinum-cobalt units (PCU).

A water year is the period from October 1 to September 30 and is designated by the year in which it ends; for example, water year 2020 was from October 1, 2019, to September 30, 2020.

Abbreviations

E. coli

Escherichia coli

MOVE.1

Maintenance of Variance Extension type 1

NWIS

National Water Information System

PO4

phosphate

PW

Providence Water

USGS

U.S. Geological Survey

WY

water year

For more information, contact

Director, New England Water Science Center

U.S. Geological Survey

10 Bearfoot Road

Northborough, MA 01532

dc_nweng@usgs.gov

or visit our website at

https://www.usgs.gov/centers/new-england-water-science-center

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Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested Citation

Smith, K.P., and Spaetzel, A.B., 2025, Streamflow, water quality, and constituent loads and yields, Scituate Reservoir drainage area, Rhode Island, water year 2022: U.S. Geological Survey Data Report 1205, 33 p., https://doi.org/10.3133/dr1205.

ISSN: 2771-9448 (online)

Study Area

Publication type Report
Publication Subtype USGS Numbered Series
Title Streamflow, water quality, and constituent loads and yields, Scituate Reservoir drainage area, Rhode Island, water year 2022
Series title Data Report
Series number 1205
DOI 10.3133/dr1205
Year Published 2025
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) New England Water Science Center
Description Report: vi, 33 p.; Data Release
Country United States
State Rhode Island
Other Geospatial Scituate Reservoir Drainage Area
Online Only (Y/N) Y
Additional Online Files (Y/N) N
Google Analytic Metrics Metrics page
Additional publication details