Streamflow, Water Quality, and Constituent Loads and Yields, Scituate Reservoir Drainage Area, Rhode Island, Water Year 2020

Data Report 1192
Prepared in cooperation with Providence Water
By:

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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 (sometimes known as 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 14 tributaries were used to calculate loads of chloride and sodium during water year 2020 (October 1, 2019, through September 30, 2020). Water-quality samples were collected by Providence Water at 37 sampling stations on tributaries to the Scituate Reservoir during water year 2020. 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 2020 in this report.

Annual mean streamflows for monitoring stations in this study ranged from about 0.32 to 26.7 cubic feet per second during water year 2020. At the 14 continuous-record streamgages, tributaries transported about 2,200 metric tons of chloride and 1,400 metric tons of sodium to the Scituate Reservoir; annual chloride yields for the tributaries ranged from 13 to 110 metric tons per square mile, and annual sodium yields ranged from 8.8 to 6 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 220 kilograms chloride per day, 10 grams nitrite as nitrogen per day, 500 grams nitrate as nitrogen per day, 290 grams orthophosphate as phosphate per day, 55,000 million colony forming units of coliform bacteria per day, and less than 900 million colony forming units of Escherichia coli per day. The medians of the median yields were 76 kilograms chloride per day per square mile, 4.1 grams nitrite as nitrogen per day per square mile, 240 grams nitrate as nitrogen per day per square mile, 100 grams orthophosphate as phosphate per day per square mile, 31,000 million colony forming units of coliform bacteria per day per square mile, and less than 260 million colony forming units of Escherichia coli per day per square mile.

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 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 subbasin, “Direct Runoff” subbasin, Moswansicut Pond reservoir subbasin, Ponaganset Reservoir subbasin, “Regulating reservoir” subbasin, and Westconnaug Reservoir subbasin. 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; sometimes known as Providence Water Supply Board), which is the agency responsible for the management and distribution of the Scituate Reservoir water supply, has been monitoring and assessing water quality in the reservoir and reservoir drainage area for more than 60 years.

6 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, in water year 2020. Modified from Breault (2009).

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 at 10 streamgages in the drainage area (table 1) since 2009 by the USGS. 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. At 13 streamgages, daily mean streamflow has been estimated 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 (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, 2022b). These equations, used together with measured (or estimated) streamflows, allow for nearly continuous estimation of chloride and sodium loads to the Scituate 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 for October 1, 2019, to September 30, 2020.

[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; M, monthly; Q, quarterly; Y, yes; N, no]

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
Barden Reservoir subbasin
24 01115190 Dolly Cole Brook 4.9 M 11 Y Continuous Continuous
25 01115200 Shippee Brook 2.37 Q 3 N Estimated None
26 01115185 Winsor Brook 4.33 Q 3 N Estimated None
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 Barden Reservoir 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
Direct runoff subbasin
1 01115180 Brandy Brook 1.57 M 12 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 11 Y Continuous Continuous
4 01115400 Kent Brook Betty Pond Stream 0.85 M 11 N Estimated None
5 01115184 Spruce Brook 1.26 Q 3 Y Estimated Continuous
6 01115183 Quonopaug Brook 1.96 M 10 Y Continuous Continuous
7 01115297 Wilbur Hollow Brook 4.33 M 12 Y Estimated Continuous
8 01115276 Westconnaug Brook Westconnaug Reservoir 5.18 M 11 Y Continuous Continuous
9 01115275 Bear Tree Brook 0.62 Q 4 Y Estimated Continuous
30 01115350 Unnamed tributary 4 to Scituate Reservoir Coventry Brook, Knight Brook 0.79 Q 3 N None None
31 01115177 Toad pond 0.03 Q 1 N None None
32 01115178 Unnamed tributary 1 to Scituate Reservoir Pine Swamp Brook 0.45 Q 3 N Estimated None
33 01115182 Unnamed tributary 3 to Scituate Reservoir Halls Estate Brook 0.28 Q 3 N Estimated None
36 Outflow from King Pond 0.76 Q 3 N None None
37 Fire tower stream 0.03 Q 3 N None None
Moswansicut Pond reservoir subbasin
19 01115170 Moswansicut stream 3.25 M 12 Y Continuous Continuous
20 01115160 Unnamed tributary 1 to Moswansicut Pond reservoir Blanchard Brook 1.18 M 9 N None None
21 01115165 Unnamed tributary 2 to Moswansicut Pond reservoir Brook from Kimball Reservoir 0.30 Q 3 N Estimated None
22 01115167 Unnamed tributary 3 to Moswansicut Pond reservoir 0.10 M 9 N None None
34 01115164 Unnamed tributary from Kimball Reservoir Kimball Stream 0.27 Q 4 N None None
Ponaganset Reservoir subbasin
23 011151843 Ponaganset Reservoir 1.92 M 9 N None None
Regulating reservoir subbasin
13 01115176 Unnamed water body at Horseshoe dam Regulating reservoir 22.1 M 11 N None None
14 01115110 Huntinghouse Brook 6.29 M 10 Y Continuous Continuous
15 01115114 Rush Brook 4.70 M 9 Y Continuous Continuous
16 01115098 Peeptoad Brook Harrisdale Brook 4.97 M 11 Y Continuous Continuous
17 01115119 Dexter Pond Paine Pond 0.22 Q 3 N None None
18 01115120 Unnamed tributary to Regulating reservoir Unnamed brook A 0.28 Q 3 Y Estimated Continuous
Westconnaug Reservoir subbasin
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 4 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 for October 1, 2019, to September 30, 2020.
1

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

In 2020, PW regularly, either monthly or quarterly, visited fixed sites on 37 tributaries within the Scituate Reservoir drainage area and collected water-quality samples. 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 (2009), Breault and Campbell (2010a–d), Breault and Smith (2010), Smith and Breault (2011), and Smith (2013, 2014, 2015a, b, 2016, 2018a–d, 2019a, b, 2021, 2022a, b), Smith and Spaetzel (2021), and Spaetzel and Smith (2022a, b).

This report presents data on streamflow, water quality, and loads and yields of selected constituents for water year (WY) 2020 in the Scituate Reservoir drainage area (a water year is the period from the previous October 1 to September 30 of the designated year). Data were collected in past studies by the USGS in cooperation with PW and the Rhode Island Department of Environmental Management. This report summarizes measured and estimated streamflows for the 10 continuous-record and 13 partial-record streamgages in the drainage area. Estimated monthly and annual loads and yields of chloride and sodium are presented for the 14 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 2020 also are presented, and 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 (2009), 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, 2018a, b, 2019a, 2022a).

Streamflow Data Collection and Estimation

Streamflow was measured or estimated by the USGS at 23 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 13 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 (2022b); data needed to estimate streamflows at partial-record sites were retrieved from the USGS National Water Information System (NWIS; U.S. Geological Survey, 2023). 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, 2019, through September 30, 2020.

[Data were collected by the U.S. Geological Survey (U.S. Geological Survey, 2023; Spaetzel and Smith, 2022b). Stations are shown on figure 1. PW, Providence Water; USGS, U.S. Geological Survey; 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)
Barden Reservoir subbasin
24 01115190 7.81 8.79 6.84 1.59
25 01115200 7.00 22.8 2.15 2.96
26 01115185 9.43 21.2 4.18 2.18
28 01115265 12.0 13.1 10.9 1.38
35 01115187 26.7 29.6 23.8 1.91
Direct Runoff subbasin
1 01115180 2.78 5.89 1.31 1.77
3 01115280 3.25 3.69 2.82 1.74
4 01115400 1.46 6.59 0.33 1.72
5 01115184 1.80 4.03 0.80 1.43
6 01115183 3.49 3.86 3.12 1.78
7 01115297 7.23 14.9 3.51 1.67
8 01115276 8.60 9.22 7.98 1.66
9 01115275 1.52 2.60 0.88 2.45
32 01115178 0.59 1.11 0.32 1.32
33 01115182 0.32 0.74 0.14 1.14
Moswansicut Pond reservoir subbasin
19 01115170 5.48 5.90 5.05 1.69
21 01115165 0.54 1.15 0.25 1.79
Regulating reservoir subbasin
14 01115110 10.5 11.9 9.08 1.67
15 1115114 7.88 9.05 6.72 1.68
16 01115098 8.69 9.85 7.54 1.75
18 01115120 0.55 2.41 0.13 1.98
Westconnaug Reservoir subbasin
10 01115274 3.25 7.78 1.36 2.20
11 01115273 1.90 5.02 0.72 2.63
Table 2.    Measured or estimated annual mean streamflow for tributary streams in the Scituate Reservoir drainage area, Rhode Island, from October 1, 2019, through September 30, 2020.

Continuous-record streamgages were operated and maintained by the USGS during WY 2020 (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 the NWIS web interface (U.S. Geological Survey, 2023). Error associated with measured streamflows was generally within about 15 percent as noted in the annual water year summary for each USGS streamgage (U.S. Geological Survey, 2023).

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 14 of the 23 streamgages. Water-quality samples were collected monthly or quarterly at 37 sampling stations in the Scituate Reservoir drainage area by PW during WY 2020 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 14 streamgages equipped with continuous specific conductance monitors in the Scituate Reservoir drainage area during WY 2020 (table 1), except from unnamed brook A, Cork Brook, and Huntinghouse Brook (USGS stations 01115120, 01115280, and 01115110, respectively; PW stations 18, 3, and 14, respectively) where samples were not collected during the summer because the streambeds were dry. Samples were collected in the centroid of the streams during the fall, winter, and summer months. Water samples were processed in the USGS New England Water Science Center laboratory in Northborough, Massachusetts, at the conclusion of scheduled sampling. After the processing, the samples were packed in ice and shipped overnight to the USGS National Water Quality Laboratory in Lakewood, Colorado. Analytical results and parameter codes for the USGS water-quality samples are available through the NWIS web interface (U.S. Geological Survey, 2023); these include specific conductance and dissolved concentrations of sodium, calcium, magnesium, potassium, chloride, and sulfate.

The USGS collected and analyzed continuous-record specific conductance data at 14 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 (U.S. Geological Survey, 2023).

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 Hirsch, 2002) on the basis of concurrent measurements of specific conductance (USGS parameter code 90095) along with chloride (USGS parameter code 00940) and sodium (USGS 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 2009–22 (table 3; U.S. Geological Survey, 2023).

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, 2019, through September 30, 2020.

[Data were collected by the U.S. Geological Survey (U.S. Geological Survey, 2023). Constituent concentrations, continuous specific conductance, and parameter codes are available in National Water Information System (U.S. Geological Survey, 2023). Locations of stations are shown in figure 1. U.S. Geological Survey (USGS) parameter codes: specific conductance, 90095; chloride, 00940; sodium, 00930. PW, Providence Water]

PW station number USGS station number Samples used in analyses Chloride Sodium Percentage of specific conductance record unavailable where streamflow is greater than zero
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 to 11/16/2021 39 1.1630 0.10812 3.3 1.1051 0.08833 5.3 16
28 01115265 03/28/2001; 03/30/2005; 01/22/2009 to 11/16/2021 40 1.1459 0.11636 3.5 1.0547 0.10919 5.6 4.5
35 01115187 03/28/2001; 03/29/2005; 01/22/2009 to 11/15/2021 39 1.1785 0.09824 4.1 1.0970 0.08864 5.4 1.1
3 01115280 03/08/2000; 03/30/2005; 01/22/2009 to 11/15/2021 38 1.1956 0.08857 3.2 1.0800 0.09416 5.0 17
5 01115184 03/05/2009 to 11/16/2021 36 1.2396 0.06766 4.0 1.0899 0.08277 4.8 4.5
6 01115183 03/08/2000; 03/30/2005; 01/22/2009 to 11/15/2021 49 1.1728 0.08720 4.2 1.1921 0.04741 5.9 0.61
7 01115297 03/28/2001; 03/30/2005; 01/22/2009 to 11/15/2021 39 1.0389 0.14192 4.0 0.85947 0.19964 5.7 2.3
8 01115276 01/22/2009 to 11/16/2021 36 1.0901 0.14386 2.7 1.0291 0.12116 3.7 0.09
9 01115275 03/08/2000; 03/30/2005; 01/22/2009 to 11/15/2021 38 1.0578 0.17900 2.3 1.0809 0.09349 3.2 1.5
19 01115170 03/08/2000; 03/29/2005; 01/22/2009 to 11/15/2021 45 1.2121 0.07585 2.4 1.2117 0.04525 2.7 2.3
14 01115110 03/28/2001; 03/29/2005; 01/22/2009 to 11/16/2021 45 0.9960 0.18085 7.4 0.9272 0.14989 7.8 0.21
15 01115114 01/22/2009 to 11/15/2021 52 1.1363 0.11603 3.8 1.0679 0.10090 5.4 2.2
16 01115098 03/28/2001; 03/29/2005; 01/22/2009 to 11/15/2021 39 1.2422 0.06467 4.2 1.0567 0.09967 6.3 3.4
18 01115120 01/22/2009 to 11/15/2021 30 1.1620 0.09866 2.7 1.1455 0.06225 3.4 21
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, 2019, through September 30, 2020.

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 2020 (table 3).

Data Collected by the Providence Water

Water-quality samples were collected by PW at 37 fixed stations on tributaries draining to the Scituate Reservoir during WY 2020. Samples were typically taken monthly at 19 stations and quarterly at another 18 stations (table 1). Water-quality samples were not collected during specific weather conditions; instead, a periodic water-quality sampling schedule was followed so that water-quality samples would be representative of various 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. Water-quality properties and constituent concentrations were measured by using unfiltered water samples. These water-quality properties included pH, alkalinity, color, turbidity, and concentrations of chloride, nitrite, nitrate, orthophosphate, and 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 PO4 unit, whereas phosphate is used to name any compound having one or more PO4 units. Analytical methods used for the determination of 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. The WY 2020 daily mean flow-duration curves for USGS streamgages Hemlock Brook (USGS station 01115265; PW station 28) and Winsor Brook (USGS station 01115185; PW station 26), are shown in figure 2. The curves represent the percentage of time that each flow duration 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 Hemlock Brook, samples were collected at flow rates that are exceeded between 0.4 percent of the time and 99 percent of the time; this range indicates that the water-quality samples collected in WY 2020 represent nearly the entire range of the flow conditions during that water year. Samples collected only on a quarterly schedule at Winsor Brook (in WY 2020, only three samples were collected) were collected during flow rates that are exceeded between 27 percent of the time and 74 percent of the time; this range of flow rates excludes the flow conditions for both the lower and upper flow range at Winsor Brook during WY 2020 (fig. 2).

12 samples were collected from Hemlock Brook and 3 samples from Winsor Brook.
Figure 2.

Graph showing flow-duration curves and streamflow on the dates (represented by points) when water-quality samples were collected at the U.S. Geological Survey streamgages Hemlock Brook (station 01115265) and Winsor Brook (station 01115185), in Foster, Rhode Island, for water year 2020. Locations of stations are shown on figure 1. Modified from Breault (2009).

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 2020. 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 four 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.

Daily loads of water-quality constituents (in samples collected by PW) were calculated for all sampling dates during WY 2020 (table 4) for which periodic- or continuous-streamflow data were available (table 1). These loads were calculated by multiplying constituent concentrations (in milligrams or colony forming units per liter) in single 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, 2019, through September 30, 2020.

[Data from Spaetzel and Smith (2021). Water-quality data are from samples collected and analyzed by the Providence Water (PW). Locations of stations shown on figure 1. USGS, U.S. Geological Survey; ft3/s, cubic foot per second; CFU×106/d; million colony forming units per day; E. coli, Escherichia coli; kg/d, kilogram per day; g/d, gram per day as N, gram per day as nitrogen; g/d, gram per day; <, less than; >, greater than]

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 phosphate)
Barden Reservoir subbasin
24 01115190 10/04/2019 0.24 6,000 59 17 0.59 <15 18
11/25/2019 17 530,000 50,000 1,000 83 <1,000 4,200
12/06/2019 6.5 93,000 1,600 430 16 <400 480
01/29/2020 12 210,000 <1,500 710 29 2,100 870
02/20/2020 8.6 120,000 <1,100 540 21 1,500 630
03/06/2020 6.0 150,000 <730 370 15 1,000 290
04/03/2020 13 410,000 <1,600 730 32 <800 1,600
05/21/2020 4.6 160,000 <570 230 23 <280 340
06/05/2020 1.9 180,000 11,000 110 14 430 180
07/16/2020 0.32 18,000 490 17 1.5 130 15
08/20/2020 0.01 1,500 36 0.61 0.024 3.7 0.49
25 01115200 11/26/2019 16 320,000 7,700 520 77 <960 1,100
03/19/2020 7.2 290,000 <870 220 35 <440 520
06/23/2020 0.15 10,000 120 4.1 0.74 36 7.4
26 01115185 11/26/2019 21 450,000 42,000 1,100 100 <1,300 1,000
03/19/2020 10 480,000 7,700 560 49 1,900 990
06/23/2020 0.26 13,000 130 20 1.3 270 32
28 01115265 10/08/2019 0.79 9,300 190 75 1.9 <48 <9.7
11/12/2019 8.3 59,000 2,000 590 41 <510 1,600
12/10/2019 71 13,000,000 420,000 3,500 520 8,700 8,700
01/23/2020 18 160,000 <2,100 1,500 86 4,900 1,700
02/12/2020 26 160,000 <3,200 1,500 130 2,500
03/10/2020 9.2 110,000 2,200 610 45 1,600 670
04/16/2020 23 330,000 <2,800 1,000 110 <1,400 4,500
05/29/2020 7.3 360,000 7,300 490 54 <450 1,100
06/10/2020 4.0 92,000 5,000 290 30 710 890
07/14/2020 0.30 7,000 75 28 1.5 150 45
08/11/2020 0.04 4,800 36 3.2 0.17 20 6.1
09/08/2020 0.05 2,500 25 3.4 0.12 26 6.2
35 01115187 10/04/2019 2.4 260,000 3,700 160 5.9 <150 1,200
11/25/2019 120 7,500,000 670,000 6,300 610 16,000 48,000
12/06/2019 24 250,000 5,800 1,300 58 3,700 2,300
01/29/2020 33 460,000 <4,000 1,700 80 7,400 1,600
02/20/2020 29 200,000 <3,500 1,700 71 7,300 1,400
03/06/2020 20 180,000 <2,400 1,100 49 4,500 980
04/03/2020 47 510,000 <5,800 2,300 120 <2900 2,300
05/21/2020 22 1,800,000 11,000 1,000 110 <1300 1,600
06/05/2020 5.3 590,000 5,300 300 26 850 510
07/16/2020 0.76 91,000 580 43 3.7 270 37
08/20/2020 0.13 10,000 97 9.4 0.31 30 9.4
Direct Runoff subbasin
1 01115180 10/01/2019 0.33 4,700 <40 16 0.81 230 65
11/05/2019 2.6 95,000 1,900 89 13 810 940
12/06/2019 3.0 100,000 730 110 22 1,300 730
01/22/2020 2.3 59,000 <280 92 11 1,400 280
02/04/2020 2.5 64,000 <310 84 12 1,400 490
03/03/2020 2.8 110,000 <350 98 14 1,600 210
04/07/2020 3.6 85,000 890 110 18 1,300 360
05/18/2020 3.9 93,000 <470 110 19 1,300 280
06/02/2020 1.3 65,000 2,700 45 9.5 860 190
07/07/2020 0.57 25,000 430 20 2.8 380 98
08/04/2020 0.08 5,300 19 3.0 0.19 47 7.5
09/01/2020 0.10 1,800 <12 3.8 0.23 69 9.3
3 01115280 10/03/2019 0.03 750 <4 5.3 0.08 5.5 6.4
11/07/2019 2.4 28,000 <300 330 5.9 380 470
12/12/2019 7.1 140,000 1,700 750 17 1,800 170
01/09/2020 3.5 45,000 <430 400 8.6 1,900 260
02/18/2020 3.5 33,000 <420 400 8.5 1,900 250
03/05/2020 2.9 31,000 <350 310 7.0 1,300 70
04/21/2020 6.9 110,000 1,700 610 17 1,500 1,000
05/19/2020 2.3 34,000 <280 190 5.5 420 220
06/04/2020 0.75 17,000 <91 68 1.8 500 91
07/02/2020 0.43 62,000 1,900 44 2.1 230 64
08/06/2020 0.06 22,000 160 8.4 0.14 42 4.3
4 01115400 10/01/2019 0.02 2,600 <2.9 0.52 0.057 <1.4 0.57
11/05/2019 0.83 19,000 200 12 2.0 <51 61
12/06/2019 1.1 7,700 <130 18 2.6 <66 79
01/22/2020 0.69 4,200 <84 13 1.7 <42 17
02/04/2020 0.79 4,200 <97 12 1.9 <49 97
03/03/2020 1.0 6,300 <120 19 2.4 <61 24
04/07/2020 1.5 61,000 <190 24 3.8 <94 300
05/18/2020 1.7 140,000 <210 22 4.2 <100 83
06/02/2020 0.25 34,000 <31 3.9 0.61 <15 18
07/07/2020 0.06 8,100 30 0.88 0.15 <3.7 5.9
08/04/2020 0.00 220 0.45 0.04 0.004 <0.11 0.13
5 01115184 10/31/2019 1.2 63,000 940 85 9.1 300 180
03/17/2020 1.6 55,000 <190 91 3.8 1,300 76
06/26/2020 0.30 19,000 230 21 1.5 190 37
6 01115183 10/01/2019 0.2 5,100 74 23 1.1 43 26
11/05/2019 1.0 49,000 1,000 150 6.8 830 510
12/06/2019 2.4 61,000 580 240 12 1,700 700
01/22/2020 2.7 35,000 2,600 270 6.6 3,400 260
02/04/2020 4.2 41,000 1,000 340 10 3,800 310
03/03/2020 4.0 30,000 <530 360 11 3,600 320
04/07/2020 4.6 130,000 7,200 350 23 2,600 450
05/18/2020 4.0 260,000 9,600 280 20 1,800 390
06/02/2020 0.77 69,000 2,100 67 5.7 590 110
07/07/2020 0.08 16,000 300 13 0.80 26 16
7 01115297 10/03/2019 0.83 33,000 <100 39 4.0 <50 61
11/07/2019 4.0 120,000 11,000 110 20 <240 690
12/12/2019 16 360,000 47,000 330 78 2,700 390
01/09/2020 11 47,000 <1,300 280 27 4,300 800
02/18/2020 10 51,000 2,400 240 24 3,700 470
03/05/2020 9.3 120,000 4,600 260 46 2,800 910
04/21/2020 15 200,000 7,200 370 36 <900 1,100
05/19/2020 7.9 530,000 10,000 170 39 <480 580
06/04/2020 2.0 150,000 6,500 66 9.8 <120 200
07/02/2020 0.74 180,000 3,200 24 7.3 <46 91
08/06/2020 0.11 6,400 54 3.9 0.27 19 11
09/03/2020 0.03 1,500 8.4 1.1 0.084 5.6 1.7
8 01115276 10/18/2019 8.1 43,000 2,000 220 20 <490 200
11/18/2019 3.7 7,300 <450 120 <4.5 710 270
12/27/2019 10 21,000 <1,200 320 24 1,500 1,200
01/24/2020 7.6 7,700 <930 260 19 <470 370
02/21/2020 11 22,000 <1,300 370 26 <650 520
03/20/2020 13 20,000 <1,600 420 31 <790 630
04/17/2020 23 47,000 <2,800 660 56 <1,400 1,100
05/27/2020 6.9 29,000 <850 200 17 <420 680
06/16/2020 3.1 38,000 <380 100 7.5 <190 150
07/24/2020 1.9 91,000 <230 61 4.6 <110 180
08/21/2020 1.6 67,000 <200 58 4.0 <100 200
9 01115275 10/31/2019 1.1 100,000 280 140 5.5 1,200 140
03/17/2020 1.3 39,000 <160 200 3.2 2,100 65
06/26/2020 0.39 46,000 300 72 1.9 700 29
08/27/2020 0.26 13,000 330 55 0.63 650 19
32 01115178 10/30/2019 0.41 14,000 <50 15 2.0 <25 70
03/06/2020 0.72 20,000 <87 26 1.7 700 52
06/22/2020 0.07 9,900 110 2.9 1.1 78 20
33 01115182 10/29/2019 0.35 14,000 260 7.5 0.85 <21 34
02/26/2020 0.28 3,000 <35 8.6 0.69 170 21
06/11/2020 0.22 5,700 <27 11 0.53 110 16
Moswansicut Pond reservoir subbasin
19 01115170 10/15/2019 0.09 2,900 <11 14 0.23 <5.7 2.3
11/19/2019 2.4 2,400 <290 340 5.8 330 120
12/19/2019 12 52,000 21,000 1,600 30 2,700 300
01/16/2020 5.8 2,900 2,900 840 14 1,400 290
02/25/2020 5.0 1,200 <610 720 24 1,300 370
03/12/2020 3.9 6,000 <470 540 9.5 1,000 190
04/09/2020 9.3 12,000 <1,100 1,300 23 1,700 690
05/22/2020 4.6 14,000 <570 640 11 870 790
06/11/2020 4.2 130,000 <510 560 21 <260 410
07/09/2020 0.97 50,000 <120 130 2.4 <60 140
08/13/2020 0.14 14,000 <18 20 0.35 <8.8 39
09/10/2020 0.10 37,000 48 14 0.24 <5.9 NA
21 01115165 12/02/2019 0.57 31,000 1,700 69 4.2 65 97
03/26/2020 0.88 28,000 <110 140 8.6 2,200 86
05/15/2020 0.77 65,000 3,800 77 5.6 1,200 94
Regulating reservoir subbasin
14 01115110 10/17/2019 52 >31,000,000 16,600,000 1,500 770 8,700 13,000
11/08/2019 7.7 230,000 5,800 320 19 <470 750
12/16/2019 32 650,000 67,000 1,100 78 6,600 2,300
01/28/2020 14 150,000 18,000 660 35 3,700 2,500
02/27/2020 24 650,000 43,000 630 120 7,300 2,300
03/13/2020 7.1 70,000 1,700 260 34 2,200 520
04/10/2020 42 1,800,000 42,000 1,000 200 <2,600 8,200
05/28/2020 4.0 380,000 5,100 130 19 1,100 390
06/12/2020 1.5 40,000 1,100 180 15 1,000 260
07/23/2020 0.026 3,900 210 0.90 0.13 8.5 1.9
15 01115114 10/17/2019 55 >33,000,000 17,600,000 4,200 1,600 17,000 19,000
11/08/2019 5.7 180,000 4,300 680 28 <350 840
12/16/2019 18 330,000 14,000 1,600 45 5,400 900
01/28/2020 11 140,000 2,700 1,000 27 3,100 1,300
02/27/2020 23 650,000 60,000 1,700 110 5,000 1,700
03/13/2020 5.9 92,000 4,300 930 29 1,800 580
04/10/2020 28 930,000 43,000 1,900 130 3,400 4,700
05/28/2020 1.9 85,000 <230 230 9.1 600 180
06/12/2020 1.4 67,000 2,400 220 10 750 170
16 01115098 10/17/2019 47 >28,000,000 3,300,000 4,700 230 5,900 3,400
11/08/2019 7.0 56,000 <860 820 17 <430 520
12/16/2019 32 740,000 <3,900 3,100 77 11,000 1,500
01/28/2020 12 110,000 5,700 1,100 28 5,200 850
02/27/2020 11 140,000 <1,300 1,100 27 4,400 1,300
03/13/2020 10 140,000 <1,200 1,000 49 4,700 490
04/10/2020 30 770,000 46,000 2,500 150 7,700 1,500
05/28/2020 7.9 670,000 <970 610 39 1,200 390
06/12/2020 3.3 280,000 1,600 280 16 <200 320
07/23/2020 0.78 50,000 190 73 1.9 150 96
08/18/2020 0.11 6,400 <13 11 0.27 <6.7 8.0
18 01115120 12/02/2019 0.33 6,500 1,100 53 0.81 350 24
01/09/2020 0.21 1,700 <25 52 0.50 240 35
05/15/2020 0.35 12,000 3,300 50 1.7 240 110
Westconnaug Reservoir subbasin
10 01115274 10/08/2019 0.17 4,700 220 5.0 0.42 <11 <2.1
11/12/2019 1.6 21,000 <200 53 3.9 <98 120
12/10/2019 34 1,100,000 52,000 1,600 160 <2,100 1,600
01/23/2020 1.5 32,000 <180 96 3.6 <90 72
02/12/2020 6.1 130,000 <740 390 15 300
03/10/2020 1.5 77,000 <180 94 3.6 <89 36
04/16/2020 8.9 82,000 <1,100 360 22 <550 1,500
05/29/2020 1.2 300,000 570 67 2.8 <71 110
06/09/2020 0.82 25,000 <100 25 2.0 <50
08/11/2020 0.00 320 1.9 0.13 0.010 <0.24 0.19
11 01115273 10/22/2019 0.68 24,000 330 17 5.0 <42 33
03/23/2020 1.8 47,000 <210 52 8.6 290 1,400
06/29/2020 0.24 38,000 690 5.6 1.7 <14 52
08/28/2020 0.01 800 46 0.09 0.047 <0.39 1.6
Table 4.    Daily loads of bacteria, chloride, nitrite, nitrate, and orthophosphate in the Scituate Reservoir drainage area, Rhode Island, from October 1, 2019, through September 30, 2020.

Streamflow

Monitoring streamflow is a necessary step to measure the volume of water and estimate 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 (USGS station 01115187, PW station 35) for the entire period of its operation (mean of the annual mean streamflows for the period of record, WYs 1995–2019) before WY 2020 was 29.0 cubic feet per second (ft3/s; U.S. Geological Survey, 2023). During WY 2020, the annual mean streamflow of 26.7 ft3/s was just above the 25th percentile (26.2 ft3/s) for the period of record (fig. 3). Daily mean streamflows were within the 10th and 90th percentile of all mean daily streamflows for WYs 1995–2019 and were similar to the median daily streamflows throughout much of the water year, departing higher in the late spring and summer months (fig. 3). The other long-term continuous-record streamgage in the Scituate Reservoir drainage area is the Peeptoad Brook streamgage (USGS station 01115098; PW station 16). The mean annual streamflow at the Peeptoad Brook streamgage for the period of record (WYs 1995–2019) before WY 2020 was 10.5 ft3/s (U.S. Geological Survey, 2023). The annual mean streamflow at the Peeptoad Brook streamgage during WY 2020 also was lower than the mean annual streamflow for its period of record at 8.69 ft3/s. The annual mean measured or estimated streamflows for the other monitoring stations in this study ranged from about 0.32 to 12.0 ft3/s (table 2).

From Oct to May there was average or high flow; after, it was low. The annual mean
                     streamflow of 2020 was about 25th percentile
Figure 3.

Hydrologic data taken at the U.S. Geological Survey streamgage on the Ponaganset River in Foster, Rhode Island (01115187); A, Graph showing measured daily mean streamflow for October 1, 2019, through September 30, 2020, and the 10th percentile, median, and 90th percentile values of daily streamflow for October 1, 1994, through September 30, 2019; and B, Boxplot showing annual mean streamflow during water year 2020 and the distribution of mean annual streamflows for water years 1994–2019. Location of station is shown on figure 1. Modified from Breault (2009).

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 (or outliers). Medians are especially important to use for summarizing a relatively limited number of values. In contrast, continuously monitored streamflow and loads of chloride and sodium (estimated from measurements of specific conductance), datasets that include a large number of values, 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 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 affect load and yield calculations more than the errors associated with measuring specific conductance or chemical analysis, and 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 (2022b) 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 7.1 to 123 milligrams per liter (mg/L) and estimated monthly mean sodium concentrations ranged from 5.1 to 70 mg/L (table 5). The highest monthly mean concentrations of chloride and sodium were estimated to be 123 and 70 mg/L, respectively, in the unnamed tributary to Regulating reservoir (USGS station 01115120; PW station 18) in both April and July 2020. The estimated monthly mean concentrations at most stations were greater during the late summer months, particularly August and September, compared with the estimated monthly mean concentrations during the winter months. Estimated monthly mean concentrations peaked in October at the beginning of the water year for Dolly Cole Brook (USGS station 01115190; PW station 24) and Moswansicut stream (USGS station 01115170; PW station 19). Peak estimated mean concentrations for Rush Brook (USGS station 01115114; PW station 15) occurred in June; and mean concentrations peaked in July for Hemlock Brook (USGS station 01115265; PW station 28), Wilbur Hollow Brook (USGS station 01115297; PW station 7), and for the unnamed tributary to Regulating reservoir (USGS station 01115120; PW station 18). The estimated monthly mean concentrations for chloride (12 mg/L) and sodium (7.6 mg/L) in Wilbur Hollow Brook were the same in July and September; likewise, monthly mean concentrations for chloride (123 mg/L) and sodium (70 mg/L) in the unnamed tributary to Regulating reservoir were the same in April and July.

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 59 and 34 mg/L, respectively, in the unnamed tributary to Regulating reservoir (USGS station 01115120; PW station 18; table 6). The relatively high annual mean concentrations of 51 mg/L of chloride and 30 mg/L of sodium in Bear Tree Brook (USGS station 01115275; PW station 9) are the result of residual chloride and sodium leaching to groundwater from a formerly uncovered salt storage pile (Nimiroski and Waldron, 2002). Annual mean concentrations of 54 mg/L of chloride and 32 mg/L of sodium estimated at Moswansicut stream (USGS station 01115170; PW station 19) also were relatively high (table 6). The stations on the Moswansicut Pond reservoir and the unnamed tributary to Regulating reservoir are in the more developed, northeastern part of the Scituate Reservoir drainage area (fig. 1.)

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, 2019, through September 30, 2020.

[Data were collected by the U.S. Geological Survey (U.S. Geological Survey, 2023). 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. PW, Providence Water; USGS, U.S. Geological Survey; Cl, chloride; Na, sodium; mg/L, milligram per liter; —, not applicable]

PW station number USGS station number October 2019 November 2019 December 2019 January 2020 February 2020
Cl (mg/L) Na (mg/L) Cl (mg/L) Na (mg/L) Cl (mg/L) Na (mg/L) Cl (mg/L) Na (mg/L) Cl (mg/L) Na (mg/L)
Barden Reservoir subbasin
24 01115190 28 17 27 17 23 14 25 15 25 15
28 01115265 31 19 23 14 21 13 27 16 24 15
35 01115187 23 14 21 13 17 11 20 12 20 12
Direct Runoff subbasin
3 01115280 41 24 43 25 36 21 40 23 35 21
5 01115184 33 19 26 16 24 14 21 13 21 13
6 01115183 32 19 36 22 27 16 30 18 32 19
7 01115297 10 6.9 9.3 6.3 7.1 5.1 8.6 5.9 9.0 6.2
8 01115276 22 14 20 13 17 11 19 12 19 12
9 01115275 53 32 52 31 45 27 51 30 53 31
Moswansicut Pond reservoir subbasin
19 01115170 56 33 56 33 56 33 55 33 55 33
Regulating reservoir subbasin
14 01115110 13 7.8 12 7.7 11 6.6 12 7.2 12 7.2
15 01115114 31 19 29 18 41 25 39 24 30 19
16 01115098 37 22 38 23 36 22 28 17 29 18
18 01115120 73 42 71 41 52 30 48 28 59 34
Mean 34 21 33 20 29 18 30 18 30 18
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, 2019, through September 30, 2020.

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, 2019, through September 30, 2020.—Continued

[Data were collected by the U.S. Geological Survey (U.S. Geological Survey, 2023). 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. PW, Providence Water; USGS, U.S. Geological Survey; Cl, chloride; Na, sodium; mg/L, milligram per liter; —, not applicable]

March 2020 April 2020 May 2020 June 2020 July 2020 August 2020 September 2020
Cl (mg/L) Na (mg/L) Cl (mg/L) Na (mg/L) Cl (mg/L) Na (mg/L) Cl (mg/L) Na (mg/L) Cl (mg/L) Na (mg/L) Cl (mg/L) Na (mg/L) Cl (mg/L) Na (mg/L)
Barden Reservoir subbasin
23 14 19 12 19 12 24 15 22 14 24 15 25 15
21 13 15 10 18 11 29 18 38 22 36 22 34 20
17 11 15 9 16 10 20 12 22 14 29 18 28 17
Direct Runoff subbasin
33 20 27 16 27 16 30 18 40 24 49 28 55 31
21 13 17 11 17 11 23 14 32 19 60 32 67 35
30 18 24 14 26 16 35 21 55 34 69 42 98 60
8.9 6.1 8.1 5.7 9.1 6.2 11 7.5 12 7.6 11 7.5 12 7.6
19 12 17 11 18 12 25 16 30 19 32 20 26 17
52 31 45 27 49 29 64 38 76 45 82 49 85 51
Moswansicut Pond reservoir subbasin
53 32 52 31 51 30 53 32 55 33 55 33 55 33
Regulating reservoir subbasin
10 6.0 8.9 5.6 10 6.4 14 8.4 15 8.9 15 9.2 34 20.8
23 14 30 18 42 26 58 35 41 24 45 26 38 23
34 21 39 23 40 24 40 24 44 25 61 35 69 40
95 54 123 70 59 34 95 54 123 70 0.0 0.0 0.0 0.0
31 19 31 19 29 17 37 22 43 26 41 24 45 26
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, 2019, through September 30, 2020.—Continued

Table 6.    

Estimated annual mean chloride and sodium concentrations, loads, and yields of streamgage stations in the Scituate Reservoir drainage area, Rhode Island, from October 1, 2019, through September 30, 2020.

[Data were collected by the U.S. Geological Survey (U.S. Geological Survey, 2023). 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; USGS, U.S. Geological Survey; Cl, chloride; mg/L, milligram per liter; Na, sodium; t/yr, metric tons per year; (t/yr)/mi2, metric tons 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)
Barden Reservoir subbasin
24 01115190 22 14 160 98 32 20
28 01115265 22 14 230 140 27 17
35 01115187 18 11 420 270 30 19
Direct Runoff subbasin
3 01115280 34 20 99 59 53 32
5 01115184 22 13 35 21 28 17
6 01115183 29 17 89 53 45 27
7 01115297 8.6 5.9 55 38 13 8.8
8 01115276 19 12 150 94 28 18
9 01115275 51 30 69 41 110 66
Moswansicut Pond reservoir subbasin
19 01115170 54 32 260 160 81 48
Regulating reservoir subbasin
14 01115110 10 6.5 98 61 16 10
15 01115114 31 19 220 130 46 28
16 01115098 36 21 280 170 56 33
18 01115120 59 34 29 17 100 60
Scituate Reservoir drainage area
Mean concentration or yield 30 18 35 22
Total load 2,200 1,400
Table 6.    Estimated annual mean chloride and sodium concentrations, loads, and yields of streamgage stations in the Scituate Reservoir drainage area, Rhode Island, from October 1, 2019, through September 30, 2020.

During WY 2020, the Scituate Reservoir received about 2,200 metric tons of chloride and 1,400 metric tons 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 2020 were estimated to be 420 and 270 metric tons, respectively, at the Ponaganset River station (USGS station 01115187; PW station 35; table 6). Monthly estimated chloride and sodium loads were both highest or tied for highest in December for 8 of the 14 stations (table 7). Monthly estimated chloride and sodium loads for Rush Brook (USGS station 01115114; PW station 15) were highest in September 2020 (table 7). Hemlock Brook (USGS station 01115265; PW station 28), Peeptoad Brook (USGS station 01115098; PW station 16), and unnamed tributary to Regulating reservoir (USGS station 01115120; PW station 18) had similar or the same loads in January as the loads in December; and Quonopaug Brook (USGS station 01115183; PW station 6) and Huntinghouse Brook (USGS station 01115110; PW station 14) had similar or the same loads in April as December (table 7). Monthly estimated chloride and sodium loads for Wilbur Hollow Brook (USGS Station 01115297; PW station 7), Moswansicut stream (USGS station 01115170; PW station 19), Westconnaug Brook (USGS station 01115276; PW station 8), and Bear Tree Brook (USGS station 01115275; PW station 9) were highest in April (table 7).

During WY 2020, estimated annual loads of chloride and sodium at the continuous streamgage stations were less than the median estimated annual loads for WYs 2009–19 for 9 of the 14 USGS stations (fig. 4) and the sum of these annual loads during WY 2020 were 37 percent lower than the sum of annual loads estimated during the previous water year (Smith, 2022b). From December through April, the sum of the monthly estimated loads of chloride and sodium for the respective drainage areas upstream from each streamgage accounted for about 60 to 80 percent of the annual load of chloride and sodium, except at Rush Brook (USGS station 01115114; PW station 15), where this period only accounted for 45 percent of the annual load for the drainage area of this streamgage (table 7). The Ponaganset River (USGS station 01115187; PW station 35), which accounts for 22 percent of the combined drainage area upstream from the 14 USGS streamgage stations, accounted for 19 percent of the combined annual load of chloride and sodium for the 14 USGS streamgage stations. Rush Brook, having about one third of the drainage area of Ponaganset River (table 1) but nearly twice the impervious area (Spaetzel and Smith, 2022b), had the second highest percentage (14 percent of the combined annual load of chloride and sodium) for the respective drainage area (table 6).

The highest annual chloride and sodium yields were 110 and 66 metric tons per year per square mile ([t/yr]/mi2; table 6), respectively, measured at Bear Tree Brook (USGS station 01115275; PW station 9) in a small subbasin (0.62 square mile). These high yields were the result, in part, from chloride and sodium groundwater contamination (Nimiroski and others, 2008). Chloride and sodium yields at 30 and 19 (t/yr)/mi2, respectively for Ponagaset River (USGS station 01115187; PW station 35), which is the largest subbasin in the Scituate Reservoir watershed, were less than one third of the yields for Bear Tree Brook. The estimated annual mean yields of chloride and sodium for the drainage area upstream from the 14 USGS streamgage stations were 35 and 22 (t/yr)/mi2 (table 6), respectively. These estimated annual mean yields of chloride and sodium for WY 2020 were substantially less than the estimated annual mean yields of chloride and sodium in the three prior water years (fig. 5) and among the lowest since WY 2009.

Table 7.    

U.S. Geological Survey monthly estimated chloride and sodium loads by sampling station, in the Scituate Reservoir drainage area, Rhode Island, October 1, 2019, through September 30, 2020.

[Data were collected by the U.S. Geological Survey (U.S. Geological Survey, 2023). Alternate station names are listed in parentheses for stations where different historical names were used for the same sampling location by the Providence Water (PW). Locations of stations are shown on figure 1. USGS, U.S. Geological Survey; Cl, chloride; t, metric ton; Na, sodium; —, not applicable]

PW station number USGS station number October 2019 November 2019 December 2019 January 2020 February 2020
Cl (t) Na (t) Cl (t) Na (t) Cl (t) Na (t) Cl (t) Na (t) Cl (t) Na (t)
Barden Reservoir subbasin
24 01115190 6.6 4.1 11 7.0 36 22 21 13 19 12
28 01115265 7.2 4.3 20 13 50 31 50 31 34 21
35 01115187 25 15 41 25 81 51 49 31 46 29
Direct Runoff subbasin
3 01115280 5.9 3.5 9.3 5.4 24 14 15 8.8 10 6.0
5 01115184 1.6 0.90 3.8 2.3 7.6 4.6 5.6 3.4 4.2 2.6
6 01115183 5.2 3.1 5.1 3.0 15 9.2 13 8.0 12 7.1
7 01115297 3.5 2.4 2.9 2.0 8.1 5.8 8.0 5.5 7.0 4.8
8 01115276 6.5 4.1 8.7 5.6 20 13 18 11 16 10
9 01115275 2.7 1.6 4.0 2.4 9.5 5.6 8.6 5.1 7.7 4.6
Moswansicut Pond reservoir subbasin
19 01115170 16 9.4 17 10 41 24 32 19 28 17
Regulating reservoir subbasin
14 01115110 4.6 2.9 7.9 4.9 21 13 10 6.5 11 6.7
15 01115114 14 8.9 44 27 27 17 28 17 28 17
16 01115098 27 16 35 21 37 22 35 22 21 13
18 01115120 2.6 1.5 2.5 1.4 3.0 1.7 6.7 3.8 3.0 1.7
Total load 130 78 210 130 380 230 150 87 250 150
Table 7.    U.S. Geological Survey monthly estimated chloride and sodium loads by sampling station, in the Scituate Reservoir drainage area, Rhode Island, October 1, 2019, through September 30, 2020.

Table 7.    

U.S. Geological Survey monthly estimated chloride and sodium loads by sampling station, in the Scituate Reservoir drainage area, Rhode Island, October 1, 2019, through September 30, 2020.—Continued

[Data were collected by the U.S. Geological Survey (U.S. Geological Survey, 2023). Alternate station names are listed in parentheses for stations where different historical names were used for the same sampling location by the Providence Water (PW). Locations of stations are shown on figure 1. USGS, U.S. Geological Survey; Cl, chloride; t, metric ton; Na, sodium; —, not applicable]

March 2020 April 2020 May 2020 June 2020 July 2020 August 2020 September 2020
Cl (t) Na (t) Cl (t) Na (t) Cl (t) Na (t) Cl (t) Na (t) Cl (t) Na (t) Cl (t) Na (t) Cl (t) Na (t)
Barden Reservoir subbasin
19 12 28 18 13 8.3 2.6 1.6 0.86 0.54 0.054 0.034 0.021 0.013
22 14 23 15 20 13 6.9 4.2 1.1 0.64 0.15 0.087 0.14 0.082
48 30 73 46 46 29 9.5 5.9 3.4 2.1 0.35 0.21 0.25 0.15
Direct Runoff subbasin
11 6.8 15 9.1 7.1 4.4 1.4 0.83 0.41 0.24 0.19 0.11 0.16 0.09
3.5 2.1 3.9 2.4 3.0 1.9 1.1 0.67 0.29 0.17 0.11 0.062 0.13 0.067
11 6.8 15 9.1 9.3 5.6 1.6 0.97 0.65 0.39 0.070 0.042 0.006 0.003
7.0 4.8 10 7.2 6.8 4.6 1.3 0.85 0.36 0.24 0.036 0.024 0.002 0.001
17 11 24 16 17 11 6.8 4.3 5.0 3.1 4.3 2.7 2.9 1.8
8.3 4.9 12 6.9 8.3 4.9 3.0 1.8 2.2 1.3 1.9 1.1 1.6 0.95
Moswansicut Pond reservoir subbasin
29 17 51 31 31 18 11 6.4 5.3 3.2 0.69 0.41 0.48 0.29
Regulating reservoir subbasin
11 7.0 20 12 12 7.3 1.2 0.72 0.17 0.10 0.002 0.001 11 7.0
36 23 22 14 4.3 2.6 0.30 0.20 13 7.8 28 16 67 40
9.4 5.7 4.2 2.5 0.18 0.11 0.02 0.01 0.68 0.39 1.3 0.73 9.7 5.6
0.10 0.10 0.005 0.003 3.0 1.7 0.090 0.052 0.005 0.003 <0.001 <0.001 <0.001 <0.001
230 150 300 190 180 110 47 28 33 20 37 22 93 56
Table 7.    U.S. Geological Survey monthly estimated chloride and sodium loads by sampling station, in the Scituate Reservoir drainage area, Rhode Island, October 1, 2019, through September 30, 2020.—Continued
The subbasins had variably low or high loads in 2020 compared to the previous year's
                        median annual loads.
Figure 4.

Graph showing annual loads of chloride and sodium estimated from streamflow and specific conductance data for water year 2020 and associated minimum, maximum, and median annual loads for water years 2009–19 at 14 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; station information is shown in table 1. Modified from Smith (2016).

2020 had some of the lowest sodium and chloride loads since 2012, and much lower than
                        2019
Figure 5.

Bar chart showing estimated annual loads and estimated annual yields of chloride and sodium estimated from continuous measurements of streamflow and specific conductance for water years 2009–20 for the area upstream from 14 Providence Water sampling stations in the Scituate Reservoir drainage area, Rhode Island. Locations of stations are shown on figure 1; station information is shown in table 1. Modified from Smith (2019a).

Physical and Chemical Properties and Daily Loads and Yields Estimated From Data Collected by the 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 E. coli 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, turbidity, alkalinity, and color were routinely measured to determine water quality in each of the six subbasins in the Scituate Reservoir drainage area (table 8) by PW. Specifically, pH is a measure of the acidity of the water; 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 8.    

Median values for water-quality data collected at Providence Water stations in the Scituate Reservoir drainage area, Rhode Island, from October 1, 2019, through September 30, 2020.

[Data from Spaetzel and Smith (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; PCU, platinum-cobalt units; NTU, nephelometric turbidity units; CFU/100 mL, colony forming units per 100 milliliters; E.coli., Escherichia coli; mg/L, milligrams per liter ; CaCO3, calcium carbonate; N, nitrogen; PO4, phosphate; <, less than; —, no data]

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)
Barden Reservoir subbasin
24 01115190 6.4 55 0.72 1,300 10 4.4 24.8 0.001 0.07 0.03
25 01115200 6.3 70 0.98 1,700 20 3.1 12.7 0.002 <0.05 0.03
26 01115185 6.4 50 0.56 1,900 31 4.5 22.5 0.002 0.08 0.04
27 011151845 6.0 25 0.27 620 <10 4.2 13.3 <0.001 0.21 0.02
28 01115265 6.3 100 0.68 760 10 5.7 28.3 0.002 0.07 0.06
29 01115271 6.6 45 0.66 460 <10 4.8 23.1 0.001 <0.05 0.03
35 01115187 6.4 50 0.89 2,500 20 4.4 22.8 0.001 0.07 0.03
Direct Runoff subbasin
1 01115180 7.0 63 1.2 1,200 <10 11 14.3 0.002 0.24 0.06
2 01115181 6.2 8 0.26 200 <15 5.3 80.6 0.001 0.45 1.24
3 01115280 6.6 45 0.37 660 <10 5.8 45.0 0.001 0.19 0.04
4 01115400 6.5 25 0.50 1,600 <10 7.2 6.50 0.001 <0.05 0.03
5 01115184 6.3 50 0.54 2,100 31 6.0 28.1 0.002 0.26 0.05
6 01115183 6.7 100 1.0 1,200 36 13 38.3 0.002 0.27 0.05
7 01115297 6.3 78 0.81 1,400 20 8.9 11.3 0.002 <0.06 0.03
8 01115276 6.4 20 0.51 86 <10 3.4 13.3 0.001 <0.05 0.02
9 01115275 6.8 43 0.69 2,800 21 9.6 67.9 0.002 0.69 0.03
30 01115350 6.2 50 0.47 2,100 120 6.3 28.6 0.001 0.09 0.03
31 01115177 6.3 120 13.00 17,000 210 10 7,000 0.018 0.99 0.03
32 01115178 6.7 130 1.3 1,400 <10 11 15.0 0.002 0.4 0.07
33 01115182 6.4 35 0.54 1,100 <10 11 12.5 0.001 0.22 0.03
36 6.5 25 0.41 640 <10 3.6 7.60 0.001 0.06 0.03
37 5.8 25 0.26 180 <10 2.5 9.60 <0.001 <0.05 0.05
Moswansicut Pond reservoir subbasin
19 01115170 7.1 19 0.6 150 <10 10 57.4 0.001 0.07 0.03
20 01115160 6.4 160 0.77 590 10 8.3 65.2 0.003 0.10 0.08
21 01115165 6.6 30 1.20 2,200 120 15 49.6 0.003 0.66 0.05
22 01115167 6.7 24 0.90 1,700 <10 15 57.7 0.005 0.9 0.06
34 01115164 6.4 93 1.30 930 <10 14 43.6 0.003 <0.05 0.03
Ponaganset Reservoir subbasin
23 011151843 6.4 15 0.48 400 <10 3.3 18.9 0.001 <0.05 0.02
Regulating reservoir subbasin
13 01115176 6.9 37 0.88 290 <10 11 39.8 0.001 <0.05 0.03
14 01115110 6.6 56 0.96 1,200 52 6.6 14.2 0.002 0.11 0.04
15 01115114 6.7 85 0.94 1,300 31 7.6 37.3 0.002 0.12 0.05
16 01115098 6.7 47 0.86 1,100 <10 13 40.3 0.001 0.08 0.03
17 01115119 6.2 56 0.46 680 10 9.0 37.7 0.001 <0.05 0.04
18 01115120 6.5 55 0.7 800 130 12 65.6 0.001 0.43 0.07
Westconnaug Reservoir subbasin
10 01115274 6.1 26 0.68 1,200 <10 4.4 18.0 0.001 <0.05 0.02
11 01115273 6.2 78 0.82 3,300 70 5.8 9.80 0.003 <0.05 0.10
12 011152745 6.2 30 0.74 320 <10 5.1 14.7 0.001 <0.05 0.06
Scituate Reservoir drainage area
Minimum 5.8 8 0.26 86 <10 2.5 6.5 <0.001 <0.05 0.02
Median 6.4 50 0.69 1,200 10 6.6 24.8 0.001 0.08 0.03
Maximum 7.1 160 13 17,000 210 15 80.6 0.018 0.99 1.24
Table 8.    Median values for water-quality data collected at Providence Water stations in the Scituate Reservoir drainage area, Rhode Island, from October 1, 2019, through September 30, 2020.

The median pH in tributaries in the Scituate Reservoir drainage area ranged from 5.8 to 7.1; the median of the medians for all stations was 6.4. Median values of color ranged from 8 to 160 platinum-cobalt units; the median for all stations was 50 platinum-cobalt units. Median values of turbidity ranged from 0.26 to 13 nephelometric turbidity units; the median for all stations was 0.69 nephelometric turbidity unit. Median alkalinity values in tributaries were low, ranging from 2.5 to 15 mg/L as calcium carbonate; the median for all stations was 6.6 mg/L alkalinity as calcium carbonate (table 8).

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 8, 9, and 10.

Table 9.    

Median daily loads of bacteria, chloride, nitrite, nitrate, and orthophosphate in the Scituate Reservoir drainage area, Rhode Island, from October 1, 2019, through September 30, 2020.

[Concentration data from Spaetzel and Smith (2021). Water-quality data are from samples collected and analyzed by the Providence Water (PW). Locations of stations are shown on figure 1. USGS, U.S. Geological Survey; (CFU×106)/d; million colony forming units per day; E. coli, Escherichia coli; kg/d, kilogram per day; g/d, grams 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 (g/d as N) Nitrate (g/d as N) Orthophosphate (g/d as PO4)
Barden Reservoir subbasin
24 01115190 150,000 <1,100 370 16 430 340
25 01115200 290,000 <870 220 35 <440 520
26 01115185 450,000 7,700 560 49 <1,300 990
28 01115265 100,000 2,200 540 43 <510 1,000
35 01115187 260,000 <4,000 1,100 58 <2,900 1,400
Direct Runoff subbasin
1 01115180 65,000 <390 87 12 1,100 250
3 01115280 33,000 <350 310 5.9 500 170
4 01115400 7,700 <97 12 1.9 49 24
5 01115184 55,000 230 85 3.8 300 76
6 01115183 45,000 1,000 260 8.4 1,800 320
7 01115297 120,000 3,900 140 22 <360 430
8 01115276 29,000 <930 220 19 <490 370
9 01115275 43,000 290 110 2.6 950 47
32 01115178 14,000 <87 15 1.7 78 52
33 01115182 5,700 <35 8.6 0.69 110 21
Moswansicut Pond reservoir subbasin
19 01115170 13,000 <490 550 10 600 290
21 01115165 31,000 1,700 77 5.6 1,200 94
Regulating reservoir subbasin
14 01115110 310,000 12,000 480 35 <2,400 1,500
15 01115114 180,000 4,300 1,000 29 3,100 900
16 01115098 140,000 <1,300 1,000 28 4,400 520
18 01115120 6,500 1,100 52 0.81 240 35
Westconnaug Reservoir subbasin
10 01115274 55,000 <210 81 3.6 89 110
11 01115273 31,000 <270 11 3.4 <28 43
Scituate Reservoir drainage area
Minimum 5,700 <35 8.6 0.69 <28 21
Median 55,000 <900 220 10 500 290
Maximum 450,000 12,000 1,100 58 4,400 1,500
Table 9.    Median daily loads of bacteria, chloride, nitrite, nitrate, and orthophosphate in the Scituate Reservoir drainage area, Rhode Island, from October 1, 2019, through September 30, 2020.

Table 10.    

Median daily yields of bacteria, chloride, nitrite, nitrate, and orthophosphate in the Scituate Reservoir drainage area, Rhode Island, from October 1, 2019, through September 30, 2020.

[Concentration data from Spaetzel and Smith (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; million colony forming units per day per square mile; E. coli, Escherichia 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 (([g/d]/mi2 as N) Nitrate ([g/d]/mi2 as N) Orthophosphate ([g/d]/mi2 as PO4)
Barden Reservoir subbasin
24 01115190 31,000 <220 76 3.3 88 69
25 01115200 120,000 <370 93 15 <190 220
26 01115185 100,000 1,800 130 11 <300 230
28 01115265 12,000 250 62 4.9 <58 110
35 01115187 19,000 <290 79 4.1 <210 100
Direct Runoff subbasin
1 01115180 41,000 <250 55 7.3 690 160
3 01115280 18,000 <190 170 3.2 270 91
4 01115400 9,100 <110 14 2.2 58 28
5 01115184 44,000 180 67 3.0 240 60
6 01115183 23,000 510 130 4.3 890 160
7 01115297 28,000 900 32 5.1 <83 99
8 01115276 5,600 180 42 3.7 <95 71
9 01115275 69,000 470 170 4.1 1,500 76
32 01115178 31,000 <190 33 3.8 170 120
33 01115182 20,000 <130 31 2.5 390 75
Moswansicut Pond reservoir subbasin
19 01115170 4,000 <150 170 3.2 180 89
21 01115165 100,000 5,700 260 19 4,000 310
Regulating reservoir subbasin
14 01115110 48,000 1,900 76 5.5 <380 240
15 01115114 38,000 910 210 6.2 660 190
16 01115098 28,000 <260 200 5.6 890 100
18 01115120 23,000 3,900 190 2.9 860 130
Westconnaug Reservoir subbasin
10 01115274 37,000 <140 54 2.4 60 74
11 01115273 43,000 <380 16 4.7 <39 59
Scituate Reservoir drainage area
Minimum 4,000 <110 14 2.2 <39 28
Median 31,000 <260 76 4.1 240 100
Maximum 120,000 5,700 260 19 4,000 310
Table 10.    Median daily yields of bacteria, chloride, nitrite, nitrate, and orthophosphate in the Scituate Reservoir drainage area, Rhode Island, from October 1, 2019, through September 30, 2020.

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 8). Median concentrations of E. coli were equal to or greater than a detection limit of 10 CFU/100 mL (which is the highest detection limit of the median values) at 19 of the 37 stations (including a single median [<15] estimated from a censored value and a noncensored value). However, only censored median concentrations of E. coli less than 10 CFU/100 mL were available from the other 18 stations. 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 1,200 CFU/100 mL for total coliform bacteria and 10 CFU/100 mL for E. coli (table 8). The highest median concentration of total coliform bacteria, 17,000 CFU/100 mL, was at the water body known locally as “Toad pond” (USGS station 01115177; PW station 31) in the Direct Runoff subbasin. Median concentrations of total coliform bacteria exceeded 2,000 CFU/100 mL at six other stations including Ponaganset River (USGS station 01115187; PW station 35), Spruce Brook (USGS station 01115184; PW station 5), Bear Tree Brook (USGS station 01115275; PW station 9), unnamed tributary 4 to Scituate Reservoir (USGS station 01115350; PW station 30), unnamed tributary 2 to Moswansicut Pond reservoir (USGS station 01115165; PW station 21), and unnamed tributary to Westconnaug Reservoir (USGS station 01115273; PW station 11). Median concentrations of total coliform bacteria were lowest at Westconnaug Brook (USGS station 01115276; PW station 8). The highest median concentration of E. coli, 210 CFU/100 mL (table 8), also was at Toad pond.

Median daily loads and yields of total coliform bac