Streamflow, Water Quality, and Constituent Loads and Yields, Scituate Reservoir Drainage Area, Rhode Island, Water Year 2020
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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.
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]
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]
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:
whereThese 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]
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).
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]
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).
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]
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]
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]
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]
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]
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).
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]
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]
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]
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 bacteria and E. coli varied by two orders of magnitude or more (tables 9 and 10). The median daily loads of total coliform bacteria for all subbasins in the Scituate Reservoir drainage area ranged from 5,700 to 450,000 million colony forming units per day ([CFU×106]/d), and yields ranged from 4,000 to 120,000 million colony forming units per day per square mile ([(CFU×106)/d]/mi2); E. coli loads ranged from less than 35 to 12,000 (CFU×106)/d, and yields ranged from less than 110 to 5,700 ([CFU×106]/d)/mi2 (table 9). The highest median daily yield of total coliform bacteria at 120,000 ([CFU×106]/d)/mi2 was at Shippee Brook (USGS station 01115200; PW station 25), and the highest median daily yield of E. coli of 5,700 ([CFU×106]/d)/mi2 occurred at unnamed tributary 2 to Moswansicut Pond reservoir (USGS station 01115165; PW station 21; table 10). 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 and Sodium
Median chloride concentrations among the PW stations ranged from 6.5 to 80.6 mg/L. The highest median concentration was collected at unnamed tributary 2 to Scituate Reservoir (USGS station 01115181; PW station 31); however, the highest single concentration (7,000 mg/L) was measured in the only sample collected at Toad pond (PW station 31; table 8). Toad pond is a small water body having a drainage area of 0.03 square mile (table 1), in the village of North Scituate, south of Route 6 and west of Route 116, in Scituate, R.I. The sample was collected in December 2019 and the high chloride concentration likely reflects salt-laden runoff entering the small pond at or before the time of sample collection. The median of median concentrations for all sites in the drainage area was 24.8 mg/L (table 8). Median daily chloride loads and yields estimated from samples collected by PW varied among monitoring stations in the drainage area (tables 9 and 10). Ponaganset River (USGS station 01115187; PW station 35) had the largest median daily chloride load at 1,100 kilograms per day (table 9). The largest median daily chloride yield was determined to be 260 kilograms per day per square mile at unnamed tributary 2 to Moswansicut Pond reservoir (USGS station 01115165; PW station 21; table 10). The median daily chloride yield for monitored areas within the drainage area was 76 kilograms per day per square mile.
Nutrients
Median concentrations of nitrite and nitrate (table 8) were 0.001 and 0.08 mg/L as nitrogen, respectively. The highest median concentrations of nitrite (0.018 mg/L as nitrogen) and nitrate (0.99 mg/L as nitrogen) were measured in a sample collected at Toad pond (USGS station 01115177; PW station 31). The median concentration of orthophosphate for the entire study area (table 8) was 0.03 mg/L as phosphate. The maximum median concentration of orthophosphate was 1.24 mg/L as phosphate measured in a sample collected at unnamed tributary 2 to Scituate Reservoir (USGS station 01115181; PW station 2). Median daily nitrite loads were largest at Ponaganset River (USGS station 01115187; PW station 35) at 58 grams per day (g/d; table 9). Median daily nitrate loads were largest at Peeptoad Brook (USGS station 01115098; PW station 16) at 4,400 g/d as nitrogen. Median daily orthophosphate loads were largest (1,500 g/d as phosphate) at Huntinghouse Brook (USGS station 01115110; PW station 14; table 9). The largest median daily yields for nitrite, nitrate, and orthophosphate were 19 grams per day per square mile ([g/d]/mi2) as nitrogen, and 4,000 (g/d)/mi2 as nitrogen, and 310 (g/d)/mi2 as phosphate, respectively, at unnamed tributary 2 to Moswansicut Pond reservoir (USGS station 01115165; PW station 21; table 10). The medians of median daily loads and yields were 10 g/d and 4.1 (g/d)/mi2 for nitrite as nitrogen, 500 g/d and 240 (g/d)/mi2 for nitrate as nitrogen, and 290 g/d and 100 (g/d)/mi2 for orthophosphate as phosphate, respectively.
Summary
Since 1993, the U.S. Geological Survey (USGS), in cooperation with the Providence Water (PW; sometimes known as Providence Water Supply Board), has maintained a long-term cooperative water-quality monitoring program within the Scituate Reservoir drainage area, Rhode Island. 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) 2020, which is defined as the period from October 1, 2019, to September 30, 2020, the U.S. Geological Survey measured or estimated streamflow at 23 streamgages; 14 of these streamgages are equipped with instrumentation capable of continuously monitoring water level, specific conductance, and water temperature. Water-quality samples, that are analyzed for dissolved concentrations of major ions (including chloride and sodium), were periodically collected by the USGS at each of the 14 streamgages. Concentrations of chloride and sodium in water samples collected during WY 2020 and in prior WYs were used to support and refine relations between each ion and specific conductance. Monthly and annual concentrations, loads, and yields were estimated for the 14 streamgages by using equations to relate specific conductance to concentrations of chloride and sodium, and with measured or estimated streamflow data.
At 14 of the 23 USGS streamgages, where both streamflow and continuous specific conductance data were available, estimated monthly mean chloride concentrations 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 in tributaries of the Scituate Reservoir drainage area. The highest annual mean concentrations of chloride and sodium were estimated to be 59 and 34 mg/L, respectively, in the unnamed tributary to the water body known as “Regulating reservoir” (USGS station 01115120; PW station 18) in the more developed, northeastern part of the Scituate Reservoir drainage area. An estimated 2,200 metric tons of chloride and 1,400 metric tons of sodium were transported to the Scituate Reservoir during WY 2020; annual chloride yields for tributaries in the drainage area ranged from 13 to 110 metric tons per square mile, and annual sodium yields ranged from 8.8 to 66 metric tons per square mile. 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 WYs and among the lowest since WY 2009.
PW collected water-quality samples at 37 sampling stations, which also include the 14 USGS continuous-record streamgages, during WY 2020 as part of their long-term sampling program in the Scituate Reservoir drainage area. Water-quality samples were analyzed by PW for pH, color, turbidity, alkalinity, and for concentrations of chloride, nutrients, and bacteria. Water-quality data collected by PW were summarized by using values of central tendency and are used in combination with periodic- or continuous-streamflow data available at 23 of the 37 sampling stations to calculate loads and yields of chloride, nutrients, and bacteria for WY 2020.
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 6.4; the median value for color was 50 platinum-cobalt units; the median value for turbidity was 0.69 nephelometric turbidity unit; and the median concentration for alkalinity was 6.6 mg/L as calcium carbonate. 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. The medians of the median concentrations for water samples from all stations were the following: 24.8 mg/L chloride, 0.001 mg/L as nitrite as nitrogen, 0.08 mg/L nitrate as nitrogen, 0.03 mg/L orthophosphate as phosphate, 1,200 total coliform bacteria colony forming units per 100 milliliters, and 10 Escherichia coli colony forming units per 100 milliliters. The medians of the median daily loads were 220 kilograms per day chloride, 10 grams per day nitrite as nitrogen, 500 grams per day nitrate as nitrogen, 290 grams per day orthophosphate as phosphate, 55,000 million coliform bacteria colony forming units per day, and less than 900 million Escherichia coli colony forming units 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 coliform bacteria colony forming units per day per square mile, and less than 260 million Escherichia coli colony forming units per day per square mile.
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Conversion Factors
U.S. customary units to International System of Units
Datums
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.
For more information about this report, contact:
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U.S. Geological Survey
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dc_nweng@usgs.gov
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Suggested Citation
Smith, K.P., 2024, Streamflow, water quality, and constituent loads and yields, Scituate Reservoir drainage area, Rhode Island, water year 2020: U.S. Geological Survey Data Report 1192, 31 p., https://doi.org/10.3133/dr1192.
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 2020 |
| Series title | Data Report |
| Series number | 1192 |
| DOI | 10.3133/dr1192 |
| Year Published | 2024 |
| Language | English |
| Publisher | U.S. Geological Survey |
| Publisher location | Reston, VA |
| Contributing office(s) | New England Water Science Center |
| Description | Report: v, 31 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 |