Streamflow, water quality, and constituent loads and yields, Scituate Reservoir drainage area, Rhode Island, water year 2018

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 the Providence Water Supply Board collected streamflow and water-quality data at the Scituate Reservoir and tributaries. Streamflow and concentrations of chloride and sodium estimated from records of specific conductance were used to calculate loads of chloride and sodium during water year 2018 (October 1, 2017, through September 30, 2018) for tributaries to the Scituate Reservoir, Rhode Island. Streamflow was measured or estimated by the U.S. Geological Survey following standard methods 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 were collected by the Providence Water Supply Board at 36 sampling stations, which also include the 14 continuous-record streamgages maintained by the U.S. Geological Survey, during water year 2018 as part of a long-term sampling program; all stations are in the Scituate Reservoir drainage area. Water-quality data collected by the Providence Water Supply Board are summarized by using values of central tendency and are used, in combination with measured (or estimated) streamflows, to calculate loads and yields (loads per unit area) of selected water-quality constituents for water year 2018.

The largest tributary to the reservoir, the Ponaganset River, which was monitored by the U.S. Geological Survey, contributed a mean streamflow of 33 cubic feet per second to the reservoir during water year 2018. For the same period, annual mean streamflows measured (or estimated) for the other monitoring stations in this study ranged from about 0.34 to about 20 cubic feet per second. Together, tributaries equipped with instrumentation capable of continuously monitoring specific conductance transported about 3,100 metric tons of chloride and 1,900 metric tons of sodium to the Scituate Reservoir during water year 2018; annual chloride yields for the tributaries ranged from 18 to 140 metric tons per square mile, and annual sodium yields ranged from 12 to 80 metric tons per square mile.

At the stations where water-quality samples were collected by the Providence Water Supply Board, the medians of the median concentrations were 25.8 milligrams per liter for chloride, 0.001 milligram per liter as nitrogen for nitrite, 0.11 milligram per liter as nitrogen for nitrate, 0.04 milligram per liter as phosphate for orthophosphate, 1,200 colony forming units per 100 milliliters for total coliform bacteria, and 10 colony forming units per 100 milliliters for Escherichia coli (E. coli). The medians of the median daily loads of chloride, nitrite, nitrate, orthophosphate, total coliform, and E. coli bacteria were 220 kilograms per day, 15 grams per day, less than 890 grams per day, 360 grams per day, 93,000 million colony forming units per day, and less than 700 million colony forming units per day, respectively. The medians of the median yields of chloride, nitrite, nitrate, orthophosphate, total coliform, and E. coli bacteria were 110 kilograms per day per square mile, 5.5 grams per day per square mile, 250 grams per day per square mile, 210 grams per day per square mile, 36,000 million colony forming units per day per square mile, and 410 million colony forming units per day per square mile, respectively.