Streamflow, water quality, and constituent loads and yields, Scituate Reservoir Drainage Area, Rhode Island, Water Year 2016

As part of a long-term cooperative program to monitor water quality within the Scituate Reservoir watershed, 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 (WY) 2016 (October 1, 2015, through September 30, 2016) 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 34 sampling stations that also include 14 continuous-record streamgages maintained by the U.S. Geological Survey during WY 2016 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 WY 2016.

The largest tributary to the reservoir, the Ponaganset River, which was monitored by the U.S. Geological Survey, contributed a mean streamflow of 18 cubic feet per second to the reservoir during WY 2016. For the same period, annual mean streamflows measured (or estimated) for the other monitoring stations in this study ranged from about 0.27 to about 12 cubic feet per second. Together, tributaries equipped with instrumentation capable of continuously monitoring specific conductance transported about 2,100,000 kilograms of chloride and 1,300,000 kilograms of sodium to the Scituate Reservoir during WY 2016; chloride and sodium yields for the tributaries ranged from 14,000 to 95,000 kilograms per square mile and from 8,600 to 56,000 kilograms per square mile, respectively.

At the stations where water-quality samples were collected by the Providence Water Supply Board, the medians of the median concentrations were 27.9 milligrams per liter for chloride, 0.002 milligram per liter as nitrogen for nitrite, 0.13 milligrams per liter as nitrogen for nitrate, 0.07 milligram per liter as phosphate for orthophosphate, and 700 and 10 colony forming units per 100 milliliters for total coliform bacteria and Escherichia coli (E. coli), respectively. The medians of the median daily loads of chloride, nitrite nitrogen, nitrate nitrogen, orthophosphate, and total coliform and E. coli bacteria were 170 kilograms per day, 8.9 grams per day, 570 grams per day, 320 grams per day, 41,000 million colony forming units per day, and 680 million colony forming units per day. The medians of the median yields of chloride, nitrite nitrogen, nitrate nitrogen, orthophosphate, total coliform, and E. coli bacteria were 53 kilograms per day per square mile, 4.7 grams per day per square mile, 130 grams per day per square mile, 165 grams per day per square mile, 23,000 million colony forming units per day per square mile, and 340 million colony forming units per day per square mile, respectively.