Streamflow, Water Quality, and Constituent Loads and Yields, Scituate Reservoir Drainage Area, Rhode Island, Water Year 2012

Abstract

Streamflow and concentrations of sodium and chloride estimated from records of specific conductance were used to calculate loads of sodium and chloride during water year (WY) 2012 (October 1, 2011, through September 30, 2012), for tributaries to the Scituate Reservoir, Rhode Island. Streamflow and water-quality data used in the study were collected by the U.S. Geological Survey (USGS) or the Providence Water Supply Board (PWSB). Streamflow was measured or estimated by the USGS following standard methods at 23 streamgages; 14 of these streamgages were equipped with instrumentation capable of continuously monitoring water level, specific conductance, and water temperature. Water-quality samples were collected at 37 sampling stations by the PWSB and at 14 continuous-record streamgages by the USGS during WY 2012 as part of a long-term sampling program; all stations were in the Scituate Reservoir drainage area. Water-quality data collected by the PWSB were summarized by using values of central tendency and used, in combination with measured (or estimated) streamflows, to calculate loads and yields (loads per unit area) of selected water-quality constituents for WY 2012.

The largest tributary to the reservoir (the Ponaganset River, which was monitored by the USGS) contributed a mean streamflow of about 26 cubic feet per second (ft³/s) to the reservoir during WY 2012. For the same time period, annual mean1 streamflows measured (or estimated) for the other monitoring stations in this study ranged from about 0.40 to about 17 ft³/s. Together, tributaries (equipped with instrumentation capable of continuously monitoring specific conductance) transported about 1,100,000 kilograms (kg) of sodium and 1,900,000 kg of chloride to the Scituate Reservoir during WY 2012; sodium and chloride yields for the tributaries ranged from 8,700 to 51,000 kilograms per square mile (kg/mi²) and from 14,000 to 87,000 kg/mi², respectively.

At the stations where water-quality samples were collected by the PWSB, the median of the median chloride concentrations was 19 milligrams per liter (mg/L), median nitrite concentration was 0.002 mg/L as nitrogen (N), median nitrate concentration was less than 0.01 mg/L as N, median orthophosphate concentration was 0.06 mg/L as phosphorus, and median concentrations of total coliform and Escherichia coli (E. coli) bacteria were 43 and 16 colony forming units per 100 milliliters (CFU/100mL), respectively. The medians of the median daily loads (and yields) of chloride, nitrite, nitrate, orthophosphate, and total coliform and E. coli bacteria were 200 kilograms per day (kg/d) (71 kilograms per day per square mile (kg/d/mi²)); 15 grams per day (g/d) (5.4 grams per day per square mile (g/d/mi²)); 100 g/d (38 g/d/mi²); 500 g/d (260 g/d/mi²); 4,300 million colony forming units per day (CFUx10⁶/d) (1,500 CFUx10⁶/d/mi²); and 1,000 CFUx10⁶/d (360 CFUx10⁶/d/mi²), respectively.

First posted January 14, 2014

Revised July 14, 2014