ABSTRACT

Streamflow and water-quality data were collected by the U.S. Geological Survey (USGS) or the Providence Water Supply Board, Rhode Island’s largest drinking-water supplier. Streamflow was measured or estimated by the USGS following standard methods at 23 streamgage stations; 10 of these stations were also equipped with instrumentation capable of continuously monitoring specific conductance. Streamflow and concentrations of sodium and chloride estimated from records of specific conductance were used to calculate instantaneous (15-minute) loads of sodium and chloride during water year (WY) 2004 (October 1, 2003, to September 30, 2004). Water-quality samples were also collected at 37 sampling stations in the Scituate Reservoir drainage area by the Providence Water Supply Board during WY 2004 as part of a long-term sampling program. 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 (loads per unit area) of selected water-quality constituents for WY 2004.

The largest tributary to the reservoir (the Ponaganset River, which was monitored by the USGS) contributed about 27 cubic feet per second (ft³/s) to the reservoir during WY 2004. For the same time period, annual mean¹ streamflows measured (or estimated) for the other monitoring stations in this study ranged from about 0.42 to 19 ft³/s. Together, tributary streams (equipped with instrumentation capable of continuously monitoring specific conductance) transported about 1,100,000 kilograms (kg) of sodium and 1,700,000 kg of chloride to the Scituate Reservoir during WY 2004; sodium and chloride yields for the tributaries ranged from 12,000 to 61,000 kilograms per square mile (kg/mi²) and from 17,000 to 100,000 kg/mi², respectively.

At the stations where water-quality samples were collected by the Providence Water Supply Board, the median of the median chloride concentrations was 24.8 milligrams per liter (mg/L), median nitrite concentration was 0.001 mg/L as N, median nitrate concentration was 0.03 mg/L as N, median orthophosphate concentration was 0.07 mg/L as P, and median concentrations of total coliform and Escherichia coli (E. coli) bacteria were 33 and 23 colony forming units per 100 milliliters (CFU/100 mL), respectively. The medians of the median daily loads (and yields) of chloride, nitrite, nitrate, orthophosphate, and total coliform and E. coli bacteria were 160 kg/d (81 kg/d/mi²), 9.1 g/d (5.2 g/d/mi²), 280 g/d (110 g/d/mi²), 760 g/d (340 g/d/mi²), and 4,700 million colony forming units per day (CFU×10⁶/d) (1,700 CFU×10⁶/d/mi²) and 1,900 CFU×10⁶/d (520 CFU×10⁶/d/mi²), respectively.

¹The arithmetic mean of the individual daily mean discharges for the year noted or for the designated period.

First posted June 1, 2010