Water Quality Monitoring in the Scituate Reservoir Drainage Area, Rhode Island Active

By New England Water Science Center June 29, 2021

The Scituate Reservoir system is the largest inland body of water in Rhode Island and the principal drinking-water supply for more than 60 percent of the State’s population. The system includes the Scituate Reservoir and five tributary reservoirs with a maximum storage capacity of 37 billion gallons. The drainage basin that contributes water to the reservoir extends across 93 square miles in northwestern Rhode Island.

Collecting an isokinetic equal-width increment water sample — Photograph of U.S. Geological Survey technician collecting an isokinetic equal-width increment water sample at USGS streamgage 01115265, Hemlock Brook at King Road near Foster, Rhode Island.

Most of the drainage basin (87 percent) is undeveloped forestland, but the basin also contains residential, commercial, and industrial development, and it is crossed by a major highway. Various contaminants associated with these land uses are a concern, including sodium and chloride from the application of deicing compounds on roads. The Providence Water Supply Board (PWSB), the agency tasked with managing the drainage basin for source-water protection, closely monitors water quality in the tributaries that drain into the basin, through a long-term sampling program and cooperative agreements with the U.S. Geological Survey (USGS) to collect streamflow data and analyze constituent loads of water-quality constituents that might lead to the degradation of reservoir water quality. This cooperative monitoring program provides PWSB resource managers with ongoing information about quality and quantity of water conditions in the basin as well as the rate of loading for constituents of interest to the reservoir.

The USGS started collecting streamflow data in the Scituate Reservoir drainage area in the 1990s.These flow data provided essential information needed to estimate the mass loads of constituents carried by the tributaries into the reservoir at the time of sampling; with data on loads, the relative inputs of constituents to the reservoir from the different tributaries can be compared. In 2009, a cooperative agreement between the PWSB and the USGS added continuous streamflow- and water-quality monitoring at 12 additional monitoring stations on tributaries to the reservoir system. Currently, the USGS operates 16 continuous monitoring stations in the Scituate Reservoir drainage area and collects periodic water-quality samples at each site. The continuous streamflow data have greatly increased the accuracy of the load estimates. Continuous water-quality monitoring data have been used to more precisely calculate loads of sodium and chloride—constituents of concern entering the reservoir. The continuous data also have allowed the PWSB to gain a better understanding of temporal changes in the overall quality and quantity of the source water to the reservoir and provide the PWSB with near real-time information about conditions within the drainage basin.

Knowledge of the temporal trends in the physical properties and constituent concentrations routinely measured in water-quality samples can be used to predict future water-quality problems or recognize improvements. Continued analysis of historic and new concentration and load data, and improvements to the precision of such estimates through investments in the current monitoring network are essential to sustaining the high-quality source of water to Scituate Reservoir and understanding how water quality in the tributaries may be changing over time.