The Scituate Reservoir is a man-made freshwater body created in the early 20^th century to supply drinking water to the residents of Providence, Rhode Island. While the majority of the Scituate Reservoir watershed is surrounded by forest, the remainder comprises residential, commercial, and industrial land uses, which are associated with water constituents, such as chloride and sodium used to de-ice roads in the winter.

Water utility company Providence Water supplies approximately 60% of Rhode Island’s population with drinking water from the Scituate Reservoir and its five tributary reservoirs. Providence Water is concerned about sodium and chloride entering the Scituate Reservoir because elevated concentrations pose potential health risks to consumers and aquatic wildlife, and these constituents are difficult to remove in the water-treatment process. A recently-released U.S. Geological Survey (USGS) report found an increase of chloride and sodium over a 10-year-period at more than 70% of the continuously-monitored sites.

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The report details water-quality and streamflow characteristics and trends within the Scituate Reservoir watershed, including looking at concentrations (amount in water body), loads (amount in water body over a specific time period), and yields (load in proportion to size of drainage area) of select constituents, such as nutrients, sodium, and chloride, from 2013 to 2019.

Chloride and sodium rose by an average of 1.35 and 0.77 milligrams per liter per year, respectively, between 2009 and 2019. The study also documented a positive trend in chloride at more than half of the monitoring stations from 1983 to 2019. Additionally, a correlation between the amount of chloride in the water and the proportion of impervious land use and road density was found, which may be indicative of the products used to melt ice on winter roads. Nevertheless, chloride concentrations in Providence Water Supply Board samples rarely exceeded the U.S. Environmental Protection Agency (EPA) secondary drinking-water regulation for chloride, which is 250 milligrams per liter.

Other U.S. Geological Survey studies in Connecticut, Massachusetts, and Rhode Island have similarly documented increasing trends in chloride concentrations and loads. Continuous concentrations in excess of 230 milligrams per liter and short-lived concentrations of 860 milligrams per liter may adversely affect aquatic life. While some exceedances of these limits were measured in this study, they did not persist over intervals that surpassed the averaging periods applied for these criteria.

Researchers also found that sodium concentrations rose over time. The daily average of sodium at half of the monitoring stations exceeded 20 milligrams per liter.  While there is no regulatory standard for sodium in drinking water, the EPA recommendation for people on no-salt diets is that it should not exceed 20 milligrams per liter.

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Long-term Monitoring, Equal Comparisons

The data analyzed for this report comes from a long-term monitoring program to assess the water quality and streamflow of the tributaries that drain into the reservoirs. This program is done in cooperation with the Providence Water Supply Board, which manages the Scituate Reservoir’s 93-square-mile drainage basin. Water-resources managers can use this data to create management plans for the Scituate Reservoir drainage area. When the physical properties and constituent loading into the Scituate Reservoir over time are known, managers can prepare for predicted water-quality issues, evaluate management decisions, and identify what water-quality improvement is needed.

In this program, USGS and Providence Water Supply Board collect streamflow data and analyze water samples for constituents that can affect the quality of the water supply. Researchers regularly sample water at 37 streamgage sites located on tributaries within the watershed on a fixed timeline. Collecting streamflow and constituent concentrations enables researchers to calculate constituent loads and yields. Accurate streamflow data when water-quality samples are taken allows for precise calculations of constituent loads carried by the tributaries and into the reservoir. With this information, USGS researchers were able to uniformly compare constituent contributions from all sampled tributaries in this study.

Other Findings

The amount of chloride, nitrite, nitrate, orthophosphate, and bacteria determined for each Providence Water Supply Board sample from 2013 to 2019 varied across the 37 monitoring stations, but yields were generally greater at stations in the Moswansicut and Regulating Reservoir subbasin. This may be because the Moswansicut subbasin has the largest percentage of commercial, industrial, and residential land use of all the subbasins within the watershed, and it drains into the Regulating Reservoir.

The pH of the tributary water within the Scituate Reservoir watershed was low, and sometimes the pH was below drinking water standards established by the U.S. Environmental Protection Agency for secondary drinking water regulations. Nevertheless, pH has been increasing in the watershed, according to study results. Similar increasing trends in pH have been connected to acid rain reduction since the 1980s. A trend of increasing alkalinity, or the water’s ability to neutralize hydrogen ions, was also found. Regionally, increases in alkalinity have historically been associated with chemical weathering. However, alkalinity levels within the watershed were also lower than EPA standards.

Looking Forward

Providence Water Supply Board will be able to continue to identify poor water-quality sources, foresee potential problems, and have the ability to rectify issues with the sustained data collection of streamflow and water-quality sampling within the Scituate Reservoir. Continuing this data collection effort and frequently analyzing new and historic data will also shed light on how the water draining into the Scituate Reservoir changes over time. 

To read the full report, click here.