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Sources of Sodium and Chloride in the Scituate Reservoir Drainage Basin, Rhode IslandWater-Resources Investigations Report 02-4149 |
The Scituate Reservoir system is the principal drinking-water supply for more than 60 percent of the population of Rhode Island. Sodium concentrations in the reservoir have been increasing slowly in recent years despite the long-term (12 years) use of a reduced-sodium deicing agent on State-maintained highways in the reservoir drainage basin. This report identifies the major sources of sodium and chloride in the drainage basin, and assesses the relative contribution of each source to the total amounts of these constituents that entered the basin from October 1999 through September 2000 (Water Year 2000).
Findings
Deicing of State and local roads was the largest source of sodium and chloride in the Scituate Reservoir drainage basin during Water Year (WY) 2000. Of approximately 1,000 tons of sodium and 2,300 tons of chloride introduced into the basin, 67 percent of the sodium and more than 90 percent of the chloride came from deicing of State highways, local roads, and other surfaces. Precipitation, individual sewage-disposal systems, and geologic weathering accounted for the remaining 33 percent of the sodium and 10 percent of the chloride inputs.
Salting of State-maintained roads contributed more sodium and chloride during WY 2000 than did salting of locally maintained roads, although there were about 45 percent more total lane-miles of locally maintained roads than State-maintained roads in the basin. Annual basin-wide application rates were estimated to be 2.02 tons per lane mile for sodium and 8.03 tons per lane mile for chloride applied to State-maintained roads. For locally maintained roads, the average application rates were 1.06 tons per lane mile for sodium and 2.12 tons per lane mile for chloride. This application-rate difference may reflect the greater volume of traffic and higher speed limits on State-maintained roads.
The median concentration of sodium measured periodically from 1983 through 2000 (calendar years) in streams that supply water to the Scituate Reservoir was positively correlated with the density of State-maintained roads in the stream subbasins. About 62 percent of the variation in median stream-sodium concentration was accounted for by variation in the density of State-maintained roads. In contrast, no correlation could be identified between median stream-sodium concentration and the density of locally maintained roads in the subbasins. This difference may result from a combination of higher road-salt application rates, greater proximity to tributary streams, and more efficient drainage systems associated with State-maintained roads as compared to locally maintained roads.
The effectiveness of a State-mandated use of a 60:40 mixture of calcium chloride and sodium chloride on State-maintained roads in the drainage basin beginning in 1990 was determined by comparing relations between subbasin-road density and median stream-sodium concentrations for the periods 1983 through 1989 and 1990 through 2000. There was no change in the slope of the regression line for these relations following the introduction of the reduced-sodium deicing agent, indicating that there was no measurable change in the contribution of State-maintained roads to the sodium loads to the streams as a result of this action. It is not known if this apparent lack of effect is due to increased use of sodium chloride for deicing by municipalities and private individuals in the basin, or to mobilization of sodium previously applied and stored in the drainage basin.
CONTENTS
Introduction
Description of the Scituate Reservoir Drainage
Basin
Sodium and Chloride Enter the Scituate Reservoir
Drainage Basin from Multiple Sources
Deicing Activities Contributed More Sodium
and Chloride to the Drainage Basin During Water Year 2000 Than Any Other Source
Tributary-Stream Concentrations of Sodium
and Chloride Are Directly Related to the Densities of State-Maintained Roads
in the Stream Drainage Areas
Sodium Concentrations in Tributary
Streams Did Not Decrease Following a State-Mandated Reduction in the Rate of
Sodium Applied to State-Maintained Roads in the Drainage Basins
Summary and Conclusions
References Cited
MORE INFORMATION BOXES
Quantifying
Sources of Sodium and Chloride
Precipitation
Road-Salt Applications
Other Deicing Activities
Septic-System Leachate
Geologic Weathering
FIGURES
Location and hydrography of the Scituate Reservoir drainage basin, Rhode Island.
Locations of Providence Water Supply Board water-quality-monitoring stations sampled from 1983 to 2000 in relation to (A) State and local roads, and (B) subbasin boundaries in the Scituate Reservoir drainage basin, Rhode Island.
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The citation for this report, in USGS format, is as follows:
Nimiroski, M.T. and Waldron, M.C., Sources of Sodium and Chloride in the Scituate Reservoir Drainage Basin, Rhode Island: U.S. Geological Survey Water-Resources Investigations Report 02-4149, 16 p.
For more information about USGS activities in Massachusetts-Rhode Island District, visit the USGS Massachusetts-Rhode Island Home Page.
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