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Open-File Report 2011–1018

Prepared in cooperation with the Federal Highway Administration and the Connecticut Department of Transportation

Preliminary Assessment of Chloride Concentrations, Loads, and Yields in Selected Watersheds along the Interstate 95 Corridor, Southeastern Connecticut, 2008–09

By Craig J. Brown, John R. Mullaney, Jonathan Morrison, and Remo Mondazzi


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Water-quality conditions were assessed to evaluate potential effects of road-deicer applications on stream-water quality in four watersheds along Interstate 95 (I-95) in southeastern Connecticut from November 1, 2008, through September 30, 2009. This preliminary study is part of a four-year cooperative study by the U.S. Geological Survey (USGS), the Federal Highway Administration (FHWA), and the Connecticut Department of Transportation (ConnDOT). Streamflow and water quality were studied at four watersheds—Four Mile River, Oil Mill Brook, Stony Brook, and Jordan Brook. Water-quality samples were collected and specific conductance was measured continuously at paired water-quality monitoring sites upstream and downstream from I-95. Specific conductance values were related to chloride (Cl) concentrations to assist in determining the effects of road-deicing operations on the levels of Cl in the streams. Streamflow and water-quality data were compared with weather data and with the timing, amount, and composition of deicers applied to state highways. Grab samples were collected during winter stormwater-runoff events, such as winter storms or periods of rain or warm temperatures in which melting takes place, and periodically during the spring and summer.

Cl concentrations at the eight water-quality monitoring sites were well below the U.S. Environmental Protection Agency (USEPA) recommended chronic and acute Cl toxicity criteria of 230 and 860 milligrams per liter (mg/L), respectively. Specific conductance and estimated Cl concentrations in streams, particularly at sites downstream from I-95, peaked during discharge events in the winter and early spring as a result of deicers applied to roads and washed off by stormwater or meltwater. During winter storms, deicing activities, or subsequent periods of melting, specific conductance and estimated Cl concentrations peaked as high as 703 microsiemens per centimeter (μS/cm) and 160 mg/L at the downstream sites.

During most of the spring and summer, specific conductance and estimated Cl concentrations decreased during discharge events because the low-ionic strength of stormwater had a diluting effect on stream-water quality. However, peaks in specific conductance and estimated Cl concentrations at Jordan Brook and Stony Brook corresponded to peaks in streamflow well after winter snow or ice events; these delayed peaks in Cl concentration likely resulted from deicing salts that remained in melting snow piles and (or) that were flushed from soils and shallow groundwater, then discharged downstream.

Cl loads in streams generally were highest in the winter and early spring. The estimated load for the period of record at the four monitoring sites downstream from I-95 ranged from 0.33 ton per day (ton/d) at the Stony Brook watershed to 0.59 ton/d at the Jordan Brook watershed. The Cl yields ranged from 0.07 ton per day per square mile (ton/d/)mi2) at Oil Mill Brook, one of the least developed watersheds, to 0.21 (ton/d)/mi2) at Jordan Brook, the watershed with the highest percentage of urban development and impervious surfaces. The median estimates of Cl load from atmospheric deposition ranged from 11 to 19 tons, and contributed 4.3 to 7.1 percent of the Cl load in streamflow from the watershed areas. A comparison of the Cl load input and output estimates indicates that less Cl is leaving the watersheds than is entering through atmospheric deposition and application of deicers. The lag time between introduction of Cl to the watershed and transport to the stream, and uncertainty in the load estimates may be the reasons for this discrepancy. In addition, estimates of direct infiltration of Cl to groundwater from atmospheric deposition, deicer applications, and septic-tank drainfields to groundwater were outside the scope of the November 2008 to September 2009 assessment. However, increased concentrations of ions were observed between upstream and downstream sites and could result from deicer applications.

Cl yields per square mile at the downstream monitoring sites at each of the four streams were compared with Cl yield estimates for 10 selected rivers in Connecticut. Four Mile River and Oil Mill Brook had low estimated Cl yields, similar to yields at Bunnell (Burlington) Brook and Shetucket River, that reflect the low percentages of developed land and impervious area. Estimated Cl yields at Jordan Brook and Stony Brook were relatively high but were not as high as those in more urbanized watersheds such as those that drain the Still River at Brookfield Center and the Hockanum River. Cl yields for these sites were positively correlated with the percentage of impervious cover and probably reflect the application of deicers to roadways, as well as sources and practices associated with greater impervious cover, such as wastewater and septic-system discharges, and leachate from landfills and salt-storage areas.

First posted April 20, 2011

For additional information contact:
U.S. Geological Survey
Connecticut Water Science Center
U.S. Geological Survey
101 Pitkin Street, East Hartford, CT 06108

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Suggested citation:

Brown, C.J., Mullaney, J.R., Morrison, Jonathan, and Mondazzi, Remo, 2011, Preliminary assessment of chloride concentrations, loads, and yields in selected watersheds along the Interstate 95 corridor, southeastern Connecticut, 2008–09: U.S. Geological Survey Open-File Report 2011–1018, 41 p. at




Purpose and Scope

Description of Study Area

Sources of Dissolved Constituents to Streams and Groundwater

Road Deicing Chemicals and their Application in the Study Areas

Methods of Data Collection and Analysis

Site Selection

Weather Data


Water Quality

Continuous Monitoring

Water-Quality Sampling and Laboratory Analysis

Quality Assurance

Data Analysis

Estimation of Chloride Concentrations

Estimation of Cl Loads and Yields

Preliminary Assessment of the Effects of Road Deicers on Water Quality

Weather Data and Storm Events

Water Quality

Base-Flow Chemistry

Stormflow Chemistry

Four Mile River

Oil Mill Brook

Stony Brook

Jordan Brook

Chloride Loads

Summary and Conclusions


References Cited

Appendix 1. Specific conductance and chloride concentrations in water samples from upstream and downstream monitoring sites at Four Mile River, Oil Mill Brook, Stony Brook, and Jordan Brook, southeastern Connecticut, November 2008 to January 2010

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