The U.S. Department of Transportation estimates that about 117,000 injuries and 1,300 deaths per year are attributable to vehicle crashes on snowy, slushy or icy pavement. A 2020 study in Ontario, Canada indicated that the risk of automobile collisions and emergency room visits for falls increase by a factor of 1.37 and 1.61, respectively, over the winter. Multiple studies have shown that applying deicing chemicals to roadways decreases subsequent collisions by 50-to-87%. For pedestrians and drivers who must travel, deicing chemicals are an unfortunate necessity.

Transportation departments, municipal workers, and many private contractors apply deicing compounds to pavement to keep roads, parking areas, and walkways safe for travelers. In developed watersheds roadways, parking lots, driveways, and sidewalks make up about 28%, 25%, 9%, and 3% of the impervious area, respectively.

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The most common deicing compound used is sodium chloride, but calcium- and magnesium-chloride salts are also commonly used. Deicers lower water’s freezing point by dissolving into the water and adding charged particles, called ions. The chloride ion is not retained by soil particles and is not lost to evaporation or chemical reactions, so it moves freely with water as it flows from the pavement over land and through aquifers to receiving streams. And, at high concentrations, chloride is toxic to freshwater animals and plants.

Nationally, about 25% of the salt we use each year is applied to pavement as deicing salt. Since the 1970s, the application of deicing salt has tripled in the United States – creating a large source of chloride that can enter surface water and groundwater. A USGS study of the northern United States that analyzed data from 1960 to 2011 found that chloride levels greatly increased in urban streams in connection with deicing-salt applications and other human-related sources.

In New England, chloride levels are highest in the shallow groundwater around cities, according to a USGS assessment of groundwater quality within the glacial aquifer. A direct tie to deicing salt, sodium levels that exceed 20 milligrams per liter for drinking water may be unsafe for residents who are on a sodium restricted diet, so chloride contamination of the aquifer or a reservoir used for drinking water can be a human health concern. Additionally, when groundwater is the dominant source to streams, elevated chloride loads can flow into streams all year, exceeding limits deemed necessary for protecting fish and other aquatic life.

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The USGS New England Water Science Center conducts a variety of research on man-made sources of chloride to surface waters, groundwaters, and, in some cases, drinking water supplies.

In Massachusetts, the USGS found that concentrations of chloride in samples from the Cambridge drinking-water source area exceeded aquatic-life guidelines for chloride and the Massachusetts Department of Environmental Protection drinking water guideline for sodium. As a result of these findings, the state of Massachusetts added the Cambridge Reservoir and several of its tributaries to the impaired waters list for chloride. Both reservoirs are in close proximity to developed areas that include major highways, parking lots, and other roadways associated with light industrial, commercial, and residential areas.

To reduce the amount of sodium and chlorides entering waterways and groundwater, the number of deicing compounds applied to roads, sidewalks, and parking lots must be reduced in a way that does not compromise public safety. State DOTs commonly use anti-icing techniques, calibrated spreaders, road-weather information systems, covered salt storage, and hindcasting with winter-severity data to minimize use of deicing chemicals. However, state DOTs manage only about 10-to-20% of the road network, with municipalities managing most of the road network, and private contractors overseeing most parking areas. New Hampshire has made significant strides in working to limit road salt through the Green SnowPro Program, which certifies winter maintenance professionals on salt-reduction “best practices” that retain public safety standards for transportation.

Take a look at some of the research and data collection efforts conducted by the USGS New England Water Science Center in relation to chloride in New England’s waters:

• Transportation-Related Water Projects in New England 
• Chloride Data for Streams in Connecticut, Massachusetts, and Rhode Island
• Mapping of road-salt-contaminated groundwater discharge and estimation of chloride load to a small stream in southern New Hampshire, USA
• Methods for evaluating potential sources of chloride in surface waters and groundwaters of the conterminous United States
• Scituate Reservoir Watershed Sees Rise in Chloride and Sodium
• Water Quality Monitoring in the Scituate Reservoir Drainage Area, Rhode Island
• Water Quality Monitoring in the Cambridge Drinking-Water Source Area, Massachusetts
• Loads and yields of deicing compounds and total phosphorus in the Cambridge drinking-water source area, Massachusetts, water years 2009–15
• Effectiveness of highway-drainage systems in preventing road-salt contamination of ground water, southeastern Massachusetts
• Robowell: Providing Accurate and Current Water-Level and Water-Quality Data in Real Time for Protecting Ground-Water Resources
• Assessing Biological Effects from Highway-Runoff Constituents
• Deicing chemicals as a source of constituents in highway runoff

Other USGS deicing studies related to New England include:

• Delineating a road-salt plume in lakebed sediments using electrical resistivity, piezometers, and seepage meters at Mirror Lake, New Hampshire, U.S.A 
• Movement of road salt to a small New Hampshire lake
• A fresh look at road salt: Aquatic toxicity and water-quality impacts on local, regional, and national scales