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Chloride, a key component of road salt, is soluble, highly mobile in water, and, at high concentrations, can be toxic to aquatic vegetation and wildlife. USGS scientists have been analyzing temporal, seasonal, and environmental trends in chloride concentrations across the U.S. to determine the effects that road salt may be having on water quality and aquatic organisms.
As part of the Milwaukee Metropolitan Sewerage District Corridor Study, USGS scientists have been analyzing temporal, seasonal, and environmental trends in chloride concentrations across the U.S. to determine the effects that road salt may be having on water quality and aquatic organisms.
Chloride, a key component of road salt (along with sodium), is soluble, highly mobile in water, and, at high concentrations, can be toxic to aquatic vegetation and wildlife. Increasing trends in chloride concentrations have been observed in water bodies of the U.S. and attributed, at least in part, to road salt influence. Road deicing by cities, counties and state agencies accounts for a significant portion of salt applications, but salt is also used by many public and private organizations and individuals to deice parking lots, walkways and driveways. All of these sources are likely to contribute to these increasing chloride concentrations.
This study has resulted in two influential journal articles. The most recent article, published in December 2014 in Science of the Total Environment, focused on defining temporal trends in chloride concentrations in relation to streamflow rates, and comparing these trends to changes in seasonality, urban land cover, aquatic life criteria, and road salt sales patterns. USGS scientists used chloride data from 30 monitoring sites on 19 streams near cities in Wisconsin, Illinois, Colorado, Michigan, Ohio, Pennsylvania, Maryland, Texas and the District of Columbia to explore the relationship between urban land cover, use of road salt, and stream chloride concentrations (fig. 1). They found that in 84 percent of urban streams studied, chloride concentrations related to road salt increased substantially over the study period (the study period was variable depending on availability at individual sites, but started as early as 1960 and ended in 2011). Numerous streams in the study exceeded concentrations that are toxic to aquatic life (figures 2 and 3). Concentrations were highest during the winter, but increased during all seasons over time at the northern sites, including sites near Milwaukee, Wisconsin; Chicago, Illinois; Denver, Colorado; and other metropolitan areas.
Other key findings from this study:
Twenty-nine percent of the sites exceeded the U.S. Environmental Protection Agency chronic water-quality criteria (230 milligrams per liter) by an average of more than 100 days per year from
2006 through 2011 (fig. 4B), including sites in on the Menomonee and Kinnickinnic Rivers near Milwaukee and Poplar Creek near Chicago.
The earlier journal article, published in September 2010 in Environmental Science and Technology, investigated the influence of road-salt runoff on surface water and aquatic organisms at multiple spatial scales: national, regional (southeast Wisconsin), and local (Milwaukee). The influence of urban development was also examined in this study.
For the national perspective, USGS scientists used historical data from over 14,000 individual chloride water-quality samples collected in 17 major metropolitan areas around the country between 1969 and 2008. Regionally, they used continuous specific conductance sensors as an indicator of road-salt runoff, monitoring 11 streams in southeast Wisconsin during both warm- and cold-weather periods from 1998 to 2008. To evaluate local conditions, they used data from 14 Milwaukee-area streams during road-salt application periods in 2007 for chloride concentrations and/or specific conductance, and performed bioassays using Pimephales promelas and Ceriodaphnia dubia.
Key findings from this study:
Nationally: During the winter, samples from fifty-five percent of northern streams in this study had chloride levels that exceeded USEPA chronic water-quality criteria, indicating potential toxicity. Samples from twenty-five percent of the streams exceeded acute water-quality criteria.
Regionally: In southeast Wisconsin, potential toxicity was found during winter at all urban streams studied, with lingering effects at some streams in the summer.
Locally: In Milwaukee, more than half of the samples collected from streams during winter deicing periods were toxic.
The Wisconsin State Laboratory of Hygiene co-authored this paper and did the bioassay testing involved. Additionally, this portion of the study was conducted in cooperation with both the Milwaukee Metropolitan Sewerage District and General Mitchell International Airport.
Impacts of deicers: Priority Substances List Assessment Report for Road Salts (Environment Canada), Effects of Road Salts on Aquatic Ecosystems (Environment Canada)
Strategies to Mitigate Impacts of Chloride Deicers on the Natural Environment (Transportation Research Board/National Cooperative Highway Research Program)
Road salt citation library (Zotero)
(updated Jan. 14, 2015)
What was the purpose of this study?
What led you to this study topic?
What geographic areas did you end up studying?
How did you study the effects of pavement deicer runoff?
What did you find?
Did any of these findings surprise you?
Could other sources be the cause of high chloride levels?
Who primarily uses deicing salt?
Are there alternative deicers that could be used?
Are there ways to reduce road salt applications and maintain the same level of safety?
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Average chloride concentrations often exceed toxic levels in many northern United States streams due to the use of salt to deice winter pavement, and...
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