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Evaluating chloride trends due to road-salt use and its impacts on water quality and aquatic organisms [1]

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Date: 2022-12

Figure 1. Road salt study site locations and watershed characteristics

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.

Trends in stream chloride concentrations due to road-salt use

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.

Figure 2. Winter (black line) and summer (grey line) flow-normalized chloride concentration trends for 30 sites in 19 streams across the United States. The background color represents watershed percent imperviousness as determined using the National Land Cover Database from 2006 (Fry et al. 2011). Sites are ordered by percent imperviousness. Seiche affected is defined by backwater influence from Lake Michigan.

Figure 3. Chloride concentration estimates at 10, 50, and 90 percentile flow rates from the WRTDS model over time and grouped by season for three Milwaukee streams. Graphs are presented in order of decreasing watershed size and increasing urban land cover from left to right. Streamflow is expressed in cubic meters per second (cms). Dashed line for USEPA chronic water quality criteria represents 230 mg/L.

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.

Streams with the lowest chloride concentrations were located in watersheds with small amounts of urban land cover, like Willamette, Oregon, or those receiving little snowfall, such as Dallas, Texas.

In 16 of the streams, winter chloride contamination concentrations increased over the study period.

In 13 of the streams, chloride concentrations increased over the study period even during non-deicing periods such as summer, suggesting that chloride infiltrated into the shallow groundwater system in the winter and was slowly released to the streams throughout the year as baseflow.

Chloride concentrations increased more rapidly than development of urban land near the study sites (fig. 4A), implying an increase in road salt application rates and not simply an increase due to an expansion of paved surfaces that required deicing.

Overall, increasing trends in chloride concentrations were likely caused by increased salt application rates, increased baseline conditions (the concentrations during summer low-flow periods) and greater snowfall in the Midwest during the latter part of the study

Sources/Usage: Public Domain. Figure 4. Average chloride concentration (A) and expected number of individual days per year with concentration exceeding the USEPA chronic water quality criteria of 230 mg/L (B) from modeling results compared to urban land cover percentage in the contributing watershed.

Impacts of road-salt runoff on water-quality and aquatic organisms

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.

During winter, 100 percent of the streams monitored had chloride levels that exceeded the USEPA chronic water quality criteria in one or more samples with fifty-five percent of samples exceeding acute water quality criteria.

Chloride levels higher than 10,000 milligrams per liter were observed at times during winter deicing periods—much greater than the chronic water-quality criteria of 230 milligrams per liter and the acute criteria of 860 milligrams per liter.

Chloride levels increased as urbanization percentage in the watershed increased.

Locally: In Milwaukee, more than half of the samples collected from streams during winter deicing periods were toxic.

Samples from seven of 13 streams collected during 2007 deicing periods were toxic in bioassay tests.

Chloride levels in 12 out of these 13 streams exceeded USEPA chronic water quality criteria; eight of 13 exceeded acute criteria.

In long-term testing of one Milwaukee stream between 1997 and 2008, seventy-two percent of the 38 samples collected during the winter were toxic in bioassay tests.

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.

Other road salt resources

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)

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[1] Url: https://www.usgs.gov/centers/upper-midwest-water-science-center/science/evaluating-chloride-trends-due-road-salt-use-and#overview

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