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

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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.

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 showing chloride concentration trends for 30 sites in 19 streams across the U.S.
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 showing chloride concentration estimates for three Milwaukee streams
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
    Figure showing relationship between urban land cover and chloride concentrations
    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)

    Road Salt Frequently Asked Questions (FAQs)

    (updated Jan. 14, 2015)

    What was the purpose of this study?

    • The primary objectives of this study were to define temporal trends in chloride concentrations in streams.
    • Define these trends in the context of chloride dependency on streamflow rates,
    • Compare temporal chloride trends among seasons, and
    • Compare these trends to changes in urban land cover, aquatic life criteria, and road salt sales patterns.

    What led you to this study topic?

    • This was a follow-up study from our previous study examining impacts of road salt on aquatic toxicity and water quality. We wanted to define how concentrations have been changing over the years and if concentrations were still increasing over time. 

    What geographic areas did you end up studying?

    • A detailed local study in the Milwaukee Metropolitan area showed that concentrations were increasing rapidly, so we also expanded the study to other sites in the northern U.S. to give a broader perspective.

    How did you study the effects of pavement deicer runoff?

    • We used the extensive historical databases from USGS and USEPA (http://www.waterqualitydata.us/) as well as Milwaukee Metropolitan Sewerage District.
    • Data was compiled from numerous streams in the northern U.S. with a range of urban influence in their watersheds. We also collected data from one site in Texas to examine the data in an area without road salt applications.
    • We analyzed water-quality data from 30 monitoring sites on 19 streams near cities in Wisconsin, Illinois, Colorado, Michigan, Ohio, Pennsylvania, Maryland, Texas and the District of Columbia.
    • Trends were determined for all sites during all four seasons of the year and put in the context of variable streamflow rates.
    • Analysis of data to determine trends was done using a modern statistical modeling technique called Weighted Regression on Time Discharge and Season (WRTDS) within the software package Exploration and Graphics for RivEr Trends (EGRET; https://github.com/USGS-R/EGRET/wiki).

    What did you find?

    • In 84 percent of urban streams analyzed, chloride levels increased substantially during the study period.
    • Twenty-nine percent of the sites exceeded the U.S. Environmental Protection Agency’s chronic water-quality criteria (230 milligrams per liter) by an average of more than 100 days per year from 2006 through 2010, which was almost double the amount of days from 1990 through 1994. 
    • The lowest chloride concentrations were in watersheds that had little urban land use or cities without much snowfall, such as Dallas, Texas.
    • In 16 of the streams, winter chloride concentrations increased over the study period.
    • The greatest chloride concentrations occurred during winter periods.
    • In 13 of the streams, chloride concentrations increased over the study period during non-deicing periods such as summer. This finding suggests that chloride infiltrates the groundwater system during the winter and is slowly released to the streams throughout the year.
    • Chloride concentrations increased more rapidly than development of urban land near the study sites.

    Did any of these findings surprise you?

    • The fact that there were elevated chloride levels was not surprising. The surprise was:
    • Chloride Concentrations appear to be increasing more rapidly than the increase in pavement that requires deicing.
      • These rapid chloride increases were likely caused by a combination of these possibilities:
        • The rate at which salt is applied on any given surface has increased.
        • The baseline conditions (the concentrations during summer and early fall low-flow periods before deicing begins) have increased over time.
        • Greater snowfall in the Midwest during the latter part of the study.

    Could other sources be the cause of high chloride levels?

    • In this study, we specifically targeted streams and time periods where the dominant source would be road salt, but in a general sense, there are other sources such as:
    • Wastewater treatment effluent
    • Septic systems
    • Farming operations
    • Natural sources
    • Geographic comparison, seasonal comparison, land use comparison, site selection (most likely streams with road salt runoff)
    • Analysis suggests road salt as the most likely source

    Who primarily uses deicing salt?

    • City maintenance crews
    • County/highway departments
    • Commercial applicators for private and public parking lots, driveways, and sidewalks
    • Private individuals for driveways and sidewalks

    Are there alternative deicers that could be used?

    • Yes, but each has their own impact.
      • Some may have toxic impacts as well
      • Some have oxygen demand when introduced to a water body causing potential for low dissolved oxygen levels that can be harmful to aquatic life

    Are there ways to reduce road salt applications and maintain the same level of safety?

    • In many cases, yes. There are a host of different salting practices that can be adopted to make most efficient use of the salt that is applied. Some examples include:
      • Pre-wetting the pavement is one example. A liquid salt brine is applied to the pavement before the storm arrives so that snow and ice does not bond as well to the pavement. This makes plowing more effective and reduces the need for chemical deicers.
      • There are also new plow-blade designs that make plowing more effective. Some act like a squeegee and others work by breaking adhesion to the pavement more effectively. The result is the same for both. After plowing, the pavement is actually cleaner, so less chemical deicers are needed.
      • Many more, some of which are included in this report: Strategies to Mitigate Impacts of Chloride Deicers on the Natural Environment (Transportation Research Board/National Cooperative Highway Research Program, http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_syn_449.pdf)