Thursday, April 20, 2017

New Analysis: Freshwater Lakes Getting Saltier

lake mononaNorth America’s freshwater lakes are getting saltier due to development and exposure to road salt. A study of 371 lakes published in the Proceedings of the National Academy of Sciences reports that many Midwestern and Northeastern lakes are experiencing increasing chloride trends, with some 44% of lakes sampled in these regions undergoing long-term salinization.

The study is the first large-scale analysis of chloride trends in freshwater lakes. It was conducted by a team of fifteen researchers as part of the Global Lake Ecological Observatory Network (GLEON) Fellowship Program, an initiative that seeks to train the next generation of freshwater scientists and practitioners.

Chloride trends in 371 freshwater lakes were analyzed. Each lake was larger than 4 hectares in size with at least 10 years of recorded chloride data. The majority of the lakes (284) were located in a North American Lakes Region that includes Connecticut, Maine, Massachusetts, Michigan, Minnesota, New Hampshire, New York, Ontario, Rhode Island, Vermont, and Wisconsin.

Since the 1940s, the use of road salt on roads has been escalating. Each year, some 23 million metric tons of sodium chloride-based deicer is applied to North America’s roads to melt away snow and ice. Much of this road salt washes into nearby water bodies, where it is recognized as a major source of chloride pollution to groundwater, streams, rivers, and lakes.

To gauge road salt exposure, the research team assessed road density and land cover within a 100- to 1500-meter buffer around each of the 371 study lakes. Roadways and impervious surfaces such as parking lots and sidewalks are reliable proxies for road salt application because as developed areas, they are susceptible to high levels of salting and runoff.

Results were clear: roads and other impervious surfaces within 500 meters of a lake’s shoreline were a strong predictor of elevated chloride concentrations. In the North American Lakes Region, 70% (94 out of 134) of lakes with more than 1% impervious land cover in their 500-meter buffer zone had increasing chloride trends. When results are extrapolated to all lakes in the North American Lakes Region, some 7,770 lakes may be at risk of rising salinity.

If current salinization trends continue, many North American lakes will surpass EPA-recommended chloride levels in 50 years. Within this study, 14 North American Lakes Region lakes are expected to exceed the EPA’s aquatic life criterion concentration of 230 mg/L by 2050, and 47 are on track to reach chloride concentrations of 100 mg/L during the same time period.

In lakes, elevated chloride levels have been shown to alter the composition of fish, invertebrates, and the plankton that form the base of the aquatic food web. Aquatic species richness and abundance can decline, and in extreme cases salinization can prevent lakes from mixing – causing low oxygen conditions that smother aquatic life and reduce water quality.

The study’s authors recommend that best lake management practices recognize that shoreline management extends well beyond a lake’s perimeter. While many states and municipalities acknowledge the importance of shoreline management, they note that zoning regulations are often only enforced within 300 meters, and many lakes lack the monitoring programs needed to adequately track lake health.

A lake’s chloride status may also provide a window into the ecological health of its watershed. Co-author Samantha Burke, a graduate student at the University of Waterloo, adds, “Unlike flowing streams and rivers, water resides in lakes for long periods of time. This makes them vulnerable to pollution from their watersheds and good early warning indicators of environmental disruption.”

Photo: Lake Monona, in Madison, Wisconsin which is experiencing rising salinity due to nearby roadways and road salt application, courtesy Hilary Dugan.

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4 Responses

  1. We recently published a report on Mirror Lake that includes chloride data dating back to 1974. Back then the surface water chloride concentration was 4 mg/L, the average surface water concentration in 2016 was 48 mg/L. We also found that sodium and chloride build up in the bottom of the lake, with concentrations exceeding 100 mg/L during the winter/spring. As a point of reference, work done by the Adirondack Watershed Institute shows that the mean chloride concentration in Adirondack lakes without roads in their watershed is 0.24 mg/L.

    You can view the report here >>

    • Boreas says:

      Brackish water pushing upstream from rising oceans is another salinity issue to be reckoned with.

    • Bill Ott says:

      Mr. Wiltse (or is it Dr.), the numbers are scary. I live near Lake Erie, and have about a hundred pounds of salt that I am never going to use. What is the best way to get rid of it. And what about all the salt we use to cook. Does it not eventually make it into the lake also?

  2. Bill,

    My best suggestion is to bring it to your local landfill or dump. Although my guess is it will end up in the environment.

    According to the USGS, in 2005 deicing topped the charts for salt use nationally, coming in at 23 million tons per year. Food products came in at 3 million tons per year. The use of salt as a deicing agent is by far the most significant contributor to elevated sodium and chloride concentrations in Adirondack lakes.


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