It’s beginning to look a lot like winter, and as the temperatures drop the snow can’t be far behind. When the snow starts to fly, skiers take to the slopes and plow trucks take to the road often bringing with them salt to help melt the accumulating inches. But have you ever wondered where all that salt goes? And once it is there, is there enough of the salt to be bad? For plants? For water? For fish?
Paper Reference: Dugan, H. A., et al. (2017) Salting our freshwater lakes. PNAS. 114(17):4453-4458. www.pnas.org/cgi/doi/10.1073/pnas.1620211114
Let it snow! Let it snow! Let it snow!
Since the 1940s, salt (solid form), and more recently brine (liquid form, applied pre-storm), has been a useful tool of highway departments across North America and Europe as they work to keep roads clear when winter precipitation threatens the forecast. But once it’s done melting ice and snow on the nation’s roadways and making it safe to motor to school, work and to the grocery store for that emergency loaf of bread and gallon of milk, road salt (measured as chloride in the environment) makes its way into ditches, streams, rivers, lakes and even groundwater.
The Dugan et al. (2017) study described here takes a regional approach and focuses on the long term effects to freshwater lakes. This builds on previous research efforts aiming to characterize the localized short term effects to freshwater.
In their study, Dugan et al. focused on lakes with at least 10 years of data that were also larger than 10 acres in size. From this sample, the researchers used a threshold approach to exclude lakes that were possibly brackish from natural sources and groups of lakes that had unique hydrology conditions that might confuse the analysis. The remaining lakes resulted in relatively dense spatial coverage in the states of Connecticut, Maine, Massachusetts, Michigan, Minnesota, New Hampshire, New York, Ontario, Rhode Island, Vermont and Wisconsin.
The researchers first considered the overall historical trend. For the 56 lakes with data from 1980 to 2010, the researchers observed three distinct trends in the chloride concentration over time: 1) neutral or decreasing in 16 lakes; 2) oscillating in four lakes; and, 3) increasing in 36 lakes.
Using the entire dataset, the researchers were then interested in the possible factors driving these trends and looked at lake, climate, and landscape characteristics. Using multiple model building approaches (classification/regression trees and random forests), the authors determined that impervious land cover and road density were the most important predictors for lake chloride trends.
Of the sites included in the study, 44% of the freshwater lakes were found to be experiencing long-term salinization, and as little as 1% impervious surface coverage within 500 m (1640 ft) of the shoreline was needed for the predictive power to be important. Looking at states with bias toward urban lakes, the trends are overwhelmingly increasing with lakes in Minnesota beginning to approach the level of chloride the US EPA considers to be hazardous to aquatic life and at which the taste of drinking water becomes perceptibly impacted. Using a linear extrapolation, at least 14 of the study lakes will be significantly impacted by 2050.
These findings have important implications for all freshwater lakes. Freshwater lakes are numerous and provide important social and ecological services to communities. Development in the vicinity of water bodies is often dictated by special “shoreline zoning” rules that can vary by state. Shoreline zoning rules can limit building construction or restrict grassy lawns that extend to the water’s edge. But while some states, including Wisconsin and Minnesota regulate near shore land use within roughly 1000 feet of a shoreline, other states have much smaller buffers including Vermont and Maine which only regulate within 250 ft from shorelines. Based on their findings, the authors suggest that shoreline zoning rules need to think in wider buffers.
Balancing the drive for development, and the desire to keep freshwater lakes “fresh” will now also need to take into account the practices we use to keep cars humming along the roadways in winter.