Salt Project Science Summary
Chloride is a naturally occurring ion in freshwater lakes that can have adverse ecological impacts if concentrations rise above natural background levels. Many lakes worldwide, including the Great Lakes, have seen increased chloride concentrations over the past few decades. Regional drivers include both natural processes (e.g., sea salt deposition and long periods of drought) and anthropogenic influences (e.g., runoff from road salt application and agricultural fertilizer and discharge of industrial effluent). While these trends and drivers have been studied at the watershed scale, there has been no large-scale analyses of chloride trends in freshwater lakes.
Here, we have compiled a dataset of long-term (> 10 years) chloride concentrations in over 500 freshwater lakes, ponds and reservoirs worldwide (>0.04 ha), predominantly in the United States, Canada, Sweden, and the United Kingdom. The geographic and morphometric range of lakes in this dataset allows us to assess the impact of ecoregion, lake area, residence time, and lake depth on relative changes in chloride concentrations. For the United States, we combined our dataset with the National Land Cover Database and the National Hydrography Dataset to quantify percent impervious surface, as a proxy for road density, within the near-shore environment of our study lakes.
We find that urban lakes, especially in the American Midwest (Wisconsin, Minnesota) have strong positive trends in chloride concentrations, whereas remote lakes, such as ponds in the Adirondack Mountains of New York, have had stable or decreasing chloride levels. In states with seasonal road-salt application, the percent impervious surface within 100 m of a lake is strongly correlated with increases in chloride. Road salt runoff is just one ecological threat to urban lakes, but can be abated with proper management.