In north temperate climates, road salt application and its associated chloride runoff can salinize freshwaters and threaten lake water quality and the many ecosystem services lakes provide. However, the extent to which lake salinity may be changing at broad spatial scales remains unknown, leading us to first identify spatial patterns and then investigate the drivers of these patterns. Significant decadal trends in lake salinization were identified from a dataset we compiled of long-term lake chloride concentrations from 529 North American and European lakes. We used both classification/regression trees and random forests to build predictive models for chloride trend.
Landscape and climate metrics calculated for each site demonstrated that impervious land cover was a strong predictor of chloride trends in Northeast and Midwest North American lakes, but not in Europe. In North America, as little as 1% impervious land cover surrounding a lake increased the likelihood of long-term salinization. Considering that 27% of large lakes in the United States have > 1% impervious land cover around their perimeters, the potential for steady and long-term salinization of these aquatic systems is high. Our results indicate that across a 10-state region, over 7000 lakes may be at risk of elevated chloride concentrations. We also predict that many lakes will exceed the aquatic life threshold criterion for chronic chloride exposure (230 mg L-1), stipulated by the U.S. Environmental Protection Agency (EPA), in the next 50 years if current trends continue.