Every winter, road crews apply sodium chloride—commonly known as rock salt—to roads to prevent ice and ensure safe driving. However, when snow melts, the salt doesn't disappear. Instead, it runs off into ditches, seeps into groundwater, and eventually flows into rivers and lakes. This runoff introduces chlorides—dissolved ions that permanently change the chemistry of freshwater systems. In Chicago, the amount of road salt used has increased by 33% over 30 years, but chloride levels in rivers have risen by 60%. Some urban lakes now have chloride concentrations up to 43% above the official level considered toxic to aquatic life, leading to oxygen-deprived "dead zones" at the bottom of the lakes. Across North America, about 23 million metric tons of sodium chloride-based de-icing materials are used annually, with much of it ending up in nearby water bodies, contributing to chloride pollution in groundwater and lakes. A recent study published in the Proceedings of the National Academy of Sciences found a strong statistical link between high chloride levels in lakes and the presence of roads or other impermeable surfaces within 500 meters of a lake’s shore. In the Great Lakes region, 94 out of 134 lakes with at least 1% of impermeable surface area within this buffer zone showed rising chloride levels. In Chicago, data from 30 years shows that while road salt use increased by 33% between 1990 and 2020, chloride levels in rivers rose by 60%. This suggests that some of the salt infiltrates the ground, eventually seeping into groundwater and being released into rivers even outside of winter. This cumulative effect impacts public water supplies and continues to pollute waterways year-round. Studies in France have found that road salt can harm vegetation along roadsides, causing burns on trees and shrubs near salted routes. In the Outaouais River basin, researchers found unexpectedly high chloride levels in water during non-winter months. Chlorides can reduce biodiversity in aquatic ecosystems by harming fish and plants, while promoting the growth of certain algae, like cyanobacteria. They also slow the natural processes that help water purify itself, such as the uptake of nutrients by plants and the breakdown of organic matter. In some urban lakes heavily affected by road runoff, chloride levels exceed legal limits. For example, in a lake near a highway in Grand Rapids, Michigan, chloride concentrations in the deepest parts of the lake have been measured at up to 331 mg/L—well above the U.S. Environmental Protection Agency’s chronic toxicity threshold of 230 mg/L. These high levels disrupt natural water mixing and create oxygen-deprived zones at depth, harming aquatic life. The presence of chlorides also complicates the work of drinking water treatment plants. Some private wells near salted roads or salt storage areas can become contaminated with sodium and chloride, with seasonal variations helping to trace the source of the problem. While alternatives to sodium chloride exist, such as brine solutions or abrasives like sand, sodium chloride remains the most commonly used due to its low cost and ease of application. Techniques like pre-wetting salt with brine can improve its effectiveness and reduce the amount needed. Some communities have begun monitoring chloride levels more closely, with the Quebec Ministry of Transportation limiting the use of de-icing salt near water bodies and groundwater sources. Scientists can distinguish between road salt and naturally occurring salts using chemical tracers, such as iodide. This helps communities understand the extent of the problem and consider changing their de-icing practices. Sodium chloride remains the dominant choice for road safety because of its affordability and logistical convenience, despite its long-term environmental impact. As awareness of the issue grows, more communities are exploring alternative methods to reduce the environmental toll of road salt.