Deicing salt exacerbates freshwater salinization under climate change and human activities

Extreme rainfall events caused by climate change are triggering the instability of water resources, while the expansion of human activities is contributing to water quality degradation, jointly intensifying the risk of water scarcity. It has been reported that 40% of the world’s population is at risk of quantity-based water scarcity for at least 1 month a year, while 26% are at risk of quality-based water scarcity.1 Increasing human activity is projected to exacerbate water quality degradation in the coming years, which poses a threat to global water security. These issues are closely related to the United Nations Sustainable Development Goals (SDGs), including human health and well-being (SDG 3), ensuring access to water resources and sanitation (SDG 6), and promoting sustainable urban and community development (SDG 11).2 Therefore, it is necessary to develop an integrated management response to these issues.

Among these threats, freshwater salinization poses a significant challenge to quality-based water scarcity. The increasing salinity of inland freshwater globally, termed "freshwater salinization syndrome," threatens both water quality and aquatic ecosystems. Previous studies have focused on natural drivers—such as climate change, seawater intrusion in coastal areas, and geological salinization in inland regions—as well as anthropogenic factors like irrigation backflow, chemical pollution, and urbanization. Recently, the role of deicing salts in freshwater salinization has received attention. The majority of deicing salts are chloride-based compounds (e.g., sodium chloride [NaCl], calcium chloride [CaCl2], magnesium chloride [MgCl2], and potassium chloride [KCl]), which lower the melting point of snow and ice to ensure road safety. For instance, the common application rate of road salt in the northeastern United States is 84 kg/km (300 lb/mile), and multiple snowfall events result in an annual application rate of 15–30 tons per mile (8.45–16.9 kg/m).3 As the snow melts, large quantities of salts are transported into nearby rivers, lakes, and groundwater, thereby increasing freshwater salinity. However, due to limited data availability, the drivers and environmental impacts of deicing salt remain unclear, posing challenges for managing lake salinization. It is crucial to reveal the migration mechanism of deicing salts—considering potential drivers such as climate patterns, population growth, and urban expansion—and characterize their temporal trends and spatial distribution under changing environments. These research gaps present significant challenges for developing and implementing effective response strategies, especially given the current imbalance in freshwater resources.

Salinization due to deicing salt threatens the environment and society

Deicing-salt-induced salinization has been proven to have a series of negative environmental and social impacts. With rainfall or snowmelt runoff during early spring and winter, large amounts of chloride ions from deicing salts are carried into nearby receiving waters, delivering a significant shock to the local water environment. Several studies indicate that once the salinity of surface water reaches a certain threshold, the ecosystem may undergo a non-linear and abrupt change, leading to ecosystem collapse, significant disruptions to food webs and ecological processes, or reduced biodiversity as sensitive species are replaced by more salt-tolerant organisms.4 This threshold may vary spatially and temporally with changing climate and intensified human activities, further complicating freshwater salinization management. Concurrently, as snow containing deicing salts melts and infiltrates the soil, substantial chloride remains, resulting in soil salinization and gradual release into groundwater during subsequent precipitation events. This process could lead to the long-term deterioration of groundwater quality, which would be detrimental to achieving SDG 6 on water resources and sanitation.