Increased Salinization Decreases Safe Drinking Water

Abstract

O decades, there has been increased salinization of fresh water in many regions of the world. Salinization was previously thought to be an environmental problem restricted primarily to arid regions. However, it is now recognized as a global environmental concern impacting humid regions due to human inputs from road deicers, sewage inputs, and water softeners (Figure 1). In both arid and humid regions, there can be “natural” variations in geologic sources of salts due to chemical weathering. Recently, it has been suggested that human activities have contributed to accelerated weathering of geologic sources and additional inputs of salinization. Thus, both human inputs and accelerated weathering are contributing to the increased salinization of fresh water. Increased salinization can impact drinking water through leaching of contaminants in soils, sediments, and water infrastructure. For example, corrosion of water infrastructure influenced by road salt has recently been linked to lead contamination in drinking water in Flint, Michigan. Deicers can also contain impurities and toxic anticaking agents. Despite these and other concerns regarding drinking water contamination, salt concentrations are not currently regulated by the U.S. Environmental Protection Agency. Given widespread increasing salinization trends, federal regulation of salt concentrations could lead to better management aimed at reducing impacts to drinking water and aging infrastructure. In northern regions, salinization of fresh water is significantly related to increasing impervious surface cover in watersheds. In addition to geologic sources and other human inputs, salinization increases as deicer is applied to impervious surfaces to promote safe transportation. During winter months, salt concentrations in urban waters can spike up to approximately 25% the salinity of seawater. It can take weeks and months for these salt pulses to diminish, and salt concentrations can remain chronically elevated during summer months. Interestingly, long-term baseline concentrations of salts have increased in fresh water, even during seasons when deicer is not applied. These widespread trends are due to the accumulation of salts stored in soils and groundwater. Thus, even if road salt applications ceased, many freshwater ecosystems would still remain chronically salinized for decades. Increased salinization of fresh water now represents a chronic environmental problem. In watersheds, salinization of fresh water enhances mobilization of contaminants from soils and sediments. It is well-known that sodium from road salts can displace toxic metals and base cations from ion exchange sites on soils and sediments. For example, toxic metals such as copper, lead, and zinc are released from soil exchange sites near streams following applications of road salt. In response to salinization, organic carbon can be released from changes in soil structure, microbial decomposition, and desorption. Salinization enhances leaching of organic carbon from soils and sediments, which can increase disinfection byproducts in fresh water. In addition, salinization enhances leaching of organic nitrogen, ammonium, and phosphorus from ion exchange sites on soils. Mobilization of nitrogen and phosphorus to drinking water supplies can further contribute to eutrophication. Overall, increased salinization of fresh water enhances mobilization of previously