From icy roads to salty streams

Abstract

For most of human history, saltwas a precious commodity. People prized it for flavoring and preserving food and for use in religious ceremonies and burials. The Roman occupation of Britain peppered the English language with a legacy of salt. We retain those Latin links in words such as ‘‘salary’’ and ‘‘salami’’ and in place names like Greenwich and Sandwich, their suffix denoting a saltworks. Today salt is no longer precious. The U.S. mines _36 million metric tons [1 metric ton _ 1 megagram (Mg)] of rock salt a year (1). Eighteen million Mg is spread on paved surfaces for deicing, making winter roads safer for people and vehicles (2). However, once the salt dissolves, it washes into streams or soil and is forgotten. A new article by Kaushal et al. (3) in a recent issue of PNAS suggested that it should not be. The use of rock salt (NaCl) on U.S. roads has skyrocketed in the last 65 years (Fig. 1), and chloride (Cl) concentrations in waters of the northeast have risen as a consequence (4–6). The mobility of salt in water leads to its potential problems in the environment. These problems include toxicity to plants and fish, groundwater contamination, and human health interactions, particularly salt intake and hypertension (7–9). In consequence, researchers have been monitoring increased salt concentrations in streams and groundwater for decades (4–6, 10). A second aspect is their intensive focus on streams in the greater Baltimore area. In this rapidly urbanizing region, they found a logarithmic relationship between the proportion of pavement in a watershed and the mean annual Cl concentration in streams observed in the northeastern U.S. and Canada (11, 12). For example, a survey of 23 springs in the greater Toronto area found Cl concentrations topping 1,200 mg_liter_1 arising from road salt use (11). This groundwater salinity is the primary concern for long-term potable water supply. Once groundwater becomes salty, it typically will take decades to centuries for the salts to disappear, even when road salt use ends.