From icy roads to salty streams

Abstract

For most of human history, salt was 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 salt-works. 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). Fig. 1. Sales of rock salt for highway use in the U.S. from 1940 to 2004 in millions of metric tons (Mg) (1, 2). The dashed line denotes our estimate of the calculated annual wet deposition of Na and Cl in the U.S., derived primarily from sea salt. The amount of Na and Cl in road salt topped Na and Cl deposition for the …