THE EFFECTS OF ROAD SALTS ON AQUATIC ECOSYSTEMS

This study is based on the Priority Substance List 2 (PSL2) assessment of the toxicity of the application of road salt to the aquatic environment. The assessment begins with a review of the PSL2 assessment process, including the formation of an Environment Resource Group (ERG) to conduct this assessment. Road salt is then characterized including its composition, properties, and application rates across Canada. This assessment focuses on the chloride salts. Sodium followed by calcium are the primary chloride salts used in the winter application of road salts. Calcium chloride is used mainly as a summer dust suppressant. Small amounts of potassium and magnesium chloride salts are also used as a winter deicer. Winter road salt is applied most heavily in southern Ontario, Quebec, Nova Scotia, New Brunswick, British Columbia, and the central (Edmonton) area of Alberta. An extensive review of the Canadian and United States literature determined that a relatively small number of studies have been conducted investigating road salt in the aquatic environment. Road salt impacts, defined as increased chloride concentrations, were most pronounced in urban areas and areas located near heavily salted highways. Furthermore, there is some evidence of more gradual increases in chloride concentrations in lakes in these regions, in part a result of road salt application. An extensive review was conducted of laboratory studies investigating the toxicity of sodium, calcium, potassium, and magnesium chlorides. By far the largest data sets were obtained for sodium chloride. Magnesium and potassium chloride appear to be more toxic than sodium chloride for all organisms investigated. Plankton and invertebrates appear to be more sensitive to calcium than sodium chloride while the reverse appears to occur for fish. It is concluded that chloride salts from road salt are toxic in the environment although in limited geographic areas where large amounts of salt are applied to highways or near leaking road salt depots. As chloride concentration gradually increases, there are unknown changes in composition and standing stock as a result of biological interactions. Because road salt has the potential to alter natural communities (like elevated phosphorous), elevated concentrations should be viewed as being potentially environmentally harmful.