The Effect of Deicing Salts on Trace Metal Mobility in Roadside Soils

Calcium magnesium acetate (CMA) (Ca₀.₃Mg₀.₇(C₂H₃O₂)₂) is being used as an alternative to NaCl for deicing highways. CMA is an effective deicer, handles well, biodegrades, is less toxic to aquatic life, and is less corrosive than Cl salts. There is some concern, however, that CMA may increase the mobility of certain trace metals from roadside soils. Roadside soils were collected from around the USA from high-traffic and high-salt use areas. These soils were characterized for total trace metal content as well as important chemical and physical parameters. Studies were carried out to determine (i) the effect of CMA decomposition on Pb and Cd concentrations in an aqueous system (no soil), (ii) changes in major and minor element chemistry in soils incubated with NaCl and CMA solutions, and (iii) the mobility of trace metals from soil columns leached with solutions of deicing chemicals and/or H₂O. Microbial decomposition of the acetate resulted in increased pH, CaCO₃ precipitation, and immobilization of Cd either through coprecipitation with CaCO₃ or CdCO₃ precipitation. There was no difference in trace metal solubility in soils treated with H₂O or 10 mmolc L⁻¹ CMA or NaCl (average runoff concentration). Trace metal concentrations were initially high in soils treated with 100 mmolc L⁻¹ solutions of deicing salts but concentrations decreased with time due to adsorption or precipitation reactions. The leachability of Pb from soil columns was strongly correlated with organic matter solubilization. Soils leached with NaCl followed by distilled water had reduced permeabilities as compared to CMA-leached soils. It appears the effect of CMA on trace metal mobility in roadside soils should generally be beneficial. Contribution from the Dep. of Soil and Environ. Sciences, Univ. of California, Riverside, CA 92521.