MOBILIZATION OF HEAVY METALS IN RESPONSE TO DEICING SALT APPLICATION

This study examined the effect of deicing salt application on the mobilization of heavy metals, particularly, As, Pb, and Hg in soil leachate and stream water through laboratory experiments and field data collection. In the soil column experiment where injection of concentrated NaCl solution was followed by flushing of low ionic strength water, significant increase of As, Pb and Hg concentrations above their background values was observed in the leachate after the peak of NaCl concentration passed through. In the NaCl injection experiment where the soil layer was mixed with arsenopyrite and galena to mimic the elevated As and Pb source, increase of As and Pb concentrations was in step with the increase of NaCl concentration in the leachate. In the mixed salt injection experiment, where CaCl₂ and NaCl were mixed at a ratio of 6:4 to mimic the deicing salt application in the state of Rhode Island, the concentration increase of As, Pb, and Hg was also in step with that of CaCl₂ in the leachate due to the strong cation exchange capacity of Ca²⁺. Alkaline fusion digestion and microwave assisted acid leaching of soil samples from two soil profiles in the studied watershed revealed that Pb and As are mainly concentrated in the A horizon due to bio-uptake and decomposition of the organic litters. These concentrated heavy metals on soil surface can be easily mobilized by the road salt facilitated dispersion of organic matter. For water samples collected from streams and lysimeters in the studied watershed, an obvious concentration increase of common metals was observed immediately after the deicing salt application, but only a slight concentration increase of Hg and Pb was observed. The concentration of As did not show any response to the application of deicing salt in stream water or runoff water along the studied transects of an interstate highway due to the source of As is dominated by groundwater baseflow. Our study concluded that the mild elevation of Pb and Hg in stream water in response to the deicing salt application, though not alarming, is more likely due to the combined effect of the dispersion of organic matter and clay particles from the elevated NaCl effect, and complexation of lead and mercury with chloride, and cation exchange on the soil surface.