The effect of salinity on tire leachate toxicity

Previous whole animal bioassays with scrap tire leachates demonstrated an antagonistic interaction between salinity and toxicity, but it is unknown if the effect is due to an interaction between salts and toxic mode of action, or release of toxicants from the rubber matrix. Microtox(R) bacterial luminescence bioassays were performed on tire leachate solutions in a gradient of salinities from 0 to 25 ppt to assess salt/toxicant interactions. Leachate solutions generated at specific salinities were amended to higher salinities to assess the effect of sea salt on toxicity vs leachability/solubility of toxicants. Saline leachate solutions were also diluted with freshwater leachate. Original leachates and samples amended with additional salt were aged for 8 days and retested to assess longer term interactions between salt and toxicants. The toxicity of leachates decreased with increasing salinity up to 15 ppt, with no significant change at higher salinities. Adding sea salt to leachate solutions did not immediately decrease toxicity significantly, except for samples starting at zero salinity. This was interpreted as an indication of interaction between sea salt and the toxic constituent(s) in the leachate. Salinity decreases also indicated a direct effect of sea salt on toxicants. Ageing samples indicated a long term interaction between sea salt and leachate toxicants, but not degradation, over at least a two week period. Tire leachates are probably a greater threat to freshwater habitats than brackish or marine habitats, but bioaccumulation of persistent organic contaminants from tires is an unknown.