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vii related substances were primarily undeteetable except for copper and zinc. Concentrations of copper and zinc were higher in the south wetland, below the highway, than in the north wetland. Mass balance studies showed, however, that the proportion of these elements that can be traced to deicing compounds does not exceed 0.002%. Other highway-derived water quality influences (wear of metallic parts, lubricants, salt and sand applications) may account for concentrations of copper and zinc reaching or exceeding the aquatic life standard, or geologic factors may be involved. 22. The overall conclusion of the study is that application of magnesium chloride deicer having a chemical composition and application rate similar to those of 1997-98 is highly unlikely to cause or contribute to environmental damage at distances greater than 20 yds from the roadway. Even very close to the roadway, the potential of magnesium chloride deicer to cause environmental damage is probably much smaller than that of other factors related to road use and maintenance, including pollution of highway surfaces by vehicles and use of salt and sand mixtures to promote traction in winter. Magnesium chloride deicer may offer net environmental benefits if its use leads to a reduction in the quantity of salt and sand applied to roadways. The environmental safety of magnesium chloride deicer depends, however, on low concentrations of contaminants and avoidance of rust inhibitors containing phosphorus. ApprOpriate specifications for vendors and routine testing can insure the continued environmental acceptability of magnesium chloride deicers.