THE INCREASING IMPACT OF ROAD SALT: WIDESPREAD AQUATIC TOXICITY AND WATER QUALITY IMPACTS ON LOCAL, REGIONAL, AND NATIONAL SCALES

While road salt influence on water quality has been documented for at least forty years, a new perspective on the severity of aquatic toxicity impact was gained by a focused seasonal analysis. Dramatic impacts were observed on local, regional, and national scales. Locally, samples from 7 of 13 Milwaukee area streams during two road salt runoff events exhibited toxicity in Ceriodaphnia dubia and Pimephales promelas bioassays with chloride concentrations as high as 6,470 mg/L, far exceeding the U.S. Environmental Protection Agency (USEPA) acute and chronic water quality criteria of 860mg/L and 230 mg/L, respectively. Twelve years of testing at Wilson Park Creek in Milwaukee resulted in chloride concentrations as high as 7,730 mg/L in 37 samples. Toxicity was observed in 72% of these samples in chronic bioassays and 43% in acute bioassays. Analysis of long-term chloride data from the Milwaukee River using weighted regressions on time, discharge, and season indicates substantial increasing concentration from 1973 to 2005. Concentrations were greatest during winter low-flow periods, but even during warm-weather months and during high-flow periods, notable increasing trends were present. Regionally in eastern and southern Wisconsin, continuous specific conductance was monitored as a chloride surrogate in 11 watersheds with urban land use from 6% to 100%. Elevated specific conductance was present during cold-weather months at all sites with effects during warm-weather months at the most urban sites. Specific conductance increased with urban land use and was measured as high as 30,800 µS/cm (Cl = 11,200 mg/L). Estimated chloride concentrations exceeded USEPA acute water quality criteria at 55% and chronic water quality criteria at 100% of these sites. Nationally, U.S. Geological Survey historical chloride data was examined for 13 northern and 4 southern metropolitan areas. Chloride concentrations exceeded USEPA water quality criteria at 25% (acute) and 55% (chronic) of the 168 northern monitoring locations during cold-weather months. Only 1% (acute) and 16% (chronic) of sites exceeded criteria during warm-weather months. At southern sites, 2% and 4% of sites had samples that exceeded chronic water quality criteria during cold- and warm-weather months, respectively; no samples exceeded acute criteria.