Effects of magnesium chloride road deicer on montane stream benthic communities

Montane streams often intercept and run parallel to roads and highways where road deicer is seasonally applied for snow and ice removal. This research used stream mesocosms to evaluate the effects of MgCl2 road deicer to a Rocky Mountain stream benthic community in Colorado, USA. Measured responses included macroinvertebrate drift, community composition metrics, and macroinvertebrate biomass after a 10-day exposure. Natural benthic communities were exposed to concentrations of liquid MgCl2 road deicer that bracketed the U.S. Environmental Protection Agency (U.S. EPA) surface water chronic chloride ‘aquatic life criteria’ (230 mg Cl−/l). Results showed no effects on macroinvertebrate drift, but significant reductions in abundance, taxa richness, and community biomass. Specifically, stonefly (Plecoptera) and mayfly (Ephemeroptera) abundance decreased at Cl− concentrations below the U.S. EPA chronic chloride water quality standard, and at concentrations substantially lower than those generated from traditional laboratory toxicity tests. However, caddisflies (Trichoptera), midges (Chironomidae) and other dipterans were tolerant to all MgCl2 treatments. We conclude that MgCl2 road deicer has the potential to impair montane stream benthic communities at relatively low ionic concentrations, and regulatory agencies should manage for and establish regionally appropriate application rates for this stressor.