The salt-strewn streets that have become a common winter sight in Minnesota now cause salt pollution severe enough that the Minnesota Pollution Control Agency (MPCA) has started to classify some Twin Cities streams as legally "impaired" from chloride contamination.
The use of road salt (mostly sodium chloride) in the United States, uncommon a generation ago, has increased eightfold since 1960. A University of Minnesota study showed that the Twin Cities metropolitan region now uses an astounding 260 pounds of road salt per person every winter.
During snow melt, peak chloride concentrations in some urban streams can approach half that of ocean water, far higher than many freshwater organisms can tolerate even for a short period. Chloride levels high enough to impair aquatic plants and animals may persist through the summer. This happens because some dissolved road salt infiltrates into groundwater, which then seeps slowly into streams, contaminating them long after the snow has melted.
Since 2002, the MPCA has designated four major Twin Cities streams -- Shingle Creek, Battle Creek, Bevins Creek and Nine Mile Creek -- as impaired because of chloride toxicity to aquatic life. For these large streams to have chloride toxicity means that numerous small streams and lakes also have toxic chloride concentrations, though they have not yet been legally designated as impaired.
The "impaired" designation means that local governments will need to find ways to reduce the contamination.
The Minnesota Department of Transportation and many local governments have sought to minimize salt use. Salt application rates are now based on real-time temperature measurement. With this information, salt can be applied just before a snow or ice event ("anti-icing"), rather than during or after the event, reducing salt application rates while improving driving conditions. New trucks have computerized controllers that deliver just enough salt to melt ice and no more. New snowplows are also equipped to apply brine ("prewetting") rather than dry salt, reducing wasted salt by 20 to 30 percent.
Use of road salt in Minnesota has been stable since 2000, after decades of increasing use, but real reductions will probably be needed to reduce chloride impairment of streams. A next step would be to install sensors directly in the most vulnerable streams. These sensors would provide additional feedback that could be used to fine-tune salting operations. For example, feedback from stream sensors might show that the normal salt-reduction strategy is not sufficient at a particular location and that alternative (but expensive) deicers are needed.
The effort to reduce road-salt use is driven largely by the legal requirement to protect aquatic life. However, as is often the case with environmental regulation, reduced road-salt pollution would have other benefits. A well-tuned adaptive management strategy could actually improve winter-driving conditions using less road salt (a shift from a brawn strategy to a brains strategy). Using less road salt would reduce corrosion of vehicles and bridge decks. According to a study by the Federal Highway Administration, bridge-deck corrosion costs $8.3 billion per year in direct costs (and 10 times more in indirect costs), much of it caused by salt. The same study reported that $23.4 billion is spent annually to avoid or repair corrosion to vehicles. The effort to reduce road-salt use would probably yield economic benefits that far exceed costs.
Lawrence Baker is a senior fellow at the University of Minnesota's Water Resources Center.