Pinpointing your environmental weak spots
Richard L. Hanneman
Over the past half-century, more than a half-trillion tons of salt have been applied to keep North American highways safe during winter ice and snowstorms.
Four truths about using salt for winter highway maintenance we know very well: (1) salt saves lives: an estimated 88% of injury crashes are prevented by judicious use of highway salt; (2) salt is the most economical deicer we have—and the most cost-effective, too, repaying its direct costs with accident prevention benefits within a half-hour; (3) keeping winter roads open and safe is essential to protect jobs and our quality of life (an investment in snowfighting that prevents even a single storm from paralyzing an area's economy, paying more dividends in terms of wage income, retail sales and tax receipts than the entire winter's snowfighting budget); and (4) unless we store and apply deicing salt sensibly, we may damage the environment. Yet, for all the hundreds of millions of tons of salt used across broad areas of the United States and Canada, recorded incidents of environmental damage are peculiarly local in character.
Much has been written about the first three "truths" and, for the fourth "truth," quite a bit of attention has been given to the general principles of "Sensible Salting"—properly storing salt and applying only the minimum amount required at the right place at the right time to keep roads safe. Less studied is the question of how we determine the time and location where "normal" salting practices overwhelm nature's adaptive capacity and produce unacceptable environmental insult.
Over the past five years, the Salt Institute has been participating in a "stakeholders group" with salt companies, public roadway maintenance agencies, government environmental regulators and environmental groups to study best practices in salt management, including pioneering the concept of identifying "salt vulnerable" areas that require special attention in creating salt management plans. The group was organized and led by Environment Canada and included a representative of the Canadian Public Works Association and also included a large number of APWA/CPWA members.
Scientists have developed an enormous volume of literature documenting the chloride tolerance of various species of plants and animals and the impacts of sodium on soil structure. We know which species and soil types are relatively "salt tolerant." We often select among salt-tolerant plant species when we build a roadway, replacing "nature" with an engineered right-of-way with accompanying stress of vehicular traffic. We have studied both chronic and acute exposure levels for virtually every species found in this man-made environment. Even when we "salt proof" the roadway environment, however, there are some spots of special environmental concern. We are now zeroing-in on how we sensitize our roadway operations practices to avoid impairing the quality of life for salt-sensitive species or damage to certain clay soils susceptible to sodium overload.
Every community that uses deicing salt can benefit by conducting a survey to identify salt vulnerable areas. Every winter roadway manager can benefit by considering the risks of current operating practices and whether special engineering or operating tactics can lower the potential for environmental damage. Even an in-house review can pay handsome dividends, though some agencies have used consulting engineers to produce high-quality overlay maps identifying sensitive areas for each environmental media (surface water, groundwater and soil/vegetation) and the species abiding there. Perhaps you have a local university with an environmental science department looking for a useful student project.
Salt can be very damaging to the environment, but damage is mainly local because salt-sensitive areas are not usually widespread and often agencies have already adopted special, protective snowfighting tactics in those areas. Mitigating potential salt damage is aided by basic highway engineering which creates clear zones for motorist safety, pushing most vulnerable native species a safe distance from runoff and traffic spray.
A salt vulnerability survey should examine areas that are known to be most susceptible to salt damage. These include groundwater recharge areas, exposed or shallow water tables with medium to high permeability soils such as medium to coarse sand and gravel, drinking water sources, communities of salt-sensitive vegetation, salt-sensitive wetlands (though salt marshes are an ecological treasure), farms and orchards growing salt-sensitive crops and, particularly, smaller/deeper ponds and lakes with low turnover or rivers with low flow rates. Of course, streams or rivers which drain an extensive highway network should be prime candidates for the survey. Environment Canada has Web-published two annexes to its Code of Practice for road salts that gives useful direction, http://www.ec.gc.ca/nopp/roadsalt/cop/en/code.htm.
In these types of areas, the survey should look for evidence of salt damage or, resources permitting, measure chloride concentrations. The U.S. EPA guideline suggests that, on average, plants should be safe if the four-day average concentration of chloride does not exceed 230 mg/L more than once every three years. Similarly, the biotic impacts would be minimal if the one-hour average chloride concentration does not exceed 860 mg/L more than once every three years. Impacts are species-specific. Whether the exposed plant or animal species is harmed is a product of the plant or animal's salt sensitivity and the duration and concentration of the exposure. If you don't have lab-favorite fathead minnows in your lakes, use data for a species that you do have.
When the survey is complete and you've identified salt-vulnerable areas on your roadway network, the best way to translate your findings into improved salt management is to map the results. This can be done with acetate overlays or simply by marking a highway map with various color felt-tipped pens, coded for the type of exposure. When you have the salt-vulnerable areas mapped and you've analyzed water or soil samples indicating problematic concentrations, you'll know where changes are required.
Depending on the local conditions, you may be able to prevent damage to salt-vulnerable areas by engineering drainage away from sensitive areas, by using your best equipment and operators to ensure the minimum salt application rates or by substituting a non-chloride chemical for salt in these local areas. Often, you'll want to upgrade and/or relocate your salt storage facility or improve its operating practices. Sometimes the answer may be to accelerate your plan to re-engineer that entire section of roadway, solving the drainage problem at the same time. There is no "one size fits all" solution; each should be custom designed.
Needless to say, the roadway environment is not pristine nature. Sunbelt or Snowbelt, roadway rights-of-way are environmentally stressed. Salt adds to that stress. Even in a hostile, engineered roadway environment, however, Mother Nature is tough and resilient. But we can do more to help. With our general practice of Sensible Salting, supplemented by special care and handling of salt-vulnerable areas, we will be able to maintain roadway safety, protect local jobs and quality of life, and ensure sustainable environmental quality—the goal we all share. The Michigan Department of Transportation studied the environmental effects of salt and concluded: "Use of road salt (sodium chloride) is both cost effective and environmentally acceptable at current levels." Environmentally acceptable, yes, but if we focus increased attention on salt-vulnerable areas, we can achieve actual environmental improvement. Let's prioritize identifying those salt-vulnerable areas and focus increased attention on protecting them.
Richard L. Hanneman can be reached at (703) 549-4648 or firstname.lastname@example.org.