Our Salty Ways

Salt residues left from last weekends snow.

There has not been much snow, ice and freezing temperatures this winter season.  Unlike the past couple years, we don’t see the plow trucks, chains on tires, frozen-over lakes, or the large amount of deicing salts spread on roads and sidewalks.  In fact the NC DOT typically uses 256,249,901 pounds of salt each year, but because of our mild winter they estimate to have used about 70% less this year. 

It sure is nice to have safe and clear roads and sidewalks after a winter snowfall.  Thanks to the crews who plow and spread salt we are able to get all around the county.  Did you know we started using sodium chloride (salt) in America in the late 1930’s to clear roads.  Back then about 5,000 tons were spread on our Nations roads, and today more than 18 million tons are spread out on our roadways.  Salt is a relatively benign material, but in such large quantities, scientists are beginning to see some impacts on the environment. 

The US Geological Survey (USGS) and other private agencies have been studying the effects of road salts on aquatic life.  High levels of salts, such as 500-1,000 milligrams per liter that would have direct mortality on aquatic life are rare in nature.  What is more likely, are concentrations of 100-200 milligrams per liter.  Long-term exposure at these low levels may not directly kill organisms in a day, but more likely to have long term effects on the food chain.  The salts affect the tiny larvae and algae that higher organisms eat.   

Professor of Biology at UNC Asheville, Peter Petranka has been conducting research on the effects of road salt on salamanders.  He tried to set up experiments in nature, but couldn’t find enough seasonal pools since North Carolina has lost 90% of its wetlands since European settlement mainly due to ditching, draining and development.  Petranka set up almost 60 kiddie pools with aged tap water, algae, protozoans, small aquatic invertebrates, leaf litter, and wood frog and spotted salamander larvae to mimic natural seasonal pools.  He added road salt mixes to the pools and found that low levels of road salt decimated the small invertebrates that the salamander larvae eat, thereby reducing salamander growth.  Seasonal pools, also known as vernal pools, typically dry out in the summer.  If amphibian growth is delayed, it puts them at greater risk of dying when the pools dry (Beeland).

The southeastern U.S. has some of the greatest diversity of salamanders in the world, and of the 102 species found here, 60 are found in North Carolina.  Salamanders breathe through their skin and are quite susceptible to contaminants found in the environment.  Petranka also found during his studies that mosquitoes did quite well in the salted pools.  He said “mosquitoes have a thick outer cuticle, and they sit just below the water’s surface, with a breathing tube sticking up – it’s most likely they are custom-built to withstand a saline environment.” 

Other researchers are finding that the salts do not flush out of a watershed quickly.  It migrates slowly to the receiving waters – such as a lake, reservoir, or larger river.  There needs to be a lot more research and evaluation, but we do know that the chloride concentrations are much higher today than they ever have been.  A study done along the I-93 corridor in New Hampshire found chloride concentrations averaged 163.2 milligrams per liter in 2007, and before salt was used as a road deicer, typical concentrations in New Hampshire were 1.3 milligrams per liter.

Other than affecting water quality and habitat, salt can have an effect on soils, reducing the capacity for absorbing other chemicals.  According to a report from the Cary Institute, there is potential for sodium ions to “knock off” captured toxins or metals sequestered on the particles releasing them back into the water.   Another possible environmental side effect of excess salts is that it can cause lakes to stratify more easily than usual.  Salty water has a higher density, which will sink to the bottom of the lake disrupting the ecosystem.   

We must have clear roads in winter, so are there alternatives to road salt?  There have been experiments with products that contain sugar beets or calcium chloride, but they are much more expensive.  Salt is still the best chemical to use to provide safe roads in freezing conditions.   Do not use fertilizer to melt snow- the nutrients in the fertilizer contribute to algae blooms and poor water quality.  

How can we have safe winter roads and reduce the impact on the environment?  We have to reduce the amount that is used and must rely on the road crew folks who maintain the road safety, according to Connie Fortin, founder of a consulting firm that offers training on winter maintenance for managers.  Her firm is not trying to “reduce the level of service, but integrate science into the tradition applying salt.”  The practices that take place across the nation, has “evolved without much science.”  

Research has shown that the best way to remove snow is with the plow.  The speed of the truck spreading salt can actually affect the amount of salt put down.  For example, according to Fortin “ a truck traveling at speeds over 30mph the salt coming off the spreader has enough momentum that with a couple of bounces, 30% of it simply caroms off the pavement and into a drainage ditch at the side of the road, creating an environmental hazard without doing a thing to enhance safety.”   

Another low cost reduction in chloride exposure can be achieved by replacing the “more is better” paradigm while filling up the truck with salt.  Studies have shown that if trucks are loaded precisely, 20% less salt is used, because drivers tend to use what they load.  These types of processes are being referred to in the industry as “sensible salting.”  

NC DOT requires all their road crew operators to go through training to learn how much salt should be applied – 250 pounds per lane mile.  They also take a day to calibrate all the equipment to ensure even spreading.  This is a proactive approach because “we need to manage ourselves before regulations are put in place.”   Knowledge is the key to lessening the impact of winter maintenance practices on the environment.   Mark Devries, chair of the American Public Works Association shares “when you tell someone a cup of salt in 5 gallons of water kills bluegills they understand.  Anyone who is over-salting is just trying to do a good job and make the road as safe as they can.”

References:

Richardson, David. Stormwater: The Journal for Surface Water Quality Professionals.  Article:  Ice School, Melding the science and craft of winter road maintenance.  Pg 14.  Issue: January/February 2012.

Beeland, DeLene. Charlotte Observer, Scitech.  Road Salts Second Sting.  November 14, 2011.