This time of year, millions of tons of salt are spread across roads and highways to keep cars and trucks from slipping. Where does all that salt go? A lot of it is washed by rain into ditches, streams, rivers and lakes.
This time of year, millions of tons of salt are spread across roads and highways to keep cars and trucks from slipping.
Where does all that salt go? A lot of it is washed by rain into ditches, streams, rivers and lakes.
That inspired a team of Ashland University researchers to try to figure out what all that salt does to wildlife.
Andy Trimble said the idea came to him a year ago while driving behind a salt truck.
The biologist and toxicologist typically studies the impact of herbicides, such as atrazine, in streams.
"It just naturally fell in line when I saw all the salts they were spreading on the roads," Trimble said. "It was that, and all the different formulations of salts that are used."
State and federal environmental-protection agencies consider salt a problem only if it affects drinking water. For example, Ohio EPA officials spent the past two years examining contaminated runoff from large road-salt storage piles and found several cases in which drinking water was affected.
In 2010, ground-water contamination from two salt piles was so bad in a Preble County village that officials were forced to drill a new well for drinking water.
Salt also is lumped into a "total dissolved solids" limit the government imposes to safeguard water quality in streams and lakes. The Ohio EPA follows the federal standard, which limits the concentration of dissolved solids to no more than 1,500 units per million units of water.
Ohio agencies have not studied toxicity and wildlife.
U.S. EPA criteria for aquatic wildlife deem sodium chloride harmful for wildlife at concentrations that exceed 230 parts per million for at least four days and 860 parts per million for at least an hour.
University of Minnesota researchers found in 2009 that salt concentrations increased in 39 Minneapolis-St. Paul area lakes over a 22-year period and tied the trend to state road-salt purchases. Their study lists road salt as a threat to frogs, fish and other wildlife.
In Ohio, the general assumption is that the salt that washes off roads is so diluted that it poses no threat.
Sodium chloride, which also is used as table salt, is not the only salt spread on pavement to melt ice. Trimble said road salt also can contain magnesium chloride and calcium chloride. Varying ratios of these three salts are sold to homeowners and to municipalities to spread on roadways.
"What we have found experimentally over many years of environmental research is that when you mix different chemicals together, you can often have different levels of toxicity," Trimble said.
The team's first round of experiments involved dissolving concentrations of individual salts in test tubes inhabited with Hyalella azteca, tiny crustaceans that form the base of the food chain in many streams, rivers and lakes.
Other crustaceans and many fish species rely on Hyalella as a primary food source. Its loss can start a chain reaction through a stream's habitat that could affect birds and other animals that eat fish. In this way, it is sort of a canary in a coal mine.
The tests followed U.S. EPA standards in which Hyalella were exposed to salt-contaminated water for four days. Concentrations that killed an average of half the Hyalella are considered toxic.
Their experiments showed that sodium chloride was toxic at a concentration of 4,606 parts per million. Calcium chloride was toxic at 9,404 parts per million, and magnesium chloride was toxic at 6,091 parts per million.
These salt concentrations are considered heavy, especially by drinking-water standards. A concentration of 250 parts per million is enough to give drinking water a salty taste.
The next round of experiments, currently under way, will subject Hyalella to different combinations of salts.
In the first of these tests, a 50-50 mixture of sodium and calcium chlorides showed a toxic concentration of 9,348 parts per million. Trimble said he and his team expected to see toxic effects at a much lower concentration.
The fact that calcium chloride somehow lowered the overall expected toxicity was surprising but not disappointing, said Shane Daugherty, the undergraduate researcher in Ashland's environmental toxicology program.
"We want to see whether it's better (for the environment) to use a single salt compound or a mixture," Daugherty said.
Future experiments will examine the effects of different combinations of salts in water.
Trimble said the lab experiments, when concluded, still won't be enough to prove that salt is toxic in streams, rivers or lakes. Future experiments will combine salt with other pollutants, including sediments and pesticides.
The overall goal is to examine how road salt could work with, or against, other pollution threats.
"If we can measure what's out there, we can determine the risks to the environment," Trimble said.
He also speculated that any effect is seasonal.
"In the spring when all the salt washes off, we would expect levels to get high," Trimble said.
Just how much salt gets into streams is a separate study that doesn't strictly relate to toxicity. Trimble said he hopes any results of the studies could be used by officials to weigh pollution threats.
Daugherty said he hopes the experiments will help reveal a formula for road salt that is the least environmentally harmful.
"Salt is very popular in Ohio," he said. "Anything you do can be toxic in the right amounts."