Aaron Stigger is a graphic and web designer born and raised in Oak Park. He caught Curious City’s attention with this question:
How does all the winter salt runoff affect Lake Michigan's water?
But he really piqued our interest after telling us the backstory.
“On my way to work everyday I pass by this gi-normous salt pile, which is kind of plopped down on some dirt and some broken-up cement,” he says. “That kind of got me thinking: Well, if it’s seeping into the ground under this big, uncovered pile, what is it doing, all the salt we distribute all around the city?”
The particular mound of salt that Aaron saw is in Dunning, a neighborhood on the city’s Northwest Side. That mound's got company: Chicago stores 19 piles of salt across the city. And that’s not counting many more spread across the suburbs and Northwest Indiana.
But is there really a wall of brine heading to the lake and, if so, should we be worried? We found out that, at least according to a few environmental standards, Lake Michigan is actually in much better shape than Stigger expected. But another waterway may have earned his concern.
Just how much salt are we talking about, anyway?
Before we get to specifics on any effects on Lake Michigan, let’s put the amounts of road salt we use into perspective, at least when it comes to Chicago.
That’s counting this winter, which has been particularly brutal. As of February 28, the city already dumped more than 370,000 tons of salt on city streets — a solid 42 percent more than the next heaviest use in the previous five years.
It’s not just a problem in Chicago. Humans move a lot of salt. A 2004 study estimated that we mobilize more than 140 teragrams — that’s 140 billion kilograms — of chlorides every year.
Salt’s destination: our streams and rivers
So, with some of these figures in mind, let’s consider the effects.
Aaron Stigger’s “aha moment” came about when he saw one of the city’s salt piles while it was uncovered. It’s a reasonable concern, given that researchers from the University of Rhode Island estimate uncovered salt piles lost about 20 percent of their salt each year. Much of it ends up in nearby waterways.
Most piles are covered during the off-season, however, so salt used for deicing is the main source of urban chloride pollution. Chemists know salt as NaCl, or sodium chloride, which breaks down in water. Hence there are pollution measurements and standards for “chlorides,” not “salt.”
But where’s this runoff headed? The hydrological lay of the land is such that most salt-laden runoff in Chicago ends up in the Chicago River and other inland waterways — not Lake Michigan.
The principal reason is that the city reversed the flow of the river more than 100 years ago, so most of our runoff ends up in the waterways that feed into the Illinois River, which flows into the Mississippi and eventually the Gulf of Mexico. A 2010 study from the University of Illinois at Urbana-Champaign found road salt runoff and treated wastewater from the Chicago region are the dominant sources of chlorides in the navigable sections of the Illinois River, and two major tributaries in the Chicago region. The study says that number has risen steadily since about 1960.
“The lake doesn’t receive very much input from stormwater from the city of Chicago,” says Scott Twait, who works in IEPA’s Water Quality Standards division. “However with all the salting, all the road salt enters into the Chicago Sanitary and Ship Canal, and the Cal-Sag channel, and flows downstream to the Des Plaines River. And collecting all the runoff, the chloride levels can spike in those areas and get quite high.”
In high concentrations, chlorides can be toxic to aquatic life. But it’s hard to tell how many times salt runoff from Chicago has caused toxic levels of chlorides in inland waterways, because the Illinois Pollution Control Board doesn’t classify those waters as “General Use” waterways. Those waters are subject to Illinois’ 500 mg/L water quality standard. Instead, IEPA regulates “total dissolved solids” in Chicago-area waterways, lumping together chlorides, sulfates and other chemicals for a single reading. Chloride levels have spiked above 1000 mg/L in some inland waterways — twice IEPA’s standard for most of the state.
Chicago-area waterways are the only ones in the state that aren’t regulated by General Use standards. As Twait explained, that’s because they were so polluted when the standards were set that they earned their own benchmarks. (You can see IEPA’s full list of impaired Illinois waterways here.)
“Back in the 70s these were the only waters that were kind of beyond repair, as to their thinking back in the 70s, so they got kind of special standards” Twait says. “They really had no hope for them in the future.”
But those waters are much cleaner now. The Metropolitan Water Reclamation District of Greater Chicago, which handles and treats the region’s combined runoff and sewer water, has improved its filtration methods. MWRD Spokeswoman Allison Fore says they’ve adopted best practices suggested by the DuPage/Salt Creek Work Group for managing their roadways and facilities.
Twait says EPA is looking to bring Chicago-area waterways in line with the rest of the state’s rivers and streams. If they update the water quality standards, he says, “one of the things we know is that we’ll have chloride issues in the winter time.”
Regulators would come up with some kind of limit for chloride in Chicago-area rivers. That could make cities think twice before spreading so much road salt. It’s much tougher for the EPA to regulate salt from so many spread-out sources (storm drains spread out across the city and suburbs) than from, say, a factory with a fallout pipe dumping salt into the river.
So our question asker Aaron Stigger is right to worry about salt runoff, but not so much in Lake Michigan. In Chicago’s case, it’s our inland waterways that are in trouble.
Corrosive chlorides and city infrastructure
Before it even gets into area waterways, salt works its way through the city’s subterranean network of pipes. That can cause problems for the city’s Department of Water Management, which provides drinking water to Chicago and 125 suburbs. They also deliver stormwater to MWRD for treatment.
Tom Powers, the city’s commissioner of water management, says chlorides are at such a low concentration in Lake Michigan that his department barely takes note.
“It would require an incredible amount of road salt to affect Lake Michigan — that’s a very robust system,” Powers says. “When we test [the water], it doesn’t even appear on what we’re testing for.”
The EPA’s national drinking water standard for chloride is 250 mg/L, some 20 times higher than Lake Michigan’s current concentration. Chicago’s Dept. of Water Management, like many such agencies, adds water softeners that can include salt. But it’s not enough to even approach the EPA limits.
But road salt can corrode the pipes that carry that water, exacerbating the stress that the winter freeze-and-thaw cycle puts on an aging network of water pipes that would stretch 4,500 miles if laid end to end. About 1,000 miles of those water pipes are 100 years old or older, Powers says. In 2009 the department had to repair 8,873 catch basins — more than twice last year’s 3,647.
Development in urban areas makes the salt corrosion problem worse, by funneling more runoff into the system. Studies have correlated growth in chloride levels with the rate of urbanization, and even with miles of road in the vicinity of the waterway in question.
“While we are right to be cautious in applying ‘common sense’ to such things,” says Stephen McCracken, who coordinates the Conservation Foundation’s DuPage River Salt Creek Workgroup, “in this case the relationship seems quite straightforward with salt being applied to road surfaces, increased road density means a larger salt total applied, even at a constant application rate.”
So more development, more impervious surfaces, more runoff.
A saltier lake?
So not much of that salt ends up in Lake Michigan. But there is enough runoff to register an increase in Lake Michigan’s chloride levels since Chicago first started spreading road salt.
Kim Biggs, a spokeswoman for the Illinois Environmental Protection Agency, says the current chloride levels in Lake Michigan are around 12 milligrams per liter.
That number has risen since widespread use of road salt began around 1960, according to the Salt Institute. Chloride levels in Lake Michigan rise about 0.1 mg/L each year, but they’re still well below the U.S. Environmental Protection Agency’s 500 mg/L standard for “General Use waters”. Nationally, EPA’s criteria for chloride toxicity are 230 mg/L over a four day average, or an hourly average of 860 mg/L. (EPA is currently reevaluating that standard, which was first set in 1988.)
If you measure chlorides in Lake Michigan in the spring, however, you pick up all that winter road ice and runoff. Since 1980, springtime average chloride levels have risen almost 50 percent:
High chloride levels choke aquatic species that depend on a certain salinity to keep their bodies in equilibrium. Amphibians, like salamanders and frogs, are especially susceptible to chloride pollution. Many of them breed in temporary vernal pools that are cut off from other bodies water, and thus have no way to flush out excess salt.
IEPA’s Biggs says chlorides in Lake Michigan aren’t threatening aquatic life. “There are not significant concerns or actions being taken to reduce chlorides in Lake Michigan as they are still reading below the water quality standard,” she wrote in an email. “We do not feel that salt runoff from the Chicago area is a major contributor to the chloride levels in Lake Michigan.”
Winter deicing is the major driver of high chloride levels in Chicago’s waterways, but wastewater treatment also contributes. In the outfall of waste water treatment plants in DuPage County, for example, chloride levels are more than ten times higher than they are in Lake Michigan. Studies by the Illinois State Water Survey and MWRD sampled the water flowing out from MWRD’s Stickney wastewater treatment (the largest such plant in the U.S.), and found median chloride levels of 145 mg/L, compared to 8-12 mg/L in Lake Michigan.
Most of MWRD’s contribution comes from human waste itself, which contains chlorides. They also use ferric chloride to help filter wastewater — the chemical is useful for, among other eyebrow-raising processes, “sludge thickening” — but are moving away from that in favor of biologically-based techniques that would replace ferric chloride.
If you can’t beet ’em ...
So what’s the city doing to cut back on its salt use?
Dept. of Streets & Sanitation spokeswoman Molly Poppe says they train salt truck drivers to spread salt judiciously — that means waiting until plows have cleared most standing snow, since salt sprinkled on top of several inches of the white stuff won’t do much. When the forecast calls for mild temperatures, salt trucks take it easy and let the weather do some of the work.
City workers move salt at the depot at Grand and Rockwell (WBEZ/Chris Bentley)
The city even enlists an unusual fruit cocktail of sorts to get more out of its salt: beet juice. It’s full of sugar, and helps lowers the freezing point of ice. Mixing salt with molasses or another sugary substance can do the same thing. Salt solutions are good too, because they spread out easier than rock salt so they’re more efficient. Wisconsin has started spraying cheese brine for similar reasons.
Typical salt (sodium chloride) is not effective in subzero temperatures, but other salt compounds can break ice crystals at lower temperatures — calcium chloride and magnesium chloride are common substitutes, but they eat into concrete and metal faster than table salt. Right now the city uses sodium chloride.
So Aaron Stigger’s salt pile is probably going to exist as long as severe winter weather visits Chicago. But if IEPA ups the standard for the metropolitan area’s inland waterways, he might start to see the salt disappear a little bit more gradually.