workers stooping over water tanks arrayed in a field.

SALTING THE EARTH

Bill Hintz imagines the small kitchen table he had in college when he thinks about salt pollution. If that table—about one meter squared—were parked in a snowy landscape, the amount of road salt applied each year is astonishing: between 3 and 18 pounds.

Averages aren’t really helpful here, because it depends if you’re talking about Ohio or Minnesota or New York. Interstates and other roads with heavy and high-speed traffic may get more salt than a small town side street.

But 70 percent of Americans live where winter weather requires some kind of de-icing treatment, and that means between 3 and 18 pounds of salt on every square meter of roadway. “Every single year, decade after decade after decade.” says Hintz. “We’re just salting the earth. It’s not going to end well.”

Hintz is a fish biologist also focused recovery of lake sturgeon. In his current position at the University of Toledo he’s working to re-establish the ancient fish in Lake Erie.

But salt pollution has grown into a significant interest since 2015when Hintz began monitoring Lake George at a field station run by the Darrin Fresh Water Institute at Rensselaer Polytechnic Institute (RPI). New York state has been generously salting its roads since the 1940s, and since 1980 chloride levels in Lake George—sometimes called the Queen of American lakes—have tripled.

Salt pollution had already flashed on New York’s radar in 1970, with salinity so high in that Irondeguoit Bay on Lake Ontario did not mix that spring. Salt seemed to vanish beneath scientific notice for a few decades, but the environmental load kept building: road salts, industrial applications, agricultural sources, even climate change induced salinization.

Hintz and his RPI post-doctoral mentor Rick Relyea started asking: What are the ecological impacts of this rising salinity? Freshwater organisms in most areas of the lakes region in North America have hardly experienced salinity for tens of thousands of years, and perhaps much longer.

Hintz got hooked by the scientific puzzle of salt pollution. Now he’s part of the “mad dash”—his term, and delivered with the tiniest note of wry humor—to understand the complex ecological impacts.

A bearded man with cap and sunglasses crouches in a boat on the water. He is holding a muskie that is probably 4 feet long
The now-pixelated photo Bill Hintz uses to grab his audiences: one big muskie in the hands of a very pleased brother-in-law. (Image courtesy Bill Hintz)

INTERROGATING THE FOOD WEB

Lakes have a lot of moving pieces, most of which we can’t even see very well. The most charismatic of these are the fish. Hintz focuses on the rare sturgeon. With its almost prehistoric reputation, these fish have plenty of intrigue and can astound you up close, particularly with a large specimen.

But when Hintz is talking about freshwater resources, he channels another amazing freshwater monster, the muskellunge—or muskie. He grew up in fishing country, and his brother-in-law who avidly chases championship muskie. One particular photo shows him cradling an impossibly large fish.

“This is the top of the food chain, and what you all love to fish for,” says Hintz when he shows the photo. Many people see a thrilling game fish. Some are rightly terrified.

This king of the food web is likely the culmination of probably millions of ecological interactions. A fish that big has to eat a lot of little fish. And those little fish need to eat a lot of zooplankton, who eat a lot of the tiny algae called phytoplankton.

As an ecologist, Hintz interrogates the weave of the food web, exploring detours, contemplating top-down and bottom-up effects. The key question: does salt pollution rewire freshwater food webs? If so, how? And how long before we see the effects?

Energy flows up and down a system in what ecologists call a trophic cascade. Organisms occupy different trophic levels—sometimes even more than one. Plants and algae convert the sun’s energy to nutrition—that’s one trophic level. Zooplankton feed on the algae, building another trophic level. Snails eat the plants—along with the algae that attaches to plants and other things—occupying an adjacent level. Smaller fish eat the zooplankton—adding one more level. Other fish eat the snails. 

A muskie starts small, eating zooplankton, but soon switches to eating small fish. As it grows, these small fish might also be a muskie. A monster muskie finally occupies a trophic level near the very top—at least until it dies, gets scavenged, and decomposes. An eagle or a bear might get lucky, downing a live muskie and building another level perched atop the already top predator.

As you can see these energy flow charts can get messy. Living and non-living things can tangle and jumble the trophic hierarchy from time to time. 

Salt, Hintz has been uncovering, can be a trophic monkey wrench, disrupting the food web. Lose a lot of one zooplankton, or maybe even lose one or more species, and that lessens the food available for young fish. When small fish decline, so do large fish.

“As things get saltier, it’s important to think about what we value,” says Hintz.

When Hintz began his investigations, only a small body of existing science looked at salt pollution. Some of that work used lab organisms that, as common lab species, may already have been more resilient or resistant to disturbance. The limited research concluded that negative effects from salinity happened at high concentrations. Existing regulations—developed in 1988—reflect that.

In 2015 Hintz and his team set up what he calls a hammer experiment, creating an array of 40 experimental food webs, each under shade cloth in their own 1210 liter cattle tank. Filled with a little sand and some leaf litter and topped off with 1000 liters of lake water, they were then seeded with zooplankton and phytoplankton collected from Lake George. All of the tanks received a second trophic level—some slightly larger crustaceans, banded mystery snails, fingernail clams. And half of them got a third trophic level—3 zooplankton-eating minnows called bridle shiners.

After the tanks acclimated they were hit with 4 different elevated salt concentrations, and closely monitored for more than two months. As expected, the fish cleared out the zooplankton and algae multiplied, a classic trophic cascade. But this cascade was much more pronounced with high salinity. “Dramatic changes are possible if natural lake communities respond similarly as our experimental communities did to high salinities,” they concluded.

4 members of Hintz's lab inside, setting up smaller experiments. They are all masked, and flashing thumbs up.
Ecological experiments happen indoors, too. (Image courtesy Bill Hintz)

WHAT STANDARD?

Worldwide, there’s not much consensus on salt regulations. In the United States, there are chronic and acute thresholds established by the Environmental Protection Agency (EPA). Chronic standards apply over time, while acute standards cover the short-term spikes you might get when it rains after a major salt application. The EPA’s chronic level is 230 milligrams of chloride per liter, while the acute red line is at 860 mg/L. Some states regulate chloride more tightly—Michigan has set the bar at 150 mg/L for chronic, and 650 mg/L for acute.

Canada uses a lower threshold concentration of 120 mg/L for chronic measurements, with 640 mg/L defining acute toxicity. 

What do these numbers mean? For comparison, most of the hundreds of thousands of lakes surrounding the Great Lakes region of the U.S. and Canada probably sat well below 10 mg/L before European settlement.

That water would be okay to drink if you were on a very low sodium diet drink, and advised to not drink water exceeding 20 mg/L. For drinking water the EPA deems sodium concentrations between 30 and 60 mg/L acceptable, a threshold set for reasons of taste.

Despite sodium’s impact on human health for some individuals, these levels are just recommendations. (Surface water standards and drinking water standards are not the same thing, but there is a relationship. Many communities that are actively working to reduce salt use are doing so because of rising salt levels in the groundwater they rely on.)

The EPA salinity thresholds are also deceiving because they’re generally not enforced. “It’s not even a regulation,” Hintz opines. “It’s not enforceable, right?” That’s because pollutants that don’t come from a pipe, what’s called non-point pollution, are very difficult to regulate, particularly when their use is widespread. Phosphorous and nitrogen—nutrient pollution that turns lakes and bays scummy—are classic examples. We’ve known for more than a half century the systemic harm they do, but we still struggle to control them.

The regulations are also very narrow, covering only sodium chloride. These are the two most common ions in seawater, and by far the biggest player in road de-icing, but magnesium, sulfate, calcium, and potassium can be pollutants of consequence. EPA regulations also don’t integrate other chloride-based salts, like magnesium chloride or calcium chloride.

Another weakness of regulations more generally is that they don’t factor in that salt is just one of many stresses on an ecosystem. Nutrient pollution, invasive species, bigger storm events and spikier droughts, overall warming. Multi-stressor ecology is difficult to regulate, and complicated by the fact that salts aren’t chemically neutral: they can activate heavy metals like mercury and lead, and also combine with pesticides for other adverse effects.

Buried in the confusion around multiple stressors is the simple fact that it provides a lever for local action. The residents or Lake George cannot, by themselves, avert the climate changes that will change their lake. But they can work to protect their lake against local threats like salt pollution, bolstering its resilience.

Imperfect as the guidelines are, many lakes, streams, and wetlands are already exceeding salt pollution thresholds. You can find these water bodies almost anywhere, but especially in urbanized areas of states like Ohio and New York.

Hintz was at the 2017 GLEON meeting, working with Shelley Arnott and a couple dozen more scientists to execute a 4 country, 16 lake comparison of salt’s impact on zooplankton biodiversity. Lakes are known for their capacity to generate a lot of contradictory data points. But the GLEON collaboration made it quite clear that even low levels of salt pollution likely pose a real threat to a lot of aquatic systems.

“The policies aren’t up to the task,” says Hintz. “It’s clear that the federal [protections] don’t apply to a lot of lake ecosystems. We need to lower those thresholds. We need to identify what systems are most susceptible.”

40 cattle tanks in an array, outside, in a fenced grassy area. More tanks are stacked in the distance. An inset photo shows the top view of one mesocosm with the shade cover off. Visible through the clear water is a sandy bottom covered with leaves.
The hammer experiment underway, in 40 models of Lake George. (Image courtesy Bill Hintz)

A SALTY LANDSCAPE OF FEAR

In nature, creatures are competing against each other, eating each other, watching each other. It’s this dance of behavior and biology and environment that creates and maintains the rhythms of life. So once the realistic threat of salt pollution was established, Hintz was free to dig a little deeper into the crazy ways that nature interacts.

“The landscape of fear” idea is one theory. “Predation in the environment is a huge driver of ecological interactions,” explains Hintz. Predators kill and consume prey. But they also have non consumptive effects, like scaring prey. That fight or flight response delivers a shot of adrenaline or other hormones. When things are scared, or at least avoiding predators, that can change their physiology, their behavior, and even their reproductive habits. “Organizing your behavior around the threat of predation has a structural impact on ecosystems,” says Hintz. For example, many zooplankton migrate to the surface of a lake after dark. One benefit: they can safely feed without highlighting themselves to predators against a bright sky.

Can salt complicate this landscape of fear? Understanding how pollutants affect organisms isn’t easy to unravel. But Hint’s mentor at RPI, Rick Relyea, had shown how predator-induced stress could interact with pesticides to make them more lethal.

If that happens with salt, that’s a threat to aquatic food webs. “Such unnatural declines in the abundance of predators or prey within a food web might decouple energy pathways,” Hintz writes with Relyea . “The interaction between natural and anthropogenic stressors like pesticides is alarming, given their continued widespread use around the world.”

Relyea and Hintz looked to the effects of salt on Daphnia, which reproduce both sexually and asexually, via parthenogenesis. In low and moderate salt waters, predatory stress increased sexual reproduction of Daphnia, but not at high salt concentrations.

Salinity already imposes a chemical fitness test on Daphnia. Could road salt limit their reproductive options as well? Once again, this would mean fewer algal grazers in the spring and summer, meaning greener waters and possibly fewer fish. 

A parking lot that has been so over salted that a sheen of white lies on the pavement.
Even in his professional backyard — Hintz snapped this pic of an over-treated parking lot at the Lake Erie Center where he works. (Image courtesy Bill Hintz)

HALT THE SALT

There’s another landscape of fear here—the fear of lawsuits. While municipalities are starting to think about the long term impacts of salt pollution, much of the problem rests with subcontractors on private sidewalks and parking lots.

If you’re chiropractor and liable for the maintenance of your sidewalks and parking lot, what’s the cost of a little more salt versus a patient taking a vicious tumble? And when that service is out-sourced, the contractor applying the salt is now liable for hundreds of parking lots and sidewalks? If they’re having trouble hiring workers in the first place, what’s the cost of a few extra bags of salt versus having to train and pay another worker?

As part of his outreach work Hintz sometimes talks to the applicators—landscapers and truck drivers—who clear streets and sidewalks and apply the salt. Two things commonly happen in these conversations: the applicators are often surprised to learn the ecological impacts, and they usually step right up to the problem, saying something like: “this is something we need to pay attention to.”

“It’s not always the applicators’ fault,” adds Hintz. Drivers have expectations—perhaps unreasonable—that we can maintain high speeds and other aggressive practices no matter the weather. And failing to keep the streets clear has brought more than one mayor down. “When roads aren’t clear, people call their local politicians. People call the Department of Transportation,” he says.

By the time Hintz left New York, the Adirondack region was waking to its salt problem. And this last December Ohio Governor Mike DeWine announced, in conjunction with the Ohio Environmental Protection Agency, a $1 million dollar grant program to get started on basic salt reduction strategies.

This basic state-level investment is an important step. “We should be concerned about the trajectory of rising salinity,” says Hintz. “I think it’s clear, we need to start rethinking chloride.”

As one of the people most closely monitoring trends and impacts, Hintz is concerned. “We need to do something now, not years from now,” he says. “Local efforts supported by state and federal efforts are really going to be needed. Obviously, it’s going to take support from the public. It’s critically important to do on a massive scale.”

There’s a lot of not-so-easy tasks left, and science will have to play catch-up. But we know enough to start reducing salt now. “We’ve got to figure out how to halt the salt,” he says. “Fresh water, man. It’s the most important thing to our existence as humans on this planet. Fresh water. Protect it.”

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