Road Salt Turns Deep Water into “Low-Sodium Soup”

Before the ice melts off these lakes here in Madison (we will get ice-off this year, right???), we thought we’d share a report from our senior research technician, Ted Bier, who you may have seen out with facilities manager, Dave Harring, drilling holes in the ice around town this winter as they collected data for the Long-Term Ecological Research network.

Ted Bier and Dave Harring take a plankton sample from an ice-covered Lake Mendota.
Ted Bier and Dave Harring take a LTER plankton sample from an ice-covered Lake Mendota. Photo: Adam Hinterthuer

The LTER is a National Science Foundation-backed effort to record ecological change throughout different ecosystems across the U.S. There are a couple dozen LTER research sites, and we here at the CFL run the one concerned with lakes – the North Temperate Lakes research site includes both Madison lakes and a handful up north in Vilas County.
Last month, Ted and Dave were out on Lake Wingra and Ted noticed that Wingra’s deeper waters were quite briny. The culprit, of course, is the salt used melt ice off of our roads during the winter.
Ted (left) and Dave (right) winter sampling on Lake Wingra.
Ted (left) and Dave (right) measuring salinity on Lake Wingra. Photo: Ted Bier

But what does this annual salinization mean for our resident lakes? We asked Ted, and here is his reply:

When salt (sodium chloride) washes into the lakes, it does
not dilute evenly through the whole water column. Water that contains dissolved salt is heavier than fresh water, so it settles in the deepest part of the lake bottom, or “deep hole,” where it forms a lens of salt water many times more concentrated than normal levels.
This is of concern to biologists because most native, aquatic
flora and fauna are not adapted to high saline concentrations. The list of consequences is long, but disruption of osmoregulation is the most severe. In short, salt-water is toxic because it interferes with freshwater organisms’ ability to balance the water concentration in their cells. In salty water, cells expel water, which can cause dehydration.
Small, urban lakes are at greatest risk, because water run-off is high with respect to lake volume. So, it is little surprise that Lake Wingra is our most affected lake in Madison. And the level is not just concerning, in the winter, it approaches levels that could be lethal. Every organism’s tolerance is different, but generally speaking 400 milligrams per liter is bad-news-bears for most critters.
The concentration I found in Lake Wingra was 600 milligrams per liter at the bottom of the deep hole. That is about the same concentration as low-sodium soup. And, in fact, if you taste the water (and I do), it is noticeably salty. Salinity decreases to 150mg/l once you get a meter off the bottom, so many fish can swim into safer water. But, plants and animals living in the muddy bottom are occupying a salinity level that freshwater scientists would predict would put them in a high stress environment.
It’s also important to note that many invasive species are adapted to saltier water. Eurasian water milfoil – a well-known invasive plant – doesn’t mind increased salinity one bit. In fact, the plant frequently inhabits brackish waters in its native habitat, so road salt tips the scales that much more in favor of a plant that already has some native species on the ropes.

It’s important to note that Ted’s not predicting cataclysmic effects from winter road salt in the Madison lakes. Once the ice comes off of Wingra and the lake has its annual turn-over, the salty bottom water will be diluted as it mixes through the water column. Then, as fresher water flows through the system in the warmer months, salinity won’t be a problem for native species.
But, placing any sorts of stressors on native species can make them less able to compete with invasives, more susceptible to disease and have other effects on the ecosystem. Road salt has been shown to be a bigger problem than first believed for aquatic life, one that’s spawned a lot of research on ways to reduce salt use while still keeping roads safe.  The City of Madison has reduced its salt use over the years, switching to sand in an attempt to keep the lakes healthy all year long.
While it has helped, salt is still part of our urban aquatic ecosystem and, next winter, the process will start all over again. For many bottom-dwelling organisms, especially the aquatic invertebrates that Lake Wingra’s fish eat to get them through the winter, the bottom of the lake will be a stressful place to call home.