Salting the Earth

Feb. 3, 2020
Measuring the impact of road-salt-laden runoff on Mirror Lake

At 6.1 million acres, Adirondack Park is the largest publicly protected area in the contiguous United States. It’s also somewhat unique in that approximately half of the park is owned by the state while the other half is privately owned. According to the Adirondack Park Agency, which is responsible for maintaining the protection of the forest preserve and overseeing development proposals of the privately owned lands, there are 101 communities within the park’s 12 counties.

“One of the side effects of having all this protected land,” says Brendan Wiltse, science and stewardship director for the Ausable River Association (ARA), “is that where there is development, it can be quite concentrated because there’s limited private land to develop.” Wiltse’s background is in limnology, the study of freshwater lakes and rivers. The ARA’s focus is on the protection of the 512-square-mile Ausable River Watershed in Upstate New York, most of which is in Adirondack Park.

The density of development in the park, combined with the wintertime application of road salt, has led to water-quality problems for Mirror Lake, which is in the village of Lake Placid, NY. Wiltse led a recent study on the reduction of spring mixing in Mirror Lake as a result of road-salt-laden runoff. The study was published in the journal Lake and Reservoir Management.

According to Wiltse, Mirror Lake has been monitored through the Citizens Statewide Lake Assessment Program for about 20 years, but that program didn’t consistently look at sodium and chloride levels, the two ions related to road salt impact. More recently, a former volunteer and ARA board member enrolled the lake in a different citizen science program managed by the Adirondack Watershed Institute (AWI) at Paul Smith’s College that does include sodium and chloride. Monitoring done by AWI in 2014 showed that something was amiss with the lake’s salt levels. Says Wiltse, “That was the first time we had data showing that the lake was in the 97th -percentile for both sodium and chloride out of all the lakes that are monitored through that program in Adirondack Park, which is just shy of a hundred.”

In hindsight, says Wiltse, it’s not surprising. “Mirror Lake probably should have stood out as one that deserved closer attention due to the amount of development, but it just sort of slipped under the radar for a really long time. Once we realized that, we stepped in.”

About 20 stormwater outfalls around Mirror Lake discharge runoff directly into lake, and in the winter, that runoff is very salty. “We’ve measured concentrations of chloride pretty consistently at 2,500 milligrams per liter or higher,” says Wiltse. “That’s above the EPA’s both acute and chronic toxicity threshold for aquatic life and it’s about 10,000 times higher than what we see in impacted streams in the area.”

The super salty runoff is denser than the lake water and accumulates at the bottom. “By the end of the winter, we have this situation where the lake water concentrations are about 45 to 50 milligrams per liter at the surface, but at the bottom, they can be as high as 140 milligrams per liter,” he says. “That density difference means that when the ice goes out in the spring and the lake would normally turn over, there’s more energy required to mix the lake than if that salt gradient wasn’t there. In Mirror Lake’s case, that’s been large enough to prevent the lake from completely mixing in some years.”

Mirror Lake is dimictic, meaning the cold water at the bottom of the lake and the warmer water near the top turn over and mix twice a year, once in the spring and once in the fall. “As the lake warms from winter to spring, right under the ice is about 0° Celsius,” says Wiltse. When the water near the surface reaches 4°C, which is when liquid water is at its most dense, it settles through the water below it. “That, in combination with wind energy, should mix the lake and redistribute oxygen and nutrients throughout the water column,” he says. “The same process happens in the fall, but it’s backwards—as the lake cools, the water gets more dense.”

Excessive chloride and sodium, however, affect the density of the water and can prevent the lake from completely mixing.

In the summer, Mirror Lake is warmer at the top and colder at the bottom. “That cold bottom water is where you’re going to find things like lake trout, rainbow trout, and a lot of other aquatic life that prefer colder temperatures and require oxygen,” says Wiltse. “With the lake not mixing completely in the spring, we don’t get as much oxygen moving into the bottom waters of the lake. In summertime when those fish are trying to find that colder water deeper in the lake, the water is cold, but there’s not enough oxygen for them.”

Wiltse says that although they don’t know the direct impact on lake trout right now, modeling shows that habitat is reduced as a result of the lack of turnover.

“There are other impacts though,” he points out. “The lack of turnover in the spring and the low oxygen also release phosphorus from the sediments on the bottom of the lake, so we see increased internal phosphorus loading. That could potentially become a concern.”

Right now, he says, the phosphorus is at the bottom of the lake where it’s cold and dark and unavailable to the phytoplankton above it in the water column. “There’s a possibility that if stratification ever broke down a little and the lake partially mixed, maybe at the end of the summer season with a big storm event, that phosphorus-rich water could get moved to the surface and trigger an algae bloom. We haven’t seen that [yet] though.”

Once ARA became aware of the salt problem, Wiltse and his colleagues began monitoring the lake in the spring of 2015. “When we went out there, we noticed higher chloride concentration at the bottom of the lake and lower dissolved oxygen than we would expect for that time of year,” he says. “That really set off this study—we didn’t have the data in 2015 to know exactly what was going on.”

The study published in Lake and Reservoir Management looked at 2016 and 2017, says Wiltse, but the monitoring has continued since then. “We sample the lake biweekly during the open water season. We’re out as soon as the ice goes out and we’re there pretty much the day before the lake freezes,” he says. Once the lake freezes, sampling is done through the ice. During the study, winter samples were taken monthly, but now they’re taken every two weeks.

“One of the things that’s challenging in limnology in general is winter sampling,” says Wiltse. “Getting the hole in the ice is pretty easy compared to standing there for an hour trying to prevent yourself and everything you’re working with from freezing.” Electronic equipment in particular is susceptible to freezing.

“Mirror Lake is probably one of the few lakes in Adirondack Park—and one of just a handful in New York State, I would guess—that has a consistent monitoring program that extends through the winter,” says Wiltse. “That data is really important for us because that’s when the salt is being applied.”

In addition to taking a 2-meter integrated surface water sample and a sample one meter off the bottom of the lake, the researchers also create a water column profile of temperature, dissolved oxygen, conductivity, and pH when they sample the lake.

“Sodium and chloride are the two ions we’re looking at relative to the salt impact,” says Wiltse, “but we’re [also] looking at phosphorus, nitrogen, chlorophyll a, alkalinity, calcium, potassium, and magnesium—the full gamut of things that you typically look for, study, or monitor in a lake water quality sample.” Although they are monitoring nitrogen and phosphorus loads, Wiltse says what stands out most significantly in Mirror Lake is salt.

In the spring of 2017, the accumulation of salt at the bottom of the lake prevented Mirror Lake from mixing completely. “There are not many lakes that have had this documented interruption of turnover due to road salt, so we were surprised,” says Wiltse. “That’s sort of an uncommon thing to find, so as a scientist, it’s exciting to have an opportunity to study a lake like this, but it’s also something we would like to see resolved, or the lake recover from.”

For the lake to recover and to prevent additional instances of incomplete mixing, the first step is to apply less road salt in the winter.

“We don’t push anyone for zero salt or for unsafe road or sidewalk conditions,” says Wiltse. Instead, he says they’re working with the state, the municipalities, and local businesses to encourage them to measure their salt application. “If we know what everyone is applying within the watershed across all these different applicators—the state, the town, the village, the commercial properties, the private properties—then we can start figuring out where we can trim back, who’s going to be able to reduce the most, and what equipment we need to get these folks if necessary.”

The State of New York has a pilot program along its section of the road near Mirror Lake exploring new plow technologies that allow them to see where the trucks are and how much salt is being applied.

“We have gotten a grant through the Lake Champlain Basin Program to put that same equipment on [the Town of North Elba and the Village of Lake Placid’s] vehicles. Through that grant, we’re also buying what are called ‘live edge plows’ for the town and the village. They conform to the surface of the road, so you can remove more snow and ice in front of the plow truck and can put less material down behind it,” says Wiltse. “Those little things like measuring what you’re applying, calibrating your truck—there’s a lot of savings in material that can be achieved by doing those small things while maintaining the same level of service on your roads.”

Lake Placid is also constructing several stormwater infiltration basins to reduce direct discharge of runoff into the lake.

At the moment, Wiltse says they’re not sure exactly how much of a reduction is necessary. “We’re working on identifying specific salt reduction targets based on the science for Mirror Lake,” he says. “We think it might be 30 to 40% based on a mild winter that occurred in the winter of 2016 to 2017.”

In the meantime, he cautions that the general salinization of surface waters is a widespread problem for streams and lakes in the US. Although the lack of turnover that occurred in Mirror Lake is likely less common, “it’s probably occurring more than we’re aware simply because we’re not looking—Mirror Lake being a great example of that,” says Witlse. He encourages monitoring any lake that receives road runoff in order to be aware of sodium and chloride levels. For lakes like Mirror Lake that are receiving direct discharge of stormwater from an area where road salt is applied, “that would be when you really want to start looking at things much closer.”

Wiltse and his fellow researchers will continue their monitoring. “We’re working on a three-year project funded by the Lake Champlain Basin Program to measure application rates within the watershed and then monitor the stormwater and track the movement of the salt from the back of the truck, through the environment and the lake, and then out of this particular watershed,” he says. During that time, the ARA will be conducting outreach to businesses and homeowners as well as carrying out training within the municipalities. “We hope that over that period of time, we will start to see a response in the lake, and we’ll be able to say how much of a reduction in salt load we’ve been able to achieve.”

Although he’s hopeful Mirror Lake will recover, Wiltse is concerned about road salt’s broader impacts. “We’re seeing more research starting to emerge that this is also impacting groundwater resources,” he says. The environmental impacts of road salt are relatively well known, but environmental impacts have a way of becoming concerns for human health. “This is not just an environmental issue—it’s a human health issue, an economic issue, and an environmental issue. There are many reasons why we need to start looking at this more closely.”

About the Author

Rachel Sim | Editor

As the editor of Stormwater magazine from June 2019 to December 2020, Rachel Sim created and curated quality content addressing the challenges faced by surface water and erosion control professionals, focusing on cutting-edge technology and the latest environmental research.