The fate of plastics in Lake Tahoe?
Special to the Tribune
If the sun hits the blue waters of Lake Tahoe just right, and it’s a calm day off the point where the Cal Neva Resort & Casino sits quiet, stand up on your paddleboard or lean over the bow of your kayak and look for plastic floating on the surface or at varying depths.
Following major wind events, you might find a mini “garbage patch” forming on the surface.
Surface currents formed by wind create two large systems of rotating currents that mimic an ocean gyre, one on the north end of the lake and one on the south. Gyres are generated by three phenomena: global wind patterns, Earth’s rotation and Earth’s landmasses. The mountains affect wind patterns in the lake allowing smaller gyres to also manifest near the shoreline, in coves or around inlets.
The large gyre near the North Shore circles counter-clockwise and on the south another circles clockwise, coaxing plastics as well as other debris to collect in specific areas of the lake. Litter on the North Shore can get carried west from Sand Harbor to Hidden Beach and continue to Kings Beach.
As a result, patches of plastics can be seen in the water within a couple of hundred yards from the shore, specifically between the spot where old hot springs sit at Brockway Springs Drive and Stateline Point near the Cal Neva.
UC Davis, Tahoe Environmental Research Center researchers created a computer simulation of surface currents for a 2-1/2 day period in August.
“The currents are ‘driven’ by the wind, which is recorded every 10 minutes on buoys and docks at 13 locations at Lake Tahoe,” explained Professor Geoffrey Schladow and a team of researches with the Tahoe Environmental Research Center.
“At the beginning of the model run (time 0) a set of colored ‘balls’ are distributed across the lake surface – a different color in each quadrant. Watch as the currents then move them about. The green arrows also show the current velocity and direction across the lake.
“While the motion may at first appear chaotic, notice that there is a lot of regularity to the motion. There are two main circles (or gyres) that the balls trace out. These gyres are usually anti-clockwise in the north, and clockwise in the south. Sometimes there are smaller gyres closer to shore, especially at the very south of the lake. Things to note are the time it takes these gyres to undergo a complete revolution (about 1.5 days) and the pulsating motion that the gyres exhibit. This is due to the daily variation in the strength of the winds at Lake Tahoe, with stronger winds typically occurring in the afternoon,” Schladow said.
Understanding surface currents is important because currents are responsible for the transport of contaminants, invasive species, urban storm water and floating debris.”
NEW STUDIES START
Heather Segale, TERC education and outreach director, talked about funding received to study the fate of microplastics in Lake Tahoe and how it will be utilized.
“The research UC Davis is doing right now is looking at the fate of the microplastics (in Lake Tahoe) … because most of the research that has been done on microplastics and plastic pollution has been based on oceans.”
Segale added, “More types of plastics will float and float longer (in salt water)” as compared to less dense fresh water where not as many types of plastic float and the ones that do, don’t float as long.
Salt, temperature and pressure affect water density. Salt water is more dense than fresh water and as the temperature or pressure increases, density increases for both salt and fresh water.
Fresh water is most dense just above freezing, becoming less dense as water temperatures rise.
“Our research right now is looking at the fate of those plastics. Are they mostly staying at the top? The answer is no, not really,” Segale said. “Are they going down… where the density gradient changes from where the warmer water sits on top of the colder water? Or are they sinking to the bottom? Or are they getting eaten up by the different filter feeders that are in the lake like kokanee salmon or zooplankton?”
At Hidden Beach in Nevada, debris gets pushed on shore after one of these wind events, Segale said, emphasizing “the amount of plastic we found these couple of times that we’ve gone after a wind event is just incredible.”
Where plastics get deposited and what effects the Lake Tahoe environment have on the plastic is something UC Davis TERC is looking to learn.
8,000 POUNDS PULLED OUT
According to the United States Department of Justice, it wasn’t that long ago that dumping became illegal in the U.S.
“Beginning in the 1970s, Congress enacted a set of laws to protect the nation’s air, water, and lands from uncontrolled pollution,” the Justice Department website states. “These laws responded to the consequences of unregulated industrial development that had fouled those resources to the point where rivers were not fishable, air was unhealthy to breathe, and land contamination put Love Canal at the forefront of national consciousness.”
Over 8,000 pounds of garbage have been pulled out of both Lake Tahoe and Donner Lake over the last two years. In addition to the obvious pieces of plastic from trash not properly disposed, smaller pieces often invisible to the naked eye are also being carried by Tahoe’s currents around the lake.
“We are seeing lots of trash from decades ago, things like a bicentennial diet Pepsi can,” said Colin West of Clean Up the Lake, which over the last two months managed crews that pulled up most of that garbage.
Polypropylene sandbags are one an example of what could be adding to the microplastics in Lake Tahoe. Tarps made of the same material are also commonly used around the lake to cover boats, kayaks and protect things from the wind and the sun’s rays, the exact thing that speeds up the deterioration of the plastic.
Ultraviolet rays “increase the degradation rate” of woven polyethylene. Sandbaggy, a high-end manufacturer, states “our sandbags have 1600 (hours) of UV protection, meaning they can last for about six months under the sun before deteriorating.” Yet, at a minimum, most sandbags used for construction on the shores of Lake Tahoe are in place for the duration of a two-year building permit.
It’s important to note that larger projects often take several years to complete.
“My research so far has not shown a definitive remove-by date,” said Jeff Cowen, public information officer with the Tahoe Regional Planning Agency. “However, since we do not allow this kind of pollution, there may be a guideline somewhere that needs to be better described.”
Michelle Gartner is a freelance writer who lives in Gardnerville, Nevada.
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