CLEVELAND, Ohio – Shut off all the water flowing into Lake Erie and it would take about 2.6 years for it to run dry.

Turn the spigots back on and dam the Niagara River so nothing flows out, and it would take the same amount of time for Lake Erie to fill back up.

That’s one way to wrap your head around the lake’s “hydraulic residence time,” or as some like to say, the average time it takes for a drop of water to go from one end to the other.

Now, consider this hypothetical.

If we could prevent any more pollution from entering Lake Erie, would the natural flushing of the lake remove all its contaminants within 2.6 years?

No, experts say, it would not. But it would happen a lot faster in Lake Erie than with the other four Great Lakes, which have longer residence times.

Each of the lakes would need more than its residence time to come clean. And even then, it would be just what’s floating in the water column that’s removed. The stuff buried in the sediment could remain there indefinitely, including a lot of the microplastics that have become increasingly worrisome in recent years.

Bucket test

Ed Verhamme, senior engineer with LimnoTech, a water research and environmental engineering firm based in Michigan, uses a classroom experiment to explain how long pollutants can remain in Lake Erie. It requires two five-gallon buckets.

Fill up one of the buckets with water and add red dye, he said. You will then have to pour three more buckets of clean water into the first bucket before the vast majority of dye splashes out.

Extrapolate that to Lake Erie and it would take much longer than 2.6 years – closer to eight to 10 years – to rid the shallowest of the Great Lakes of its pollution. And that’s only if all the water coming into it is clean, which it is not.

Still, that timeline is far shorter than in other, deeper lakes, where water — and what it carries — can linger for far longer.

Part of a chain

Lake Erie is one link in a continuously flowing chain of lakes and rivers that begins at the western edge of Lake Superior and ends when the St. Lawrence River empties into the Atlantic Ocean.

All the other Great Lakes have much longer hydraulic residence times – calculated by dividing the volume of each lake by its flow rate – largely because they hold significantly more water.

Lake Superior has a residence time of 191 years, according to the U.S. Environmental Protection Agency. Lake Michigan’s is 99 years, followed by Lake Huron at 22 years and Lake Ontario at 6 years.

Triple those time periods and that’s theoretically how long it would take for each lake to drain or refill.

Those differences mean Lake Erie would be better positioned than the other lakes to shed pollutants from the water column once sources are reduced, even though the process would still unfold slowly.

Different journeys

Lake Erie gets most of its inflow from the Detroit River, which is part of the waterway that drains Lake Huron. But a molecule of water that enters Lake Erie does not actually take 2.6 years to reach Niagara Falls. It could, but that would be sheer happenstance, because after entering the lake, water takes a variety of pathways dictated mostly by wind but also by temperature. Some molecules will cross the lake much faster than 2.6 years. Others will take a lot longer.

For example, surface water driven by wind can reach the eastern end of Lake Erie in a matter of days or months, while water closer to the bottom might take decades to reach Niagara Falls, according to Eric Anderson, associate professor in the civil and environmental engineering department at Colorado School of Mines.

Water can also move up and down on the journey east, and sometimes reverse itself. In April, after ice cover has melted, the lake is the same temperature throughout — about 39 degrees Fahrenheit. As summer takes over, the surface water warms. But then it cools in the fall and begins to sink. That allows for the upwelling of warmer, often oxygen-depleted water from below, explains Sherri Mason director of Project NePTWNE, a water quality research initiative at Gannon University in Erie, Pennsylvania.

So, water that was once on the fast track to Niagara Falls is now in the slow lane far below, and vice versa.

Contaminants in the water

As for contaminants in the lake, they can float, sink, get dumped onto a beach somewhere or buried in the lake bottom. Large numbers can get caught up in circulating currents called gyres.

Unlike the permanent gyres found in the oceans, the ones in Lake Erie are seasonal, Mason said. For example, a prevailing current in summer flows along the southern shore of Lake Erie and then spirals out into the lake toward Canada, Mason said.

As a result, sections in the eastern part of the lake tend to be hotspots for microplastics on the surface.

“They will kind of congregate there until the current patterns change,” she said.

Not all microplastics in Lake Erie float along the surface, however. Most of them are bits of polyethylene, which is found in plastic bags and bottles, and polypropylene, used to make food packaging and other products. Both tend to be light and buoyant, Mason said. But they may also attract microorganisms that feed on the plastic, creating a biofilm that increases density and causes them to sink.

Polyester, which is prominent in many textiles, is denser than other plastics and without a lot of movement on the lake, specks of such material tend to sink on their own, Mason said.

Let them sink

As it is, Mason believes there are more microplastics in the sediment of Lake Erie than there are floating on its surface, and that they can remain there for a long, long time.

But whether they are in the water column or on the bottom, Mason and other experts believe it’s futile to try and remove microplastics from the lake. The best thing is to let what’s there sink to the bottom and get buried, and for people to stop using so much plastic, so less of the waste gets released into the environment.

“And then Mother Nature takes care of herself quite well,” she said.