Invasive creatures have fouled lawn-sprinkler systems. Fire departments are on the alert.
Zebra mussels are clogging lawn-sprinkler systems for lakeshore homes that draw water directly from Lake Minnetonka, and there's concern that larger pipes in the lake used to fight fires also may be blocked.
The problems are the latest nuisance caused by the fingernail-sized mussels, better known for littering beaches with razor-sharp shells and out-competing small fish and other native species for food.
They were discovered in Lake Minnetonka in 2010 and since have spread to nearly all of the lake's 26 bays and 125 miles of shoreline.
Dean Holasek, president of Aqua Engineering of Eden Prairie, said his firm serves about 200 customers with sprinkler systems that pull water from Lake Minnetonka. "We're seeing zebra mussels that just jump onto the suction screens and hang on for dear life," Holasek said. "If they get on there heavy enough and long enough, they restrict the flow of water going into the lawn-sprinkler systems."
The typical system has a pump on land that draws water from the lake through a small-diameter hose with a mesh-covered basket on the end. An underwater screen prevents weeds and other debris from being sucked into system.
Holasek said there are probably a couple thousand homes that draw water from the lake. Many homeowners leave hoses in the water year-round, increasing the likelihood of zebra encrustation.
Lakeshore owners should be aware that equipment can be damaged, perhaps severely enough to be replaced, said Eric Evenson, administrator for the Minnehaha Creek Watershed District. "It could be expensive if you start getting these clogged lines and you're running a pump without any water going through it," he said.
"These little critters are going into these pipes and dark nooks and crevices, wherever they feel the safest and can latch on to something."
Fire departments rely on submerged lake pipes in locations where communities don't have pressurized hydrants. In those cases, firefighters hook up to "dry hydrants" and suck lake water through them to battle blazes.
Most communities around Lake Minnetonka have municipal water systems, but Excelsior Fire Chief Scott Gerber said that two homeowner associations in Deephaven rely partly on lake water for fire protection.
"Are zebra mussels on our radar screen? Absolutely," said Gerber, whose department also serves Tonka Bay, Greenwood and Shorewood.
The two pipes that service the hydrants in Deephaven are not encrusted enough to restrict flow, he said, but it's a potential problem. The associations own and operate the dry hydrants, and Gerber said he has requested time at their annual meetings in June to explain concerns about maintenance.
In some parts of the metro, divers have already been busy scraping zebra mussels off underwater pipes.
Tom Suerth, president of Waterfront Restoration in Long Lake, said his firm cleared zebra mussels last year from all four intake pipes in a North Oaks lake that are used by the Lake Johanna Fire Department. The mussels were discovered in Pleasant Lake in 2007 and have formed dense clusters on many hard surfaces, including the pipes.
"They weren't 100 percent clogged, but it was pretty bad," Suerth said.
Chipping and scraping
Divers use metal brushes, scrapers and smaller tools to remove the mussels, a process that Suerth said is "simple but arduous." Each of the pipes took a team of divers about two hours to clean, he said.
"In cases where it gets particularly crusted up, it's almost like chipping away cement because the mussels have calcified and hardened" on the pipe surface, Suerth said.
Suerth's company also cleaned intake pipes that supply North Oaks Golf Course with water from Pleasant Lake. Mussels returned so quickly that the 2010 cleaning had to be repeated in 2011, he said, and may need to be done twice this year.
Dick Osgood, executive director of the Lake Minnetonka Association, said that zebra mussels can accumulate by the thousands in dense clusters on hard surfaces.
Their expansion in Lake Minnetonka is likely to continue for the next two to five years, he said.
"These things will explode in the lake, and people with irrigation pipes and docks and boats are going to be experiencing greater and greater problems in the next few years," he said.
Tom Meersman • 612-673-7388