The “pool closed” sign will stay up at least until Tuesday at the Webber Park facility in north Minneapolis after tests showed the water quality remained too poor to allow swimming.

The naturally filtered pool has not been open since June 24 because of contaminants in the water. A worker took water samples Thursday morning. Results came back Friday showing improvement but not enough to open the pool, according to city Park and Recreation Department spokeswoman Dawn Sommers.

The pool is normally closed on Monday for testing and maintenance, so the earliest it can reopen is Tuesday. By then, the pool will have been closed 13 days this summer, more than double the six days it was closed in the summer of 2017.

The Park Board posted a news release about the problem, saying it has worked tirelessly to research and implement best practices for cleaning. But now the pool will be closed for the second consecutive weekend of temperatures pushing 90 degrees.

The pool opened in July 2015, the first of its kind in the nation to be cleaned by cycling water through an adjacent pond. Unlike most pools, no chlorine is used to cleanse the water.

The natural cleaning process, however, has proved to be less efficient than chlorine. Heavy rainfall challenges the pool’s ecosystem. If a swimmer urinates, vomits or deposits fecal matter in the pool, the delicate balance is upset and the pool must be closed. A regular pool can get an extra dose of chlorine to take on the contamination, Sommers said.

“There’s a ton of benefits of a natural swimming pool. The downside is, it’s a small lake,” she said.

When the 21,000-square-foot Webber Park pool debuted, the project was over budget and overdue by almost two years. Before it ever opened, there were problems with ducks and toads depositing fecal matter in the cleansing pond. The price tag climbed to $7 million from the initial estimate of $4 million.

Still, it received national acclaim for the innovative engineering and cleaning system. And it was a neighborhood hit from day one, often reaching its capacity of 500. The pool, at 4330 Webber Pkwy., has a lap area with lane lines, a deep area with a diving platform, a zero-depth entry for youngsters and grassy areas for sunbathing.

Its 500,000 gallons of water gets circulated between the pool and the pond, which has some 7,000 aquatic plants rooted in limestone and granite gravel, according to the Park Board’s website.

Unlike natural lakes, which are bigger and allow more dispersal, contaminants get concentrated at Webber Park. Sommers also pointed out that the city is following stricter, European standards of water quality there.

An explanation of the Webber Park tests and most recent findings is online.