ST. PAUL — Officials with the Minnesota Pollution Control Agency are working to identify the effects of failing septic systems across the state.

There are about 500,000 homes in Minnesota from which wastewater flows into buried septic tanks — systems that are maintained, and often ignored, by homeowners. The Minnesota Pollution Control Agency estimates that more than 100,000 of those septic systems are too old or so close to the water table that they’re putting groundwater at risk, and that 25,000 of those septic systems are so degraded that they pose an immediate threat to human health.

The impact and cost of failing septic systems are front and center in a far northern Minnesota community, but they’re less clear across the rest of the state.

In International Falls, along Minnesota’s border with Canada, at least 200 failing septic systems line a 15-mile stretch from a neighborhood along Rainy Lake east to Voyageurs National Park.

“Anywhere where you have wastewater going into a lake, you’re going to get extra weed growth,” Koochiching County Environmental Director Dale Olson said. “You’re gonna get extra algae. And that can kill off certain species or make them move to different areas.”

Olson has been working for the last 20 years to extend a sewer line from International Fall’s water treatment plant to those failing septic systems. He recently secured funding for the $17 million needed to install a 6-inch pipe channel through more than 15 miles of solid granite.

The septic problems are obvious in Rainy Lake, where sewage laces homeowners’ pieces of shoreline, but faulty systems are harder to identify across most of Minnesota.