One of the rainiest Junes on record has led to about 189 releases of untreated sewage from Minnesota communities and businesses — a "very high number," according to Wendy Turri, program manager for the Minnesota Pollution Control Agency (MPCA).
"And we're not done," she said, as rising rivers and a saturated landscape continue to work against the region's wastewater drainage systems.
Sewage releases — also called "overflows" or "bypasses" — almost always go into rivers, and infrequently into lakes, where they can shut down beaches. Health officials have continually warned residents during the past week to avoid flooded waters. But of the 31 beaches that Hennepin County tests each Monday, none was closed from contamination this week.
Overflows occur because pipes that carry human waste and other used water get overloaded by stormwater, which isn't supposed to be in those pipes in the first place. The stormwater enters though illegal connections from sump pumps and other building drainage systems, as well as through faulty manhole seals and cracks in the underground sanitary pipes themselves. The overloaded wastewater systems then threaten to back up into homes, so cities will dump the sewage without treating it to avoid such backups.
In such cases, bacteria and other pollutants are very diluted by stormwater. They still carry a public health risk, but sun, evaporation and other factors can reduce the risks "within a short time," Turri said.
Of the bypasses this month, about 45 occurred Thursday alone, when rainfall of 2 to 4 inches — more in some areas — fell across much of the southern half of Minnesota. Comparisons aren't easy, Turri said, because the most recent summer month as rainy as this one was July 1997, and MPCA records from that far back aren't reliable.
Most of the overflows have occurred outstate, with just 26 of the releases this month in the seven-county metro area. Three beaches on Lake Minnetonka were closed June 2 and one on Lake Gervais in Little Canada last Thursday so swimmers would not encounter possible E. coli contamination.
Officials for the Metropolitan Council, which collects wastewater from municipal systems and delivers it to seven regional treatment plants, say bypasses could be becoming less frequent after heavy rains, due to several strategies. First and foremost was the epic effort to install separate sanitary and stormwater sewer systems in Minneapolis, St. Paul and South St. Paul in the 1980s.
More recently, the agency has been working with local communities, aided by $7 million in state money since 2012, to reduce the amount of stormwater getting into the wastewater system, through the process of seepage and illegal drainage known collectively as "inflow and infiltration."
Bryce Pickart, assistant general manager for technical services for the Metropolitan Council, noted that peak flow in the agency's wastewater system was less after Thursday's 4.13-inch rain than it was after a 4.6-incher on Oct. 4, 2005. The Met Council has determined that a long-term program to reduce inflow and infiltration would cost about $150 million, while treating excessive flow, much of it clear stormwater, would cost about $1 billion. Pickart went further, saying it would be "impossible" to build a wastewater system that could handle stormwater flow.
Pickart said the main target of the programs is sump pump connections. But because many home drainage systems are invisible or inaccessible, many homeowners, he said, probably aren't even aware they might be pushing clear stormwater that collects around their homes into the wastewater system, illegally and at considerable taxpayer expense. A sump pump from one home can generate the equivalent of 40 homes' wastewater flow, according to the Met Council.
Mound vs. Met Council
Mound has bypassed its treatment plant twice this month. Mound officials have blamed the Met Council regional collection system for being inadequate to handle Mound's flow, but the Met Council said Mound simply had too much stormwater in its wastewater system.
Carlton Moore, Mound's public works director, said the city has spent about $100,000 annually for the past decade to reduce inflow and infiltration, and rebuilding some sanitary lift stations. He said he believes the city has found and eliminated most illegal sump pump connections, but that other drainage systems, such as those installed decades ago around home foundations, continue to be harder to track.
In nearby Deephaven, City Administrator Dana Young noted that cities around Lake Minnetonka are more prone than many to inflow and infiltration because of the high water table, numerous wetlands and hilly terrain. Like their neighbors, Deephaven public works crews clean out their wastewater systems each year and run television cameras through the pipes, requiring the city engineer to watch "hours and hours or boring tape," Young said.
"For the most part, I think everybody is conscientious about their sewer lines," he said. "You just really want to stay on top of it. The worst phone call you can get is from a resident who's had a backup."
Turri noted that the wastewater bypasses that occurred this month resulted from rainfalls that weren't close to the upper limits of those they're expected to respond to effectively. But the dynamics have been complicated by ground that's been pre-soaked for weeks, as well as inflow and infiltration, which can be unpredictable.
Another key variable, she said, has been climate change. According to the National Climate Data Center, the amount of rainfall coming in the most intense storms increased 45 percent in the Upper Midwest from 1958 to 2011. That was the second-steepest increase among nine U.S. regions.
"We need to be building for those levels," Turri said. "But the design criteria are chasing the climate."