MANKATO — Rising nitrate levels in the Minnesota and Blue Earth Rivers are forcing Mankato to adjust how it provides drinking water to residents and potentially threatens the city's attempts to limit the amount of water it takes from the region's most critical aquifer.
"The fix is going to be big," City Manager Pat Hentges said of the looming cost of reacting to the rising pollution levels in the rivers.
Options range from new water treatment systems or drilling another well into the Mt. Simon aquifer, which provides all or part of the drinking water for more than a million Minnesotans from southern sections of the state to the Twin Cities metropolitan area.
The Department of Natural Resources has been concerned for more than a decade that the amount of water being pulled from the Mt. Simon aquifer by municipal and industrial users might be exceeding the amount of water seeping back into the aquifer.
Mankato has traditionally attempted to minimize the quantity of water it draws from Mt. Simon to about 25 percent of its total, relying on shallow wells near the Blue Earth and Minnesota rivers for the majority of its drinking water.
Those wells draw water from just a few dozen feet below the rivers, so nitrates in the surface water percolate quickly through the sand and silt to the underground water. The Mt. Simon is hundreds of feet below the surface and is less vulnerable to surface pollution.
Recently, the nitrate level in the shallow wells has been rising as rainy weather boosted the erosion from river banks and farm land into the Blue Earth and Minnesota rivers.
"We're in that high-flow level, have been for two years," Hentges said.
The increased nitrate levels from the shallow wells below the Blue Earth River were no surprise to city officials.
"That has traditionally been a problem to us because the Blue Earth system — the Cobb, the Le Sueur that flow into it — has traditionally had nitrate issues," he said.
In the past, the Minnesota River hasn't seen similar spikes in nitrates — even when the river is high — so Mankato would shift from the Blue Earth River well to the Minnesota River well to keep nitrate levels in the drinking water at acceptable levels. In the last year, however, nitrate levels in the Minnesota River well have also surged, forcing the city to shift production more to its deep wells tapping the Mt. Simon aquifer.
Looking for solutions that don't rely on the potentially overused aquifer, the city has hired consulting engineers Short, Elliott and Hendrickson, Inc. to explore the options. SEH, among other things, will look at the feasibility and cost of increased treatment of the water from the shallow wells to remove the nitrates.
That most likely would cost several times the expense of a new well into the Mt. Simon aquifer.
"If it's a new well, it's a couple of million dollars," Hentges said. "Treatment? It might be $4 million or $5 million. ... The council may pick a more expensive option just to be more sustainable."
Doing nothing isn't an option when nitrate levels rise too significantly. Federal drinking water standards limit the contaminant to 10 parts per million, because high nitrate levels are a health hazard, particularly for infants.
Mankato water is tested daily for nitrate levels, said Public Utilities Director Mark Winson. And the strategy of blending water from various wells has continued to keep the nitrate levels well below the federal standards.
"We average between zero and 4.2 parts per million," Winson said. "... It's not an emergency, and we anticipate going forward that we can continue to blend."
Longer term, that's not guaranteed. There have been days when Winson's staff has shut down the Blue Earth River well completely and has still had to blend deep-aquifer water with Minnesota River well water.
"We've seen higher nitrate levels for a while from the Blue Earth well, and now we're seeing higher levels in the Minnesota," Winson said.
Relying more heavily on the Mt. Simon wells is not just an environmental concern, it's a financial one. Pumping that deep water to the surface consumes more energy, and it requires more chlorination because the Mt. Simon water is heavier in ammonia and ammonia undermines the effectiveness of chlorination.
By late summer, the $63,600 SEH report will present the City Council with the cost and feasibility of a new Mt. Simon well, of reusing water from a private well into the Mt. Simon aquifer owned by the CHS soybean processing plant, and of new treatment options such as reverse osmosis to remove nitrates from the shallow well water.
"Then we can start looking at those alternatives and funding sources," Winson said.
And the City Council can start weighing their conflicting responsibilities: spend more dollars on treatment to address the problem or save some money by consuming more of the limited Mt. Simon aquifer water.
If the city decides to spend more taxpayer/ratepayer funds to preserve the Mt. Simon resource, Hentges would like to see the state of Minnesota show good faith in addressing the causes of rising contaminants in the Blue Earth and Minnesota rivers.
"You've got to really think long-term about cleaning up the river," he said. "... We have to be assured that the state of Minnesota is doing their share."