With 589 storm water ponds, the southeast metro suburb wonders how many more will need attention.
Inver Grove Heights has begun grappling with polluted sediments in storm water collection ponds.
Carrying out a state directive to keep storm ponds in good working order, Inver Grove discovered a pond with contaminated sediments that will have to be scooped out and disposed of in a landfill.
The cost of this scoop-and-dispose exercise: $450,000.
Using a state grant of $75,000 and spending $80,000 from city funds, Inver Grove plans to remove about a third of the sediments this fall or winter.
"This is the first bite of a large elephant," said city engineer Tom Kaldunski. "It's a lot of money that gets spent on this stuff. We are going to do enough work to improve the functionality of the pond."
The sobering fact for Inver Grove Heights is that melting glaciers left the city with 589 such natural drainage ponds, and so far the city has inspected just 12.
"Envision the last glacier: That big chunk of ice stopped and sat there and melted and left a depression in the ground. That happened in 589 cases in the city of Inver Grove Heights," Kaldunski said.
The city found that storm pond collection of storm water is far cheaper for the developing northwest part of the city than installing storm water sewers to carry the water to the Mississippi River.
But because the depressions are landlocked basins that do not connect to the river, "What flows into the ponds has to sink in," Kaldunski said. And what is left has to be scooped out.
Of the dozen ponds inspected, about 25 percent had contaminants at a level that required landfill disposal, he said. The city trucked away the sediment from smaller ponds.
About 7,000 cubic yards must come out of the larger pond. A typical dump truck carries about 12 cubic yards.
To begin this removal, the city has hired a contractor who will use a backhoe to scoop out the mud, pile it up to dry, and then haul it to a landfill. "The landfill is planning to use the material as cover over the mixed waste," Kaldunski said.
The city is hoping that the Legislature will give the Minnesota Pollution Control Agency more money to help cities pay the enormous costs associated with the disposal of storm sediments.
In the larger pond, the city found arsenic and PAHs -- polycyclic aromatic hydrocarbons -- compounds that are considered harmful to humans and aquatic life.
The MPCA has identified coal tar pavement sealants as one source of PAHs, and it has encouraged cities to ban their use on driveways and parking lots. Inver Grove Heights last year banned the sealants to become eligible for the MPCA grant it is applying to the pond cleanup.
Of the dozen ponds the city has inspected so far, about a quarter of them had PAHs.
The question is, what expenses lurk in the more than 500 ponds remaining to be inspected?
Laurie Blake • 952-746-3287