Crews last week began the first phase of restoration work in the Monterey lagoon: building an embankment to separate the lagoon from the Rock River.
The embankment will allow Bjoin Limestone to work on the rest of the lagoon in spring even if the river rises, said Paul Woodard, public works director.
The city has made changes to its plans for the lagoon to accommodate concerns brought forward by a former contractor and initially ignored by city officials.
The lagoon still will become a detention pond with surrounding picnic and lawn space. But instead of picnic space on a peninsula in the center of the pond, it will be along the river near the Monterey Rock, Woodard said.
The embankment, or berm, will be built from a combination of rock and fill from a local quarry, not from materials scooped from the bottom of the lagoon, Woodard said.
Soil in the lagoon was exposed after the Monterey Dam was removed and the lagoon drained in summer 2018.
An earlier contractor, Drax, had told the city the “organic muck” at the bottom of the lagoon is not suitable for piling into a berm or embankment as the city had planned. Drax also expressed concern about contamination in the muck.
The city dropped Drax for Monterey restoration work, saying the company breached its contract by refusing to complete the lagoon project as specified, according to a Sept. 4 letter from the city.
In October, Woodard told The Gazette the city still planned to make a berm out of materials in the lagoon and intended to do so by flattening slopes of the berm and planned for less water in the detention pond.
On Friday, Woodard said the city switched its plan from using materials in the lagoon to using outside materials because new material will be more stable.
Costs will remain “neutral” because the city reduced the amount of earthwork to offset costs of new materials, Woodard said.
The city estimates it will import 1,500 cubic yards of material to make the embankment.
None of the existing lagoon materials will be removed. The detention pond will be made by placing excavated materials around the perimeter of the pond, Woodard said.
No peninsula into the pond will be constructed.
Material excavated from the lagoon and placed at the perimeter of the detention pond will be covered with 6 inches of topsoil, Woodard said.
A 6-inch topsoil cap is mandated by the state Department of Natural Resources to prevent people from coming in contact with the contaminated muck from the lagoon.
Levels of contamination are “assumed to be low,” according to the DNR.
The city and the DNR have been aware of what the DNR classifies as “minor exceedances” of contamination limits since 2015, when Inter-Fluve, a Madison company asked to provide the city plans prior to dam removal, completed a sediment report in preparation for the dam’s removal.
Officials speculate contamination comes from the former General Motors Assembly Plant across the river or other industrial sites.
The detention pond is intended to have a permanent pool of water, Woodard said.