Two months after a rainstorm caused a former gravel pit on the edge of a St. Croix River bluff to overflow, sending a torrent of water downhill, additional information about the incident is available.

The damage is better understood, initial steps have been taken to increase how much water can be held at the site, and authorities continue efforts to figure out what happened and how to prevent it in the future.

“Right now my main concern is this doesn’t happen again next spring with the winter runoff,” said Jim Shaver, administrator of the Carnelian-Marine-St. Croix Watershed District.

To that end, the company responsible for stormwater management has performed work to improve the site, including pumping out existing ponds so they can hold new runoff and excavating a new retention basin.

The land is owned by Dr. James Zavoral, but has been leased for the Argo Navis solar farm by BHE Renewables. It was previously operated as a gravel mine by Tiller Corp.

Details about impacts

On Oct. 8, a group of representatives from several government agencies visited the site and assessed the damage. They met again on Oct. 22 to discuss their findings, and then drafted a brief report. The group included the Minnesota Pollution Control Agency (MPCA), Department of Natural Resources (DNR), Board of Water and Soil Resources, and the National Park Service.

The panel found that the incident had affected four primary areas: the bluff, a wetland, spring-fed Middle Creek, and the St. Croix River.

The incident created a sandbar in the river at the mouth of the creek about 52 feet wide and extending 35 feet into the river, covering about 1,700 square feet. It is also assumed that some amount of finer sediments was discharged and was carried downstream.

In the creek, sand was deposited up to two-feet deep on the stretch between a railroad grade and the river. Approximately 6,500 square feet of sand was deposited in the wetland at the base of the bluff, the bluff slope was laid bare, and trees were knocked down.

Inadequate infiltration

A key question is why the holding basins couldn’t handle four inches of rainfall, when engineers had earlier said only back-to-back six-inch rainfalls would cause an overflow.

There is little soil at the site, due to its recent history as a gravel quarry, so water must soak directly into the sandstone bedrock. To ensure that would happen, the solar company was instructed to remove water from the retention ponds and scrape away accumulated sediment.

A stormwater management permit for the site was approved by the Carnelian-Marine-St. Croix Watershed District in August 2017. The watershed district engineer, Carl Almer, who works for environmental engineering firm EOR, said at the time that the company would restore the ponds.

“Mr. Almer also clarified that the basins will be regraded and sediment removed,” district board meeting minutes read. “The basins should hold all water in this closed water system.”

At the watershed district’s Oct. 10 board meeting, engineer Almer said he had asked the company if that had happened.

“Answer he received was that Argo-Navis did not fully dewater the ponds, they did it just enough to install the solar garden,” meeting minutes read.

When the officials visited the site in October, the MPCA observed that the site’s infiltration basin had a one-inch layer of silt on the surface, as well as standing water.

Remediation and prevention

The experts who visited the site have written draft recommendations for addressing the damage.

They suggest that attempting to remove the sediment from the St. Croix River “could cause additional harm,” though whether or not it could be attempted may be decided based on rare and endangered species, such as the river’s important mussel population.

Likewise, the experts say removing sediment from Middle Creek could cause “more harm than good.” But the wetland should be restored as much as possible, with sediment removed and reseeding native plants. And the bare bluff slopes should be stabilized, both in the short and long term.

With the site’s limited holding capacity now so clear, officials also want to make sure next spring’s snowmelt doesn’t cause another overflow.

Before winter set in, the company dug out new parts of the solar farm to hold water, and pumped water out of existing ponds so they could scrape away silt.

“I am trying to get as much volume as we can for the spring runoff, make changes to the permit and maintenance agreement, until the vegetation gets fully established and the infiltration is working,” said administrator Shaver.

Another update is expected at the district’s monthly board meeting, which is open to the public, on Dec. 12. The Minnesota Pollution Control Agency may still issue fines, but no determination has yet been made.