The contaminants were discovered during an analysis being done as part of the preliminary work to determine the future of the Monterey Dam.
GM is undertaking a separate environmental study of the property, officials told city leaders last month.
The city received the report in mid-December from Inter-Fluve, a river restoration engineering and design firm, and notified the Wisconsin Department of Natural Resources, according to a city of Janesville news release.
The city, DNR and Inter-Fluve staff met Dec. 2 to discuss the results of the study. DNR staff indicated that addressing the contamination near the GM facility would be the responsibility of GM.
The Rock River is under jurisdiction of the DNR, which is responsible for overseeing the river sediment management process. The DNR told city officials that there is no immediate risk to the public health from either fish consumption or direct contact.
GM also is reviewing the report and will conduct evaluation of the river sediments in the vicinity of its plant that it will share with the DNR and the city of Janesville.
The DNR will work with the city, GM and any other relevant parties to determine the next course of action and to identify appropriate solutions for managing the issue, according to the news release.
The city early last year began discussions about the repair or removal of the Monterey Dam after the DNR ordered the dam spillway repaired.
The order came after a 2012 DNR inspection showed significant cracking and breakup of concrete portions of the spillways and deep pits of scoured-out riverbed undermining both sides of the spillway, according to the Gazette.
Ayres Associates, the city’s consultant for the repair project, estimated it could cost $250,000 to repair the dam. But three bids for the project came back at more than twice that cost. The bids ranged from $500,000 to $830,000, according to city records.
Now, the city’s plan to either repair or remove the dam may be delayed to allow time for the river sediment issue to be addressed, according to the city.
Site plan up for debate
GM officials revealed Dec. 17 during a visit with Janesville City Manager Mark Freitag, other leaders from the city and county as well as representatives of Forward Janesville, their plans to sell the property, according to a city news release.
The plant was moved from standby status to closed in November as part of a new contract between GM and the United Auto Workers.
The site is zoned industrial and has excellent road and rail transportation as well as a pretreatment plant for wastewater, an electrical substation and underground electric, gas and water lines, Freitag said.
However, residential and parkland uses could conceivably become a part of a redevelopment plan, he said, adding that much depends on the developer.
The plant has not produced any vehicles since 2009. Most production ended in 2008 with the layoffs of hundreds of workers at the plant and at its local suppliers.
The Inter-Fluve report on the sediment contamination stated that the Monterey Dam was constructed in the mid-1800s.
Potential sources of pollutants include agricultural runoff, stormwater discharge, treated municipal, industrial wastewater discharges and legacy contamination.
GM produced automobiles at a facility immediately adjacent to the impoundment between 1918 and 2009.
The report noted that sediment near GM had a distinct odor and its proximity to the industrial area led to suspicion that contamination levels might be higher at this location. Sample analysis confirmed this.
Among the contaminants discovered were chromium, lead and mercury concentrations that exceeded both the probable effects concentration and 20 times what the report called the toxicity characteristic leaching process limits.
Most PCBs were below detection limits, but one type was found at a concentration that exceeds the threshold effects concentration for total PCBs.
Inter-Fluve recommends the sediment of the contaminated area be removed mechanically. DNR staff indicated it is GM’s responsibility to take care of it, according to the report.