DENVER (AP) Construction crews returned to the scene of a massive mine-waste spill in southwestern Colorado to stabilize the mine opening with steel and concrete, the U.S. Environmental Protection Agency said.

The EPA said the work is designed to keep rock and dirt from collapsing at the entrance to the Gold King Mine and to make sure it's safe to enter during future cleanup efforts. The stabilization work will last through October.

An EPA-led contractor inadvertently triggered a spill of 3 million gal. of wastewater from the Gold King last August while doing preliminary cleanup work. The blowout tainted rivers in Colorado, New Mexico and Utah with potentially toxic heavy metals, prompting utilities and farmers to stop drawing from the waterways.

The water quality quickly returned to pre-spill levels, the EPA said.

In an email to The Associated Press, the EPA said it is very unlikely the work being done at the mine this year would trigger another spill. “The EPA has taken precautions to prevent any unanticipated discharges,” the agency said.

The contractor hired to do this summer's work, Environmental Restoration LLC, was also on the scene at the time of the August blowout. But the EPA and outside investigators have said it was government officials, not the contractor, who made the decision to begin the work that led to the spill.

The EPA pledged to alert downstream communities if anything goes wrong this summer, using a notification plan put in place after the August blowout. The agency was widely criticized for not alerting all the tribal, state and local governments affected by the spill.

Wastewater is still running from the mine, and if the rate increases during this summer's work, a temporary treatment plant installed last fall can handle a higher flow, the EPA said.

The $1.8 million plant went into operation in October. Officials said at the time it could handle 800 gal. per minute, while wastewater was flowing from the mine at about 560 gal. per minute.

The plant is scheduled to run through November of this year. Colorado lawmakers have urged the EPA to keep it operating, and the agency said it is looking into that.

The August blowout triggered lawsuits and intense criticism of the EPA, both for causing the spill and for its handling of the aftermath.

The agency has proposed using the federal Superfund program to fund a long-term cleanup of the Gold King and other nearby mining sites.