The Environmental Protection is admitting to a spill from a treatment plant it set up after it dumped 3 million gallons of toxic wastewater into a Colorado river last year.

The EPA said Thursday night that the spill happened on Tuesday, and officials are still attempting to determine how much and what metals were contained in the sludgy discharge, according to the Associated Press.

The spill occurred near the site of last year's spill at the abandoned Gold King Mine in Silverton, Colo., where agency contractors didn't adequately check the mine's pressure before attempting to open it up after several years of being idle. The result was a massive mine blowout that sent 3 million gallons of metal-tainted water into the waterways of three states.

The Navajo Nation sued the agency over the spill last week after the EPA inspector general and the Justice Department opened a criminal investigation into the incident a few days before the Aug. 5 anniversary of the 2015 spill. The Navajo argue in their lawsuit that the spill significantly harmed the tribe's primary source of revenue from crops and other agricultural products.

Local officials said this week's release was not large enough to warrant a public advisory.

Last year's spill sent nearly 1 million pounds of metals into the waterways of the Animas and San Juan rivers, which traverse three states. The metals include arsenic, cadmium, copper, lead, mercury, nickel and zinc.

This week's spill came from the treatment plant that the EPA set up near the mine to filter water coming from the mine before releasing it into the creek and river systems. A large amount of rain in Colorado caused the treatment facility to overflow and some of the untreated water to spill into the waterways.

EPA said the water that spilled from he plant was partially treated, and the metals present in it should quickly settle to the bottom of waterways where they are less harmful.