Last week, contractors from the Environmental Protection Agency (EPA) accidentally spilled 3 million gallons of mine wastewater into Colorado’s Animas River. Now, toxicologists at the site are warning that even though the bright yellow of the pollutants has faded from the river, the health problems associated with the disaster could remain for years to come, CNN reports.

At the time of the disaster, the EPA contractors were implementing a plan to stop contaminated water from the abandoned Gold King Mine near San Juan, Colorado, from reaching local waters. Instead, the workers accidentally released a torrent of toxic sludge into one of the river’s main tributaries.

According to the report, toxicologists confirmed levels of mercury, beryllium, cadmium, arsenic and lead that were many times higher than the EPA’s official “maximum contaminant levels” for these heavy metals.

Health experts noted that exposure to even a fraction of the concentration of heavy metals reported by the EPA are known to cause a wide array of long-term health problems, such as cancer and kidney disease, and even developmental problems in children. Scientists also warned that the health effects will be hard to trace, as they will likely take years to develop.

The EPA claimed that heavy metal levels in the water “began to return to pre-event conditions” very shortly after the spill occurred, and the Animas River has since been re-opened for recreational use. But the agency wouldn’t comment on how many residents used the river for drinking water or to irrigate farmlands during that period.

As a result, many environmental and health experts are still worried about this massive toxic-waste spill.

For more information about how heavy metals can negatively affect human health, click here.