Hurricane Harvey’s unprecedented rains and flooding last month caused a leak from a heavily polluted site along the San Jacinto River east of Houston, according to the US Environmental Protection Agency (EPA).

The so-called San Jacinto River Waste Pits, one of several Superfund sites flooded during the storm, contain cancer-causing waste from a paper mill. Harvey’s rains damaged the protective cap that was supposed to hold in the waste, exposing the “underlying waste material,” the EPA says.

Some of the highly toxic chemicals found include dioxins; they’re known to cause reproductive and developmental problems, damage the immune system, and cause cancer, according to the World Health Organization. A sediment sample at the site showed dioxins at 70,000 ng/kg — orders of magnitude higher than the recommended level at the site, which is 30 ng/kg. That’s already trouble, but it gets worse: Certain types of dioxins can be very hard to dispose of, increasing the risks of contamination. The dioxin in the Superfund site waste doesn’t dissolve easily in water, but it can seep into the surrounding sediments, the EPA says.

Following the storm, several chemical plants in and around Houston suffered leaks and even explosions. Several Superfund sites were also flooded, raising fears that the toxic materials could spread to surrounding areas where people live.

In an investigation of the federal clean-up sites in the area, the Associated Press revealed earlier this month that the San Jacinto River Waste Pits were completely covered with floodwaters. The protective cap damaged during the storm was installed in 2011, but required repairs on at least six occasions because parts of it were displaced or went missing, according to the AP. Environmentalists had warned for years that the 34-acre San Jacinto River Waste Pits, and other sites, could be flooded during storms, spreading dangerous chemicals around the Houston area, the AP says.

After confirming the spill, the EPA directed the parties responsible for the Superfund site to test more sediment samples, “to ensure that the exposed waste material is isolated,” the EPA says. The additional sampling will determine whether the dioxins and other toxic sludge has contaminated surrounding sediments.