Mercury levels remain up to 20 times above natural levels in Ontario First Nation tribe Grassy Narrows’ territory, according to a report released on Monday, that was commissioned by the provincial government and the tribe in response to a decades-long push to clean up contamination in the area.

The study, which identified gaps in research that should be addressed in future studies, found that although mercury levels in Clay Lake had declined since the 1970s, levels remained elevated above natural levels by up to 20 times. The study, written by Patricia Sellers, a biology professor at the University of North Carolina, added that mercury levels have increased in lakes downstream from Dryden, where 9,000 kilograms of mercury was dumped from a paper mill in the 1960s.

“When we shared our land and water we expected it to be kept pristine, but they have failed and destroyed our culture as a result,” Chief Roger Fobister Sr. of Grassy Narrows said in a press release on Monday. Fobister went on hunger strike in July 2014 over mercury contamination.

Since the mercury was released in Dryden, the contamination has traveled downstream along the Wabigoon River to Clay Lake, to Ball Lake, to the English River, and further downstream to Separation Lake, the report said. Ball Lake was one of two sites downstream from Clay Lake that showed an increase in mercury in surface sediment, the report added. “This pattern of increase can be best explained by the accelerated erosion, transport downstream, and deposition of (upstream) particles richer in mercury than those already there,” the report said.

The report warned that mercury pollution could be released and sent downstream even more rapidly if the contaminated area is logged. Last year, a 10-year logging plan for Whiskey Jack Forest was approved by the government, which would affect large swathes of Grassy Narrow's territory. Logging that has already been carried out on Grassy Narrows territory “undoubtedly contributed to degraded downstream water quality,” the report said.

The Grassy Narrows tribe has long protested against the government’s approval of what they call “clear-cut logging” on their traditional territory — warning it could exacerbate the mercury contamination.

Grassy Narrows said in its release that all downstream basins and lakes have the potential for increased contamination if the government-approved logging in the Whiskey Jack Forest go ahead as planned.

The study makes 40 recommendations, including long-term monitoring of mercury levels in the lakes the tribe uses for fishing, the potential impacts of logging on mercury contamination, and a comprehensive health analysis of the impact of eating local fish — especially with regards to child development and aging-related neurodegenerative diseases.

No official cleanup has been carried out to address the mercury contamination, the report said, meaning only “natural recovery” has been underway on the Wabigoon-English River system since the 1970’s.

Many members of Grassy Narrows, from ages 16 to 76, showed signs of mercury poisoning, according to a Japanese research team that visited Ontario in 2014. An earlier study by Canada’s Mercury Disability Board had showed that adults there had been diagnosed with symptoms consistent with mercury poisoning, the report said.