Crops grown near the Chernobyl nuclear site in Ukraine are still contaminated with radiation from the explosive 1986 disaster.

In a new study, researchers found that wheat, rye, oats and barley grown in this area contained two radioactive isotopes — strontium 90 and cesium 137 — that were above safe consumption limits. Radioactive isotopes are elements that have increased masses and release excess energy as a result.

"Our findings point to ongoing contamination and human exposure, compounded by lack of official routine monitoring,” study author David Santillo, an environmental forensic scientist at Greenpeace Research Laboratories at the University of Exeter, said in a statement, referring to the fact that the government suspended its radioactive goods monitoring program in 2013.

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Santillo and his colleagues, in collaboration with researchers from the Ukrainian Institute of Agricultural Radiology, analyzed 116 grain samples, collected between 2011 and 2019, from the Ivankiv district of Ukraine — about 31 miles (50 kilometers) south of the nuclear plant.

This area is outside of Chernobyl's "exclusion zone," which is a 30 mile (48 km) radius around the plant that was evacuated in 1986 and has remained unoccupied. They found radioactive isotopes, predominantly strontium 90, were above safe consumption level in 48% of samples. They also found that wood samples collected from the same region between 2015 and 2019, had strontium 90 levels above the safe limit for firewood.

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The researchers believe that the lingering radiation in the wood, in particular, may be the reason for the continued contamination of crops, almost 35 years after the disaster. When analyzing the wood ash from domestic wood-burning ovens, they found strontium 90 levels that were 25 times higher than the safe limit. Locals use this ash, as well as ash from the local thermal power plant (TPP), to fertilize their crops, which continues to cycle the radiation through their soil.

However, computer simulations suggest that it could be possible to grow crops in the region at "safe" levels if this process of repeated contamination ceased. The researchers are now calling for the Ukrainian government to reinstate its monitoring program and create a system for properly disposing of radioactive ash.

"Contamination of grain and wood grown in the Ivankiv district remains of major concern and deserves further urgent investigation,” study author Valery Kashparov, director of the Ukrainian Institute of Agricultural Radiology, said in the statement. "Similarly, further research is urgently needed to assess the effects of the Ivankiv TPP on the environment and local residents, which still remain mostly unknown."

The findings were published on Dec. 17 in the journal Environment International.

Originally published on Live Science.