Dangerous levels of cancer-causing chemicals have been found in flowback fluid from hydraulic fracturing operations in California.

According to the Center for Biological Diversity, an environmental group, benzene levels over 1,500 times the federal limits for drinking water were found in fracking flowback fluid tests dating back to April 2014, which were obtained and analyzed by the Center. The organization said that benzene in excess of federal limits was found in 320 tests, and chromium-6, another chemical that can cause cancer, was detected 118 times.

Wastewater from fracked oil wells, which contains flowback fluid, is commonly disposed of in injection wells. These often feed into aquifers, including some that could be used for drinking water and irrigation.

Oil wastewater is also dumped into open pits, the conservation group said.

Earlier this week California's oil and gas regulators announced changes in the state's regulation of oil and gas injection wells to protect underground drinking-water supplies. The announcement came after a review found that more than 2,500 underground injection wells had been permitted into aquifers that were supposed to be protected as current or potential sources of water for drinking and agricultural use.

"Cancer-causing chemicals are surfacing in fracking flowback fluid just as we learn that the California oil industry is disposing of wastewater in hundreds of illegal disposal wells and open pits," commented Hollin Kretzmann, the Center for Biological Diversity lawyer who conducted the analysis.

She called on Jerry Brown, the governor of California, to shut down all the illegal wells immediately and ban fracking to help address this threat to California's water supply.