Oil and gas waste injection has been halted at 11 sites in California following orders issued by the state's Division of Oil and Gas and Geothermal Resources. State officials believe that fracking wastewater injections might be putting drinking water aquifers at risk of contamination.

The emergency shutdown order affects seven energy companies operating in California's Central Valley. Regulators have also opened a review into more than 100 other sites, suspecting that oil and gas companies may be pumping fracking fluids into drinking water aquifers. In previous years, many underground water reservoirs were exempted from environmental regulation because they were considered too hard to access or contained water of poor quality, but it is possible that companies have also injected wastewater in protected aquifers.

Although state officials say there is no direct evidence of underground water contamination at the moment, their investigation will continue and tests will be carried out to look for the presence of chemicals in aquifers. Regulators have also requested data from companies on their wastewater injection practices, the Bakersfield Californian reported.

The problem of potentially polluted water reserves is particularly pressing for California, which already faces serious water stress due to severe drought. If drinking water resources are contaminated, the state has very few options to replace or replenish the affected aquifers, media reports say. Dealing with the drought is expected to cost the state about $2.2 billion this year, according to a report by scientists at the University of California in Davis.