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Prepare for a fight over fracking for black gold in the Golden State.

California is emerging as the new battleground over the fracking process – in which water, sand and chemical additives are injected into rock formations to release the hydrocarbons trapped within. It is a fight that could become a political lightning rod, as the sites earmarked for development include densely populated urban areas.

So far the US controversy over fracking has been most intense in Pennsylvania, New York and other north-eastern states, where natural gas lies in shale formations in mostly rural areas.

Fracking is also responsible for an – bringing prosperity for some, but raising concerns about pollution.

The vast Monterey Shale, which lies beneath central and southern California, is – more than 2.5 times as large as the deposits in North Dakota. Many more people will be affected by its development, because oil fields lie near heavily populated areas of Los Angeles county, including , formerly the home of MGM Studios and still the base for Sony Pictures.

Air-quality fears

Last week, a coalition called Californians Against Fracking to governor Jerry Brown, asking him to ban fracking in the state. Meanwhile, a bill that would have imposed a moratorium until new regulations were in place was .

Campaigners vow to keep up the pressure on Brown and note that two other bills seeking to impose a moratorium, which stalled this year, could come back for consideration in January.

While protests in the north-east US have focused on fears about contamination of water supplies, in California concerns about volatile chemicals released into the atmosphere may come to the fore.

“These areas already have some of the worst air quality in the country,” says of the San Francisco branch of the Center for Biological Diversity, which is part of Californians Against Fracking.