By Keith Coffman

DENVER (Reuters) - Voters in three Colorado communities have decided to suspend or ban an oil and gas production process known as hydraulic fracturing, or fracking, according to unofficial election returns on Wednesday.

Boulder, Lafayette and Fort Collins passed measures with solid margins, but a fourth community, Broomfield, about 12 miles east of Boulder, narrowly rejected a fracking moratorium.

Seventy-eight percent of Boulder residents voted to suspend fracking within city limits for five years, while a similar measure won 56 percent support in Fort Collins.

In Lafayette, a city ordinance that permanently bans fracking within the city passed with 59 percent of the vote.

Broomfield defeated the measure by just 13 votes out of more than 20,500 cast, according to unofficial returns posted on the city's website.

Fracking involves pumping millions of gallons of pressurized water, sand and chemicals underground to fracture shale rock and release oil or gas.

Much of that water returns to the surface and is stored in lined pits or closed tanks for recycling or injection in underground storage caverns offsite.

Environmental groups say fracking can contaminate water supplies, but the industry has argued that it does not hurt the environment.

Fracking activity has increased in Colorado over the past decade, as it has in much of the United States. A drilling boom is under way and in June this year, oil production rose nearly 30 percent compared with the same period in 2012, to 161,000 barrels of oil each day.

(Editing by Karen Brooks and Maureen Bavdek)