Topeka ? A method of prying more oil from rock layers, called hydraulic fracturing — or fracking — is fueling controversy in Kansas, with opponents saying it could threaten the water quality of the Ogallala Aquifer and supporters arguing that it has been used for years without any evidence that it pollutes water.

The method involves drilling deeply into the rock and then injecting water filled with chemicals to open seams in the rock. It has been used for up to 60 years but is attracting new attention because of increased use in northwest Kansas, where analysts expect to find shale associated with oil deposits.

More fracking activity also is reported in the south-central portion of the state, particularly in the Oklahoma border counties of Harper and Barber, The Topeka Capital-Journal reported.

The Sierra Club of Kansas contends the process uses unhealthy chemicals, requires heavy consumption of water and could contaminate water supplies.

Critics are particularly concerned about possible damage to the Ogallala Aquifer, the underground reservoir that provides water for much of western Kansas.

The process, which started in Kansas in 1947, involves drilling deep vertically and horizontally before injecting with tremendous pressure water with a chemical brew to open seams in the rock to free oil and gas. The mixture can include compressed gases, including nitrogen, as well as acid. Sand or ceramic material is then moved into fluid-driven channels.

In Kansas, it involves drilling about 4,000 feet below the surface and nearly that distance horizontally to reach reserves. The approach requires multiple layers of pipe and cement to encase well bores, which shield groundwater at shallow depths. The Ogallala runs 500 to 1,000 feet deep in Kansas.