After the drilling began on Terry Greenwood's farm, he says, "the water went bad -- it looked like iced tea."

In 2008, he recounted, "10 of my cows died. I don't want my cows to drink the water from the pond anymore, and we drink trucked water from a tank."

Greenwood, a white-bearded, grizzled former truck driver of 64, said he and his family used to use the water from a pond and a well on the property, but since the takeoff of natural gas fracking in the southwest corner of the state, "I've had nothing but trouble."

Fracking, the technique of hydraulically fracturing shale rock deep beneath the surface to release natural gas, has upended lives in the area, for the good and the bad.

In the past four years, drillers have pumped out billions of dollars worth of natural gas from the Marcellus Shale deep underground, revitalizing a region once known mainly for poor farms and coal mining.

Farmers earned sometimes tens of thousands of dollars letting exploration companies drill on their land.

But many now worry it has unleashed environmental and health threats they never imagined.

High up on a hill near Waynesburg, Consol Energy, a huge regional coal and gas company with 9,000 employees, is laying in yet another fracking well. In sight of the 50-meter (150-foot) drilling rig are two other sites leveled and prepared for wells.

"Safety is our first value," says Jeff Boggs, who heads drilling operations in the area for Consol.

Six fracking well bores extend down and then horizontally from the wellhead, so that the shale strata below can be tapped to the maximum extent, intended to release enough gas to flow for many years.

The operation is highly automated, but still takes nearly 75 workers in small teams that operate on shifts around the clock for the different stages from preparation to production.

Boggs says that his company adheres to federal and state environmental laws.

But activists say producing natural gas by fracking is more environmentally dangerous than the drillers admit.

The main worry is the chemicals used in the drilling, which may leak into the water table from below or from leaky holding tanks on the surface.

Some studies have shown higher risks of cancer and non-cancer health impacts from emissions for people living closer to wells, likely from the release into the air of gas from the pockets around the wells.

Other research indicates hydraulic fracturing itself may not pollute groundwater but suggests other parts of the process may leak contaminants into water supplies.

Boggs admits drillers "probably have some culpability in some cases" of pollution.

But he adds, "They are blaming us as an industry for the ills of the past," a reference to the damage done in the region by two centuries of coal mining.

"They say we don't have regulation, but we have plenty of regulation, and one of them is to protect the drinking water."

"We comply with all of them. We do not leave anything anymore on site in terms of drilling fluids," he adds.

But scientists, the energy industry and the government continue to battle over whether fracking is fundamentally a health and environmental threat -- and whether any such dangers might be worth supplying the cheap gas to the nation's energy supply.

Greenwood, speaking outside a Waynesburg event organized by an environmental group to educate local people about the impact of fracking on water, said since the fracking began his cows seem to have become infertile and he can't make any money with them.

But he has gotten little support from the government's Environmental Protection Agency.

"The EPA agents told me I had to prove it was from the fracking fluids. They told me to find a good attorney. I'm starting with a third one," he told AFP.