DIMOCK, Pa. — Tugging on rubber gloves, a laboratory worker kneels before a gushing spigot behind Kim Grosso’s house and positions an empty bottle under the clear, cold stream. The process is repeated dozens of times as bottles are filled, marked and packed into coolers.

After extensive testing, Grosso and dozens of her neighbors will know this week what may be lurking in their well water as federal regulators investigate claims of contamination in the midst of one of the nation’s most productive natural gas fields.

More than three years into the gas-drilling boom that’s produced thousands of new wells, the U.S. Environmental Protection Agency and the state of Pennsylvania are tussling over regulation of the Marcellus Shale, the vast underground rock formation that holds trillions of cubic feet of gas.

The state says EPA is meddling. EPA says it is doing its job.

Grosso, who lives near a pair of gas wells drilled in 2008, told federal officials her water became discolored a few months ago, with an intermittent foul odor and taste. Her dog and cats refused to drink it. While there’s no indication the problems are related to drilling, she hopes the testing will provide answers.

“If there is something wrong with the water, who is responsible?” she asked. “Who’s going to fix it, and what does it do to the value of the property?”

Federal regulators are ramping up their oversight of the Marcellus with dual investigations in the northeastern and southwestern corners of Pennsylvania. EPA is also sampling water around Pennsylvania for its national study of the potential environmental and public health impacts of hydraulic fracturing, or fracking, the technique that blasts a cocktail of sand, water and chemicals deep underground to stimulate oil and gas production in shale formations like the Marcellus.

Fracking allows drillers to reach previously inaccessible gas reserves, but it produces huge volumes of polluted wastewater, and environmentalists say it can taint groundwater. Energy companies deny it.

The heightened federal scrutiny rankles the industry and politicians in the state capital, where the administration of pro-drilling Gov. Tom Corbett insists that Pennsylvania regulators are best-suited to oversee the gas industry.

Caught in the middle of the state-federal regulatory dispute are residents who don’t know if their water is safe to drink.

EPA is charged by law with protecting and ensuring the safety of the nation’s drinking water, but it has largely allowed the states to take the lead on rules and enforcement as energy companies drilled and fracked tens of thousands of new wells in recent years.

In Pennsylvania, that began to change last spring after The Associated Press and other news organizations reported that huge volumes of partially treated wastewater were being discharged into rivers and streams that supply drinking water.

The EPA investigations are being conducted amid reports of possibly drilling-related contamination in several Pennsylvania communities.

In recent years, methane migrating from drill sites into private water supplies has forced scores of residents to stop using their wells and rely on deliveries of fresh water. Some residents complain the state has failed to hold drillers accountable.

In heavily drilled Washington County, near the West Virginia border, EPA staff are inspecting well pads and natural gas compressor stations for compliance with water- and air-quality laws. In Dimock, a village about 20 miles south of the New York state line, EPA stepped in after a gas driller won the state’s permission to halt fresh water deliveries to about a dozen residents whose wells were tainted with methane and, the residents say, heavy metals, organic compounds and drilling chemicals.

Dimock holds the distinction of being Pennsylvania’s top gas-producing town, yielding enough gas in six months to supply 400,000 U.S. homes for a year. Some residents contend their water wells were irreversibly contaminated after Houston-based Cabot Oil & Gas Corp. drilled faulty gas wells that leaked methane into the aquifer, and spilled thousands of gallons of fracking fluids that residents suspect leached into the groundwater.

Twice a day, six days a week, Carter Road resident Ray Kemble drives about eight miles to a hydrant in Montrose, fills a 550-gallon tank strapped to the back of a donated truck, and delivers water to as many as five homes. including his own. Anti-drilling groups are footing the bill, estimated at $500 per week.

Kemble said his well water turned brown and became unusable in 2008, shortly after the gas well across the street was drilled and fracked.

At his home, he filled a large plastic container dubbed a water buffalo from the tank on the truck.

“Never had a problem before until Cabot came in,” he said.