Sample results from case study sites in Pennsylvania and across the country are expected to be released in weeks as part of an ongoing federal study of the potential impact of hydraulic fracturing on water supplies.

The U.S. Environmental Protection Agency issued a progress report for the closely watched study on Dec. 21, but the update primarily described the framework of the broad investigation and drew no conclusions about whether the oil and gas extraction process poses a risk to drinking water.

The progress report also did not include results from two rounds of sampling the agency completed at each of five case study sites where residents suspect drilling has impacted their drinking water in Bradford, Susquehanna and Washington counties in Pennsylvania and Colorado, North Dakota and Texas.

During an online presentation about the progress report on Friday, an EPA official said those sample results have been analyzed and the agency plans to post them on its website in weeks.

"I don't have a specific date, but it's going to be a number of weeks from now," Jeanne Briskin, the agency's hydraulic fracturing research coordinator, said.

Conclusions from the sampling and more than a dozen other related research projects that make up the study are expected to be released in draft form in December 2014, she said.

The study is evaluating several phases of the hydraulic fracturing process, from river water withdrawals to the treatment and disposal of wastewater. The EPA is reviewing past studies and existing data on the chemicals that are mixed with water and injected underground at high pressure to crack rock formations and release the oil or gas. It is also conducting new studies on how well common treatment options remove contaminants from wastewater, assessing the toxicity of the chemical additives used in the process, and modeling potential passageways for gas or fluid to mix with fresh water supplies.

The case studies in Pennsylvania and elsewhere are meant to determine if reported water changes can be attributed to oil and gas development or some other cause.

In Northeast Pennsylvania, the EPA sampled 34 water wells, two springs, a pond and a stream in October and November 2011, and April and May. Most of the sampling in the first two rounds and all of the sampling in future rounds will be in Bradford County. Only four wells were sampled in Susquehanna County because the agency collected extensive information during a separate investigation of Dimock Twp. water supplies last year.

That investigation ended with the EPA stating it found no cause to take further action.

Some information related to the first round of sampling in Bradford County was released in May by natural gas operator Chesapeake Energy. The company split samples from 15 water sources with the government team during the October and November sampling outing. An analysis by a Chesapeake contractor found that natural gas development had not affected the water supplies.

Contact the writer: llegere@timesshamrock.com