More than 200 private drinking water wells have been contaminated by natural gas drilling activity in Pennsylvania since 2007, according to documents released late Thursday by the Pennsylvania Department of Environmental Protection.
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The DEP released a list of 243 well contamination complaints that led to investigations and, in most cases, some form of resolution.
The number of complaints amounts to 2.8% of the 7,536 unconventional wells drilled since 2009.
Between 2009 and 2010, during the land rush to the Marcellus, water complaints nearly doubled from 34 in 2009 to 61 in 2010, the record shows.
About half drinking water wells were damaged by conventional drilling and half by unconventional, or horizontal, drilling, DEP Deputy Secretary for Oil and Gas Management Scott Perry said in a statement.
Complaints declined 41% in 2011 to 36 contaminated wells and remained at that level until dropping again this year to 1.3% of the 810 horizontal wells drilled through July, DEP records showed.
Common complaints that were confirmed include elevated levels of methane in the well water, wastewater and chemical spills on the surface, and wells whose water became undrinkable. None of the complaints could be linked directly to hydraulic fracturing fluids leaving the wellbore.
As the letters are all to individual well owners, sometimes a single well generated multiple complaints.
Many complaints were temporary and resolved by the operator, a survey of the 243 documents showed.
Drinking water well complaints were made most frequently in the dry gas portion of the Marcellus in northeast Pennsylvania counties such as Susquehanna, Bradford and Tioga.
The head of the Marcellus Shale Coalition, a trade group for Marcellus drillers, said the numbers only confirm that Pennsylvania -- with large amounts of methane close to the surface and no regulations on water well construction -- has "long-standing water well-related challenges."
"Pennsylvania's shale regulations have been dramatically strengthened over the past several years -- from overall well construction practices, including enhanced casing and cementing requirements, to expanded pre-drill testing, and heightened surface containment measurements -- aimed at protecting groundwater, as well as the reality that these issues -- many related to surface spills -- were not tied to the hydraulic fracturing process," MSC President Dave Spigelmyer said.
The DEP gave no reason for the timing of the data release. The agency has been under pressure, including lawsuits by the Associated Press and other media outlets, to release the complaint files.