A new report warns that natural gas drilling in the Marcellus Shale might pose a threat to Chesapeake cleanup efforts, and called for a number of actions to better assess impacts and reduce any risk from drilling activities.

Natural gas drilling in the Marcellus Shale of Pennsylvania has already resulted in the loss of forested tracks, and the expectation is that tens of thousands of additional acres will be cleared in the next few decades to make way for drilling pads, pipelines and other infrastructure, according to the report from the Bay Program's Scientific and Technical Advisory Committee.

It is difficult to determine the potential impact on the Bay because of a lack of information, said the report, which stemmed from a two-day workshop last spring. As the report summed up, "there are some things known regarding shale gas development, but more importantly...there are many more things unknown."

The loss of forest from drill pads, pipeline construction and other natural gas infrastructure creates the potential for the increased runoff of sediment, nutrients and chemicals. Forests typically absorb more nutrients and retain more sediment than other land uses, so when forests are lost, nutrient runoff can increase.

The report said more research was needed on the effectiveness of best management practices used to control runoff from drilling activities. Many of those practices used vegetated cover to control runoff, but drilling activities often takes place during winter months when vegetation cannot be established.

Another concern is about the effect of gravel access routes, which can act like streams during rains and increase sediment loss from drilling sites.

In addition, the report noted that air pollution from drilling activities can increase local nitrogen deposition.

Drilling activities could increase runoff to local streams — especially for sediment — enough to affect the ability of local cleanup plans to achieve nutrient and sediment goals under the Chesapeake Bay Total Maximum Daily Load, the report said.

The report said it is unclear whether the Bay Program or Pennsylvania have the information with which to quantify the nutrient and sediment impact of the land use changes associated with drilling to adequately offset their impact, as required under the TMDL.

While the individual impact of local drilling activity may appear small, they can pose threats to local streams, the report said, and the cumulative effects of thousands of drilling sites could add up to have an impact on the Bay.

The report said the Bay Program needs to begin collecting better information about drilling impacts and increase monitoring to fill in information gaps. Addressing the issue now is important, it said, because there could be a "time lag" between when activities occur in the upper watershed, and when they have downstream effects.

The report said the Bay Program needs to determine whether assumptions it uses in computer models accurately reflect the impact of forest cleared for drilling activities and related infrastructure, and to ensure that land use changes related to drilling are fully accounted for in future nutrient and sediment projections.

It also expressed concern that metals from drilling activities "are extremely important and will become an issue." Metals are not part of the Bay TMDL.

The EPA's Bay Program office said in a statement that it would consider any potential land use impacts from drilling activities, as well as related changes in nutrient and sediment pollution, as part of a planned midpoint assessment of TMDL goals to be completed in 2017.

"Work is under way to determine how best to account for any new land uses to ensure efforts remain on track to achieve all required actions are in place prior to 2025 to restore water quality to the Chesapeake Bay," it said.

The full report, "Exploring the Environmental Effects of Shale Gas Development in the Chesapeake Bay Watershed," is found at www.chesapeake.org/pubs/297_Gottschalk2013.pdf.