CHARLESTON, W.Va. -- Researchers have begun to identify ways that wastewater and runoff from the natural gas production boom in the Marcellus Shale region is making its way into area rivers and streams.A new study released this week documents increases in mud and silt downstream from gas drilling and production operations and higher levels of another key pollutant downstream from water treatment plants that handle Marcellus wastes.The study, published in the Proceedings of the National Academy of Sciences, compared the results of more than 20,000 surface water monitoring reports with the locations of 5,000 wells across the gas-producing areas of Pennsylvania.Researchers found a 10 percent increase in chlorides concentrations for every 1.5 wastewater treatment facilities located upstream. They found a 5 percent increase in total suspended solids for every additional 18 well pads upstream."These are not dramatic changes that they found, but they're not inconsequential either," said Duke University biologist Robert Jackson, who has studied gas-production impacts, but was not involved in the new research.The study did not look at potential impacts on groundwater supplies -- a matter of much public concern and scientific debate. And it did not look for increases in any other toxic chemicals that might be discharged from gas production. But the study did confirm some previous findings about the drilling boom's effects on streams and rivers."The nature of surface water contamination from shale gas development considered here is qualitatively different from the groundwater concerns explored in the literature," says the study, written by Sheila Olmstead and others from the Washington, D.C., think tank Resources for the Future. "Although groundwater concerns may have primarily to do with contamination directly from wellbores or shale formations, surface water concerns may have primarily to do with off-site waste treatment and above-ground management."The study concluded, "These results can inform future voluntary measures taken by shale-gas operators and policy approaches taken by regulators to protect surface water quality as the scale of this economically important activity increases."Olmstead said that researchers found a statistically significant increase in solids pollution downstream as the number of well sites increased, but were not able to pinpoint which part of the gas-production process caused the pollution. Testing did not support their hypothesis that runoff impacts would be greater during heavy rain events or during the construction of well pads, or both, she said."So it may be that we are picking up on other aspects of infrastructure development (pipelines, roads), which are correlated with well pad construction," Olmstead said in an email. "But lacking data on these other types of infrastructure, we don't test that directly. So the question remains an open one."Jackson said the large scope of the study -- looking at drilling impacts across the state -- provided important broad-brush information, but also did not allow for a closer look at on-the-ground impacts.Brian Lutz, a Kent State University scientist who has also studied gas-drilling impacts, said the scope of the study was such that it didn't answer how pollution might impact different types of waterways."An 11 percent increase in chlorides in a pristine headwater stream might have quite an impact, but an 11 percent increase in chlorides in a major, developed river might not," Lutz said.Reach Ken Ward Jr. at firstname.lastname@example.org or 304-348-1702.