PITTSBURGH (AP) — High levels of an ultra-salty compound that could be linked to oil and gas drilling persist in the Allegheny River’s Pittsburgh-area watershed, while the levels declined in the nearby Monongahela River, recent research shows.

Officials at public water utilities in both watersheds grew concerned in 2009 and 2010 when bromide levels soared during a surge of Marcellus Shale gas drilling. Although not considered a pollutant by themselves, the bromides combine with chlorine used in water treatment to produce compounds that can threaten public health.