Washington’s Department of Ecology is escalating its cleanup strategy for a once-prosperous pulp mill outside of Aberdeen, while its latest owner continues to push back against regulators. The first order of business is finding out just how bad pollution spread from the Cosmo Specialty Fiber mill, about 100 miles southwest of Seattle, is. State environmental regulators know the place is leaking acid and other toxics, sometimes in residential neighborhoods or into the Chehalis River, but they say the true scope of the contamination remains unknown. …The defunct mill’s current owner, Richard Bassett, has proved a difficult partner for state and federal officials, increasingly defiant as he struggles to reopen the site while arguing about the conditions there. Whether he can reopen the mill or not, Ha Tran, Ecology’s project coordinator for the site, said Bassett and past owners will be expected to clean up the site in Cosmopolis.