Environmentalists are suing to stop the flow of 1,4-dioxane into the drinking water supply, which one local water utility found at concentrations 17 times higher than the EPA’s health advisory goal.By Lisa SorgASHEBORO, N.C.—Boxy, gunmetal gray buildings loom over a labyrinth of ducts and tubes and catwalks, beyond which 100 train cars loll on their tracks. Smokestacks wait to exhale.