Strictly speaking the ground portion of the spill would indeed run downhill or down in the ground. But this smoke plume is going at least to Lake Erie, and other parts East and West, so far. The plume contains Hydrogen chloride, among other nasty stuff. If that HCL gets hit with rain or any other wetness it turns to hydrochloric acid and that, worst case, could turn that whole watershed into a desert.
You're right. I saw a chemical engineer later who explained the conversion to hydrochloric acid in the airborne plume.
Bad ju-ju.