Looks like a BC smelter has been dumping mercury into the Columbia River for decades. The most damning sentences:

In 1994 and 1995, the discharges exceeded the cumulative totals for all U.S. companies for copper and zinc. Mercury discharges were less than the U.S. total, but they were equivalent to 40 percent, 20 percent and 57 percent of all the U.S. releases to water in 1995, 1996 and 1997, the report notes.

Because the pollution crossed from Canada into the US, it apparently escaped the notice of the regulators in both countries. All the more reason to believe that the real boundaries that matter aren’t political ones, but instead are the ones defined by the landscape itself.