Actually, I spoke too soon. I should have read the article before posting. I stand by what I say about mercury, but I am familiar with the Lake Roosevelt project area.
I've actually visited and sampled some of the mine sites in the area, on a couple Patty Stone actually came with and observed. Most of what we found at our sites was lead....huge hits of lead in soils. The refinery in Canada dumped tons and tons of lead slag into the Columbia just before it flowed into the USA. I sampled at an old lead mill in Colville, WA where we found huge levels of lead...to the point that the county health department moved renters with children off the site the day after we informed them of the analytical results.
It wouldn't surprise me to find mercury at the mills in the area that processed gold ore.