I think they accumualte for a few years and then start to attenuate over time.
Methyl mercury is a big issue due to the breakdown of organic material flooded in the reservoir. At least for the latest big dam in BC (Site C) it was supposed to go up for about about 50 years and then start to receded back to background levels.
I'd say negative imapcts to fisheries are probably a more significant effect for dams.
There was a link that showed all the higher costs that the people of the Cape will be paying but it was all messed so I deleted it.
On calm days all the greenies can just turn off their AC units. Should prove interesting inside their sealedconcrete towers. Stick with hydro.
Nothing like paying more for crap than quality. Wind power kills electronics and pretty much any electrical device that relies on a steady 60Hz