It is not a mystery where the mercury is coming from.  It is unfortunate this very good journalist omitted any mention of coal in her story.  The combustion of coal is the leading cause of mercury contamination.  When metallic mercury is deposited in the atmosphere, (along with lots of other toxic metals) it get taken into the food chain, and methylated.  As it moves up the food chain, it poisons the animals and humans who are exposed to it.

Smart people no longer eat fish.  Almost all fish, and especially the top predator fish, are loaded with mercury.

Coal kills.  It is not a cheap source of energy.  When the external costs are considered, it is the most expensive form of energy.