Over the span of its operation, a typical mercury cell chlor-alkali facility would have released somewhere in the neighborhood of 10 tons of elemental mercury into the aquatic environment. There are (or were) an unknown number of these facilities around the world, but a reasonable estimate might be 250 and includes locations everywhere: in the U.S., in Canada, in many of the countries of the European Union. To this day, nobody knows how many the Soviets built. Or where. They were built on rivers, lakes, fjords, in estuaries, in lagoons, and on harbors; by 2019 the vast majority have been abandoned, closed or converted to other technologies. The process itself begins with NaCl brine and ends with caustic soda and chlorine. Both caustic soda and chlorine have many uses; the chlorine, as example, to make pesticides, bleach for the pulp and paper industry, and PVC.
In the period from roughly the middle of the 20th century until its end, chlor-alkali production relied globally on mercury and electricity to split that Na from the Cl. This process wasn’t a closed loop and elemental mercury was cheap: if the process was run efficiently, production might release ~10 lbs of mercury into wastewater a year. Depending on when you were on that 20th century arc, that wastewater could have been discharged into the river, lake, fjord, estuary, lagoon, or harbor. Or it could have been discharged into a treatment pond. Depending on when you were on that 20th century arc, the bottom of that pond might have been sealed with fine clay or a membrane liner. If the process was run inefficiently – although not necessarily in violation of any discharge regulations and as appeared to have happened in many locations in the initial years of facility operation – then over the life of the facility, production of chlor-alkali likely released somewhere in the neighborhood of 10 tons of elemental mercury into the aquatic environment.
By volume, this much elemental mercury would overflow 3 × 55 gallon drums. Maybe this seems like a lot of mercury and maybe it doesn’t. The spectrum of ecological and human health impacts that result from this overflow depends on a long list of factors. Fundamentally, the type of ecosystem that that mercury was released into matters significantly. Even more fundamentally, the extent to which people living in the vicinity of that release have had to rely on that aquatic ecosystem for food and livelihood matters even more significantly. Critically, it is the type of environment – the river, lake, fjord, estuary, lagoon or harbor – that determines how far from the facility you have to be living for ‘in the vicinity’ to not include your community.