Legacy and ongoing human activity has concentrated and released unnaturally large amounts of mercury into the environment. Source examples include historic mercury, gold, and silver mining in the 1800s, fossil fuel combustion, concrete production, and consumer goods. Mercury washes or seeps into local waterways and is then turned into its toxic form (methylmercury) by bacteria that live in wet soils and aquatic environments.
Methylmercury is a neurotoxin. Mercury is contained in batteries, fluorescent lightbulb filaments, older thermostats, thermometers, electrical switches, old paint cans, and other consumer goods. If mercury is disposed of in trash, it will go to a landfill where it can potentially seep into groundwater.