Arsenic is naturally found in most water to varying degrees. It is an essential but poorly understood micronutrient. Its toxicity is similar to selenium, anothe

Arsenic is naturally found in most water to varying degrees. It is an essential but poorly understood micronutrient. Its toxicity is similar to selenium, another essential micronutrient, so it isn't unique in terms of the dose response. Some plants concentrate it but that usually isn't bioavailable. Background lead levels are sometimes high naturally but those geographies are relatively rare. Lead is usually industrial pollution. Mercury chemistry in nature tends to have very poor bioavailability. Dietary mercury is almost exclusively industrial pollution where mercury is integrated into molecules that are much more bioavailable.