Generally, sources of drinking water include rivers, lakes, streams, natural springs and wells. As water travels over the surface of the land or under the ground, it dissolves naturally occurring minerals and radioactive material. It also picks up substances left by animal or human activity as it travels to its destination. For instance, microbial contaminants may come form sewage treatment plants, septic tanks, livestock operations and wildlife pesticides and herbicides come from agricultural runoff and excess residential use. Other contaminants come from urban runoff, petroleum products, mining and industrial wastewater.
Radioactive materials can occur naturally or can come from oil and gas production and mining.
MCLG's allow for a margin of safety. The highest level of a contaminant that is allowed in drinking water. MCL's are set as close to the MCLG's as feasible using the best available treatment technology. A required process intended to reduce the level of a contaminant in drinking water. The concentration of a contaminant which, if exceeded, triggers treatment or other requirements which a water system must follow.