Action Level (AL): The concentration of a contaminant which, if exceeded, triggers treatment or other requirements that a water system must follow.
Locational Running Annual Average (LRAA): The arithmetic average of analytic results for samples taken at a specific monitoring location during the previous four calendar quarters.
Maximum Contaminant Level or MCL: The highest level of a contaminant that is allowed in drinking water. MCLs are set as close to the MCLGs as feasible using the best available treatment technology.
Maximum Contaminant Level Goal or MCLG: The level of a contaminant in drinking water below which there is no known or expected risk to health. MCLGs allow for a margin of safety.
Maximum Residual Disinfectant Level or MRDL: The highest level of a disinfectant allowed in drinking water. There is convincing evidence that addition of a disinfectant is necessary for control of microbial contaminants.
Maximum Residual Disinfectant Level Goal or MRDLG: The level of a drinking water disinfectant below which there is no known or expected risk to health. MRDLGs do not reflect the benefits of the use of disinfectants to control microbial contaminants.
No Detection (ND): Indicates the substance was not found by laboratory analysis.
Parts per million (ppm) or Milligrams per liter (mg/l)—one part by weight of analyte to 1 million parts by weight of the water sample.
Parts per billion (ppb) or Micrograms per liter (μg/l)—one part by weight of analyte to 1 billion parts by weight of the water sample.
