A huge percentage of the general public does not know the difference between MCL, MCLG, MRDL and MRDLG. They ought not feel ashamed, though, because we took a short survey of people connected to water treatment, water testing, water processing, etc. and found that many of them had just as little knowledge on the topic as the general public.
Therefore we decided to visit the United States Environmental Protection Agency web site to find clear, concise definitions of MCL, MCLG, MRDL and MRDLG. See below:
Maximum Contaminant Level (MCL) – The highest level of a contaminant that is allowed in drinking water. MCLs are set as close to MCLGs as feasible using the best available treatment technology and taking cost into consideration. MCLs are enforceable standards.
Maximum Contaminant Level Goal (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 and are non-enforceable public health goals.
Maximum Residual Disinfectant Level (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 (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.
( source )