Which of the following describes the difference between a "Primary" MCL and a "Secondary" MCL?

• Primary MCLs apply to industrial water; Secondary MCLs apply to drinking water.
• There is no difference; both are legally enforceable by the EPA.
• Primary MCLs are health-based and enforceable; Secondary MCLs are aesthetic-based and non-mandatory.
• Secondary MCLs are for surface water only; Primary MCLs are for groundwater.