Distinguish between water quality (numeric criteria) standards and maximum contaminant levels (MCLs).

Water quality standards are adopted to ensure that the water is fit for the designated applications such as:
Recreation
Public drinking water supply
Agricultural, industrial, navigational uses
Protection and propagation of fish and wildlife
These standards can be numeric or narrative. The numeric standards consist of numerical values of the various physical, chemical, and biological parameters of water quality and are developed to ensure that the water is suitable for the intended use. Narrative criteria are qualitative criteria (for example, water should be toxin-free, etc.).
Maximum contaminant levels (MCL) are set by the EPA to protect drinking water quality. Technological feasibility and cost considerations are included when deciding upon the values of MCLs. Additionally, MCLs do not consider any other exposure pathway apart from the drinking water.
Hope this helps.
https://www.epa.gov/sites/production/files/2014-10/documents/handbook-chapter3.pdf