The objective of the 1972 Clean Water Act is to restore and maintain the chemical, physical, and biological integrity of waters in the US. Water quality standards define clean; therefore, how standards are set is important for policy and regulatory decisions.
Standards based on maximum concentration limits (MCL) of toxic chemicals apply to potable waters but not to aquatic life, wildlife, livestock, human recreation, irrigation, or industrial uses. MCLs provide no knowledge of the physical or biological integrity of the water body. Each use requires appropriate standards. Biological standards are more suitable for these uses because the pattern of aquatic community function reflects water quality integrated over time and space. Biotic communities reflect the physical, chemical, and biotic integrity of water bodies.