The following was adapted from information taken from the website of California's State Water Resources Control Board (SWRCB). Water quality control agencies in other states should have similar programs.
Section 303(d) of the federal Clean Water Act (CWA) requires States to identify waters that do not meet water quality standards (called "impaired water bodies") after applying effluent limits for point sources other than publicly-owned treatment works (POTWs) that are based on the best practicable control technology currently available and effluent limits for POTWs based on secondary treatment. States are then required to prioritize waters/watersheds for total maximum daily loads (TMDL) development (see below). States are to compile this information in a list and submit the list to USEPA for review and approval. This list is known as the 303(d) list of impaired waters (303(d) list).
The State Water Resources Control Board (SWRCB) and Regional Water Quality Control Boards (RWQCBs) have ongoing efforts to monitor and assess water quality, to prepare the Section 303(d) list, and to develop TMDLs. The Clean Water Act does not require the implementation of TMDLs, only the development of the document. A federal regulation established in August 2000 that became effective in October 2001 requires that implementation plans be developed along with the TMDLs.
The SWRCB has developed a new program to monitor the quality of the State's Waters. The Surface Water Ambient Monitoring Program (SWAMP) will assess impacts on beneficial uses, the locations of polluted sites, the areal extent of pollution, and trends in water quality.
California's most recent 303(d) list was approved in 2010. The list is updated every two years. Priorities (high, medium, low) are assigned to each pollutant causing impairment at each listed water body, and TMDLs are developed for each pollutant according to a schedule shown on the 303(d) list.
TMDLs are documents that describe a specific water quality attainment strategy for a water body and related impairment identified on the 303(d) list. TMDLs may include more than one water body and more than one pollutant. The TMDL defines specific measurable features that describe attainment of the relevant water quality standards. TMDLs include a description of the total allowable level of the pollutant(s) in question and allocation of allowable loads to individual sources or groups of sources of the pollutant(s) of concern.
Typical categories of pollutants include metals, pesticides, pathogens (bacteria and viruses or sometimes specified as "high coliform count"), nutrients, priority organics, sedimentation/siltation, salinity/TDS/chlorides, and trash.