Surface Water Quality Bureau
Monitoring, Assessment & Standards Section
Total Maximum Daily Loads

Under Section 303(d)(1) of the federal Clean Water Act, each state is required to develop a list of waters within that state which are not in compliance with current water quality standards and establish a total maximum daily load (TMDL) for each pollutant.

A TMDL is defined as “a written plan and analysis established to ensure that a waterbody will attain and maintain water quality standard including consideration of existing pollutant loads and reasonably foreseeable increases in pollutant loads” (USEPA 1999). TMDLs have become an integral part of New Mexico’s Water Quality Management Plan.

Through the NM Water Quality Act, the State Legislature has empowered the Water Quality Control Commission with the authority to create such planning documents. The NMED is the agency responsible for implementing and enforcing the regulations adopted by the WQCC, including the development of TMDLs.

According to 40 Code of Federal Regulations (CFR) Part 130, TMDLs are the sum of the individual Waste Load Allocations (WLAs) for point sources and Load Allocations (LAs) for nonpoint sources (NPSs) and background conditions, and includes a margin of safety (MOS). A point source is a discrete discharge of pollution through a pipe or similar conveyance. A nonpoint source is any non-specific source of pollution (e.g. agricultural/rangeland runoff). Current estimates indicate that nonpoint sources are the cause of approximately 95% of the state’s water quality problem. The MOS accounts for uncertainty in the loading calculation. The MOS may not be the same for all waterbodies due to differences in the availability and strength of data used in the calculations. TMDLs are not regulatory documents, but they can be used to issue or modify permits for point sources. Non-point source pollution problems are addressed through non-regulatory programs including CWA §319(h) grants.