Region 1: EPA New England
EPA New England Total Maximum Daily Load (TMDL)
What is a Total Maximum Daily Load or TMDL?
A Total Maximum Daily Load (TMDL) is an estimate of how much of a pollutant, or group of pollutants, a water body (lake, pond, river, stream, or estuary) can absorb without becoming polluted. In this situation, being polluted means a water body is not meeting the water quality standards that have been assigned to it by the state. TMDLs are developed for a pollutant (or a group of pollutants) in water bodies that are listed in each state's list of impaired waters, known as the 303(d) list. The 303(d) list is named for the section of the Clean Water Act which requires its development every two years.
In the calculation of a TMDL, pollutant loads from point sources (permitted discharges from identifiable points, such as industrial or municipal discharge pipes) are called "waste load allocations," or WLAs, and loads from nonpoint sources (diffuse sources, such as urban, residential, or agricultural runoff) are called "load allocations," or LAs. The TMDL calculation includes a margin of safety (MOS) to account for uncertainty in the TMDL calculation.
The TMDL is the sum of the wasteload and load allocations, and the MOS:
TMDL = WLA + LA + MOS
The calculation also includes consideration of seasonal variation in water quality, as well as important assumptions, such as future growth, made in developing the TMDL.
EPA requires the development of TMDLs in its regulations -- known as the TMDL regulations -- which were issued in 1985 and 1992 to implement section 303(d) of the Clean Water Act (CWA).
Why are TMDLs important?
Developing TMDLs is an important step in solving water-quality problems. A TMDL provides (1) analysis and estimates of pollutant loadings from all significant point and non-point sources, (2) a link between pollutants sources and their impacts on water quality, (3) an allocation of allowable pollutant loads among sources, (4) identification of control actions for achieving WQSs, and (5) an opportunity for public participation.
Who is responsible for developing TMDLs?
States are responsible for developing and submitting TMDLs to EPA. EPA is responsible for approving TMDLs and completing any TMDLs that are disapproved. In some cases, EPA may develop a TMDL if requested by one or more states.