Total Maximum Daily Load (TMDL) Definitions

Load Allocation (LA): The portion of a receiving water's loading capacity that is attributed either to one of its existing or future nonpoint sources of pollution or to natural background (40 CFR 130.2(g))

Loading Capacity (LC): The greatest amount of loading that a water can receive without violating water quality standards (40 CFR 130.2(f))

Margin of Safety (MOS): The "MOS" accounts for uncertainty in the loading calculation. The MOS may not be the same for different waterbodies due to differences in the availability and strength of data used in the calculations.

Nonpoint Source: A nonpoint source is essentially any source of pollutant(s) that is not a point source. Examples are sheet flow from pastures and runoff from paved areas.

Point Source: A point source is simply described as a discrete discharge of pollutants as through a pipe or similar conveyance (e.g., a ditch). A technical definition exists in federal regulation at 40 CFR 122.2.

Total Maximum Daily Load: The sum of the individual wasteload allocations for point sources and load allocations for nonpoint sources and natural background (40 CFR 130.2(I))

TMDL "budget": The allocations are simply the amounts of pollutants that can be discharged from each category. The TMDL does not specify how the dischargers must attain their particular load allocation. In other words, the TMDL will not set best management practices for a discharger or otherwise tell the discharger how to meet their goal, it merely sets their goal.

Water quality-limited segments: Those water segments that do not or are not expected to meet applicable water quality standards even after the application of technology.

Wasteload allocation (WLA): The portion of a receiving water's loading capacity that is allocated to one of its existing or future point sources of pollution. WLAs constitute the type of water quality-based effluent limitation. (40 CFR 130.2(h))