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Water: Water Quality Standards Academy

Basic Course: Supplemental Topics (h)

Listing Impaired Waters and Developing TMDLs

« Page 8 of 14 »

TMDL Development: Introduction

Point source discharging into water body.

A TMDL is a calculation of the maximum amount of a pollutant that a water body (or segment) can receive and still meet the state/tribe’s water quality standards—and an allocation of that amount to the pollutant’s sources. In this way, the TMDL provides the basis for establishing water quality-based controls that should provide the pollution reduction necessary for a water body to meet the standards.

A TMDL is the sum of the allocations to sources that cause or threaten a water body with impairment, and includes a margin of safety to account for uncertainty in the response of the water body to loading reductions. Pollutant sources include both:

  • Point source (PS). Pollutant loads discharged at a specific location from pipes, outfalls, and conveyance channels from either municipal wastewater treatment plants or industrial waste treatment facilities. Point sources can also include pollutant loads contributed by tributaries to the main receiving water stream or river.

  • Nonpoint source (NPS). Pollution that is not released through pipes but rather originates from multiple sources over a relatively large area. Nonpoint sources can be divided into source activities related either to land or water use, including failing septic tanks, improper animal-keeping practices, forestry practices, and urban and rural runoff.

For informational purposes only–Not official statements of EPA policy.

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