Total Maximum Daily Load

A Total Maximum Daily Load (TMDL) is a regulatory term in the U.S. Clean Water Act (CWA), describing a value of the maximum amount of a pollutant that a body of water can receive while still meeting water quality standards. [CWA sec. 303(d), USC|33|1313(d).] Alternatively, TMDL is an allocation of that pollutant deemed acceptable to the subject receiving waters.

TMDLs have been used extensively by the United States Environmental Protection Agency (EPA) and state environmental agencies in implementing the CWA by establishing maximum pollution limits for industrial wastewater dischargers. EPA published regulations in 1992 establishing TMDL procedures. [ [http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=3ee6fd22bec20f63d483d8f874834d64&rgn=div8&view=text&node=40:21.0.1.1.17.0.16.8&idno=40 "Water Quality Planning and Management."] "Code of Federal Regulations," 40 CFR 130.7.] Application of TMDL has broadened significantly in the last decade to include many watershed-scale efforts. This process incorporates both point source and nonpoint source pollutants within a watershed.

tate Inventories

The CWA requires states to compile a lists of water bodies that do not fully support beneficial uses such as aquatic life, fisheries, drinking water, recreation, industry, or agriculture. These inventories are known as 303(d) Lists and characterize waters as fully supporting, impaired, or in some cases threatened for beneficial uses.

TMDL Planning Process

Beneficial use determinations must have sufficient credible water quality data for TMDL planning.

Throughout the U.S., data are often lacking adequate spatial or temporal coverage to reliably establish the sources and magnitude of water quality degradation.

TMDL planning in large watersheds is a process that typically involves the following steps:
# Watershed Characterization Report--understanding the basic physical, environmental, and human elements of the watershed.
# Impairment Status Report--analyzing existing data to determine if waters fully support beneficial uses
# Data Gaps and Monitoring Report--identification of any additional data needs and monitoring recommendations
# Source Assessment--identification of sources of pollutants, and magnitude of sources.
# Load Allocation--determination of natural pollutant load, and load from human activities (i.e. diffuse nonpoint sources and point discharges).
# Set Targets--establishment of water quality targets intended to restore or maintain beneficial uses.
# TMDL Implementation Plan--a watershed management strategy to attain established targets.

Water Quality Targets

The purpose of water quality targets is to protect or restore beneficial uses and protect human health. These targets may include state/federal numerical water quality standards or narrative standards, i.e. within the range of "natural" conditions. Establishing targets to restore beneficial uses is challenging and sometimes controversial. For example, the restoration of a fishery may require reducing temperatures, nutrients, sediments, and improving habitat.

Necessary values for each pollutant target to restore fisheries can be uncertain. The potential for a water body to support a fishery even in a pristine state can be uncertain.

Load Allocations

Load allocations are equally challenging as setting targets. Load allocations provide a framework for determining the relative share of natural sources and human sources of pollution.

The natural background load for a pollutant may be imprecisely understood. Industrial dischargers, farmers, land developers, municipalities, natural resource agencies, and other watershed stakeholders each have a vested interest in the outcome.

Implementation of TMDLs

To enforce TMDLs with point sources, water quality-based effluent limitations (WQBELs) must be developed and incorporated into discharge permits for these sources. [U.S. EPA. [http://cfpub.epa.gov/npdes/writermanual.cfm?program_id=45 "NPDES Permit Writers' Manual."] Chapter 6. December 1996. Document No. EPA-833-B-96-003.] The permits are issued by EPA and state agencies under the National Pollutant Discharge Elimination System (NPDES). Nonpoint source discharges (e.g. agriculture) are generally in a voluntary compliance scenario. The TMDL implementation plan is intended to help bridge this divide and insure that watershed beneficial uses are restored and maintained. Local watershed groups play a critical role in educating stakeholders, generating funding, and implementing projects to reduce nonpoint sources of pollution.

ee also

*Clean Water Act
*Wastewater quality indicators
*Water pollution
*Water quality
*Water quality modelling

References

U.S. Environmental Protection Agency. Washington, D.C. [http://www.epa.gov/OWOW/tmdl/decisions/ "Guidance for Water Quality-Based Decisions: The TMDL Process."] Doc. No. EPA 440/4-91-001. April 1991.

External links

* [http://www.epa.gov/owow/tmdl/ EPA TMDL Program]
* [http://www.epa.gov/waters/tmdl/expert_query.html EPA's WATERS Expert Query Tool (data on TMDLs)]
* [http://www.epa.gov/waterscience/standards/ EPA Water Quality Standards Program]
* [http://www.ttwater.com/ Tetra Tech Water Resources and TMDL Center]
* [http://www.tmdl.bse.vt.edu/ The Center for TMDL and Watershed Studies at Virginia Tech]