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A '''total maximum daily load''' (TMDL) is a regulatory term in the U.S. Clean Water Act, describing a plan for restoring impaired waters that identifies the maximum amount of a [[Chemistry:Pollutant|pollutant]] that a [[Earth:Body of water|body of water]] can receive while still meeting water quality standards.<ref>United States. Clean Water Act, sec. 303(d), {{USCSub|33|1313|d}}</ref><ref name ="NRC2001">{{cite report |author=National Research Council |date=2001 |title=Assessing the TMDL Approach to Water Quality Management |location=Washington, DC |publisher=National Academies Press |doi=10.17226/10146|isbn=978-0-309-07579-4 }}</ref><ref name ="NASEM2019">{{cite report |author=National Academies of Sciences, Engineering and Medicine |date=2019 |title= Approaches for Determining and Complying with TMDL Requirements Related to Roadway Stormwater Runoff |publisher=National Academies Press |doi=10.17226/25473|isbn=978-0-309-49376-5 }}</ref>

==State and federal agency responsibilities==
The Clean Water Act requires that state environmental agencies complete TMDLs for impaired waters and that the United States Environmental Protection Agency (EPA) review and approve / disapprove those TMDLs.<ref name="EPA program overview">{{cite web |title=Overview of Identifying and Restoring Impaired Waters under Section 303(d) of the CWA |url=https://www.epa.gov/tmdl/overview-identifying-and-restoring-impaired-waters-under-section-303d-cwa |author=<!--Not stated--> |date=2021-09-20 |website=Impaired Waters and TMDLs |publisher=EPA}}</ref> Because both state and federal governments are involved in completing TMDLs, the TMDL program is an example of [[Philosophy:Cooperative federalism|cooperative federalism]]. If a state doesn't take action to develop TMDLs, or if EPA disapproves state-developed TMDLs, the EPA is responsible for issuing TMDLs. EPA published regulations in 1992 establishing TMDL procedures.<ref>EPA. "Water Quality Planning and Management: Total maximum daily loads (TMDL) and individual water quality-based effluent limitations". ''Code of Federal Regulations,'' {{uscfr|40|130|7}}.</ref> Application of TMDLs has broadened significantly in the last decade to include many watershed-scale efforts, including the Chesapeake Bay TMDL.<ref>{{cite report |title=Chesapeake Bay TMDL Executive Summary |url=https://www.epa.gov/sites/production/files/2014-12/documents/bay_tmdl_executive_summary_final_12.29.10_final_1.pdf |date=2010-12-29 |publisher=EPA}}</ref> TMDLs identify all point source and [[Earth:Nonpoint source pollution|nonpoint source]] pollutants within a [[Earth:Drainage basin|watershed]].<ref name="EPA program overview"/>

==State inventories==
The Clean Water Act requires states to compile lists of water bodies that do not fully support beneficial uses such as aquatic life, fisheries, drinking water, recreation, industry, or [[Earth:Agriculture|agriculture]]; and to prioritize those water bodies for TMDL development. These inventories are known as "303(d) lists" and characterize waters as ''fully supporting'', ''impaired'', or in some cases ''threatened'' for beneficial uses.<ref name="EPA TMDL overview">{{cite web |title=Overview of Total Maximum Daily Loads |url=https://www.epa.gov/tmdl/overview-total-maximum-daily-loads-tmdls |date=2021-09-20 |publisher=EPA}}</ref>

==Planning process==
Beneficial use determinations must have sufficient credible [[Earth:Water quality|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—understanding the basic physical, environmental, and human elements of the watershed.
# Impairment status—analyzing existing data to determine if waters fully support beneficial uses
# Data gaps and monitoring report—identification of any additional data needs and [[Earth:Water quality#Sampling and measurement|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.<ref name="EPA TMDL overview"/>

===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.<ref name="EPA TMDL overview"/>

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.

===Background===
Calculating the TMDL for any given body of water involves the combination of factors that contribute to the problem of nutrient concentrated runoff.  Bodies of water are tested for contaminants based on their intended use.  Each body of water is tested similarly but designated with a different TMDL. Drinking water reservoirs are designated differently from areas for public swimming and water bodies intended for fishing are designated differently from water located in wildlife conservation areas.  The size of the water body also is taken into consideration when TMDL calculating is undertaken. The larger the body of water, the greater the amounts of contaminants can be present and still maintain a margin of safety. The ''margin of safety'' (MOS) is numeric estimate included in the TMDL calculation, sometimes 10% of the TMDL, intended to allow a safety buffer between the calculated TMDL and the actual load that will allow the water body to meet its beneficial use (since the natural world is complex and several variables may alter future conditions). TMDL is the end product of all point and non-point source pollutants of a single contaminant.  Pollutants that originate from a point source are given allowable levels of contaminants to be discharged; this is the ''waste load allocation'' (WLA). Nonpoint source pollutants are also calculated into the TMDL equation with ''load allocation'' (LA).<ref name="EPA TMDL overview"/>

===Calculation===
The calculation of a TMDL is as follows:<br />
:<math>TMDL = WLA + LA + MOS</math>
where WLA is the waste load allocation for point sources, LA is the load allocation for nonpoint sources, and MOS is the margin of safety.<ref>{{cite journal |author1=A. Shirmohammadi |author2=I. Chaubey |author3=R. D. Harmel |author4=D. D. Bosch |author5=R. Muñoz-Carpena |author6=C. Dharmasri |author7=A. Sexton |author8=M. Arabi |author9=M. L. Wolfe |author10=J. Frankenberger |author11=C. Graff |author12=T. M. Sohrabi |title=Uncertainty in TMDL Models |journal=Transactions of the American Society of Agricultural and Biological Engineers |year=2006 |volume=49 |issue=4 |doi=10.13031/2013.21741 |pages=1033–1049 |url=https://naldc-legacy.nal.usda.gov/naldc/download.xhtml?id=1403&content=PDF |hdl=10919/48202 |hdl-access=free}}</ref><ref name ="NRC2001" /><ref name ="NASEM2019" /><ref>{{cite journal |last1=Granato |first1=G.E. |last2=Jones |first2=S.C. |date=2017 |title=Estimating Total Maximum Daily Loads with the Stochastic Empirical Loading and Dilution Model |journal=Transportation Research Record |volume=2638 |publisher=Transportation Research Board; National Academies of Sciences, Engineering and Medicine |id=2638 |pages=104–112 |doi=10.3141/2638-12|s2cid=116016432 }}</ref><ref>Lantin, A., Larsen, L., Vyas, A., Barrett, M., Leisenring, M., Koryto, K., and Pechacek, L., 2019, Approaches for determining and complying with TMDL requirements related to roadway stormwater runoff: National Academies Press, National Cooperative Highway Research Program Research Report 918, 133 p. [Also available at https://doi.org/10.17226/25473.]</ref><ref>Granato, G.E., and Friesz, P.J., 2021, Approaches for assessing long-term annual yields of highway and urban runoff in selected areas of California with the Stochastic Empirical Loading and Dilution Model (SELDM): U.S. Geological Survey Scientific Investigations Report 2021–5043, 37 p., https://doi.org/10.3133/sir20215043</ref><ref>Granato, G.E., Spaetzel, A.B., and Jeznach, L.C., 2023, Approaches for assessing flows, concentrations, and loads of highway and urban runoff and receiving-stream stormwater in southern New England with the Stochastic Empirical Loading and Dilution Model (SELDM): U.S. Geological Survey Scientific Investigations Report 2023–5087, 152 p., https://doi.org/10.3133/sir20235087.</ref>

===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, [[Engineering:Land development|land developers]], municipalities, natural resource agencies, and other watershed stakeholders each have a vested interest in the outcome.<ref name="EPA TMDL overview"/>

==Implementation==
To implement TMDLs with point sources, wasteload allocations are incorporated into discharge permits for these sources.<ref>{{cite report |publisher=EPA |chapter-url=https://www.epa.gov/npdes/npdes-permit-writers-manual |title=NPDES Permit Writers' Manual |chapter=Chapter 6: Water Quality-Based Effluent Limitations |date=September 2010 |id=EPA 833-K-10-001}}</ref> The permits are issued by EPA or delegated 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 ensure 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.<ref name="EPA TMDL overview"/>

==See also==
*[[Wastewater quality indicators]]
*[[Earth:Water pollution|Water pollution]]
*[[Earth:Water quality modelling|Water quality modelling]]

==References==
{{Refbegin}}
* {{cite report |publisher=U.S. Environmental Protection Agency (EPA) |location=Washington, D.C. |url=http://nepis.epa.gov/EPA/html/DLwait.htm?url=/Exe/ZyPDF.cgi/00001KIO.PDF?Dockey=00001KIO.PDF |title=Guidance for Water Quality-Based Decisions: The TMDL Process |id=EPA 440/4-91-001 |date=April 1991}}
{{Refend}}
{{Reflist}}

==External links==
*[https://www.epa.gov/tmdl EPA TMDL Program]
*[https://www.epa.gov/waterdata/ask-waters EPA's AskWATERS Expert Query Tool] - data on TMDLs
*[https://www.epa.gov/wqs-tech EPA Water Quality Standards Program]
*[https://doi.org/10.5066/F7BG2M33 Stochastic Empirical Loading and Dilution Model Documentation Page] - US Geological Survey
*[https://web.archive.org/web/20060911175035/http://www.tmdl.bse.vt.edu/ The Center for TMDL and Watershed Studies at Virginia Tech] 

{{US Environmental law}}

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[[Category:Hydrology]]

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