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National Pollutant Discharge Elimination System (NPDES)

Permit Limits-Permitting to Meet a Total Maximum Daily Load (TMDL)

A TMDL is a calculation of the maximum amount of a pollutant that a waterbody can receive and still safely meet water quality standards. Generally, TMDLs are made up of wasteload allocations (WLAs) for point sources, load allocations for nonpoint sources, a margin of safety, and possibly a reserve allocation. The WLAs are then used to develop NPDES permit limits.

The process of translating WLAs into NPDES permit limits that are consistent with the assumptions and requirements of TMDLs is not always straightforward. During the development of the permit, it is not uncommon for the permit writer, and often the TMDL writer, to be asked to interpret aspects of the TMDL that are not clear-cut. These FAQs try to assist permit writers in navigating through some of these scenarios. These questions represent specific scenarios and are not representative of either the TMDL or NPDES program as a whole.

Frequent Questions

When should WLAs be expressed in permits as mass-based Water Quality Based Effluent Limits (WQBELs), concentration-based WQBELs, or both?

Applicable regulatory requirements:

NPDES regulations specify that mass-based WQBELs are required in permits except when pollutants cannot appropriately be expressed in terms of mass; the applicable standards are expressed in terms of other units; or limits expressed in terms of mass are infeasible because the mass of a pollutant cannot be related to a measure of operations and permit conditions ensure that dilution will not be used as a substitute for treatment. See 40 CFR 122.45(f)(1). The regulations also give permit writers the discretion to include other limits, such as concentration-based limits, to supplement mass-based WQBELs. 40 CFR 122.45(f)(2). NPDES regulations further require that WQBELs be consistent with the assumptions and requirements of any available wasteload allocation in the TMDL. 40 CFR 122.44(d)(1)(vii)(B).

Where the WLA is expressed in terms of mass:

TMDL WLAs are generally expressed in terms of total mass of a pollutant per unit of time. In such situations, it should be fairly straightforward for a permit writer to implement those mass-based WLAs as mass-based WQBELs, thus meeting the regulatory requirement to include WQBELs consistent with the assumptions of the WLA (40 CFR 122.44(d)(1)(vii)(B)).

The permit writer should also consider whether concentration-based limits are needed in addition to mass-based WQBELs to be consistent with the assumptions and requirements of the WLA. In doing so, the permit writer should first examine the TMDL documentation and any TMDL implementation plan, to determine whether they reflect an assumption that concentration-based limits may be necessary in addition to the mass-based WQBELs. If so, then the permit must include concentration-based WQBELs to ensure consistency with the assumptions of the WLA, as required by 40 CFR 122.44(d)(1)(vii)(B). When possible, the permit writer should also consult the TMDL writer to confirm any interpretation of the TMDL documents.

Moreover, even if the TMDL documentation or implementation plan does not specifically reflect an assumption for concentration-based WQBELs, permit writers should consider using their discretionary authority under 40 CFR 122.45(f)(2) to include supplemental concentration-based WQBELs in permits where appropriate. For example, because TMDL WLAs are often based on watershed loadings analyses and may not have considered localized impacts, supplemental concentration-based WQBELs may be appropriate to prevent a discharge from causing localized impacts in critical low-flow stream segments within the watershed. Similarly, supplemental concentration-based WQBELs may be appropriate for non-continuous discharges, such as stormwater, which tend to be of a relatively short term, sporadic, and highly variable. Mass-based limits are typically a less effective means of regulating such discharges, where the operator has limited control over the volume of the discharge, which is often directly a function of the amount of precipitation falling on the site. Operators do, however, have control over pollutant sources that could come into contact with precipitation and directly affect the concentration of pollutants in stormwater; therefore, concentration-based limits are also typically better for evaluating the effectiveness of pollutant abatement activities for stormwater discharges.

Where the WLA is expressed in terms other than mass:

If the TMDL WLA is not a mass-based expression, permit writers can establish WQBELs expressed in terms other than mass if any of the exceptions found in 40 CFR 122.45(f)(1)(i – iii) apply.

The permit writer should first consider whether the pollutant at issue can be appropriately expressed by mass; if not – as would be the case, for example, for temperature and pH – then the permit writer can include a WQBEL expressed in terms other than mass, as provided in 40 CFR 122.45(f)(1)(i).

If this exception does not apply, the permit writer should then consider whether the “applicable standards and limitations” are expressed in terms other than mass. 40 CFR 122.45(f)(1)(ii) allows for non-mass based limits if standards or limitations are expressed in other units. Most water quality criteria are expressed in terms of concentration; therefore, if a WLA is expressed as a concentration, the underlying standards (i.e. criteria) are also likely to be expressed as a concentration. In this case, the exception in 40 CFR 122.45(f)(1)(ii) would allow for use of a concentration-based WQBEL in lieu of mass based limits. However, because mass based effluent limits prevent complying with permit limits through dilution in lieu of treatment, it is recommended that permit writers also include mass based limits in permits where appropriate. Also, even where the WLA is expressed as a concentration, permit writers should consult the available TMDL documentation to determine if mass based limits are necessary to ensure consistency with the assumptions of the WLA., as required under 40 CFR Part 122.44(d)(1)(vii)(A), or otherwise necessary to meet applicable water quality standards under CWA Section 301(b)(1)(C). The TMDL writer will also be able to provide helpful information and should be consulted by the permit writer when practicable.

The third exception for non-mass based limits, found at 40 CFR 122.45(f)(1)(iii), is when mass-based limits are infeasible because they cannot be related to a measure of operation. One example, provided in the regulation, is TSS limits at facilities like quarries. Most discharges from these types of facilities have generally been limited on a concentration basis. The volume of drainage discharged typically results from a combination of hydrogeology and precipitation and not production levels of a rock or mineral. Operators are able to control the concentration of pollutants in drainage through a combination of Best Management Practices and treatment; however, they are not normally able to control the volume of mine drainage and thus have little control over the mass of pollutants discharged. This exception to the requirement to include mass based limits in NPDES permits is often also applicable to stormwater discharges.

If none of these three exceptions apply, permit writers will need to calculate a mass-based WQBEL using all the relevant available information including information provided in the TMDL document and any implementation documents. In some cases, water quality model input and output files may provide pertinent information, such as the effluent concentrations and discharge rates that can be used to calculate mass-based limits, as required by 40 CFR 122.45(f)(1). In this situation, because the WLA is expressed as a concentration, the permit writer may need to include a concentration-based WQBEL in addition to the mass-based WQBEL, as authorized under 122.45(f)(2). This approach will help to ensure that the permit contains WQBELs that are consistent with the assumptions of the WLA, as required under 40 CFR 122.44(d)(1)(vii)(B).

What options are available to the permit writer when a source has been treated in the TMDL as an insignificant, negligible, or minor contributor of the pollutant of concern?

Where the TMDL considers the contribution of the pollutant of concern from a regulated source to be insignificant, the permit for that source must still include a WQBEL that is consistent with the assumptions of the relevant WLA as per 40 CFR 122.44(d)(1)(i) and (d)(1)(vii)(B). To the extent that the source is contributing some amount of the pollutant of concern to the impaired water body and has “reasonable potential” to cause or contribute to the impairment, a WQBEL must be included in the permit, as per 40 CFR 122.44(d)(1)(i), even if the loading is minimal. Some TMDLs that address insignificant sources do so in a way to suggest that current loads from particular discharges do not warrant more restrictive effluent limits to implement the applicable WLA. In these situations, the permit writer’s task is to establish a WQBEL that keeps pollutant discharges at levels consistent with the assumption in the TMDL that the load from this source is insignificant.

Here are some of the available WQBEL options that the permit writer may have depending on the way the TMDL is worded:

  • Set the WQBEL equal to the existing effluent limits, or if known, the current actual discharge of the pollutant from the source. In some instances, where the TMDL considers some sources to be negligible based on their current loadings, the TMDL will recommend that the appropriate WLA is for these sources to maintain that level. Therefore, including a WQBEL in the permit that is essentially the same as the current effluent limit or actual discharge level will be consistent with the assumptions of the WLA on which it is based, i.e. that discharges at this negligible level do not need to be further controlled in order to meet water quality standards. Note that this is not the same as saying that there is no WQBEL. A WQBEL is still needed in the permit to ensure that the facility continues to discharge no more of the pollutant of concern than the current performance level, consistent with the assumptions of the WLA.
    Examples of TMDLs where this approach might be appropriate:
  • Develop a WQBEL based on water quality criteria at the “end-of-pipe”. If WLAs in the TMDL are expressed in concentrations of the pollutant discharged or can be appropriately calculated as such, and information suggests the source that is treated in the TMDL as insignificant is discharging at a level that is already at or below the applicable water quality criterion, the permit could apply the criterion at the “end-of-pipe.” Use of an “end-of-pipe” concentration-based WQBEL would, in such cases, help ensure that the discharger continues to control its discharges as necessary to meet applicable water quality standards.

In conjunction with the options above, it will typically be appropriate to require the permitted source to conduct effluent monitoring for the pollutant of concern to verify the TMDL’s findings that the source is contributing negligible loadings of the pollutant. The monitoring frequency should vary based on the potential to exceed current loading levels or applicable water quality standards. This approach is particularly appropriate where there is some uncertainty regarding the assumed negligible loadings from the source.

Does the permit writer need to conduct a reasonable potential analysis for a discharge when the TMDL has assigned WLAs?

A reasonable potential analysis is used to determine whether or not pollutants “are or may be discharged at a level that will cause, have the reasonable potential to cause, or contribute to an excursion above any [s]tate water quality standard.” 40 CFR §122.44(d)(1)(i). During the development of the permit, if the permitting authority determines that the discharger has “reasonable potential,” then the permit must include a water quality-based effluent limit (WQBEL) for the discharge. 40 CFR 122.44(d)(1)(iii) – (vi). In cases where an EPA-approved TMDL includes a WLA for a point source, either specifically or within a categorical allocation, the permitting authority may make a reasonable potential determination based directly on the fact that an allocation has been established and assigned to the point source. .In this circumstance, it is unnecessary to conduct a separate quantitative reasonable potential analysis during the development of the NPDES permit for that point source, unless there is new information to suggest that the source’s levels of discharge of the pollutant of concern differs significantly from the information used to establish the TMDL WLA.

Do Clean Water Act (CWA) technology standards continue to apply where TMDL WLAs are established for pollutants of concern?

The NPDES regulations at 40 CFR 125.3(a) require NPDES permit writers to develop technology-based treatment requirements, consistent with CWA section 301(b), that represent the minimum level of control that must be imposed in a permit. Additional or more stringent effluent limitations and conditions, i.e., WQBELs, are imposed when technology-based effluent limits are not sufficient to meet applicable water quality standards. CWA section 301(b)(1)(C); 40 CFR 122.44(d)(1).

Where the WQBEL is more stringent than the technology-based effluent limit, the WQBEL will be the limit established in the permit for the pollutant of concern. In this situation, using the “most stringent” limit in the permit ensures compliance with all applicable technology and water quality standards. Although the more stringent water quality-based level of the pollutant is the enforceable limit in the permit, the technology-based limit remains the “floor.” As such, an explanation in the permit’s administrative record as to how the technology- and water quality-based effluent limits apply may assist the permittee’s and the public’s understanding of the interplay between these two different types of limits.

Under what circumstances is it appropriate to grant a compliance schedule in a permit, and how should the permit writer determine an appropriate length of time for the compliance schedule?

EPA's regulations at 40 CFR § 122.47 govern the use of compliance schedules in NPDES permits. Permittees must meet final effluent limitation(s) "as soon as possible." 40 CFR § 122.47(a)(1). EPA interpreted these requirements in a 2007 memo, “Compliance Schedules for Water Quality-Based Effluent Limitations in NPDES Permits (PDF) (3 pp, 56K, About PDF),” which clarified that compliance schedules may extend beyond the permit term so long as the permit includes the final effluent limitation and an enforceable sequence of actions leading to its achievement. See also 40 CFR § 122.47(a)(3).

In order to include a compliance schedule in an NPDES permit the use of a compliance schedule must be authorized in the state water quality standards. If the state water quality standard allows for a compliance schedule, then the permitting authority must make a reasonable finding, adequately supported by the administrative record and described in the fact sheet (40 CFR § 124.8), that a compliance schedule is “appropriate” and that compliance with the final WQBEL is required “as soon as possible.” See 40 CFR §§ 122.47(a), 122.47(a)(1). Factors relevant to whether a compliance schedule in a specific permit is “appropriate” under 40 CFR § 122.47(a) include: how much time the discharger has already had to meet the WQBEL(s) under prior permits; the extent to which the discharger has made good faith efforts to comply with the WQBELs and other requirements in its prior permit(s); whether there is any need for modifications to treatment facilities, operations or measures to meet the WQBELs and if so, how long would it take to implement the modifications to treatment, operations or other measures; or whether the discharger would be expected to use the same treatment facilities, operations or other measures to meet the WQBEL as it would have used to meet the WQBEL in its prior permit.

Where a TMDL has been established and there is an accompanying implementation plan that provides a schedule to implement the TMDL, the permitting authority should consider that schedule as it decides whether and how to establish enforceable interim requirements and interim compliance dates in the permit.

In cases where extended compliance schedules are not authorized (by state water quality standards, for example), it is often possible to issue a companion administrative order as an alternative to a compliance schedule in the NPDES permit. These orders can provide additional time for the facility to come into compliance with an NPDES requirement and specify interim actions needed to achieve eventual compliance. Where an extended time frame is necessary to achieve compliance, enforcement orders should provide schedules for CWA requirements that prioritize the most significant human health and environmental needs first.

How can TMDL writers identify point sources in a watershed when developing WLAs?

There are several ways to search for NPDES permit data online. The information below lays out how to get to and use some of EPA’s tools to find this type of information. Note that each of these tools search data from EPA’s national NPDES database, ICIS-NPDES.

IMPORTANT: Due to current reporting requirements, some facility information may not be available in ICIS-NPDES. For instance, HUC and other facility location information may be absent for some facilities. Additionally, some general permit-covered facilities may not be included in the database. To ensure that information from your search is complete, please contact the applicable state permitting authority.

Enforcement and Compliance History Online (ECHO)

  • From the ECHO homepage, click on “Explore Facilities”
  • Under “Evaluate Compliance” click on “Water”
  • The search page allows users to enter many different search criteria
  • To obtain a list and map of permits in a specific geographic area, use the “Geographic Location” section to limit your search by watershed or various other factors
  • You may also choose to limit your search in other ways in the additional sections. For example, under Facility Characteristics, you may want to exclude unpermitted facilities in the “Permit Type” drop down
  • After selecting all of the relevant criteria, click “search” on the right side of the page
  • On the search results page, you can choose to display additional data using the “Customize Columns” button just under the map at the top of the page


  • From the Envirofacts homepage, click on the “System Data Searches” tab
  • On this tab, in the “System Search” column click on “ICIS”
  • Use the search page to enter relevant search criteria, including locational information, and then click “Search” at the bottom of the page
  • In addition to this search feature, the user may also click on the “Widgets” tab at the top of the page, then click on “Water.” In the PCS/ICIS search box enter a geographic location, such as ZIP code or city and state, and click the search symbol. On the next page click “List and Map Facilities Reporting in This View”

Clean Water Act DMR Pollutant Loading Tool

  • From the Loading Tool homepage, click on the “Advanced Search” tab
  • Various search criteria may be entered, including several fields under the category “Receiving Watershed”
  • After entering your search criteria, click “Select Output Fields” at the bottom of the page
  • Select the fields you would like to include in the results and click “Search” at the bottom of the page to download a CSV file of the requested data


  • On the MyWATERS Mapper homepage, enter locational information in the box on the upper left of the screen to zoom to your area of interest
  • Expand the “Other EPA Water Data” section on the right side of the screen
  • Scroll down and check the box next to “NPDES Permitted Facilities” (Note – if this option is grayed out, you may need to zoom in further)
  • Small squares will appear on the map, which you can click to get information on the facilities in the area

How can permit writers identify applicable Total Maximum Daily Load wasteload allocations for the permittees covered by a permit?

To identify applicable Total Maximum Daily Load (TMDL) wasteload allocations (WLAs), follow this link to conduct a simple query of the EPA’s Assessment TMDL Tracking & Implementation System (ATTAINS) database.

ask waters tool screen shot

Click on “What TMDLs are associated with NPDES-permitted facilities?” This will do a search of all TMDLs that could include WLAs that apply to a given facility. The search can be done by a known permit number or alternatively by geographic location (defined by state, EPA Region or nationally).

ask waters search screen shot

The search will generate a table that includes the following information:

  • State
  • EPA Region
  • TMDL Name
  • Pollutant Name
  • Other Point Source ID
  • Wasteload Allocation (Note that there may be a WLA in the TMDL even if this field appears blank, please refer to the TMDL document)
  • Units
  • TMDL Status
  • TMDL Date
  • TMDL Documents (where available, this will provide a link to the TMDL document)

To save this table for future reference, click on “Download CSV” at the end of the page.

User Note: This query conducts a search by permit number of two fields in ATTAINS: (1) NPDES ID and (2) Other Point Source ID. When entering a TMDL into ATTAINS, if the permit number is not in the pick list for the NPDES ID field, you can enter it manually into Other Point Source ID and it would still be captured here. As with any data system, the output is only as good as what information is entered. States are encouraged to be as complete and accurate as possible when entering permit numbers associated with the TMDL.

How can permit writers identify whether or not there are any impaired waters that should be considered in their analysis?

The Discharge Mapping Tool is designed to help users determine the receiving waters to which a facility discharges, and whether the waterbodies are considered "impaired" under Section 303(d) of the Clean Water Act. The Discharge Mapping Tool uses the National Hydrologic Dataset Plus (NHDPlus) catchment data layer, and NHD waters indexed with Section 303(d) listing and TMDL information, to analyze whether a project is located with the catchment of an impaired water, and therefore discharges to that impaired water.

To use the Discharge Mapping Tool, follow these steps:

  • Enter the location of the site/facility. In the box below, enter the address or latitude/longitude of the site/facility and then click "Go."
  • Draw the site/facility or discharge point locations on the map. On the map that appears, zoom in to the site/facility location. Using the “Draw Fixed Polygon” or the “Draw Freehand Polygon” button at the top of the map, draw an outline of the site/facility. Or, if you know the site/facility's exact discharge point(s) (i.e., outfalls), draw the point(s) using the “Draw Point” button where the site/facility will discharge.
  • Click "Execute Query" at the bottom of the map to obtain discharge information. Once you click "Execute Query," the mapping tool will provide an output table that includes information about the surface waters to which the facility discharges, including surface waters that are impaired and the pollutants for which they are impaired, and surface waters that have a TMDL and the pollutants for which there is a TMDL.

User Note: If multiple points or polygons are selected, all of them will be processed upon clicking "Execute Query." To delete a point or polygon, select the point or polygon and then click "Delete Shape." Alternatively, to remove all points and polygons, click on "Clear All Points/Polygons". To save the map, click on "Save Map for Later." This will create a unique link that can be bookmarked or shared with others that contains all the points and polygons that were defined. Upon navigating to that link, the defined points and polygons will be automatically populated on the map.

NOTE: EPA anticipates that the information in FAQs will need to be updated to include new examples or modified information. EPA has an interest in ensuring the accuracy of the information contained in these FAQs, and therefore welcomes input on any aspect of these answers at any time; the Agency will update the FAQs as needed based on the comments received. EPA notes that the inclusion of any particular permit example should not be read as an Agency endorsement of the entire approach taken in that permit. Additionally, this document does not impose any new legally binding requirements on EPA, States, or the regulated community, and does not confer legal rights or impose legal obligations upon any member of the public.

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