Protecting Public Health and the Environment.

Frequently Asked Questions about Subbasin Assessments and TMDLs

What is a Pollutant?

A pollutant is a substance, such as bacteria or sediment, that is identifiable and in some way quantifiable. Some unnatural conditions that impair water quality—such as flow alteration, human-caused lack of flow, and habitat alteration—are considered pollution but are not caused by quantifiable pollutants (and therefore do not require a TMDL). Temperature, while not a substance, is considered a pollutant, as changes in water temperature are quantifiable.

The specific pollutants of concern vary from subbasin to subbasin. In much of rural Idaho, water quality concerns center around excess sedimentation, elevated stream temperatures, and nutrient overloading. In developed areas, concerns often focus on bacteria, oil and grease, and dissolved oxygen. In industrial or mining areas, heavy metals are often at the top of the list.

What Exactly is a TMDL?

Simply put, a TMDL is a pollutant budget. A TMDL is a calculation of the maximum amount of a pollutant that a water body can receive and still meet water quality standards. This budget is expressed in terms of loads: the amounts of pollutants added to a water body during a given time or per a volume of water. For example, a load allocation to a water body might be 5 kilograms of phosphorus per day from a given source.

A TMDL budget takes into account loads from the following sources:

  • Point: A point source is a single, identifiable, localized source of pollutants. Point sources are typically associated with industrial discharges, municipal waste treatment facilities, and confined animal feeding operations. The effects can be directly traced to a particular source or facility (a "point"). Point source pollutants can often be measured at an outfall or pipe.
  • Nonpoint: A nonpoint source is a diffuse source of pollution. It includes, among other things, the cumulative effects of fertilizers and pesticides that farmers and homeowners may use; oil that is carelessly poured down storm drains; and various land use practices including urban development, agriculture, and forestry. Due to the state’s rural nature, most pollution in Idaho is nonpoint source.
  • Natural Background: Natural background conditions exist when there is no measurable difference between the quality of water now and the quality of water that would exist with no human-caused changes in the watershed. Some pollutants are naturally occurring and must be factored in to the total pollutant budget.

The load assigned to point sources is called a wasteload allocation; the load assigned to nonpoint sources is called a load allocation. The budget is balanced at the point where water quality standards are just being met and loads are allocated among all the various sources. Like keeping money in the bank for a rainy day, some of the budget is set aside as a margin of safety. And like cash flow in a business, the pollutant budget must take into account the seasonal or cyclic nature of pollutant loads and the receiving water's capacity so a temporary exceedance does not occur.

The TMDL budget can be summarized as follows:

Load capacity = margin of safety + natural background + wasteload allocation + load allocation = TMDL

The equation is written in this order because it represents the logical order in which a TMDL is developed. First, the load capacity is determined. The load capacity is the quantity of a pollutant a water body can receive over a given period without causing violations of state water quality standards. Then the load capacity is broken down into its components. After the necessary margin of safety and natural background, if relevant, are quantified, the remainder is allocated among pollutant sources (i.e., the wasteload allocation and load allocation). When the breakdown and allocation are complete, the result is a TMDL, which must equal the load capacity.

In addition to being a pollutant load, "TMDL" also refers to the written, quantitative assessment of water quality problems and contributing pollutant sources. DEQ has the authority and the responsibility to ensure that TMDLs are completed and submitted for EPA approval. On tribal lands, EPA is likely to lead TMDL efforts with considerable help from the state, tribes, and other agencies. EPA has the responsibility to approve or disapprove all TMDLs. If EPA formally disapproves a state TMDL, it is obligated under the Clean Water Act to issue a new TMDL within 30 days.

What is a Subbasin Assessment?

A subbasin assessment is the first step in developing a TMDL or recommending removing a water body from the list of impaired waters in the Integrated Report. Conducting the assessment entails analyzing and integrating multiple types of water body data, such as biological, physical, chemical, and landscape data. A subbasin assessment describes the affected area, the water quality concerns, the status of beneficial uses of individual water bodies, the nature and location of pollutant sources, and past and ongoing pollution control activities. Its main purpose is to determine the causes and extent of the impairment when water bodies are not attaining water quality standards.

While a subbasin assessment is not required by the Clean Water Act, DEQ completes the assessment to ensure impairment listings are up to date and accurate. The subbasin assessment is usually part of the TMDL document but may be prepared separately.

How Are Pollutants Measured?

DEQ collects data on individual water bodies in Idaho through the Beneficial Use Reconnaissance Program (BURP) and solicits data from outside sources such as other government agencies. These data are compared to Idaho's water quality standards to determine if a particular water body is meeting standards and supporting beneficial uses, and the findings are documented in the Integrated Report. While DEQ frequently is able to determine what pollutants (if any) are impairing water quality from these data, this determination is not required at this stage. A water body may be listed in DEQ's Integrated Report as not supporting its beneficial uses or meeting standards but be listed with the pollutant as "unknown."

In the subbasin assessment phase of writing a TMDL, more data about the water body are collected. At this stage, DEQ determines what pollutants are causing the impairments and the sources of those pollutants. Because a TMDL is a total maximum daily load, pollutants are generally measured and allocated as loads (a concentration [mass/volume] or the amount of a pollutant measured over time [mass/time]).

However, it is not always feasible to measure pollutant loads directly or to compare a pollutant load to beneficial use support. In those situations, surrogate measures are used. A surrogate is simply one measurement substituted for another. Surrogates are typically used because the substitute is more easily measured or used than the attribute of real interest. A surrogate is often a more practical gauge of progress in reaching water quality objectives than a load by itself.

An example of a common surrogate used in TMDLs is assigning a target of a certain percent shade to a stream segment instead of assigning an actual temperature "load." A temperature load (such as kilocalories/mile/day) is too abstract to provide a useful or easily understandable allocation. However, a surrogate allocation of a certain percent shade cover over a stream is much easier to measure and understand. The appropriate percent shade cover would provide the conditions necessary to meet water quality standards (in this case, a maximum temperature), just as a temperature load would.

While surrogates can be used as targets, they cannot, by themselves, be used to determine if a stream is in compliance with water quality standards or is meeting beneficial uses. For example, if a stream is assigned a surrogate target of 50% shade and it reaches that target, that fact alone cannot be used to say that the stream now meets its beneficial uses or water quality standards. Stream temperatures must still be taken to compare to standards, and biological data must be gathered to determine beneficial use support.

How Does the TMDL Process Work?

Once a water body is listed as impaired in the Integrated Report, it is given a ranking of high, medium, or low priority for TMDL development based on a suite of factors including staffing resources, pollutant type, severity of impairment, pollutant source, and permitting requirements. The priority list is dynamic, and DEQ makes determinations as to the ultimate scheduling of the TMDL for development. It takes approximately two years to write a TMDL; however, this process can take longer if the subbasin is highly complex. The following are the general steps involved in writing a TMDL:

  • DEQ prepares a draft subbasin assessment with input and advice from a watershed advisory group (WAG).
  • DEQ presents the draft subbasin assessment to the WAG, basin advisory group (BAG), or both.
  • DEQ revises the draft subbasin assessment and determines water quality targets.
  • DEQ develops draft total maximum daily load.
  • DEQ presents the proposed load and wasteload allocations to the WAG, BAG, or both.
  • DEQ incorporates WAG and BAG comments and prepares the TMDL for formal public comment.
  • A formal 30-day public comment period occurs. Click here for current public comment opportunities.
  • DEQ responds to public comments.
  • DEQ submits a final TMDL to EPA for approval.
  • Concurrent with submittal to EPA, DEQ publishes notice in the Idaho Administrative Bulletin that the TMDL has been submitted to EPA. Those affected by the TMDL may file an appeal within 35 days of publication in the bulletin.
  • EPA approves or disapproves the TMDL; if EPA disapproves a TMDL, it has 30 days to issue a new one.

When a TMDL is completed, the next task is to implement its recommendations and meet its goals. An implementation plan, guided by an approved TMDL, is written and provides details of the actions needed to achieve load reductions and a schedule of those actions. It also specifies the monitoring needed to document action and progress toward meeting water quality standards.

State and federal government agencies can often assist in this process by providing technical assistance and grants, but it is the residents, businesses, and landowners within the watershed who determine success. WAGs are in a key position to lead the implementation plan. Of course, it is not the plan but the results of that plan that will return waters to a healthy state that will support swimming, fishing, and other uses. Local people need to be involved in solving the problems; they are also the main beneficiaries of the end results.

I'm a Landowner. How Does the TMDL Process Affect Me?

If you own or manage land and have runoff from your land that enters a stream, river, lake, or agricultural drain, your land management activities will likely be subject to the TMDL and its implementation plan. One thing you can do in anticipation of the TMDL and implementation plan is to develop a OnePlan or a Natural Resources Conservation Service conservation plan. Such a plan will include recommended practices for minimizing water quality impacts. These practices, commonly referred to as best management practices (BMPs), are based on the specific conditions in your area.

Staff Contacts

Water Quality Coordinator
Graham Freeman
DEQ State Office
Water Quality Division
1410 N. Hilton
Boise, ID 83706
(208) 373-0461

Related Pages

Water Quality Standards

Beneficial Use Reconnaissance Program

Integrated Report

TMDL Implementation Plans

Basin Advisory Groups

Watershed Advisory Groups