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 from human-caused sources 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 point, nonpoint, and natural background sources. The load assigned to point sources is called a wasteload allocation (WLA); the load assigned to nonpoint sources is called a load allocation (LA). The budget is balanced at the point where water quality standards are just being met and is 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 pollution 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:

Load capacity (LC) = margin of safety (MOS) + natural background (NB) + allocations to pollutant sources (WLA and LA) = 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. First, a margin of safety is determined and subtracted; then natural background, if relevant, is quantified and subtracted; and then the remainder is allocated among pollutant sources. When the breakdown and allocation are completed, 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, the EPA is likely to lead TMDL efforts with considerable help from the state, the tribes, and other agencies. The EPA has the responsibility to approve or disapprove all TMDLs. If the EPA formally di sapproves a state TMDL, it is obligated under the Clean Water Act to issue a new TMDL within 30 days.

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 pollution 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.

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. Temperature, while not a substance, is considered a pollutant, as changes in water temperature are quantifiable.

A TMDL is only required to address pollution that is caused by pollutants; therefore, if a water body is impaired because of pollution, but not a pollutant, that water body will not receive a TMDL for that pollution. If a water body is impaired by pollution and a pollutant (e.g., habitat alteration and bacteria), it will still receive a TMDL for the pollutant (in this case, bacteria).

In general, watersheds are subject to two broad categories of pollution sources: point and nonpoint. Point source pollution is 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 pollution can often be measured at an outfall or pipe.

Nonpoint source pollution, on the other hand, is more difficult to identify. It includes, among other things, the cumulative effects of fertilizers and pesticides that farmers and homeowners may use; the oil that is carelessly poured down storm drains; and various land use practices including urban development, agriculture, and forestry. Due to its rural nature, most pollution in Idaho is nonpoint source pollution.

The specific pollutants of concern vary from watershed to watershed. 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.

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 this 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 m eet 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.

Once a water body is listed as impaired in the Integrated Report, it is placed on the TMDL schedule. Each TMDL must be submitted to the EPA by December 31 the year it is due. 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 allocations.
• DEQ presents the proposed load 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 the EPA for approval.
• Concurrent with submittal to the EPA, DEQ publishes notice in the Idaho Administrative Bulletin that the TMDL has been submitted to the EPA. Those affected by the TMDL may file an appeal within 35 days of publication in the bulletin.
• The EPA approves or disapproves the TMDL; if the 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. Watershed advisory groups 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.

 BAGs

 WAGs

The BAGs recommend people for the DEQ director to appoint to WAGs. As the name implies, WAGs provide guidance on specific watersheds, whereas BAGs provide guidance on much larger basins. Watershed advisory groups provide local public input and guidance to DEQ when developing a TMDL. The WAG does not typically write the TMDL document, but is an integral part of the process and its input is given great weight in TMDL development and implementation. The WAG is also very involved in writing and implementing the implementation plan that follows the TMDL.

The WAG provides an opportunity for concerned and involved citizens to see the TMDL process through from start to finish. Members of each WAG represent the industries and interests affected by the management of that watershed. The WAG also includes representatives of local government and the land management or regulatory agencies interested in the management of water quality in the watershed. The WAG also provides the leadership to implement the TMDL and has the potential to shape the final outcome of a TMDL in ways that go beyond the public comment process. Citizens not involved in the WAG can get involved in the TMDL development process; however, this involvement tends to be limited to formal public comment periods and public hearings. Learn more about serving on a WAG here.

 TAGs