Restoring impaired waters one

Although the acronym TMDL refers to the “total maximum daily load” of a pollutant, in Clean Water Act parlance, the expression has come to represent both a pollutant cap and a restoration or management plan.

The cap is a calculation of the maximum amount of a pollutant that a water body can receive and still safely meet water quality standards. The cap is focused on one pollutant (such as bacteria or sediment) usually in one particular part of a water body, though sometimes it addresses an entire river or lake.

The plan describes how the cumulative contribution from every source of the pollutant (plus an allowance for a margin of safety) must be reduced to a level that is less than the pollutant cap. This reduction is based on restoring or maintaining the water body so it is safe for people and wildlife. The plan may address more than one pollutant, and frequently it covers more than one impaired or threatened reach or segment of the water body. More TMDLs are being developed on a watershed basis.

TMDLs are required to address impairment and threats identified on the 303(d) list. Therefore, the plans are needed both to restore and protect our water bodies. For the purposes of explanation, however, most of the language in this chapter focuses on restoration.

TMDL components

A TMDL is made up of point sources, known as the Waste Load Allocation (WLA), nonpoint sources, known as the Load Allocation (LA) and a margin of safety (MOS) to account for the uncertainty in predicting how much pollution reduction will result in meeting water quality standards. In allocating pollutant loads, it is required that background conditions (before human impacts) and seasonal variation of the pollutant levels are taken into account (40CFR130.7(c)(1)). It is recommended by the EPA that TMDLs specifically account for future growth as well.

Triggering TMDL development

According to the Clean Water Act, each state must develop TMDLs for all the waters on the 303(d) list. However, the EPA has developed a guidance document that recommends placing all waters in one or more of five categories when developing the 303(d) list, with only one category requiring TMDLs (Guidance for 2006 Integrated Report). Insist that your state follow the CWA regulations for listing.

Timelines for the completion of TMDLs vary across the country. Most states are under court orders and consent decrees for TMDL completion; required timelines range from five to fifteen years. In reality, if states list waters because they are threatened rather than impaired, they are likely to be assigned the lowest priority. TMDLs may not be completed for many years, if ever.

The steps of the TMDL process

When the TMDL development begins, the pollutant cap must be set based on what the water body can handle and still meet water quality standards, not what is already there. The sources of the pollutant(s) are identified, and they are each allocated a portion of the pollutant cap that usually requires a reduction in their pollution discharge in order to help solve the problem.

Some argue that the Act doesn’t require anything beyond the identification of the cap and assignment of the responsibilities to meet the cap. Without full implementation of the TMDL, however, it cannot be an effective tool to address problems in our threatened and impaired waters.

Setting the TMDL goal

The whole point of this exercise is to end up with a plan to meet water quality standards (uses, water quality criteria and the antidegradation policy) and restore or protect each water body.

Every water body has a limit of how much pollution it can take in (assimilate) and still support all its legally protected existing and designated uses. At some point, for each pollutant, the amount coming into a water body will become too much, and aquatic life will be affected, recreation will be compromised, or the water supply will be jeopardized.

But what is that point? Agencies depend on existing research, data collected, models and/or “best professional judgment” to determine the allowable contribution or load for each pollutant in each segment of a water body. These allowable loads must be protective of all uses of the water body during the most critical conditions, such as low flow. To determine allowable loads, “site-specific information should be used whenever possible” (40CFR130.7(c)(1)(i)). Documentation of the calculation of allowable pollutant amounts should be available to the public. Local technical experts (e.g., university researchers or consultants) may be willing to help review these calculations.