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NAVIGATING THE TMDL PROCESS: ESTIMATING BACKGROUND POLLUTANT LOADS

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The TMDL program has likely generated as much controversy as any program in EPA's history. To help water quality professionals navigate the TMDL process, the Water Environment Research Foundation funded a project to identify and address some of the technical issues associated with conducting TMDL studies. The project is a collaborative effort among four firms, each focusing on specific issues or elements of the TMDL process which that firm is uniquely qualified to address.

The objective of this ongoing research element is to develop a working definition of background loads and to recommend appropriate methods to estimate the loads for different types of pollutants. Pollutant loads to receiving systems are contributed from three sources: Point sources, nonpoint sources, and background sources. Background sources, in most instances, are not anthropogenic in origin. Background sources can not be controlled and represent sustained pollutant loadings to the system. Unrealistic nonpoint source loading reductions and load allocations can result if background sources are not estimated accurately. Alternatively, failure to adequately account for background concentrations can impact the probability that the TMDL will result in attainment of standards.

A review of State Programs revealed that background pollutants were not adequately considered in many TMDLs. Over 30% of the TMDLs reviewed did not include background sources in loading estimates even though the TMDL was addressing pathogen, nutrient or sediment pollutants with non-anthropogenic sources.

States indicated that there was not a workable definition of background sources in the EPA rules. EPA's definitions include “levels representing the chemical, physical, and biological conditions that would result from natural geomorphological processes such as weathering or dissolution”. No consideration to uncontrollable anthropogenic sources is provided in this definition. Background sources can be categorized into external sources such as atmospheric deposition, surface runoff, groundwater inputs, and internal sources such as bottom sediments.

Based on this research, a revised definition of background sources is proposed. Methods to estimate background sources are also reviewed and recommended. These include regional reference waterbodies relatively undisturbed by human activities, modeling studies to separate background from nonpoint contributions, ecoregion estimates, and literature values. Guidance on the use of these methods will be provided in this presentation.
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Document Type: Research Article

Publication date: 2002-01-01

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