Total Maximum Daily Load Analysis and Modeling: Assessment and Advancement

The special collection on Total Maximum Daily Load Analysis and Modeling: Assessment and Advancement is available in the ASCE Library (https://ascelibrary.org/jhyeff/maximum_load_analysis_modeling).

In a regulatory context, total maximum daily load (TMDL) is used in the United States to describe a document or plan of action to manage both point and nonpoint source (PS and NPS) pollution of water bodies. In this sense, a TMDL is a set of actions to restore clean water by examining water quality problems, identifying sources of pollutants, and specifying actions that create solutions. It is a written, quantitative plan and analysis for attaining and maintaining water quality standards in all seasons for a specific water body and pollutant. TMDLs are intended to be part of a comprehensive watershed strategy to restore ambient water quality to meet its designated use (DU) and should involve collaboration with a wide variety of stakeholders.

The US Congress enacted the Water Pollution Control Act in 1948 to “enhance the quality and value of our water resources and to establish a national policy for the prevention, control, and abatement of water pollution.” This act was amended in 1972 and became commonly known as the Clean Water Act (CWA) [33 U.S.C. §1251 et seq.]. The CWA is the primary legislative driver that requires the responsible parties—states, territories, and tribes—to develop plans and remediate or maintain the nation’s water bodies. There have been several amendments to the CWA since its passage, for example, in 1987 the CWA was amended to establish stormwater permitting and nonpoint source pollution control. To translate the broad goals of the CWA into water-body-specific objectives, water quality standards (WQSs) are developed by states, territories, and tribes. WQSs involve three major components: DUs, water quality criteria (WQC), and antidegradation policies. The DUs of a water body are those uses that the water body should be able to satisfy at all times. WQC are numeric and narrative descriptions of the conditions of a water body necessary to support the DUs. Antidegradation policies establish a set of rules that should be followed when addressing proposed activities that could improve the quality of surface waters. Although WQSs are not directly enforceable, state regulatory agencies are required to place water bodies on the state’s CWA Sec. 303(d) list if the WQSs are not met and develop TMDL(s) for each pollutant exceeding WQCs.

Quantitatively, the term $TMDL$ is used to refer to the maximum amount of a pollutant a water body can receive and still meet the applicable water quality standards. This maximum allowable pollutant load is defined aswhere LC = load capacity or waste assimilative capacity of the water body; $\sum \mathrm{WLA}$ = sum of all waste load allocations from the PSs discharged into an impaired or threatened water body; $\sum \mathrm{LA}$ = sum of all load allocations from the NPSs (diffused sources) entering the water body; and MOS = margin of safety. Point sources are well understood as a cause of impairment. Pollutant load from a point source is relatively easy to calculate or estimate; but not from nonpoint sources. Estimation of pollutant loads entering water bodies from nonpoint sources involves the application of hydrologic models with pollutant transport modeling capabilities. Margin of safety (MOS) is a part of TMDL attributed to uncertainties.

The minimum elements in a TMDL required by the USEPA (n.d.) are

As is the case with most natural systems, there are no experimental or direct methods to derive TMDL estimates. Developing TMDLs is a complex process in which watershed and receiving water quality modeling is vital. As found in USEPA’s Assessment, Total Maximum Daily Load Tracking and Implementation System (ATTAINS), 76,127 TMDLs have been developed throughout the United States during the period 1975–2017 using various analytical procedures, simple models, watershed models, and receiving water quality models chosen by the individual TMDL developers because there exists no clear guidance regarding analysis procedures or models to be used. There are widespread concerns over the current practices of TMDL analysis and modeling in terms of analysis technique and model selection, determining data requirements and processing, calibration, validation, and uncertainty analysis and reporting.

The TMDL Analysis and Modeling Task Committee of the ASCE Environmental and Water Resources Institute (EWRI) was formed to address the preceding concerns and to develop a manual of practice (MOP) on TMDL analysis and modeling. The task committee conducted extensive reviews on various TMDL analysis and modeling topics and documented its findings in a book, Total Maximum Daily Load Analysis and Modeling: Assessment of the Practice (ASCE-EWRI TMDL Analysis and Modeling Task Committee 2017).

Papers in this special collection are state-of-the-art and state-of-the-practice reviews of various TMDL topics. These peer-reviewed papers are expected to provide a sound basis for the development of the MOP. The collection consists of 15 papers as listed in the following. The first 10 papers, listed in a logical sequence pertaining to the collection theme, are contributed by members of the task committee, and the last five papers are from external contributors on topics of significance to the theme.

The guest editors recognize this collection is only a small subset of the topics in the field of TMDL analysis and modeling, assessment, and advancement. Nevertheless, the limited number of papers in this collection presents a review of the current state of the practice and state of the art in TMDL analysis and modeling. The guest editors wish to thank all the authors and reviewers of this special collection of papers for their contributions. It is quite clear from the papers that more guidance and research is needed for TMDL analysis and modeling, particularly in the areas of uncertainty analysis, model integration for complex systems, watershed or receiving water body model selection, and integrating GIS and remote sensing within the TMDL analysis and modeling frameworks.