Defining the Critical Condition in the TMDL Development Process: Continuous, Statistical or Event-Based Approach

One of the key challenges in the TMDL development process is how to define the critical condition for a receiving waterbody impacted by both point and nonpoint sources. Knowledge of the critical condition could help identify the feasible allocation scenarios needed to be taken to meet water quality standards. However, EPA guidelines thus far do not specify how to define the so-called design or critical conditions. When point sources and nonpoint sources are considered an “integrated” manner, the worst-case scenario is not obviously known because nonpoint sources are mostly storm-driven. Therefore, the objective of this paper is to illustrate the strength and weakness of available methods in defining critical condition during the TMDL development process and highlights the applicability of a practical event-based approach.The main concern of using continuous simulation approach is that there is no guarantee that the most critical condition will be captured during the selected representative hydrologic period. In addition, the risk (or reliability) associated with continuous simulation cannot be estimated. Statistically-based load duration curve method involves developing a TMDL for a period that encompasses all possible critical conditions, for example, covering all flow conditions. However, it is difficult to derive explicit percentage reduction of source categories in the TMDL allocation. In contrast, an eventbased Critical Flow-Storm (CFS) approach explicitly addresses the critical condition as a combination of stream flow and magnitude of the storm event, both having certain frequency of occurrence. One of the major advantages is its ability to assess the risk associated with a certain load reduction scenario and examine impacts of allocation options during TMDL development, which makes nonpoint source control more consistent with the traditional approach to point source management. Key factors influencing critical condition and TMDL allocation were discussed, for example, stream flow, precipitation and antecedent condition. In summary, rigorous analysis for addressing critical conditions is technically challenging, time-consuming, and typically costly. However, the experience, case studies, and technical guidance for evaluating the critical conditions related to nonpoint source pollution will continue to evolve.