DEFINING THE CRITICAL CONDITION FOR AMBIENT WATER QUALITY-BASED LOADING CAPACITY DETERMINATION

摘要

Ambient water quality-based loading capacity determination, Total Maximum Daily Load (TMDL) development, is critical to the attainment of water quality goals in watershed management. USEPA regulations require that in developing TMDLs, one must take into account the critical conditions for stream flow, loading, and water quality parameters. The intent of this requirement is to ensure that the water quality of the receiving water body is protected during times when it is most vulnerable. Knowledge of the critical condition may help identify the actions 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. Under the "point source only" situation, this "worst-case" scenario is traditionally defined as a low flow (e.g., 7Q10), combined with a high water temperature. 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 objectives of this paper are to clearly address the critical condition issue and to compare continuous and event-based approaches in defining critical condition during the TMDL development for a waterbody impacted by both point and nonpoint source pollution. An analytical conceptual model is derived, which utilizes the event-based method to integrate both point and nonpoint source loads and link their impacts to the pollutant concentration of a receiving water body. The conceptual model includes the most significant physical and chemical processes during wet weather condition and a combination of empirical and physically based algorithms. By analyzing the representative synthetic storms, the derived model provides the insight for defining the critical flow and storm conditions during wet weather flow in the TMDL development. In addition, a preliminary analysis on USGS sample file using HSPF is included as example. In summary, the CFS approach, which is based on CFS concept, is scientifically sound and explicitly addresses the critical condition as a combination of stream flow and magnitude of the storm event. It could be considered as an alternative method in the ambient water quality-based TMDL development.