Development of a Linked Watershed and Receiving Water Modeling System of Los Angeles and Long Beach Harbors for TMDL Development

This paper describes the development and application of a modeling system to support regulatory water quality decision making in Los Angeles and Long Beach Harbors. Large urban harbor systems, such as Los Angles and Long Beach, pose significant challenges with respect to mitigating water quality impairment required under the U. S. Environmental Protection Agency's (EPA's) Total Maximum Daily Load (TMDL) and Superfund programs. Water quality impairment in Harbor systems typically includes failure to meet both traditional nutrient and bacteria water quality standards and standards associated with sediment and water and sediment contamination associated with hydrophobic organic compounds and heavy metals. Linked or coupled hydrodynamic and water quality models provide powerful tools for evaluating the transport and fate of contaminants in surface water systems as well as identifying contaminant sources and evaluation of source control alternatives leading to establishment of TMDLs and selection of Superfund site remediation alternatives. Modeling of harbors has traditionally focused on hydrodynamic and sediment transport processes associated with navigation. The study described herein extends this traditional modeling framework to address the transport and fate of dissolved and sediment sorbed contaminants in a regulatory framework. The selection and site specific application of a modeling system for use in regulatory decision making not only requires consideration of technical aspects of the model's capabilities but also establishment of consensus as to the appropriateness of the model and application plan among regulatory agencies and major stakeholders. Selection and application of the approach for this study included input from the EPA, California Regional Water Quality Board, Port of Los Angeles, Port of Long Beach, City of Los Angeles, Los Angeles County, City of Long Beach, and various cities with jurisdictions in the watersheds of the harbor complex. Because the region to be modeled includes both the interconnected harbors, a portion of adjacent San Pedro Bay and three bay tributaries (Dominguez Channel, Los Angeles River, and San Gabriel River), a multidimensional modeling system applicable for range of hydrodynamic environments was selected. The Environmental Fluid Dynamics Code (EFDC) model was selected based on its proven ability to simulate the complex hydrodynamic and sediment transport processes responsible for moving organic pollutants and sediment adsorbed metals environments ranging from the coastal ocean to rivers. The public domain status of the model and its support by EPA further strengthened consensus that the model would be available to stakeholders. This paper describes the current status of the development of the EFDC Harbor model and the linkage to the US EPA's Loading Simulation Program C++ (LSPC) models of the harbors major watersheds. Results of model calibration for hydrodynamics, sediment transport and fate and transport of sorptive contaminants will also be presented, as well as examples of model use to identify contamination source distribution uncertainty and control strategies necessary for TMDL implementation. Additional modeling results and conclusions will be presented at the WEF conference in June 2007.