Boundary Schematization in Regulatory Mixing Zone Analysis

Water quality regulations often permit a "mixing zone" or "allocated impact zone" in which the initial dilution of a wastewater occurs rather imposing strict end-of-pipe criteria. Since wastewater treatment typically entails high marginal costs for removal of the last few percentages of contaminants from wastewater, the mixing zone concept allows for economically efficient use of the natural assimilative capacity of a water body. In practice, the regulatory mixing zone can encompass either the near field where discharge characteristics control mixing behavior or the far field where ambient conditions predominate the mixing process. Plume boundary interaction controls the transition between near-field and far-field mixing processes and strongly influences mixing behavior. Simulation of plume boundary interaction is accounted for in the schematization processes, a technique whereby ambient bathymetric geometry data is simplified while preserving the consequences of local boundaries on mixing behavior. Definitions of near-field and far-field mixing are given, followed by the current regulatory context of mixing zone analysis. Several examples of the schematization process are presented. The paper concludes with a case study example of the Cornell Mixing Zone Expert System (CORMIX) application in regulatory mixing zone analysis. CORMIX is a comprehensive rule-based decision support system and hydrodynamic simulation model developed to assist water quality managers with mixing zone analysis. The case study example highlights schematization and plume boundary interaction, shows graphical representations of the plume within the mixing zone, and illustrates the importance of ambient density stratification and density-current buoyant spreading in mixing zone regulatory analysis.