Modeling procedures for the implementation of dissolved oxygen criteria to

protect the Fish and Wildlife Propagation beneficial use Additional technical guidance for modeling procedures may be found in the CPP. (1) Model types. Many types of water quality models for DO are available. Professional judgment is exercised to determine the appropriate modeling strategy for a particular situation. A simple model should only be used for small, non-complex systems with discharge flows under 1.0 MGD or as an initial screening tool for larger systems. A complex model requires a large amount of field data and should be calibrated and/or verified with observed conditions and then used to make predictive decisions. Complex models are appropriate for complex systems, multiple discharges, or large systems involving point source discharges of 1.0 MGD or more. (2) Target DO concentrations. Some of the DO criteria in the WQS are minimum values while others allow for a 1.0 mg/L excursion from the criterion for up to eight hours during any 24-hour period. Dynamic models can predict these time-dependent conditions and the WQS criteria may be used directly. Since steady state water quality models cannot represent such a time-based excursion, appropriate average DO target values are used to protect the minimum DO criteria. See OAC 252:730 Appendix G for target DO concentrations. A target average DO concentration of 2.0 mg/L will be utilized to prevent nuisance conditions. 31 (3) Margin of safety. To compensate for uncertainty, a margin of safety is required for all models. The CPP provides guidance for recommended margins of safety for various types of models. As model complexity and use of actual data increase, the recommended margin of safety decreases. The margin of safety is determined by comparing the unallocated load to the maximum assimilative capacity of the system as predicted by the water quality model.