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Probabilistic Analysis of Acute Mixing Zone Limits
Christopher C. Obropta, Sajan Thomas, Richard I. Hires, and Robert Miskewitz

The most common method for defining acute mixing zones is via spatial criteria. The concentrations at the edges of these regions are used to determine dilution factors for effluent discharges. These dilution factors tend to be overly conservative due to the fact that they are based on a set of conditions that are unlikely to occur simultaneously, and the mixing zone is very small. An alternative method is the time of exposure method; it involves the calculation of the average effluent concentration experienced by a floating organism over a specified time interval. Although these dilution factors are typically higher than those calculated for the spatially defined acute mixing zone, they are still overly conservative because they are calculated using a combination of unrealistic critical conditions. A more realistic method consists of using a probabilistic approach with real observations of ocean currents, effluent flow rates, stratification, and water depth to define the dilution factor for a temporally defined acute mixing zone. This method was used to predict the tenth and fifth percentile lowest dilutions for three ocean outfalls over a 55-day period. Based upon the probabilistic analyses for the tenth percentile, the acute dilution factors for the Avalon, Ocean City, and Wildwood outfalls are 336.9, 344, and 259, respectively. These represent 46%, 82%, and 32% increases over the worst-case scenario for the temporally defined mixing zone, and are greater than the spatially defined mixing zones by factors of 15.2, 13.5, and 12.6.

Keywords: acute mixing zone, Criterion Maximum Concentration (CMC), spatial criteria, acute dilution factors, time of exposure, probabilistic approach.

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