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I I I I I I I I I I I I I I I I I I I f'\r-~!, 1 I") CONCLUSIONS Water quality modeling conducted as part of this study effort verified what the District already knows from its on-going monitoring efforts: -the DO standard in Segment IS of the river is IIOt being met with the District's North Complex nitrification/denitrification facilities operating. -If the District were .1lso to construct and operate nitrification/denitrification facilities at the South Complex to achieve a 2 mglL ammonia (NHl-N) concentration in the plant effluent, DO standards throughout Segment IS would still not be met. This is the case for both current (138 mgd) and ultimate (185 mgd) plant discharge conditions. The model also determined that the District can comply with in-stream chronic and acute ammonia water quality standards with existing plant facilities for both current and ultimate flow conditions. There are limitations to the Segment IS Water Quality Model that may impact the reliability of its use in selecting one alternative over another at this time. Of particular concern is the lack of knowledge regarding the role of sediments in oxygen consumption and the nitrogen transformations within the sediments. Hydrologic and water quality data pertaining to seepagetlow in Segment IS and flow contributions from Sand Creek and Big Dry Creek need to be collected to determine both flow and water quality contributions to Segment IS. Further, additional 30-hour data sets are needed to complete the model validation process. During the final assessment of the eight preferred alternatives, it became apparent that none of these alternatives should be recommended for development into a District Effluent Management Plan for 'egment IS. Each alternative contained techniques in various combinations that may be beneficial for the District to consider in further detail. These beneficial techniques include: stream channel .modifications, diversion of effluent to irrigation ditches, artificial reaeration, and filtration of plant effluent. Further conceptual analysis and design should be conducted to investigate the technical details and to revise the costs of these techniques. As this process evolves, the DiStrict can define further potential implementation and institutional issues through ongoing discussions with regulatory . agencies, ditch companies, and other affected parties. The practical combination of these techniques into a viable Effluent Management Plan could save the District $40 to $70 million in capital costs and $1.0 to $3.5 million in annual operating costs over conventional in-plant nitrification improvements. D-8