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0 <br /> INFORMATION SHEET ORDER NO. 97-115 (revised) -7- <br /> CITY OF MANTECA AND CITY OF LATHROP <br /> SAN JOAQUIN COUNTY <br /> • The cumulative cross sectional area of the 12 plumes is 39 square feet, and would cover a maximum <br /> of 1.74% of the river's cross sectional area. <br /> • At the location of maximum plume diameter, approximately 98% of the river's cross section would <br /> have ammonia concentrations below acute toxicity levels. <br /> • An aquatic organism floating with the river current would take approximately 42 seconds to pass <br /> through the maximum length of the acute toxicity zone, assuming the river velocity of 0.69 fps. <br /> Chronic Toxicity Mixing Zone <br /> Chronic toxicity calculations used a river pH of 8.0, and a background river ammonia concentration of <br /> 0.07 mg/l. Plant effluent discharge pH was 7.3, and effluent ammonia concentration was 30 mg/l. <br /> The chronic mixing zone was predicted to be: <br /> • Cigar-shaped plumes emanating from each diffuser port, extending a maximum of 40 feet down <br /> river. The plumes gradually widen as the effluent is mixed with river water to a maximum of 2.37 <br /> feet in diameter. The ammonia concentration decreases along the length of the plume. (Attachment <br /> G) <br /> • The cumulative cross sectional area of the 12 plumes is 53 square feet, and would cover a maximum <br /> of 2.5% of the river's cross sectional area. <br /> • At the location of maximum plume diameter, approximately 97% of the river's cross section would <br /> have ammonia concentrations below chronic toxicity levels. <br /> • An aquatic organism floating with the river current would take approximately 44 seconds to pass <br /> through the maximum length of the acute toxicity zone, assuming the river velocity of 0.69 fps. <br /> MIXING ZONE ANALYSIS CONCLUSIONS <br /> The Discharger has concluded that construction of an effluent discharge diffuser and effluent storage <br /> basins will minimize the acute and chronic ammonia mixing zones during worst case flow periods. <br /> The longest mixing zone is predicted to occur during the summer months at low river flow periods (641 <br /> cfs). For these conditions,the acute toxicity plumes are predicted to be 40 feet long. The cumulative <br /> cross sectional area of the plumes at the widest point would cover 1.4% of the river channel. The <br /> chronic toxicity plumes are predicted to be 51 feet long. The cumulative cross sectional area of the <br /> plumes at the widest point would cover 3.16% of the river channel. <br />