SITE INFORMATION AND CORRESPONDENCE_CASE 2

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ntUJUNAL vvv,i tK Liu, HU Kt6l UNSE (SWRCE/OCC File A-1846(a) and A-1846(b)) -19- PETITIONS FOR REVIEW OF V TE DISCHARGE REQUIREMENTS ORDER NO. R5-2007-0036 (NPL~'5 NO. CA0079154) AND TIME SCHEDULE ORDER NO. R5-2007-0037 CITY OF TRACY, WASTEWATER TREATMENT PLANT concentration, for which the acute criterion is designed to protect. The 90th percentile of receiving water pH was 8.5. In evaluating the effluent, the maximum allowed effluent pH is used for calculation of the acute criterion, which is also 8.5. Therefore, the acute criterion for ammonia was determined by using a pH of 8.5, resulting in a CMC of 2.14 mg/L, ammonia as nitrogen, calculated with salmonids--present. .rix u<•' For the chronic criterion, the median pH observations were used. The median was chosen for chronic toxicity, because the CCC is a 30-day average criterion and over a period of time receptors would be exposed to a more or less average ammonia concentration. Using this approach, the chronic design pH was calculated as 7.8 using the receiving water data and 7.4, based on the effluent data. The higher pH of 7.8 results in a more stringent CCC, therefore, the design pH based on the receiving water data was used in the calculation of the CCC. The chronic criterion decreases (i.e. is more stringent) as temperature increases. Since the effluent and receiving water temperatures vary seasonally, a chronic criterion was calculated for both winter (Nov 1 — May 31) and summer (June 1 — Oct 31). The effluent temperature exceeds the receiving water temperature, therefore, the effluent temperature data were used in the calculation of the chronic criterion. The maximum winter 30-day average effluent temperature was 23.3°C and the maximum summer 30-day average effluent temperature was 26.5°C. Using the chronic design pH of 7.8, results in a summer chronic criterion of 1 .47 mg/L, ammonia as nitrogen, and a winter chronic criterion of 1.81 mg/L, ammonia as nitrogen, calculated with early life stages of aquatic species present. Although the use of seasonal effluent limitations was considered due to the seasonal variation of temperature of the effluent, the acute criterion, which is not dependent on temperature, controls the effluent limitation derivation. Therefore, for the application of the acute condition (1-hr duration), the ammonia effluent limitations apply year-round. This methodology for developing the WQBELs for ammonia has been reviewed and approved by USEPA Region IX. It is the same procedure used in development of WQBELs for ammonia in the City of Stockton Regional Wastewater Control Facility (RWCF) NPDES permit renewal (see Order No. R5-2002-0083, located in the record[Reports and Reference Documents Binder 1 of 2]). USEPA Region IX provided the following comments regarding the methodology in its 22 March 2002 letter regarding the tentative Stockton permit, "We are pleased to see that limits have been established for ammonia...EPA concurs with the methodology staff has used in the selection of pH values both for the determination of RP, and for the establishment of WQBELs." The receiving water conditions in Old River for the City of Tracy discharge and the San Joaquin River in the vicinity of the Stockton RWCF discharge are very similar. Therefore, staff used the same procedure for calculating WQBELs for ammonia.