SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

L � RESPONSE TO WRITTEN COMMENTS 8_ a, City Of Stockton-Regional Wastewater Control Facility NPDES No. CA0079138 26 April 2002 Board Meeting periods, using ammonia measurements, was also confirmed by Jones & Stokes (2001). Thus, all acute calculations should have assumed at least 1:1 dilution. RWQCB Response: The reason for not granting an acute dilution credit is that during above average - wet years there have been low flow periods that would result in minimal dilution. Therefore, it is reasonable to assume that during critically dry conditions there would be little or no dilution. The City's comment states that Dr. Brown has shown at least a 1:1 acute dilution and has shown dilution during slack tide periods. There are many concerns with the results of Dr. Brown's box model, as discussed in Attachment B. Also, with regards to the demonstration of dilution during slack conditions, the sampling performed by Dr. Brown was performed when the SJR flow at Stockton was estimated to be 1,250 cfs. Dilution would likely be less at low flow conditions. Stockton Comment No. I.D (7)(b): Board staff evaluated chronic dilution for ammonia by comparison of the actual effluent flow (35.7 cfs) that occur-red simultaneously with the minimum 30-day average flow observed by the UVM (233 cfs). This analysis yields an available chronic dilution of about 6:1. The Findings acknowledge that ammonia is exempt from the SIP guidance and should be evaluated according to the 1991 USEPA TSD. The TSD does not support the pairing of conditions (e.g., the permitted effluent flow and the critical low flow) that are not expected to occur simultaneously for the development of permit limitations. In keeping with the staff's intent to rely on real-time data, the available chronic dilution of about 6:1 should be applied for ammonia, as this is the only"real-time data" available to determine chronic dilution for ammonia. RWQCB Response: The minimum 30-day average flow observed by the UVM was during above average wet years. The 30-day average SJR flow at Vemalis was 1,626 cfs during the same time period. In contrast, the minimum 30-day average SJR flow at Vernalis during 1991 and 1992 (i.e. during drought flow conditions) was only 445 cfs from 23 June through 22 July 1992. The minimum 30-day average SJR flow at Vernalis during the winter months was 750 cfs from 29 January through 27 February 1991. Therefore, it is reasonable to assume that there could be minimal flow at Stockton during a critically dry year. The City comments that it is inappropriate to pair the permitted effluent flow with the critical river flow. Due to the storage provided by the 640 acres of ponds, and intermittent operation of the tertiary facility, the City has the ability to discharge 55 mgd at any time. II. Mixing Zones Stockton Comment No. II.: The discharge satisfies all the conditions specified in Section 1.4.2.2 of the SIP, and therefore a mixing zone should be granted for all pollutants. A 125-foot radius acute mixing zone is adequate. The chronic mixing zone should extend 2500 feet in either direction from the outfall.