SITE INFORMATION AND CORRESPONDENCE_1985-2004

Home Browse Search

INFORMATION SHEET ORDER NO. 97-115 (revised) ' -3- CITY OF MANTECA AND CITY OF LATHROP SAN JOAQUIN COUNTY SUBSTANTIATION OF THE PERMIT REVISION REQUEST The Discharger used computer modeling to predict whether the mixing zone in the San Joaquin River has enough assimilative capacity to mitigate the toxic effects of the ammonia in their discharge. Worst case and average case conditions in terms of river and wastewater effluent characteristics were examined. Acute and chronic mixing zones were predicted using the Comel Mixing Zone Expert System (CORMIX) computer model, which is one of US EPA's approved mixing zone models for near-field analysis. The Discharger proposed the following mitigation options for effluent disposal: • Maximize land discharge to crop irrigation during the summer months and for as much of the rest of the year as crop irrigation demand allows. From April to October 1999, the average land disposal rate is 1.77, 2.91, 2.11, 3.19, 2.23, and 2.05 mgd each month respectively • Discharge any effluent in excess of agronomic capacity to the San Joaquin River. Based on past disposal data, it is predicted there will be times during wet weather or crop harvesting that the entire plant effluent will be discharged to the River. • When discharging to the San Joaquin River, utilize effluent storage basins to mitigate toxic impacts of ammonia to aquatic life during periods of low river flow. • The current configuration of the Discharger's side bank outfall likely causes adverse toxicological effects to the benthic organisms along the river bottom. A mixing zone using the current outfall design will continue to adversely affect benthic organisms, and could also impact fish habitat on the riverbank. As mitigation, the Discharger will install a 12-port diffuser designed to eliminate the effluent's affect on benthic organisms and fish habitat. The conceptual design of the diffuser is shown in Attachment C. The twelve 8-inch diameter discharge ports are angled 15 degrees from horizontal and are located three feet above the river bottom. The ports are at eight-foot centers and are grouped 105 feet from both river banks to prevent toxic concentrations of ammonia from contacting the river banks. The discharge diffuser configuration was used in the mixing analyses discussed below. MODEL INPUT PARAMETERS For modeling purposes, river flow conditions were based on San Joaquin River flows measured at Vernalis modeled by the Department of Water Resources using historic precipitation from 1922 to 1994, and flow and operation criteria from the VAMP agreement(now part of the SWRCB Decision 1641). Vernalis is approximately 11 miles upstream of Manteca's discharge point. The Discharger assumed seasonal variations of river characteristics in their models. For the worst case model, river flow data from the DWR Vernalis station was compiled and the low river flows of 641 cfs