SITE INFORMATION AND CORRESPONDENCE

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INFORMATION SHEET CITY OF STOCKTON REGIONAL WATER QUALITY CONTROL PLANT SAN JOAQUIN COUNTY are wired so that detection of pH excursions at the inlet to the siphon will break the siphon. The City monitors pH continuously, and estimates that from detection of an excursion to complete breaking of the siphon takes approximately 30 seconds. Stockton submitted data regarding pH excursions for 1993. The data showed that there were 13 excursions during the year, 62% occurred for less than a minute, and the remaining excursions were during times when there was no effluent discharge into the river. The data indicates that the excursions are short-term and of small magnitude. The Order requires that the discharge not have a pH less than 6.0 nor greater than 8.5, and that excursions above or below the limits not exceed 60 minutes, respectively. Also, the total duration of excursions may not exceed 1% of the discharge time within a reporting period. A provision is also included to require the pH control system and the siphon discharge shutoff be operated to prevent excursions outside the range of 6.0-8.5, to the extent it is operationally feasible. The Discharger is also required to conduct an internal review and report the reasons for any excursion lasting more than 30 minutes. Settleable Solids- The permit requires that effluent settleable solids not exceed a daily maximum of 0.1 mg/1. The limit is an extension of the Basin Plan requirement that "Waters shall not contain substances in concentrations that result in the deposition of material that causes nuisance or adversely affects beneficial uses." The 0.1 mg/l limit represents a numerical value which, if not exceeded, guarantees compliance with the narrative standard. This limitation is included in the permit as allowed by state regulations. The settleable solids limitations are based on professional judgement, having been found to be achievable at a properly operating treatment facility. Facilities complying with this standard have not been found to have caused deposition of waste materials within the receiving waters. Reasonable Potential to Exceed Federal regulations require effluent limitations for all pollutants that are or may be discharged at a level that will cause or have the reasonable potential to cause, or contribute to an in-stream excursion above a narrative or numerical water quality standard. The City of Stockton has been collecting water quality samples of the effluent and receiving water monthly since July 1993. The constituents evaluated included those which were regulated by the Inland Surface Waters Plan. To date results of twelve sample events have been received. Of the results reviewed, all constituents appear to meet applicable water quality objectives, taking into account dilution and receiving water concentrations, with the exception of mercury. The water quality objectives for mercury include an acute 1-hour average of 2.4 µg/1 to protect freshwater aquatic life, and a 30-day average of 0.012 µg/1 to protect human health (due to bioaccumulation of