SITE INFORMATION AND CORRESPONDENCE

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INFORMATION SHEET CITY OF STOCKTON REGIONAL WATER QUALITY CONTROL PLANT SAN JOAQUIN COUNTY want to be held liable for temperature violations if it is due to other dischargers, the p:rmit requires the City to evaluate how to best monitor compliance with the limits. pH- EPA regulations (40 CFR 401.17) state that allowable pH effluent limitations under continuous monitoring may allow for excursions from the pH range given that: (1) the total time during which the pH values are outside the required range of pH values shall not exceed 7- hours and 26 minutes in any calendar month; and (2) no individual excursion from the range of pH values shall exceed 60 minutes. This regulation essentially allows pH limitations to be exceeded 1 percent of the time. Effluent from the plant is chlorinated to meet the coliform limitation, and then the plant strategy is to dechlorinate with sulfur dioxide to eliminate all chlorine, and leave a residual of dechlorination chemical. However, overdosing with dechlorination chemicals can occasionally depress the effluent pH below the minimum 6.0. Caustic can be added to bring the effluent pH back into compliance. The effluent is discharged to the San Joaquin River through two siphon pipes which run from the chlorine contact channel over the levee to the river. Control systems 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 3 minutes, respectively. Also, the total duration of excursions may not exceed 1% of the discharge time within a reporting period. This limit is more restrictive than allowed by EPA, but, with operation of the siphon shutoff control system, should easily be achieved. 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. 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/1 limit represents a numerical value which, if not exceeded, guarantees compliance with the narrative standard.