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Geofogica(Technics Inc. 2741 River Road M,&r sto,California 95351 (209)538- 424/Fax#538-5852 RECE�VED December 7, 2000 DEC 1 % 2000 Project No.: 894.2 ENvypOvum NEwh Project Name: City of Escalon (WDR) pERfv mSER�hcc` Mr. Doug Stidham, City Engineer City of Escalon 1854 Main Street Escalon, CA 95320 RE: Work Plan: Disinfection of Groundwater Monitoring Wells City of Escalon- Waste Water Treatment Plant, 25100 River Road, Escalon, CA Dear Mr. Stidham: Geological Technics Inc. is pleased to present the attached Work Plan as requested by Tim O'Brien of the RWQCB during a recent phone conversation. This request was made due to the high total coliform counts in the groundwater samples collected during the fall 2000 groundwater monitoring event. This work plan addresses the disinfection procedure for the groundwater monitoring wells at the City of Escalon Waste Water Treatment Plant located at 25100 River Road in Escalon, CA. The information used to develop the following procedures were taken from Driscoll, 1986, Groundwater and Wells, Second Edition, Chapters 18 & 19. Typical chlorine concentrations used to disinfect wells range from 50 to 200 mg/L of available chlorine. Household bleaches, such as Chlorox, contain 5 to 5.25 percent available chlorine. It is proposed to use a solution containing 200 mg/L of available chlorine since some of the chlorine will be lost into the formation and to volatilization. Table 1 was produced which shows the volume of Chlorox that will need to be added to each well to obtain the target concentration. The annual groundwater monitoring event will be performed this month as documented in GTI's July 28, 2000 Groundwater and Surface Water Monitoring Work Plan. However, Coliform samples will not be collected. After the groundwater samples have been collected from each well, the following procedure will be performed: The required amount of Chlorox will be added to the monitoring well. Each well is equipped with a dedicated Waterra purging system, which consists of a check-ball assembly at the bottom of 3/8" diameter poly tubing. The Waterra system is operated by an up and down motion of the tubing and check-ball assembly. After the well has been chlorinated, the Waterra purging system will be started. The discharge tube will be placed back in the top of the well so the chlorinated water will run down the inside of the casing to disinfect the portion of the well above the water table. The up and down motion of the check-ball assembly will act to surge the well and filter pack, assuring disinfection. Each well will be purged for 1/2-hour. After purging, the wells will be allowed to sit for a period of 2-weeks.