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Water Boards %a Central Valley Regional Water Quality Control Board 21 October 2015 Michael Riddell City of Riverbank 6707 Third Street Riverbank, CA 95367 CONDITIONAL APPROVAL OF GROUNDWATER MONITORING WELL INSTALLATION WORKPLAN, CITY OF RIVERBANK WASTEWATER TREATMENT FACILITY, STANISLAUS COUNTY The City of Riverbank Wastewater Treatment Facility (WWTF) is regulated by the Central Valley Water Board under Waste Discharge Requirements (WDRs) Order 94-100. Board staff has reviewed a 16 July 2015 Groundwater Monitoring Well Installation Workplan prepared by your consultant, Kjeldsen, Sinnock & Neudeck, Inc. The workplan describes the proposed destruction and replacement of Monitoring Well (MW) 5. The workplan was prepared to address coliform levels in MW 5 that may be the result of defects in the existing well. The workplan states that MW-5 will be destroyed by overdrilling and grouting techniques, and the replacement well will be installed using hollow stem auger drilling methods. Board staff grants the City of Riverbank a conditional approval as long as the work in conducted and completed in accordance with the workplan and the below comments. • The workplan shows that the replacement groundwater monitoring well will be installed approximately 50 feet west of the MW-5. Board staff prefers that the well replacement is no more than 25 feet from the original location. • The workplan indicates that the replacement monitoring well will be installed with two inch diameter schedule 40 polyvinyl chloride casing with 25 feet of 0.020 inch slotted screen from approximately 17 to 35 feet below ground surface (bgs). Be advised that the replacement monitoring well shall be constructed with a screen not to exceed 15 feet in length. A portion of the well screen shall be installed above the groundwater table to account for any seasonal water level fluctuations. • In addition, the workplan indicates that the well seal will consist of bentonite with a minimum thickness of 10 feet that is overlain by neat cement. Please be advised that the California Water Well Standards Bulletin 74-94 states that the sanitary well seal shall be no less than 20 feet in length. We prefer the deepest sanitary seal possible for the first encountered groundwater, even if the top of the well screen is below the water table. In addition, the sealing material shall consist of neat cement, sand cement, or bentonite clay. KARL E. LONGLEY SOD, P.E.,CHAIR I PAMELA C. CMEEDON P.E., BCEE,EXCOMIVE OFFICCA 11020 Sun Canter Clive 0200,Rancho Cordova,CA 95610 1 www.wat.rboarde.ce.gov/centralvall.y