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SFPUC June 10, 2020 Page 9 between uses but are not intended to maintain high water quality condition. Thus, the baker tanks are not improving overall discharge water quality. The grit pond's "wetland" delineation was verified by the U.S. Army Corps of Engineers in 2001 (LSA 200 1) as a non-jurisdictional feature and subsequently confirmed by a follow-up delineation and evaluation by May & Assoc., Inc. (2008). Since it has been confirmed to be non- jurisdictional, this basin may be maintained by removing plants and accumulated grit. The grit pond has adequate volume to accommodate the discharge volume associated with a UV lamp break: an 11,000-gallon discharge would only fill that area about 4 inches deep, which would evaporate within a few days (under normal dry conditions). Soil samples collected by AEW Engineering(2018) reported mercury at concentrations ranging from below the detection(< 0.017 mg/kg) to 0.42 mg/kg with an approximate average concentration of 0.07 mg/kg, which is well below the lowest respective threshold(4 mg/kg). If all the mercury from discharges (assuming 11,000 gallons at 3.0 ug/L total Hg based on trench water monitoring data) is sequestered in the upper one inch of soil in Grit Pond#1, it would take about 680 breaks for the soil to reach the lowest threshold. Even if the entire 2.3 g from each broken UV lamp were discharged to Grit Pond #1, it would take about 33 events to reach the minimum screening-level threshold. Thus, use of Grit Pond #1 is considered a best management practice, providing on-site treatment with no off-site discharge needed and no measurable accumulation in surficial sediments to be expected. Controlling The following subsections represent a recommended Interception Plan, revised and simplified from the version submitted to the Regional Board on August 1, 2018. Each step is in chronological and/or upstream-to-downstream order. Preventing UV Lamp Breaks Appendix E of USEPA(2006)provides direction on how to prevent breakage of UV lamps during transport/handling and operation. The following such measures are already in place: • Minimize debris: The Hetch Hetchy Reservoir source water is exceptionally low in debris. Clogging of valves and physical impinging has never been problematic at the Tesla Facility. • Handle and maintain lamps carefully: Lamps and lamp sleeve damage or improper setting are prevented by proper operator training, handling lamps with clean, lint-free cotton gloves to prevent etching and storing lamps horizontally in protective sleeves. • Orient UV lamps perpendicular to the flow path: This orientation provides for uniform heating. • Stabilize water flow: Lamp intensity is regulated as a function of water flowrate, to minimize lamp temperature spikes. Lamps are allowed to cool for a minimum of 5 minutes before removing. • Stabilize hydraulic pressure: Pressure is regulated through the 12-reactor system through valve controls.