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Sediment samples were collected from the trench drain adjacent to the weir, and Grit Pond #1, including a sample below the discharge/inlet, which primarily consisted of decaying organic matter, and clayey soil surrounding the armored area and approximately 5 feet away. The maximum concentration of mercury was detected in the trench drain at a concentration of 2.2 mg/kg. Very low (<0.5 mg/kg) concentrations of mercury are present in sediment at the decaying organic matter by the inlet and in the sediment immediately surrounding the armored area; however, at five feet beyond the edge of the armored area, no mercury was detected (Attachment E). Conclusions The current mercury response procedure was proven to be effective in minimizing worker exposure to mercury and preventing elevated concentrations from reaching downstream consumers. SFPUC is currently working with Calgon Corporation to determine potential root causes of the lamp rupture and changes to standard operating procedures during routine calibration activities. Mercury concentrations in water quality samples collected from the trench drain and grit pond indicate that some mercury was released to the grit pond, but that these concentration are not sustained over time. Current concentrations are less than the surface water criteria of 0.77 µg/L, based on the California Environmental Screening Level (ESL) for Freshwater Aquatic Habitat (Cal -EPA, 2016) and the National Recommended Water Quality Criteria for Aquatic Life (USEPA, 1995). Mercury is very insoluble, and quickly forms very minute clusters that settle quickly (Stantec, 2010). Once the discharge reaches the grit pond, the mercury settles quickly, within a few feet of the grit pond inlet. Some of this mercury has settled within the trench drain behind the weir, accumulating in low concentrations in the sediment, which was also observed to contain small shards of quartz glass. As a conservative approach, the concentration of 2.2 mg/kg was compared to the California Human Health Screening Level (CHSSL) of 180 mg/kg (Industrial/Commercial Exposure) and ESL of 44 mg/kg (Construction Worker Direct Exposure) and therefore does not pose a risk to site workers. At the concentrations present in the trench drain and Grit Pond #1, no further abatement is required. References California Environmental Protection Agency (Cal -EPA), 2016, Environmental Screening Levels — Interim Final. http•//www.waterboards.ca.gov/sanfranciscobay/water issues rograms/esl.shtml Calgon Corporation, 2012. Sentinel' UV Disinfection System Operation and Maintenance Manual. September. Stantec, 2010. Tesla Treatment Facility— Mercury Fate after Lamp Breakage. May. SFPUC, 2013, Draft Mercury Release Response Plan — Tesla UV Treatment Facility, version 0.0. January. USEPA, 1995. National Recommended Water Quality Criteria - Aquatic Life Ambient Water Quality Criteria for Mercury, htt s: www.e a. ov w c national -recommended -water- ualit -criteria- aquatic -life -criteria -table. LJ U 1 l ?017 F' NO`(� (Elly �5L �� rva�c��ufiaP chi �^e�-0 e1rP = T3 rn�9 I �-