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SFPUC June 10, 2020 Page 8 • The time period from UV lamp break/discharge to a post-event, clean building floor(i.e., after facility staff have cleaned up the discharge) is on the order of 30 minutes. If Hg0 is present in the water, that short time window is insufficient for significant loss by volatilization. • Water drained from the reactor pipeline after it is isolated(valves closed after a break) has never had detectable concentrations of mercury (<0.2 ug/L for all samples) even though UV lamp glass has been found in that drainage (as well as on the building floor). UV Reactor Trench/Sump Storage Grit Pond CRaw,in Volatilize? All, Volatilize? JoRelease Pond Trench , CRaw,out Pump Baker Tanks Adsorb Adsorb? CRaw,in <0.0008 ug/L Ctrench&sump =<0.2-13.9 ug/L Ct,nk =0.08 ug/L APond=0.1 acre VRelease =11K gal. Cai,=<0.5-7.7 ug/m3 Csoil =0.07 mg/kg Cpipe drain<0.2 ug/L mRelease =2.3 g? CRelease =55 ug/L? CRaw,out =<0.2 ug/L Figure 3.Pathways for Hg transport for a UV lamp break and measured concentrations. Discharge Management Prior to September 2018, UV lamp breaks were discharged directly through the emergency overflow culvert to Grit Pond#1 (surface area=440 square meters, or about 0.1 acres), in which water percolates and evaporates. The overflow culvert is currently sealed by an inflatable plug. Instead of discharging to Grit Pond#1, water is pumped from the sump into a pair of baker tanks. The pump removing water from the drain sump is sized conservatively. Basically, the pump removes water as fast as it's spilled(1,000 gallons per minute). Each baker tank's capacity is 20,000 gallons'. Although closed except for a roof vent, they are exposed to the area's intense heat/sun and so can become quite hot inside. The tank sampled on 10/24/19 had a cloudy suspended material (likely iron oxides) but its total Hg concentration of 0.066 ug/L was still less than typically measured in the sump. The tanks are spray-washed ' Technical Information Manual 2.3.8;Flip Top Weir Tank Product Data Sheet,January,2007.