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Privileged and Confidential—Attorney Client Communication Geosyntec"; ctaMtaIUH It A flow analysis was also completed in October 2017 to assess the future water system residence times at three flow rates: • Design low: the water treatment system design low flow rate is 100 gpm based on historical flow rates from the portal • Design high: the water system high flow rate is 1,215 gpm based on the CDPS Permit 30-Day Average flow rate • Historical high: based on the Mine's historical flow data, the instantaneous water flow rate out of the portal has been as high as 1,366 gpm in June 2017 (this flow rate is an estimate) The residence times for each component of the system are shown in Table 5. Table 5: Water System Residence Times 1,366 gpm(highest Flow 100 gpm 1,215 gpm historical flow estimated (design low) (design high) from the Mine portal— instantaneous flow rate Mine Water Discharge 47.5 3.9 3.5 Pond(hr)* Pond#2 (hr)** 38.7 3.2 2.8 Pond#2 with 52.7 4.3 3.8 expansion(hr)** Pond#3 (hr)** 13.5 1.1 1.0 Pond#3 with expansion(hr)** 46.3 3.9 3.4 *Assumes MWP is same size as it is today—this assumption will need to be reevaluated during detailed design of pond improvements **Assumes porosity for the bioreactor configuration of 0.34(EPA,Sept. 2008) OSMI has provided Geosyntec with the following baseline assumptions regarding potential modifications to the existing water system: • MWP: the geomembrane pond liner will be removed and replaced by an impermeable clay basin liner. The newly constructed hybrid MWP will be designed with some or all of the following options: expansion of extents to the south and west, installation of baffling to enhance flocculation/settling, construction of an engineered settling zone that would allow for the periodic manual removal of solids, and the potential conversion of some pond area to aerobic wetland treatment near the pond effluent. Additional treatment may also Treatability Study Work Plan 7 February 8,2018