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Privileged and Confidential—Attorney Client Communication Geosyntee° <br /> consultants <br /> A flow analysis was also completed in October 2017 to assess the future water system <br /> residence times at three flow rates: <br /> • Design low: the water treatment system design low flow rate is 100 gpm based <br /> on historical flow rates from the portal <br /> • Design high: the water system high flow rate is 1,215 gpm based on the CDPS <br /> Permit 30-Day Average flow rate <br /> • Historical high: based on the Mine's historical flow data,the instantaneous <br /> water flow rate out of the portal has been as high as 1,366 gpm in June 2017 <br /> (this flow rate is an estimate) <br /> The residence times for each component of the system are shown in Table 5. <br /> Table 5:Water System Residence Times <br /> Flow 100 gpm 1,215 gpm historical flow estimated <br /> (design low) (desi ,high) from the Mine portal— <br /> instantaneous flow rate <br /> Mine Water Discharge 47 3.9 <br /> Pond(hr)* <br /> Pond#2(hr)** 38.7 3.2 <br /> Pond#2 with 52.7 4.3 <br /> expansion(hr)** <br /> Pond#3 (hr)** 13.5 1.1 1.(1 <br /> Pond#3 with 46.3 3.9 3.4 <br /> expansion(hr)** <br /> *Assumes MWP is at current size—this assumption will need to be reevaluated during detailed design of <br /> pond improvements <br /> **Assumes porosity for the bioreactor configuration of 0.34(EPA,Sept. 2008) <br /> OSMI has provided Geosyntec with the following baseline assumptions during the <br /> development of the Treatability Testing Plan(Feb. 2018)regarding potential <br /> modifications to the existing water system: <br /> • MWP: the geomembrane pond liner will be modified to expand the pond and <br /> hide the membrane edges from sight. The newly constructed hybrid MWP will <br /> be designed with some or all of the following options: expansion of extents to <br /> the south and west, installation of baffling to enhance flocculation/settling, <br /> construction of an engineered settling zone that would allow for the periodic <br /> manual removal of solids, and the potential conversion of some pond area to <br /> Treatability Testing Report 7 December 11,2018 <br />