2016-07-28_REVISION - M1980244 (2)

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RESPONSE. Yes. These are the EMPs currently constructed in Grassy Valley in addition to several sediment basins. There was an additional EMP (EMP-010) that was mined out by the WHEX operations. The stormwater management design for Grassy Valley was updated as part of Amendment 11 due to the proposed modifications to ECOSA. Please see Appendix 10 of Amendment 11, Design Report for Stormwater Management Amendment No. 11 (December 2015)for details. 4.6.3.1 Monitoring Program • Page 29: The text lists 3 surface water monitoring stations in this drainage, including GV- 01, GV-02,and GV-03, and refers to Figure 2-1. However,only two of these stations are shown on Figure 2-1 (GV-02 and GV-03). Please revise Figure 2-1 to show locations of all monitoring stations discussed in this section. RESPONSE. GV-01, GV-02, GV-03 were all once surface water compliance points. The proximity of GV-01 to the WHEX active mining area and the lack of flow through the flume resulted in abandoning GV-01 as a monitoring location (as approved by DRMS). Figure 2- 1 has been updated to show the location of GV-01 although it is no longer an active monitoring location (See Attachment 2). 4.6.3.3 Discussion • Page 30: The text states "Flow rates (when reported for the same day) increase into a downstream direction from GV-01 to GV-02, but often decrease from GV-02 to GV-03." Do you suspect this has anything to do with GV-03 being located within the diatreme? RESPONSE. Yes. The flow in Grassy Valley, and the basin yield to which it is related, is closely associated with the geology underlying the portion of the basin in which the measuring point is located. The geology and the locations of the Grassy Valley surface water points are shown in Plate 17 (Rev.]) and the water table and groundwater capture/discharge areas are shown in Plate 9 (Rev.]), which are attached to these responses. The catchment of GV-01 is predominantly drained diatremal and Precambrian rock, resulting in a low flow and basin yield. GV-02 is located over undrained Precambrian rock and has more of its catchment outside of the diatreme, which reduces infiltration and increases basin yield, although the flow at that location is still low, and there is normally no baseflow. GV-03 is located over drained diatremal rock, and its catchment has more drained diatremal rock, resulting in Grassy Creek being a losing stream in this reach, and the flow and basin yield reducing compared with GV-02, and with GV-03 being normally dry, and having limited flow. GV-04 and GV-05 are located east of the diatreme, with undrained Precambrian rock beneath them, and a greater proportion of undrained Precambrian rock in their catchments. This hydrogeological setting causes a small upwelling of water from the diatreme into Grassy Creek immediately upstream of the diatreme/Precambrian boundary [Plate 9 (Rev.])], which is added to by shallow infiltration on top of the Precambrian seeping into the creek downstream of the diatreme/Precambrian boundary.Both locations show a small baseflow, which results from this hydrogeology. 4.6.3.4 AM-I I Impacts to Surface Waters • Page 31: The text states"any infiltration from the ECOSA will be captured by the Diatreme drainage system". However, as noted previously, water infiltrating the ECOSA during high precipitation events has been seeping from its toe. Because the northern portion of the ECOSA is located outside of the diatreme, and the ECOSA underdrains do not appear to be functioning properly,there is potential for impacted waters to enter the Grassy Valley stream system. How do you intend to minimize impact to Grassy Valley from ECOSA seepage?Will the proposed AM-11 raise for the ECOSA intensify the seepage? RESPONSE. Impacts to Grassy Valley are currently minimized by the designed and Page 21 of 28