2017-03-20_PERMIT FILE - C1981019A (47)

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EXHIBIT 7, ITEM 23, PART J Sediment Control System For the Initial Facilities Disturbance Area Near the beginning of construction activities for the Collom facilities area, a short term condition is expected to occur within the watershed and associated ponds for the Eastside model. The three ponds affected by this temporary condition are the Section 36 Pond, the Middle Pond, and the lowermost Section 25 Pond. All three ponds are expected to be completed during the first year of construction activities. However, due to a construction delay in the Section 25 Pond until July 15, this pond will not be completed until later in the summer or fall. This delay is a result of the need to avoid activities near a Greater Sage Grouse lek within 0.6 miles of the Section 25 Pond during the lekking and brooding season. Prior to construction of the Section 25 Pond, but after completion of the Section 36 Pond and the Middle Pond, it is expected that ground disturbing activities within the Collom facilities area will occur. This includes the Collom Haul Road construction, the shop facility, water line and tank construction, the primary crusher area, and other disturbances associated with develop of the Collom facilities. Assuming that these ground disturbing activities will need to occur as soon as the Section 36 Pond and the Middle Pond are constructed, this analysis evaluates the effectiveness of those two ponds to be able to adequately treat the runoff from the disturbed area without the use of the Section 25 Pond. Note that this is a short term condition and that will end following the completion of the Section 25 Pond later in the summer or fall. Three SEDCAD models are provided for this case. The first provides sedimentology for the resultant 10 year, 24 hour storm event, while the third model evaluates the effectiveness of the pond's spillway systems under the 25 year, 24 hour storm. Exhibit 7-23, Figure J1 shows the location of the disturbances, curve numbers, drainage areas, watershed names, structure numbers, and other information. As shown, not all of the areas naturally drain to the Section 36 pond, but surface water runoff will be routed from the disturbed areas back to the Section 36 Pond watershed. All of the undisturbed areas are modeled at a curve number of 57 while all disturbances are modeled at a curve number of 85, based on Volume 21), Exhibit 7, Table 1 values. Exh. 7-23, Figure J2 shows the design information for the Coal Stockpile Pond, which will be built downgradient from the coal stockpile area to collect runoff and coal fines from the stockpile area. As planned, it will be built early in the initial construction phase. It is a containment pond designed to completely store the runoff from the 10 year, 24 hour storm event. As such, it effectively cuts off all upstream runoff from the contributory area. However, in order to evaluate whether or not this structure needs to be completed prior to other activities, a second SEDCAD model is provided. This second model is identical to the first model except that the Coal Stockpile Pond (Structure 45) has been reduced in capacity. This was done by lowering the spillway elevation to simulate a much smaller pond, such as would be the case if the Coal Stockpile Pond were only partially completed. In this case, the spillway was lowered to elevation 7,212.0, corresponding to a pond capacity of less than 2 acre feet. Results of this second model show that the effluent concentration at the outlet of the Middle Pond produces 0.22 ml/l, which indicates that the Coal Stockpile Pond is not needed prior to Exh. 7-23J-1 Revision Date: 12/30/16 Revision No.: TR -108