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Existing stormwater on the terrace either infiltrates through the gravel or runs off through the un- <br />named drainages. Surface water within the disturbed area will be retained during mining and will <br />infiltrate, with each of the pits and the facilities area acting as large sumps. No sediment will be <br />allowed to leave the site and cloud any downstream waters. The site will not affect existing water <br />rights, since the pit will not expose groundwater and will not store stormwater for more than 72 <br />hours. <br />After reclamation, stormwater encountered in the mining area will continue to infiltrate or <br />evaporate. <br />The existing surface drainage is through numerous dry ephemeral drainages of various sizes. The <br />drainages transport water from the upper portion of the watershed and sides of the watershed to the <br />flat terrace area. The smaller drainages dissipate which causes any water to sheet flow and infiltrate <br />into the ground. The water eventually becomes part of the Colorado River groundwater system. The <br />larger drainages have a defined channel leading directly to the river. There are numerous culverts <br />under the railroad to allow the water to continue to the river (the railroad runs parallel to the river). <br />Ephemeral drainages will be reconstructed as shown on Map F. The channels will be engineered to <br />convey a 100 -year, 24-hour storm event. The restored channels will be stable and sufficiently <br />designed to trap sediment and prevent excess sediment from entering the river. The reduced <br />gradient of the restored streams will also increase infiltration and reduce sedimentation. Appendix 2 <br />contains designs for the restored channels and associated culverts. <br />2. Surface Water <br />The terrace deposit for proposed mining is located east of the Colorado River. The permeable <br />nature of the gravel and its height above the river preclude any aquifers from being present in the <br />material that would be mined. The phreatic surface is likely to be only slightly above the river, <br />keeping it well below the middle portions of the terrace deposit. The alluvial nature of the deposit <br />will allow for the rapid infiltration of any stormwater that is collected in the disturbed areas. <br />Arroyos between the pits will remain in place and will pass under the mine access road. The pits <br />will be designed so that no disturbed areas will drain to these arroyos (Map C -2A). <br />Rincon Materials G-2 <br />Greg I.ewirki and Asmriatei. PIA .(' <br />February 2020 <br />