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M-11R can be calculated using the flow net shown in Figure 3-4. The resulting travel time is <br />approximately 1.5 yeazs. Thus, the minimum time required for rinsing the alluvial aquifer is <br />approximately 1.5 years. More likely, sulfate, manganese, and TDS will be transported through <br />the alluvial aquifer at slower rates than the groundwater and a tailing effect will occur. <br />Therefore, the rinsing time for constituents in the groundwater will be probably be greater than <br />1.5 yeazs, possibly three to four times as long. <br />6.4 Monitoring Plan <br />As discussed throughout this text, investigations by BMRI (1998, 1999) have identified the <br />primary source of solutes as the dissolution of soluble weathering products that accumulated on <br />backfilled waste rock and the previously de-saturated pit walls of the West Pit. The hydraulic <br />conductivity of the pit backfill is high compazed to the hydraulic conductivity of either the <br />Precambrian gneiss or the Tertiary Santa Fe Formation (BMRI, 1999, Section 3.1.2). The next <br />most permeable unit in the system is the Quaternary alluvium of Rito Seco. According to the <br />current conceptual hydrologic model, when the groundwater level in the re-saturating, backfilled <br />pit reached the elevation of the contact between Precambrian rock and alluvium of Rito Seco at <br />the southeast edge of the pit, water from the pit began to flow into the alluvial groundwater <br />system. (BMRI, 1999, Section 3.1.4). The contact between the backfilled pit and the alluvium <br />constitutes a hydrogeological "window" bordered by lithologies of much lower permeability. <br />In the alluvial system, groundwater from the pit mixes with alluvial groundwater flowing down- <br />gradient (i.e., from east to west past the West Pit) and disperses in the alluvial groundwater <br />system on the north bank of the stream. Some of the alluvial groundwater recharges Rito Seco <br />close to the West Pit, at and in the vicinity of the now-controlled seep, and some of the alluvial <br />water flows in the poorly-consolidated alluvial apron of the Rito Seco, eventually discharging to <br />the creek further downstream. Before reaching the "hogback" neaz the BMRI property boundary <br />(~3 miles), where the alluvium has narrowed and thinned to a negligible component of the total <br />flow system, essentially all the alluvial groundwater has dischazged to the surface-water system. <br />~~ <br />Mountain Resources, /nc. <br />p.l/001671reporlslmnrchrp(Itrl6wtrmnglmarch.doc 69 <br />Mnrch ?I. ! 999 <br />