2010-09-08_REVISION - M1977342 (3)

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PROPOSED SYSTEM COMPONENTS The following descriptions of the system components start at the seep water pipeline and progress upstream to the actual point of collection. Discharge Piping and Collection Vault (Ref. Drawing No. 3DS-3) Based upon the topographic survey and As Built drawings of the No. 3 Dam Seep Water pipeline, there is only about ten feet of fall from the lowest seepage point to the invert of the seep water pipeline. When the depth of the collection vault is included the slope of the pipeline will be only 0.41 percent, as shown on the Profile Detail 2. That would normally be enough slope for clean water. However, the amorphous iron hydroxide that precipitates from the seepage makes that slope marginal. Indeed, if the flow of 5-7 gpm is continuous through the pipelines it would not be long before the precipitates plug the pipeline. A better method of delivering the seepage flow is to gather a suitable volume of the seepage at the upstream end and to force it through the pipe as a slug-flow. The pipe will then either be empty (between slug-flow events) or flowing at a rapid rate, so that there won't be as much time for iron precipitate to form in the pipe. The frequency of pipe cleaning can therefore be lengthened as much as practical. The most certain alternative to provide slug-flow capability would be by pumping. Unfortunately, electrical power is not readily available at the site. The proposed design incorporates a method that is commonly used for Individual Sewage Disposal Systems (ISDS) called dosing siphons. A cross section of the upstream collection vault is depicted on Detail 3. The vault will have two chambers, with approximately 2/3rds of the volume in the first. A conventional 1500 gallon septic tank is shown. The intent is for iron hydroxide sludge to form and settle in the first chamber from which the sludge can be periodically removed with a VacTruck . Removal of a portion of the iron will diminish (but not totally eliminate) the potential for precipitation in the pipeline. Water from the first chamber will decant into the second chamber which will be fitted with a dual auto-siphon system. The auto-siphons work like a flush toilet. The second chamber fills to a predetermined level that initiates siphoning. The siphoning continues until the water level drops to a vent that allows air to enter, breaking the siphon action. There will be Henderson Operations, Mill Site 9/7/10 No. 3 Dam Seep Mitigation, Design Report Page 4