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`~ <br />-~- <br />It is for these reasons that the Division requires a dewatering device at the <br />100% sediment storaye elevation for ali. mine sedimentation ponds in the State <br />of Colorado. An easy way to comply c:ith this requirement would be to use a <br />"drop-inlet" principle spills/ay with a one or two inch hole placed in the risor <br />at the 100%o sediment storaye level. This would allow the principle spillway <br />to function as you have designed it, but still allow a slow, total des/atering <br />of the pond through the "dewatering hole". If a small enough orifice is used, <br />detention time wilt remain long, and effluent limitations will be met. For <br />your reference, I have attached typical automatic dewatering device drawings. <br />6. The original application submitted February 17, 1982 states, on page v-32, <br />that additional baseline data is needed and will be collected in the following <br />months. The applicant should supply all the data that has been collected to <br />date. This information should include surface and ground water quality and <br />quantity, as well as grab samples of seeps and springs along with flow <br />approximations and source characteristics. The data should be organized and <br />displayed graphically to allow easy interpretation and comparison. The data <br />should be analyzed through comparisons with Colorado Receiving Stream Standards, <br />EPA Quality Criteria for water, and any other indicator standards on record. <br />Qualitative comparisons of water quality should be presented through the use <br />of Piper Trilinear diagrams, vector, polygon, or bar graphs where appropriate. <br />7. The applicaht's statement of probable hydrologic consequences is incomplete <br />and lacking in specific analysis. This section is one of the most important <br />sections of the application. [without a good discussion backed vp by specific <br />data or modelling, it will be difficult to assess cumulative impacts and make <br />a statement of compliance. <br />The statement needs to be organized as follows: <br />a) Individual discussions of alI anticipated potential hydrologic impacts <br />resulting from the operation c/hether beneficial or detrimental. <br />b) Of the impacts identified, their significance to the local and <br />regional hydrology should be discussed. <br />c) Data and/or models should he presented to support the above state- <br />ment. <br />d) Measures that will be taken to minimize these impacts are then <br />discussed. <br />e) Finally, the duration of these impacts should be predicted and justified. <br />An example of a common impact that is discussed is sediment yields: <br />a) The applicant would discuss how sediment yields increase at mines <br />and how this could affect surface water. <br />b) The significance of this impact is discussed in regards to the <br />magnitude of the increase and baseline conditions. For example, if the <br />receiving stream normally flows muddy, the significance is minor or <br />moderate. If the stream is usually clear, the impact may he very significant. <br />