2009-06-01_ENFORCEMENT - M1983176

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Bird, David From: Bird, David :/ Sent: Monday, June 01 2009 4:34 PM To: 'Dan Hunt' Cc: ?IVlarJohnson; Rob Laird; Waldron, Tony Subject: r/ M-1983-176 Clarification of Dewatering Requirements Mr. Hunt: Thank you for the prompt response. The target is not specifically the amount of water that needs to be pumped. The target is to lower the water level sufficiently so that the offsite damage is mitigated in the short term and prevented in the long term. The dewatering needed to provide the short term mitigation will probably involve measuring water levels at relevant locations near the pit and in the neighborhood during the dewatering. However, if your short-term mitigation is to be a dewatering well, the water levels in your monitoring wells MW-3 and MW-9 will probably fall within the cone of depression during the dewatering and will probably not be indicative of the water levels distal from the pumping well and in the vicinity of the citizens' homes. You will likely have to be in contact with the neighbors to monitor the water levels in their immediate vicinities. If the long-term plan is for a French Drain, then a short term dewatering well will serve the dual function of lowering the water table to mitigate the off-site damage, as well as providing dry conditions for installation of the French Drain. A qualified hydrologist should be able to provide you with a relatively quick estimate of the short-term dewatering pumping rate that must be sustained to meet the goals stated above. The fact that Kirk Kirby was able to maintain dry conditions in his basement with 250 gpm may be a good starting point. The mathematical equations that would be used for such an estimate are readily available and well-validated. The South Platte alluvium is a relatively well-studied environment and the aquifer parameters needed for the equations are either published or can be estimated with a reasonable level of uncertainty. However, this is not an exercise that should be attempted by a person not trained in hydrology. A critical factor that needs to be determined will be the elevation of the base of the French Drain if that is the chosen long term mitigation method. The base of the drain must be high enough that shallow domestic wells in the area are not permanently dried up, and low enough so that off-site damage is prevented. A ground water model may be necessary to make these determinations. Again, close communication with the neighbors may be necessary. Feel free to contact us if you have further questions. David Bird Principal Scientist Division of Reclamation, Mining & Safety Department of Natural Resources 1313 Sherman Street, Rm 215 Denver, CO 80203 Tel: 303-866-3567 x8108 Fax: 303-832-8106 email: david.bird@state.co.us http://minin14.state.cO.us/