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Mine Dewatering


Greg
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Hi All,

I am working with a client to simulate a mining project where we want to construct a regional model to evaluate mine dewatering effects on the local surface water bodies and groundwater sources in the area, and evaluate recovery of the groundwater system once mining is complete. The mine is designed to use cut and fill techniques with the fill material consisting of different hydrologic properties. There are multiple lithologic layers beneath the water table that will be removed from the cuts. I am considering using GMS/MODFLOW for this analysis, however I am concerned how it will handle the dewatering of the multiple layers and how to limit instability in the model due to the drying of cells and then rewetting of cells with the new material properties. I have tried to simplify the problem in some initial modeling efforts by use of a 2D model and averaging hydraulic properties; however the regulatory review would like a more complete analysis. So I have a few conceptual ideas on how to handle this, and was hoping I could get some input.

  1. To handle the changing material properties I would most likely use a cascading model scheme where you would set up multiple models with different cuts and change the material properties for the appropriate cut as the mine advanced. Using the heads from the previous model run as the starting heads for the subsequent run. This seems fairly straight forward although it will be cumbersome.
  2. Use of rewetting has always been an issue with instabilities in MODFLOW models, however I have not tried them since GMS 6.5 so I may need to brush up on any improvements. If rewetting is a viable option for this analysis, can someone point me to some documentation that I can review to see how it has been improved. I will continue to conduct web searches, but in my initial reviews this topic is hard to find a good reference.
  3. The other possibility I have contemplated is modeling the system as confined and applying unconfined storage parameters to the alluvial systems and where the cuts will be applied. I have used this technique with success in transport modeling of steep/thin water tables; however in a dewatering project it seems that the change in thickness could be significant. I would assume the significance of the change in thickness would be greatest near the dewatered areas and it would become less significant at areas where there is limited dewatering.

Any advice or constructive criticism of my conceptual plan would be appreciated.

Thanks in advance - Greg

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