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Post Closure Open Pit Mine - Groundwater Level Recovery


Theo
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Good day GMS community and other.

So am I modelling a typical open pit mine scenario, however I am struggling to model groundwater recovery (rebound) to natural (calibrated head) conditions. - please see attachment. I did the follow steps using GMS 9.

1. Developed conceptual model;

2. Used drain package to represent the bottom of the open pits;

3. Calculated (using zone id's) pit inflows;

4. Modelled the groundwater level during mining (Life of Mine). Thus bottom of the open pits represents my post mine heads.

My objective is to calculate how long the groundwater level (post mine head) will take to reach natural conditions (calibrated head).

My problem is as follow:

1. I can't use the Lake Package as this requires an initial stage. In my scenario (please see attachment) the groundwater level change throughout my open pit area. Using an initial stage (in the Lake package) model the groundwater level to the initial stage therefore flooding my whole model domain.

Is there a way to model groundwater recovery?

How can I calculate how long it will take to reach the calibrated conditions?

Hope to hear from you soon.

Regards,

Theo (Hydrogeologist)

post-10539-0-03341600-1370419438_thumb.j

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Good day Sean,

Thank you for you reply.

Maybe my approach is wrong!?

The open pit areas will be backfilled after mine development and therefore I still need to give a representative hydraulic conductivity (e.g. 0.5m/d) to the excavated areas (backfilled). So, the recovered groundwater level "recovers" to the 'initial stage' as set in the LAK package. It would have been great if the 'initial stage' in the LAK package could be set according to a TIN.

The problem is that the "model" I am working with has very high topography. The pits are located in the top and sloped parts of mountainous areas. (as seen in the attachment). Therefore, using an initial stage "recovers" the groundwater level far above certain regions in my model (causing flooding).

So my main objective is to get an idea on how long it will take the groundwater to recover to calibrated conditions (transient).

So rather let me ask:

How can I approach this?

Is there any other way I can model recovery of groundwater levels? (even if it means starting from scratch)

(I attached updated figures).

Any input will be much appreciated.

post-10539-0-05636600-1370430506_thumb.j

post-10539-0-07217500-1370436770_thumb.j

post-10539-0-45086700-1370436787_thumb.j

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I think you may be overthinking this (or else I am "underthinking" it or missing something in your explanation). If you are attempting to determine how long it takes to recover, you would use the Post Mine Head as your starting head in your post mine model (which includes whatever fill you add to replace the removed material). The recharge you apply to the model will bring the groundwater level up towards your original water level over time (run your transient model for a year to see how it does). You would have to use rewetting (not desirable, especially with your topography, but if your time steps are small enough, you might be successful).

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