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Hydro_Kalispell

Modeling an aquitard that pinches out.

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I am trying to model two sand aquifers separated by a clay aquitard. The aquitard pinches out and the two sand aquifers become one. Where the aquitard is present, the head in the overlying sand is approximately 13 feet higher than the head in the underlying sand. There are five layers: one for the overlying sand, three for the clay aquitard, and one for the underlying sand. I am utilizing a specified head boundary condition on the upgradient side of the model and am inputing a higher head in Layer 1 (upper sand) than in Layer 5 (lower sand).

There is seven orders of magnitude difference between the vertical hydraulic conductivity of the sand aquifers and the clay aquitard and flow is still going through the clay as shown by particle tracking. The clay is four to five feet thick.

I am utilizing the LPF flow package in MODFLOW 2005 with the PCG2 solver.

Any thoughts on how to get MODFLOW to represent this conceptual model?

Greg

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What is your downgradient boundary condition? If you don't have one, the water only has one place to go - from the higher head to the lower head. It will definitely go through the clay to get there. Also, if you are running a steady-state simulation, it wouldn't be a shock that water goes through the clay over time. However, a transient run could show that it takes a long time. You could set time breaks on your particle pathlines to show this as well.

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I am also utilizing a specified head boundary condition on the downgradient end--set at one value for all layers (the aquitard does not exist at the downgradient end of the model). Yes, I have been running steady-state simulations.

How do I simulate the observed head difference between the two sand aquifers? Currently, the two sand aquifers equilibrate across the aquitard.

Greg

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I'm assuming that your downgradient head is lower than both the upper and lower upgradient heads (to make sure flow is all going from upgradient to downgradient). What are your K values (horizontal and/or vertical) in the sand vs. the clay? I know you said 7 orders of magnitude difference, but if the clay isn't low enough, water could get through. If it were me, the first thing I would do is make that clay value as low as if it were rock - that should limit the water going vertically. Then raise the value to see when it starts being too porous.

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I found an error (not sure if it was something I selected or a problem in GMS) with the vertical K value for the clay. In the LPF properties, if the option to "Specify anisotropy factors" was selected instead of "Specify Kv" for a given layer, the value that was being brought from the Materials table to the LPF array was the Vertical k value instead of the Vertical Anisotropy value; therefore, the calculation to assign the Vertical k was incorrect. In my case, this resulted in a Vertical k for the clay that was much larger than what I had specified in the Materials table.

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