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Strange Model Response

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I am seeing a strange response in my model to a pumping well that is being turned on and off. I have an observation well in the cell with my pumping well. I also have one in a cell 6 cells away from my pumping well. When the well is turned off, the gw rises in the cell with the well as expected, but I am seeing a sudden drop in the groundwater elevation in the cells around the well. This sudden drop propagates pretty far throughout the model. I have attached a jpg that shows the response that I am seeing.

In the plot, the black line is the pumping well cell. The red line is the observation well 6 cells away. As you can see in the plot, the water level in the pumping cell drops quickly from elevation 321 to 282, then is turned off for several hours and starts to rebound. As the pumping cell starts to rebound, the observation cell takes a quick drop in elevation of 3 feet. The observation well starts to rebound only when the pumping well is truned back on. This response is the opposite of what should be happening.

A similar response is seen at date 39912.5, when the pumping well is finally turned off. The immediate response of the observation well is to take a sudden drop for about 6 hours, then start to rebound. It seems that whenever the pump is turned on or off, the model is responding in the opposite direction, and at an accelerated rate.

The model is run with a 10-minute time step, and all fluxes and observation wells are in Layer 1. This response is seen in all of the cells around the pumping well, and gradually diminishes as you get around 200 cells away.

Has anybody seen this response before? I am trying to calibrate my model to a pump test and with this type of response, I am not having much confidence in this model.

post-108-1240423981_thumb.jpg

Edited by Hydrology

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It looks like you have some other change in your model during those stress periods besides just turning off that pumping well. Is it possible that your model recharge turns off at the same time.....or that you have other remnant sources/sinks that you forgot about in other areas of the model?

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No, I don't have any other stresses in the model. The BC's are static, there is only one stress in the model, and that is the single well. I am modeling the well as a drain that exists in a single cell. The drain elevation time series is the measured water level from a pump test that was conducted last week. There is a definite problem here. There seems to be a weird shock that is transmitted through the model whenever the well flow rate changes.

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Wierd. I know that the developers would usually say "let us see the files" - I am curious enough to ask if you would be willing to let me take a look, but I don't want you to pass along anything confidential. Your call.

The only other thought I have without seeing this is that transient stresses need to be entered as steps in the model such that each stress period has both the start and stop of the stress period at the same flow rate. Sometimes this gets messed up when importing data. You have probably already looked at this, but it is just a thought.

Sean Czarniecki, P.E.

AECOM

Wakefield, MA

sean.czarniecki@aecom.com

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