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hydrogeol

by Fleurant (Submitted: 03/30/2012)

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## WHAT IS IT?

This model computes the 2D underground flows through a porous aquifer.

## HOW IT WORKS

The study area consists of an aquifer (top view) in which a well was located (the user can choose its X and Y coordinates). According to the aquifer properties, the user can:
- To watch the hydraulic head properties (Darcy law)
- To watch the water pathways
- To create clouds of pollutant (the user can choose X and Y coordinates of the source as well as the pollutant properties)

Assumptions:
- The aquifer has uniform properties and isotropic characteristics ;
- A river (coordinates with Y < 0) induces Dirichlet boundary conditions: a constant hydraulic head (h = 0 m) along the entire river.

## HOW TO USE IT

Here is a rough explanation of the model variables:

- "r" is a delay factor (no dimension) compare to groundwater flow. If "r" = 1, the cloud of particles (pollutant or not) has a hydrodynamic behavior like groundwater (basically the same velocity and dispersion). If "r" > 1, particles are constrained through porous media (it is called chromatographic effect). If "r" < 1, particles go faster than groundwater (in average)! This last case is observed with colloidal particles for instance.

- "alpha" is the aquifer dispersivity (m). The dispersivity is a scale-dependant parameter of the dispersion in porous media.

- "dt" is the time step (days).

- "n" is the number of particles (no dimension) when user create a cloud.

- "q" is the well's flow rate (m3/s). Increasing "q" will create a depression cone of the hydraulic head values.

- "T" is the transmissivity (m2/s) which is computed by T = k * ep, with "k" the aquifer permeability (m/s) and "ep" the aquifer thickness (m).

- "ep" is the aquifer thickness (m).

- "a" is the slope of the hydraulic head's aquifer when the flow rate of the well is nil. Then "a" is the slope of the aquifer in natural conditions.

- "p" is the aquifer porosity (no dimension).

- "xs" and "xy" are the X and Y coordinates (m) of the well respectively.

- "xu" and "yu" are the X and Y coordinates (m) of the particles' source.

Here is a quick description of the functions (buttons on the right):

- "Clear all": clear all!

- You can choose the type of the aquifer view by clicking "Upper aquifer view", then you see a geographic view: positions of the well (a black dot), of the waste repository (a red dot) and of the river (blue rectangle at the bottom of the screen). If you click on "Hydraulic head view", then you see how hydraulic head is computed according to the set of parameters (e.g. "q", "T" or "a").

- To see how groundwater goes through the aquifer according to the computed hydraulic head, click on "Setup water pathways" (uniform distribution of water particles over the aquifer area) and then on "Watch water pathways" (water particles move along the flow lines).

- Add a cloud of particles by clicking the button "Create a cloud of particles". The particles move according to the simulation parameters by clicking "Simulate". The concentration (C/C0: the actual concentration divided by the initial concentration) is then computed when particles reach the river. You can follow the particles' pathways by choosing "On" in the switch button.

## THINGS TO TRY

- The number of particles "n": for a given set of parameters (default), only change the number of particles "n" = 100, "n" = 1000 and "n" = 10000. Then you can see how the output concentration curve becomes smoother.

- The well's flow rate value "q": for a given set of parameters (default), only change the flow rate of the well "q" = 0.0 m3/s, "q" = 0.005 m3/s and "q" = 0.01 m3/s. Then you can see that output concentration is more and more delayed and dispersed. You can also observe on the view how the cloud of particles behaves spatially.

## CREDITS AND REFERENCES

Copyright 2011 Cyril Fleurant.

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