CE En 547 - Midterm Exam #1 KEY

Fall Semester 2017

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26. (modflow) The following diagram represents an industrial site with shallow groundwater and a contaminant plume caused by a chemical leak. The ambient groundwater conditions result in groundwater flow from west to east with an observed head on the west side of the property at elevation = 541 m and an observed head on the east side at elevation = 537 m. The site is relatively flat with a ground surface elevation = ~545 m. As part of a cleanup team, you are exploring various options for cleaning up the site. One of the options you consider is a flushing system consisting of a injection trench on the west side of the plume and an extraction trench on the east side of the plume. To keep the water focused on the plume and to ensure there is concentrated flow from the injection trench to the extraction trench, there is also a slurry cutoff wall north and south of the plume. Each of these trenches is keyed into a flat clay layer that underlies the entire site at an elevation of 510 m.

One of the questions associated with this design is how much water would be required on a daily basis to maintain the target water levels in the trench and flush the plume. In order to explore this an other design questions, you have been asked to build a one-layer MODFLOW model of the site. As you build the model, consider the following:

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1) Assume that K = 0.5 m/day for the upper sand layer at the site.

2) Rainfall in the area is about 1 m/year. Assume that about 10% of this amount results in recharge to the groundwater system.

3) For the slurry trench cutoff wall, assume that K = 1e-6 cm/sec and that the thickness of the trench is 1.0 m.

4) Determine what package you will use for the boundary conditions and for the trenches. Enter the appropriate inputs for each package.

5) For your units, use meters for lenght and days for time.

6) Use a local coordinate system and let the lower left corner of your grid = the origin (i.e., x,y = 0,0).

7) For the numbers of rows and columns, use the numbers shown above on the site dimensions.

 

Grading:

(a) Grid with the proper dimensions and number of cells.(2 pts)

(b) Units and selection of packages. (1 pt)

(c) Array values for starting heads, top and bottom elevations, K, and recharge. (6 pts)

(d) Boundary conditions on perimeter of model. (2 pts)

(e) Representation of injection/extraction trenches (4 pts)

(f) Representation of slurry trenches. (4 pts)

 

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