# Fall Semester 2017

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33. Assume you have been hired to build a MODFLOW model of the groundwater flow conditions in "Bear Valley". A simplified representation of the site is shown below. The boundary of the site to be modelled is shown with a black line corresponding to a groundwater flow divide. The blue lines in the middle represent a river. There are three wells and a low permeability fault shown by dashed lines. We will build a one-layer steady state MODFLOW model and do a capture zone analysis for the three wells.

Click here to download a zip archive of a GMS project. Unzip the archive and load to GMS. Note that the project file references a background image that has been pre-registered. Use this as a base map to build your model.

Create and run a MODFLOW model using the conceptual model approach in GMS. As you build your model, consider the following data/instructions:

Grid

Create a one-layer 3D grid with 100 cells in the x-direction and 65 cells in the y-direction.

General properties

Import the following two scatter point files from the zip archive:

ground surface.txt
bedrock.txt

Interpolate the top and bottom elevations of the MODFLOW grid using the two scatter point sets.

Assume that K is uniform and = 6 m/day.

Recharge

Compute an appropriate recharge rate assuming that the annual rainfall in the area is 0.8 m/yr and about 10% of that amount reaches the water table.

River Network

Assign the following elevations to the junctions/end points of the river:

 Junction Stage (m) Bottom Elev (m) R1 175 172 R2 168 165 R3 158 155 R4 145 142

Calculate an appropriate conductance for the river arcs assuming the following.

 Average width: 3.5 m Average vertical thickness: 2.0 m Average vertical K: 0.5 m/day

Wells

Create three wells at the locations shown. Enter the following values for the pumping rates:

 Well Q (m^3/day) Q1 -2500 Q2 -1700 Q3 -3100

Low K Fault

Create a barrier arc at the location shown by the dashed lines. Compute and assign the hydraulic characteristic assuming the following for the fault:

 Average thickness: 2.0 m Average horizontal K: 0.001 m/day

MODFLOW Solution

After setting up your model, save and run your simulation to get a MODFLOW Solution. Check your solution to make sure it looks reasonable

MODPATH

Find the capture zones for each of the three wells using MODPATH. Change the particle tracking settings to find the capture zones corresponding to a travel time of 20,000 days.

a. Conceptual model and coverage setup/organization (2 pts)

b. Computational grid (dimensions, location, etc.) (2 pts)

c. Recharge and K (2 pts)

d. Grid elevations (top, bottom, starting head) (4 pts)

e. Representation of fault (3 pts)

f. Representation of river (5 pts)

g. Wells (3 pts)

h. Capture zones (2 pts)