Objectives 20

Department of Earth Science |Northeastern Illinois University

PRINCIPLES OF HYDROGEOLOGY
ESCI 337
Fall, 2007

Daily Objectives #20 (November 6, 2007) Dr. Sanders

By the end of today's class, you should be able to do the following:

v Ground Water Modeling: Practice

Use Graphic Groundwater to create a model or a given study site. Review
Assign reasonable values to the variables in the model, and support your choice for each variable. Review
Explain what the results of your model are (i.e. tell what the direction of flow is, and how pumping affects the flow system).
Explain/defend the results of your model, telling if/why it is a reasonable representation of the flow system.
Explore what happens to the modeled flow systems when you change values for variables.
Evaluate and describe the degree of uncertainty in your model.
Do a "sensitivity analysis" for your model.

Today's Agenda:

v Ground Water Modeling: Practice

(Continued from last time) How do we come up with "hard numbers" for each variable in a ground water model? Find reasonable values or ranges of values for these variables:

rate of pumping from wells (Use this figure: 69.582 million gallons per year for all three CWS wells combined.)
particle tracking (and delineation of the 5-year recharge area)

What do the results of the model tell us? What is the direction of flow? How does water enter the flow system, which way does it move, and how does it exit the flow system?

At a minimum, your model should be running before you leave today! If you already have a running model, spend your time today working on evaluating it (below). Take good notes on your work! You will need them when you write up your results.

v Ground Water Modeling: Evaluating the Model

How good is your model? First, apply the "Is this reasonable?" test. Does it show water flowing in what appears to be a reasonable direction? Are there good reasons for the gradients to be as indicated on the map? Does the five-year recharge area look reasonable?

Second, try changing some variables and see what happens. Consider how good your estimates are. How reliable is your estimate of, for example, hydraulic conductivity? Could the actual value reasonably be higher? Lower? By how much? Change the values and re-run the model; see how the results differ. Here are some suggested approaches

top elevation of the aquifer: how close to reality do you think your input data are? what happens if you change them?
bottom elevation of the aquifer: what if these data were changed? what would the results look like?
hydraulic conductivity (horizontal): what's a reasonable range of possible values? How much difference does it make?
vertical hydraulic conductivity: how does it change the results if the Kh/Kv ratio is 5:1, or 20:1, instead of 10:1?
recharge to the aquifer through infiltration: what if recharge is doubled? Halved?
porosity: what difference would it make if the porosity were 30%? 20%?
initial head: how does changing your initial heads change the model results?
rate of pumping from wells: what if two of the wells were shut down for maintenance, and the third well had to do all the work? What if more people move into the area, and the rate of pumping increases by 10%?

Finally, do a "sensitivity analysis": Now that you know the effect of changing each variable, rate the effect as "strong", "moderate", or "minimal".

Department of Earth Science | Northeastern Illinois University