CE En 547 - Final Exam KEY

SAMPLE

Essay Questions

Click here to access the Equation Sheet.

25. (2 pts) (transient) Specific storage represents the volume of water lost from a unit volume of the aquifer due to a unit drop in head. The loss of water is due to two factors. Describe the two factors.

Compression of the soil matrix from an increase in the effective stress resulting in loss of void space, and expansion of the water due to a reduction in pore pressure.

26. (3 pts) (transient) Suppose you are building a MODFLOW model for a one year transient simulation. The purpose of the model is to simulate the effect of a new well. The well starts out in the off position (Q = 0). After four months, the well turns on. The well is on for four months and is then off for the last four months. Discuss how many stress periods you would use in this simulation. Also describe how you would distribute time step lengths within each stress period.

You would need three stress periods: one for the first stage when the well is on, one for the second stage when the well is off, and one for the last stage when the well if off again. I would distribute a set of time steps evenly within each stress period or I would bias the time steps for the second and third stress periods to be biased towards the beginning of the stress periods.

27. (3 pts) (modflow) With both the river and the general head packages in MODFLOW, the flow rate is a function of the head difference between the aquifer and the external source/sink. What is the key difference that distinguishes the river package from the general head package?

With both packages, the flow rate is a function of the conductance times the head difference. With the river package, when the head drops below the river bottom elevation, the head difference is limited to a maximum value of the river stage minus the river bottom elevation. With the general head package, there is no such limit on the head difference.

28. (3 pts) (bc) Suppose you are asked to review a ground water model developed by another company. The model is a local scale model to be used in designing a remediation system. You notice that the boundary conditions at the upstream and downstream ends of the model are specified head boundary conditions and the prescribed head values were interpolated from heads at observation wells. The other two boundaries are parallel flow boundaries. What would you discuss with the company regarding the model conceptualization/setup?

I would warn them that they will have a problem with model calibration. The model should calibrate quite well but it will not be a unique model. In other words, they will be able to put in just about any value for K and they will get the same answer. If they are modeling recharge, there will be many combinations of K and R that will calibrate the model. Therefore the flowrates in the model will most likely be too large or two small.

29. (3 pts) (stochastic) What is the objective of a Monte Carlo simulation? What are the basic steps involved?

With a Monte Carlo simulation, one generates an entire ensemble of models rather than a single model representing the best estimate of the solution. Each of the models in the ensemble is constructed such that is considered to be "equally probable" to the other models. The model for all N models is then analyzed for the question under consideration (travel time, drawdown, etc.) and the probability of a particular outcome can be determined.

30. (3 pts) (calibration) Discuss the problem of model non-uniqueness. What can be done to prevent model non-uniqueness?

Non-uniqueness is when you have many combinations of parameter values, each of which results in a low value of model residual error. To make a model more unique, one could obtaining more observations or combine flow observations with head observations.

31. (3 pts) (sustain) Describe the dynamics of what happens when a new set of wells is introduced. Explain how achieving a new state of equilibrium is a requirement for a sustainable aquifer management plan. Include a definition of transitional storage and groundwater mining.

When a new set of wells is added to a groundwater basin in a state of equilibrium, the wells create a new discharge to the system the water table drops until the the pumping rate is balanced by increased recharge or reduced discharge and a new state of equilibrium exists. The water drawn from the aquifer as this change happens is called transitional storage. If a new state of equilibrium is never achieved, the aquifer is said to be in a state of groundwater mining.