Summative Assessment: Flow in Aquifers (In-Person Lab for Blended Class)

Throughout Module 6.1, we have focused on the physical properties of aquifers that control the storage and movement of groundwater. Today's "laboratory" activity is designed to give you a first-hand look at how hydrogeologists actually quantify the ability of aquifers to transmit water.

As you've already seen in the section of the Module on "Darcy's Law", Henri Darcy conducted a set of famous experiments in the mid-1850's that are widely regarded to mark the beginnings of modern hydrogeology. Darcy recognized the connection between how fast water percolates, or seeps, through an aquifer, and the combination of driving forces and aquifer properties. Today you will recreate these experiments, and explore these relationships first-hand.

Pre-experiment Questions:

1. What is an aquifer?
2. What physical characteristics of a rock do you think would make it a good aquifer?

Darcy Experiment:

This experiment is designed to investigate the relationships between flow rate, aquifer material, fluid properties, head gradient, and flow area. These relationships are described formally by Darcy's Law, which you will use to quantify the hydraulic conductivity of each material type.

Experimental Equipment:

• Darcy Tube
• Graduated cylinder or beaker to measure Q
• Ruler, measuring tape, or yardstick
• Stopwatch
• Water

Instructions:

1. Begin with one of the two Darcy Tubes. Find the cross sectional area of the tube and record it.
2. Measure the length of the column and record it in the table. This is the flow path length, ΔL .
3. Set up your tube with ring clamps to maintain a constant Δh . Measure the head difference from that mark to the bottom of the porous material, and record it in the table.
4. Measure the flow rate of water through the column as follows:
   1. Pour water into the funnel end of the tube, while maintaining a constant water level at the upstream side of the tube. Start the stopwatch whenever you are ready.
   2. Measure the amount of time needed for a fixed amount of water (e.g., 50 cc's) to flow through the column in to the cylinder. (The larger volume you use, the smaller your experimental error will be. Why do you suppose that is?)
5. Repeat step #4 three times, and determine the average flow rate from your three trials. Record it in the table.
6. Repeat steps #2-5 with the other tube.
7. Pool data from the two groups.

Files to Download

Download instructions (Microsoft Word 2007 (.docx) 134kB Mar28 17) on how to make a Darcy Tube

Download the assignment worksheet (Microsoft Word 45kB Mar28 17) to use when submitting your assignment.

Data (Excel 2007 (.xlsx) 42kB Mar28 17)

Assignment to be handed in:

1. Using your data, plot Q/A vs. Δh/Δl . For some materials, you will have several data points on your graph, whereas for other materials you may have only 1 or 2 data points. Discuss, briefly, the meaning of these graphs – do your data points follow any relationship or trend? If so, what does the trend tell you?.
2. Calculate the hydraulic conductivity (K) for each tube. Be sure to include proper units as part of your answers. To do this you will need to use Darcy's Law:
   
   Q = KA Δh/Δl
   
   Based on your measurements, there is only one thing in the equation we don't know, so you should be able to plug in values after re-arranging the equation a bit!
3. Note possible sources of error in the experiment. Based on those sources of error, how many significant digits do you feel comfortable reporting for K?
4. Plot Darcy's original dataset in the same way (Q/A vs. Δh/Δl ). Calculate K from these graphs for each of the sand packs he used just as you did for your own data.

Submitting Your Work

You will be working on this lab in class and handing it in at the end of the session.

Grading and Rubric

Each assignment will earn a maximum of 100 points, as described below.