Earth 111 Aquifers group activity



Earth 111 Module 6.1 Summative Assessment

Groundwater and aquifers: Darcy Tube Activity

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 (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.

Grading and Rubric

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

|Work Shown |Possible Points |

|Pre-experiment questions answered correctly |20 points (10 each) |

|1. Graph  | |

|a. Accurate labeling of axes and plotting of data points  |20 points |

| | |

|b. Recognition of the nature of the relationship between flow rate and hydraulic gradient |10 points |

|2. Calculation of hydraulic conductivity  | |

|a. Correctly setting up the problem to calculate K. |15 points |

| | |

|b. Correct values. |10 points  |

|3. Note several potential sources of experimental error. |15 points |

|4. Use the relationship between hydraulic conductivity and grain size to accurately predict experimental grain size. | 10 points |

Pre-experiment Questions:

1. What is an aquifer?

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

Assignment to be handed in:

1. Use Darcy's data set, downloaded from above, to plot Q/A vs. Δh/Δl. The spreadsheet includes data from four experiments. For some experiments, 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 (linear, exponential, parabolic, etc)? If so, what does the trend suggest to you about the relationship between these variables?

2. Calculate the hydraulic conductivity (K) for each experiment. Be sure to include proper units as part of your answers. To do this you will need to use Darcy’s Law:

Q = K*A*(dh/dl)

Based on the data from Darcy's experiments, there is only one thing in the equation we don’t know. You should be able to re-arrange the equation to find K and plug in the values from the data sets. Find the average K for each of the four experiments.

3. All experimental data are subject to experimental error (uncertainty). This uncertainty may be due to limitations on our ability to accurately measure variables in the lab, variations in environmental conditions or the physical properties of our materials, or other factors beyond our control. List some possible sources of error in Darcy's experiments. Do you think the K values you calculated for each of the four experiments were significantly different from one another?

4. Based on what you have learned about the relationship between hydraulic conductivity and grain size, which experiment do you think had the coarsest sand pack?

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