Chapter 11 Answers

Review Question Answers:

1. Porosity is the proportion of open space (space that can be filled with water), within a rock or unconsolidated sediment. Permeability is an expression of the ease with which water will flow through that material.
2. Clay deposits have low permeability because of the small size of the clay fragments. Water is tightly held to the grains by surface tension, and in the very small spaces between grains in clay there is virtually no water that is not able to flow.
3. From least to most permeable: unfractured gneiss, mudstone, sandstone, fractured granite, limestone in a karst region.
4.

• Sue’s well accesses an unconfined aquifer with low permeability.
• Frank’s well accesses a confined aquifer with high permeability.
• Sue’s low capacity aquifer acts as a (leaky) confining layer to Frank’s high capacity aquifer.

5. V = (Ki)/n
i = the gradient which is the elevation difference (83 − 77 = 6 meters) over the distance (70 meters) = 0.09, n is the porosity, therefore V = (0.003 * 0.09)/0.2 = 0.00135 meters per second.
6. After a drop of 9 meters (from 83 to 74 meters), and assuming that the other well did not drop at all, the gradient direction will have changed and the groundwater should flow toward the well that now has a level of 74 meters.
7. Governments have the responsibility to protect our resources and to do their best to make sure that individuals and industry can access the groundwater that they need. Without observation well networks governments will have no independent information on how water levels are changing, and will be unable to make decisions on what might need to be done to ensure an adequate water supply for all.
8. Natural groundwater contamination originates from the natural reactions between the groundwater and the aquifer minerals. Anthropogenic groundwater contamination typically comes from human-sourced chemicals at or near to surface that are allowed to leak into the aquifer.
9. Water travels faster through a highly permeable aquifer and thus can spread the contamination further than in a less permeable one.
10. Livestock wastes are rich in nitrogen compounds, and these most commonly lead to nitrate contamination within the groundwater. Livestock wastes may also contain pharmaceuticals, which could contaminate groundwater.
11. The mineral pyrite is most likely to be responsible for acid rock drainage.
12. The waste water in a septic field needs to percolate slowly through the ground in order for natural processes to break down the contaminants. If the permeability is too low the waste water could come to surface. If the permeability is too high it could contaminate groundwater.

Exercise Answers:

Exercise 11.1 How long will it take?

$i=\frac{37\text{m}-21\text{m}}{80\text{m}}=0.2,K=0.0002\text{m/s},$

and $n=0.25,$ so $V=\frac{0.0002\text{m/s}\times 0.2}{0.25}=0.00016\text{m/s}.$

At that rate, it will take 500,000 seconds for the groundwater to flow from the gas station to the stream. That converts to 138 hours, or about 6 days.

Exercise 11.2 Cone of depression

The cone of depression increases the gradient of the water table in the area around the well. That should increase the rate at which water flows towards the well.

Exercise 11.3 What is your water table doing?

The water-level for a random observation well in BC is shown above. The water table is slowly rising at this location. Since 2004 the lowest water level has risen from just above 4 m below surface to around 3.6 m above surface and the highest level has risen from around 0.3 m below surface to nearly at surface (0 m). Prior to 2004, where the points are not joined with lines, the trend appears to be similar.

Exercise 11.4 What goes on at your landfill?

Responses will vary.

Exercise 11.5 Finding a leaking UST in your community

Responses will vary.

Exercise 11.6 Manipulating a contaminant plume

What could you do at wells A and C to prevent this? Explain and use the diagram below to illustrate the expected changes to the water table and the movement of the plume.

Possible Answer: Injection into well A will cause water table to rise there (like the reverse of a cone of depression), thus reversing flow direction to the right of well A and moving the plume towards Well B. Extraction from Wells B and C will cause cones of depression and help to reverse the flow and pull the plume back from the stream. Both wells B and C may receive contaminants and so the water from both may need treatment.