2. What is the mechanism that causes land subsidence?
Choice 1 A reduction in subsurface fluid pressures within sedimentary layers.
 Choice 2 An increase in weight at the land surface.
 Choice 3 A heavy rainfall for a prolonged time period.
 Choice 4 The dissolution of soluble bedrock (e.g., limestone) exposed at the land surface.
REVIEW QUESTIONS FOR LAND SUBSIDENCE

 

 

 

 

 

 

 

 

 

 

1. Why does land subsidence tend to be a problem in the southwestern United States?

Choice 1 Because this region is underlain by limestone bedrock.
 Choice 2 Because the average annual rainfall in this region is relatively high.
 Choice 3 Because urbanization in this region has added considerable weight to the land surface.
 Choice 4 Because the rates of groundwater recharge in this region are low.
Choice 4 None of the above is correct.

Instructions: underlined and colored text is linked to an image that will help explain the concept in question. These images are popup windows that should be closed when you are done. To choose an answer, click the mouse on a blue button. A popup message will appear to indicate whether your choice is correct or not. Sometimes a popup message will also take you to another image when you click on OK. To escape these images, you must close the popup window.

 

 

 

 

 

 

 

 

 

 

3. What can be done to counteract subsidence over oil and natural gas fields?
Choice 1 Remove buildings from the area affected by land subsidence.
 Choice 2 Construct recharge basins to collect surface water runoff .
 Choice 3 Only pump oil wells during the summer when recharge rates are high.
 Choice 4 Inject water into the reservoir fromation as oil and gas are pumped out.
 Choice 4 Nothing can be done.


 

4. In what way(s) can a sinkhole develop?
Choice 1 Sinkholes can form at the land surface if soluble bedorck is exposed.
 Choice 2 Sinkholes can form as surface sediments erode into a subsurface cavity.
 Choice 3 Sinkholes can form as the roof of a cave collapses into the subsurface cavity.
 Choice 4 Sinkholes can form as a subsurface cavity is dewatered, allowing surface sediments to sink down.
 Choice 4 All of the above are correct.

 

 

 

 

5. Withdrawing large volumes of groundwater from a water table aquifer:
Choice 1 could lead to land subsidence if the aquifer is composed of sediments.
 Choice 2 could lead to sinkhole collapse if the aquifer is composed of limestone.
 Choice 3 could lead to hydrocompaction if the aquifer is composed of sand.
 Choice 4 All of the above are true.
 Choice 4 None of the above is true.

 

 

 

6. Collapse sinkholes:
Choice 1 can form naturally as subsurface cavities grow larger through time.
Choice 2 can form as a result of human activities, such as groundwater withdrawal.
 Choice 3 can form naturally during periods of below-average rainfall.
 Choice 4 All of the above are true.

 

7. Why might sinkholes not form in an area underlain by caves?
Choice 1 Because the caves are too deep in the subsurface.
 Choice 2 Because the caves are overlain by strong, insoluble rock.
 Choice 3 Because the caves are filled with groundwater under pressure.
 Choice 4 All of the above are true.
8. What causes hydrocompaction?
Choice 1 Withdrawing groundwater from an artesian (confined) aquifer.
 Choice 2 Withdrawing groundwater from a water table (unconfined) aquifer.
 Choice 3 Adding water to deposits of highly porous sediments whose structure is preserved by clay "cement"
 Choice 4 The injection of water into an oil and gas reservoir formation.