PBL: Sinkholes

Part 1: Sinkholes near Orlando, Florida

The Orlando metropolitan area is situated where sinkholes can develop unexpectedly as land surface collapses into underground cavities. These cavities occur in the limestone bedrock, which is covered by 90-150 feet of sandy clay and an additional 30-60 feet of sand (fig. 1). The limestone bedrock also serves as an aquifer, called the Floridan Aquifer, for the residents of this area.

At least 140 new sinkholes formed in the Orlando area (Orange and Seminole counties) between 1961 and 1986, with an average of 11 new sinkholes appearing each year. Data show that the appearance of new sinkholes is not equally distributed throughout the year (fig. 2).

Fig. 1. Geologic cross-section showing entry points to

groundwater contamination.

Fig. 2. Number of new sinkholes in Orange and Seminole Counties, Florida by month of the year fro the years 1961-1986. Note that the month of the year is rfepresented with a number, starting with January as 1 and ending with December as 12.

Questions:

1. During what months of the year are sinkholes most likely to form in the Orlando area?

2. What hypothesis (es) can you suggest to explain this pattern of sinkhole development?

3. What data would you need to test your hypothesis (es)?

Part 2: Sinkholes in Action Laboratory

Sinkholes form in carbonate terraces when acidic groundwater dissolves the underlying rock. They are typically closed depressions, in which water drains down into the underlying rock rather than over a surface stream or gully. Sinkholes are common in a type of topography called a karst, which is characterized by abundant sinkholes, caves, springs, and disappearing streams. Although common in many parts of the world, such as Southeast United States, karst is uncommon in the western United States.

Sinkholes, caves, and other karst features form when carbonate rock dissolves in acidic groundwater. Normal rainwater becomes acidic as it percolates through the soil and picks up carbon dioxide (CO2) produced by organisms in the soil. The CO2 dissolves in the water and forms carbonic acid:

(CO2 + H2O = H2CO3), which dissociates into a hydrogen cation (+) and bicarbonate anion (-) to form carbonic acid: H2CO3 = H+ + HCO3-. The hydrogen cation then attacks the calcium carbonate of which limestone and marble are composed: CaCO3 + 2H+ = Ca++ + 2HCO3-. The Ca++ and the HCO3- ions flow away in the ground water.

This process can form underground caves and passageways. If one of these underground cavities collapses, a sinkhole forms. Groundwater flows along joints and fractures dissolving the marble and forming sinkholes, caves and other karst features. With time, the joints and fractures widen and turn into cracks and canyons.

Procedures:

1) Obtain the following materials from your instructor.

  • Large pan
  • Balloon
  • Soils samples
  • Pin
  • Water

2) Place around 4-5 cm of gravel in the bottom of the pan.

3) Blow the balloon up (not to full size) and tie it. Place the balloon on top of the gravel.

4) Next, place a layer of soil around the balloon until it is almost covered.

5) Finish the layering by placing sand on top and a few cm. Above the balloon.

6) Take some water (500ml) and pour over the profile. Record your obervations.

7) Take the pin and gently insert into the balloon. Record your observations.

Questions:

1. Did the water you poured in reach the bottom of the pan? Why or why not?

2. What type of geologic formation did the balloon represent?

3. How would water effect this formation?

Part 3: Mining Company Sued for Causing Sinkhole

During the spring of 1994, a man driving his minivan near Westminster, Maryland, plunged to his death in a sinkhole that suddenly opened up in the road. His widow sued a local mining company for $13.5 million, claiming that it was responsible for the development of this sinkhole.

Read the Article “Mining Firm Sued for Millions Over Sinkhole” which appeared in a Maryland newspaper, and answer the questions below. As is common with newspaper articles, this one does not provide all of the information we might like to have to evaluate the validity of the widow’s claim. However, using the geologic information available for this region and your understanding of how sinkholes form, it is possible to at least have an informed opinion about what happened and what additional information would be needed to further evaluate this case.

Questions:

1. Westminster is situated near the center fo Carroll County in east central Maryland. Are geologic conditions in this part of the state appropreate for sinkhole formation? To answer this question, refer to the following geologic maps of Maryland at:

a)

b)

What other information might you want to answer this question more fully?

2. What is the widow of Mr. Knight basing her suit on? What evidence would she need to prove her case?