That Sinking Feeling

Students will:
- analyze satellite imagery of Central Florida
- compare and contrast geologic differences over a twenty year period
- assess the role of water's impact on the local geologic features

Materials:

Background:

What does water quality have to do with rocks and the development of Central Florida?  More than the average person might think.  This activity will guide students to the understanding that there is a significant link between water quality and the geologic features of Florida.

Chemical sedimentary rocks form when minerals are precipitated from a solution or are left behind when a solution evaporates.  Deposits of minerals that precipitate out of solution, or remain after evaporation, form rocks.

Calcium carbonate is a compound that is carried in solution in ocean water.  When the calcium carbonate precipitates out of solution as calcite, many crystals grow together and form the chemical sedimentary rock known as limestone.  Limestone may also contain other minerals and sediments, but it is comprised of at least 50 percent calcite.  Limestone is usually deposited on sea or ocean floors.  Large areas of the United States, including Florida, are underlain by limestone because oceans once covered much of the country for millions of years.

This is significant because limestone is subject to chemical weathering, resulting in rock changes.  Water is an important agent of chemical weathering.  When the hydrogen and oxygen atoms in water mix with carbon dioxide molecules in the air, a weak acid, known as carbonic acid, reacts with minerals such as calcite, the main mineral in limestone.  The product of this reaction then dissolves and can be carried away.  Over thousands of years, carbonic acid has weathered enough limestone to create caves.  Inside the caves, water often drips slowly from cracks in the cave walls and ceilings.  If enough water seeps through, it can eventually dissolve the underground rock.  If the rock is dissolved near the surface, a sinkhole may form.  A sinkhole is a depression that forms when the roof of a cave collapses.

Most of Florida is prone to sinkhole formation because it is underlain by thick carbonate deposits that are susceptible to dissolution by the circulating ground water.  Florida's principal source of freshwater, groundwater, moves into and out of storage in the carbonate aquifers. 

Development of groundwater resources for municipal, industrial and agricultural water supplies creates regional groundwater level declines.  The decline of available groundwater plays a role in accelerating sinkhole formation, thereby increasing susceptibility of the aquifers to contamination from surface water drainage.  

In some areas of Florida, sinkholes are used as storm drains, and because they are a direct link with the underlying aquifer systems it is important that their drainage areas be kept free of contaminants.  Conversely, when sinkholes become plugged, they can cause flooding by capturing surface water flow and can create new wetlands, ponds, and lakes.

Increased sinkhole development and property loss are strongly correlated to human activity and cultural development.  There are several reasons for this correlation.  First, the rapid growth and development makes it more likely that new sinkholes will be reported, and the construction of roads and industrial or residential buildings increases exposures to the rise of property  damage.  Second, Land-use changes in rapidly developing areas are often loosely controlled and include altered drainage, new collection areas for surface water, and new construction in sinkhole-prone areas.  Finally, the changing land use is often associated with population increases and increasing demands for water supplies, which leads to the lowering of regional ground-water levels, risking salt water intrusion and increases run-off, diminishing water quality, which only accelerates the calcium carbonate dissolution process.

Procedure:

1.  Have students access the USGS - Earthshots: Orlando Change webpage.  Have them toggle through the images available from varying years.  It might be helpful for some students to print the images or have printed images available to them for analysis.

2.  Ask the students if they notice any significant changes to the area in the satellite imagery.  Ask the students if they are aware of any man-made changes to the area over the years that the images were captured.  Might there be any correlations?

3.  Ask the students or have them research the topic of sinkholes.  What does a sinkhole have to do with water?  Do the recent man-made developed areas and need for water correlate with the increase of sinkholes?

4.  From the satellite imagery, have the students estimate approximately how many karstic lakes have been created over the past 20 years.  What impact will these lakes have on local water quality?

Assessment:

References
USGS - Sinkholes
Sinkholes of West-Central Florida
USGS - Earthshots: Orlando Change