Abstract

The area of municipal well fields north of Tampa is densely pitted with natural sinkholes and sinkhole lakes that have resulted from collapses of surficial sand and clay into solution cavities in the underlying carbonate rocks of the Floridan aquifer. Although solution of the underlying rocks is the ultimate cause of sinkholes, some have been induced by abrupt changes in groundwater levels caused by pumping. Declines in water levels cause loss of support to the bedrock roofs over cavities and to surficial material overlying openings in the top of bedrock. Alternate swelling-shririking and support-withdrawal caused by seasonal fluctuations in water level tend to disrupt the cohesiveness of unconsolidated surficial material and promote collapse./p>

The volume of calcium, magnesium, and carbonate (the constituents of limestone and dolomite) in solution in the water withdrawn from four well fields near Tampa totaled about 240,000 cubic feet in 1978. Most induced solution takes place at the limestone surface however, and the area of induced recharge is so extensive that the effect of induced limestone solution on sinkhole development is negligible.

Alinement of established sinkholes along joint patterns in the bedrock suggests that a well along these lineations might have direct hydraulic connection with a zone of incipient sinkholes. Therefore, pumping of large-capacity wells along such lineations would increase the probability of sinkhole development.

Although sinkholes generally form abruptly in the study area, local changes such as vegetative stress, ponding of rainfall, misalinement of structures, and turbidity in well water are all indications that preco11apse subsidence may be taking place.

First posted August 15, 2011