Thursday, March 7, 2013

Florida Sinkholes Also Pose A More Subtle Danger

Last week, a fatality, as the ground gave way under a Tampa Bay area home.  However, the more common concern regarding sinkholes is their connection to groundwater quality. Despite its reputation as a great tourist destination, Florida is also a major agricultural state. The soil is reenforced with fertilizers, and crops are sprayed extensively with pesticides. Environmental managers worry that these chemicals might find their way into groundwater.

This concern is not unique to Florida but is amplified due to the presence of karst topography.  Extensive limestone formations underlie Florida. Limestone reacts with the weakly acidic groundwater and over the last few million years these limestone beds have dissolved away in varying degree to create an underground network of caves and pipes. The overlying strata may bear its own weight for long periods depending on how thick it is and how porous the underground limestone has become, but occasionally the cover collapses into the underlying cavity forming surface craters or sinkholes.

Fatalities are rare, but sinkhole formation is quite common over geological time.  The map below shows an area just east of Gainsville.

Notice the circular and irregular shaped lakes. Most of them are sinkholes now filled with water. This surface water makes its way through fractures and pipes into the underlying limestone which make up the Floridan aquifer system. Sinkholes thus provide a direct and quick connection for potential pollutants to contaminate groundwater which is the main source of drinking water in Florida.

The question is then: what combination of geological conditions will make aquifers at one location more vulnerable to contamination than at some other location? The Florida Department of Environmental Protection (FDEP) has been wrestling with this question for quite some time.  The 1996 amendment of the Safe Drinking Water Act required each state to have a Source Water Assessment and Protection program to proactively protect the State's water resources from potential contamination.

As one approach to better understand aquifer vulnerability, the FDEP came up with a Geographic Information Systems methodology known as the Florida Aquifer Vulnerability Assessment model. I contributed to this work during my time as a researcher with FDEP some years ago.  This model uses the Weights of Evidence method which is a Bayesian statistical method based on probability. The basic idea was to create GIS layers representing different geological conditions or predictors of vulnerability such as a) density of/proximity to sinkholes b) soil permeability c) thickness of formations d) depth to water table/confined aquifer. Known occurrences of contamination (example: levels of nitrate above a certain baseline measured from monitoring wells) were used to develop a spatial correlation between contamination and a geological condition. Each unit area in each layer was weighted based on the strength of the correlation. Then the layers were added in a GIS (representation on left above: Source- FDEP)  As the same geographic location containing different geological indicators was superposed, individual weights from the different layers got added to give a cumulative weight for that location.

The spatial distribution of cumulative weights was represented by a probability map of relative vulnerability.

The poster below summarizes some results from a selection of Florida counties.

 Source: FDEP

The tool is now being used by environmental managers and regulatory and planning professionals to better protect Florida's groundwater quality.

Coming back to sinkholes the results of this long drawn out GIS project were what common sense would expect. Proximity to sinkholes was a positive predictor of aquifer vulnerability, meaning, the closer fertilizer and pesticide use is to a sinkhole, the greater the chances that the groundwater would get contaminated. That does offer environmental managers a firm statistical footing to put in place best land use practices such as having conservation buffers i.e. strips of vegetation around the sinkhole that will stop agricultural runoff, prohibiting dumping of any sort in the vicinity of sinkholes and even fencing the sinkhole to prevent animal waste from making its way into the groundwater.

No comments:

Post a Comment