ASSESSMENT OF KARST SURFACE FEATURES IN PINELLAS COUNTY

Robert Brinkmann; Department of Environmental Science and Policy, University of South Florida
H. Leonard Vacher, Lee J. Florea; Department of Geology, University of South Florida

PROJECT BACKGROUND

Pinellas County, in West Central Florida, is located in one of the more active and least understood karst regions of the world. Some of the more common karst features found in this area are sinkholes. The topographic expression of these sinkholes is partially masked by Holocene sand deposits and recent urban development. Urbanization results in filled depression features and modifications to others for storm water retention. Mapping and analysis of pre- and post-development sinkholes have not been completed to assess the distribution of sinkholes and document changes in topography due to urbanization in Pinellas County. This is problematic in that Pinellas County, like much of the Tampa Bay region continues to urbanize rapidly and is likely to undertake significant construction projects that have the potential to directly impact the karst landscape. Without a clear understanding of where karst processes are active, it is difficult to implement appropriate land use and management decisions suitable for the geology and populace of the region.

It is clear from studies of sinkholes in undeveloped portions of Florida that they occur in particular clusters or "sinkhole regions". Regions of past sinkhole formation are locations where modern sinkhole formation may also occur. Certainly there are sinkhole regions that are hidden by the urbanized nature of areas such as Pinellas County. Mapping karst surface features using historic air photos and maps will be a useful exercise that will assist our scientific understanding of karstification in Florida and the nature and extent of karst processes that have acted in the pre-urbanized past.

In addition, Pinellas County has acquired data that have not been analyzed to assess the county geomorphology. LIDAR has been used previously to map existing depressions that exist below the contour-interval resolution of available topographic maps. We propose to use the existing LIDAR dataset to map surface depressions in the county to a resolution applicable to a countywide karst analysis.

BACKGROUND:

Karst, a landscape and collection of landforms produced from the dissolution of limestone and other soluble rocks, is present in many regions throughout the world and is of distinct interest to individuals and organizations associated with these landscapes. This is due to unique engineering difficulties and contaminant transport properties of karst aquifers. This project will occur within a portion of the unconfined Floridan Aquifer System (hereinafter referred to as the UCFAS) in Pinellas County of West Central Florida.

Like many carbonate terrains, the UCFAS is modified by dissolution processes resulting in karst (White, 1988; Miller, 1986; Copeland, 1991, White, 1970; Lane, 1986, Schmidt and Scott, 1984). Karstification has developed significant solution porosity throughout the stratigraphic framework of the platform and is a dominant component of groundwater flow. The complex nature of conduit systems in the UCFAS is not well understood (Wilson, 2002). This is primarily because of the difficulty nature of locating and exploring submerged conduits.

Voids and conduits within the vadose zone do exist throughout the UCFAS and are concentrated primarily in the uplands of the panhandle, along the Cody Scarp and the Ocala Uplift at the boundary between confinement and unconfinement. These dissolution features can be subdivided into two primary categories: those that form in vadose conditions and those that are a reflection of a prior phreatic surface. These latter voids may serve as a key to understanding present speleogenetic processes below the present water table and may also help link speleogenetic processes to sea-level change.

The integration and morphology of conduits is an essential element of any groundwater flow model. In the UCFAS, conduit integration and morphology is potentially a reflection of the young diagenetic state of the host rock, or eogenetic karst (Vacher and Mylroie, 2002), mixing zone dissolution (Plummer, 1975), biogeochemical processes (Garman, 2002), and glacio-eustatic overprinting (Whitaker and Smart, 1997a; Brinkmann and Reeder, 1994).

In Pinellas County, surficial expression of karst is limited to sinkholes, sinkhole-associated features, and springs. These features provide distinctive problems for planners and environmental scientists in the regions. Land stability is perhaps the largest issue for citizens, although contaminatant transport and water quantity are also important issues.

Sinkholes can form from a variety of processes (White, 1988). Those found in Pinellas County are cover-collapse sinkholes that form when Pleistocene sands above limestone ravel into void space in bedrock. The formation of these sinkholes appears linear on many Florida maps, mainly due to the fact that sinkholes typically form along linear joint patterns in the limestone bedrock.

There have been attempts to map or describe karst features in Pinellas County in the past. For example, Sinclair and others (1985) discussed sinkholes in west central Florida and described their formation based upon cover and bedrock type. They also reviewed data on recently formed sinkholes in the region. However, they did not map the sinkholes of the region. Frank and Beck (1991) assessed the causes of subsidence damage in northern Pinellas County in Dunedin. They examined the mechanisms behind the formation of subsidence in the area in the years 1990-1991. They found that the damage was caused by a variety of factors including sinkhole activity, soil compaction, and shrink/swell clays.

In 1991, Beck and Sayad (1991) published a summary of the sinkhole hazard in Pinellas County. They utilized reported sinkhole events to assess the overall hazard to the area from subsidence. Mark Stewart and others (1995) also examined foundation failures in Pinellas County to assess the causes of failures. More recently, the Florida Geological Survey has mapped closed depressions on USGS topographic maps.

Although all of the past research is useful, to date, there has not been a concentrated effort to map the array of sinkholes that are present in Pinellas County. This project will complete this effort by mapping sinkholes using historic air photos and LIDAR data.

LIDAR (Light Detection and Ranging) can be used for a variety of purposes in geomorphic and vegetative studies (Woolard and Colby, 1998; Brock and Sallenger, 2001; Carusso, 2001; and Hansen and others, 2002). Recently, there has been interest in the use of LIDAR in mapping sinkhole occurrence (Montane, 2002). This report will build upon previous work in LIDAR by using LIDAR information to map depressions within Pinellas County.

In summary, it is the purpose of this project to map sinkholes covered through the process of urbanization that are visable on 1926 air photos. These data will be compared with sinkholes mapped on more recent air photos and with depressions mapped using LIDAR data.