Sinkhole and buried sinkhole densities and new sinkhole frequencies in karsts of northwest Peninsular Florida

In karst terranes of the northwest quarter of the Florida Peninsula, topographic maps having contour intervals of 1.5 or 3 m (5 or 10 feet) at a scale of 1:24,000, depict sinkholes (SH) having modal densities in the 0–3 SH/km² (0–9 SH/mi²) class in the Gulf Coastal Lowlands, 4–7 SH/km² (10–19 SH/mi²) in portions of the Lake Wales and Mt. Dora Ridges, and 15–19 SH/km² (40–49 SH/mi²) in the Western Valley. All karsts in the northwest quarter of the Florida Peninsula contain sand-filled, buried sinkholes (BSH) that perforate the subsurface, clayey, uncemented formations and/or soils above the Tertiary carbonates. Ground penetrating radar surveys mapped 56–250 BSH/km² (145–650 BSH/mi²) at 5 upland sites and 460–3,400 BSH/km² (1,200–8,700 BSH/mi²) at 10 lowland sites. The average buried sinkhole density is 150 BSH/km² (400 BSH/mi²) for upland sites and 1,200 BSH/km² (3,100 BSH/mi²) for lowland sites. Dissolutional openings (DO) in the top of the Floridan Aquifer occur with densities ranging from 12,300– 730,000 DO/km² (32,000–1,900,000 DO/mi²). New sinkholes (NSH) develop at minimum documented frequencies ranging from less than 0.0004 NSH/km²/yr (0.001 NSH/mi²/yr) to a maximum of 1.9 NSH/km²/yr (4.8 NSH/mi²/yr). The typical NSH frequency is approximately 0.04 NSH/km²/yr (0.1 NSH/mi²/yr), but, after adjustment for under-reporting, it is thought to be approximately 0.4 NSH/km²/yr (1 NSH/mi²/yr). The sand-filled buried sinkholes are high-permeability pathways through the clay layer. Commonly, contaminant migration from the surficial sand is concentrated into the buried sinkholes and passes through the sand-filled throats into cave networks. This contaminant dispersion pattern must be considered for the design of effective ground-water quality monitoring networks, but it is commonly and erroneously ignored. The density of buried sinkholes and dissolutional openings demonstrates why the entire karst terrane must be considered vulnerable to ground-water contamination and sinkhole development, rather than merely the areas immediately above and within obvious topographic sinkholes.