Lee J. Florea

Sinkhole distribution based on pre-development mapping in urbanized Pinellas County, Florida, USA

Abstract Locating sinkholes in Pinellas County, Florida, is confounded by the presence of a cover of Quaternary sediments that mute the surface appearance of these sinkholes. As a first step in addressing the sinkhole hazard in the county, we analysed aerial photographs from 1926 and 1995 that covered the entire county. We digitized all identifiable sinkholes in each set of photographs in a GIS (Geographical Information System) using a set of criteria established to differentiate between karst depressions and depressions resulting from other geological processes. The 1926 photographs, although of low quality, helped to establish a baseline prior to urbanization. The 1995 photographs provided a post-urbanization distribution of natural sinkholes and man-made depression features (e.g. retention ponds). From these two data sets, we are able to assess natural and anthropogenic changes in the karst landscape of the study area. In particular, we discovered that 87% of the sinkhole features identified in the 1926 photographs are no longer present in the photographs from 1995. Many of the lost depressions have been incorporated into retention ponds.

Stable isotopic and geochemical variability within shallow groundwater beneath a hardwood hammock and surface water in an adjoining slough (Everglades National Park, Florida, USA).

Data from a 10-month monitoring study during 2007 in the Everglades ecosystem provide insight into the variation of δ18O, δD, and ion chemistry in surface water and shallow groundwater. Surface waters are sensitive to dilution from rainfall and input from external sources. Shallow groundwater, on the other hand, remains geochemically stable during the year. Surface water input from canals derived from draining agricultural areas to the north and east of the Everglades is evident in the ion data. δ18O and δD values in shallow groundwater remain near the mean of−2.4 and−12 ‰, respectively. 18O and D values are enriched in surface water compared with shallow groundwater and fluctuate in sync with those measured in rainfall. The local meteoric water line (LMWL) for precipitation is in close agreement with the global meteoric water line; however, the local evaporation line (LEL) for surface water and shallow groundwater is δ D=5.6 δ18O+1.5, a sign that these waters have experienced evaporation. The intercept of the LMWL and LEL indicates that the primary recharge to the Everglades is tropical cyclones or fronts. δ deuterium to δ18O excess (Dex values) generally reveal two moisture sources for precipitation, a maritime source during the fall and winter (D ex>10 ‰) and a continental-influenced source (D ex<10 ‰) in the spring and summer.

Sedimentation and porosity enhancement in a breached flank margin cave

San Salvdor Island, Bahamas, provides unique opportunities to study modern geologic processes on carbonate platforms as a result of constraints in time and space. The time span of exposed geology is limited to the middle Pleistocene through Holocene (<500 ka), and the island lies on an isolated platform (12 by 19 km). Altar Cave, formed within an oxygen isotope substage 5e eolianite (approximately 125 ka) of the Grotto Beach Formation on San Salvador, is a classic example of a flank margin cave that has been exposed during hillslope retreat. The nature of Altar Cave (restricted entrance, simplistic morphology, and easy access) facilitates a sedimentation study. Sediment profiles from trenches dug at three locations in Altar Cave show that the deposits in the cave formed as an early stage of development of a Holocene strand plain that is present today between the cave and the beach. Altar Cave was breached by Holocene coastal processes;14C dates show sand fill deposits in the cave to be Holocene (4.7 ka).14C dates, XRD, and geochemical analyses show the surficial sediment to be recent (0.6 ka), and that leaching has altered the bedrock floor of the cave. Petrologic study of the floor rock has provided evidence of autogenic sedimentation prior to breaching of the cave in the form of dissolution residuum accumulating during cave development. Petrologic analysis shows that this leaching has resulted in increased bedrock porosity below the sediment profile. Also, introduced organics have contaminated the late Pleistocene bedrock with young carbon, resulting in14 C ages of 14 ka at 0.3 m in depth and 28 ka at 1.3 m in depth. The results of this study demonstrate a potential method of porosity enhancement in young carbonates by vadose leaching. Porosity-enhanced zones have implications for our understanding of recharge to fresh-water lenses on carbonate islands.

NMR Imaging of Fluid Exchange between Macropores and Matrix in Eogenetic Karst

Sequential time-step images acquired using nuclear magnetic resonance (NMR) show the displacement of deuterated water (D(2)O) by fresh water within two limestone samples characterized by a porous and permeable limestone matrix of peloids and ooids. These samples were selected because they have a macropore system representative of some parts of the eogenetic karst limestone of the Biscayne Aquifer in southeastern Florida. The macroporosity, created by the trace fossil Ophiomorpha, is principally well connected and of centimeter scale. These macropores occur in broadly continuous stratiform zones that create preferential flow layers within the hydrogeologic units of the Biscayne. This arrangement of porosity is important because in coastal areas, it could produce a preferential pathway for salt water intrusion. Two experiments were conducted in which samples saturated with D(2)O were placed in acrylic chambers filled with fresh water and examined with NMR. Results reveal a substantial flux of fresh water into the matrix porosity with a simultaneous loss of D(2)O. Specifically, we measured rates upward of 0.001 mL/h/g of sample in static conditions, and perhaps as great as 0.07 mL/h/g of sample when fresh water continuously flows past a sample at velocities less than those found within stressed areas of the Biscayne. These experiments illustrate how fresh water and D(2)O, with different chemical properties, migrate within one type of matrix porosity found in the Biscayne. Furthermore, these experiments are a comparative exercise in the displacement of sea water by fresh water in the matrix of a coastal, karst aquifer since D(2)O has a greater density than fresh water.