10-12. Diagrams showing: 10. Idealized fifth-order cycles for the Fort Thompson Formation and Miami Limestone showing relations between lithofacies, depositional environments, porosity (pore classes
Combined analyses of cores, borehole geophysical logs, and cyclostratigraphy produced a new conceptual hydrogeologic framework for the triple-porosity (matrix, touching-vug, and conduit porosity) karst limestone of the Biscayne aquifer in a 0.65 km 2 study area, SE Florida. Vertical lithofacies successions, which have recurrent stacking patterns, fit within high-frequency cycles. We define three ideal highfrequency cycles as: (1) upward-shallowing subtidal cycles, (2) upward-shallowing paralic cycles, and (3) aggradational subtidal cycles. Digital optical borehole images, tracers, and flow meters indicate that there is a predictable vertical pattern of porosity and permeability within the three ideal cycles, because the distribution of porosity and permeability is related to lithofacies. Stratiform zones of high permeability commonly occur just above flooding surfaces in the lower part of upward-shallowing subtidal and paralic cycles, forming preferential groundwater flow zones. Aggradational subtidal cycles are either mostly high-permeability zones or leaky, low-permeability units. In the study area, groundwater flow within stratiform high-permeability zones is through a secondary pore system of touching-vug porosity principally related to molds of burrows and pelecypods and to interburrow vugs. Movement of a dye-tracer pulse observed using a borehole fluid-temperature tool during a conservative tracer test indicates heterogeneous permeability. Advective movement of the tracer appears to be most concentrated within a thin stratiform flow zone contained within the lower part of a high-frequency cycle, indicating a distinctly high relative permeability for this zone. Borehole flow-meter measurements corroborate the relatively high permeability of the flow zone. Identification and mapping of such high-permeability flow
Proposed expansion of extractive lime-rock mines near the Miami-Dade County Northwest well field and Everglades wetland areas has garnered intense scrutiny by government, public, environmental stakeholders, and the media because of concern that mining will increase the risk of pathogen contamination. Rock mines are excavated to the same depth as the well field's primary producing zone. The underlying karst Biscayne aquifer is a triple-porosity system characterized by (1) a matrix of interparticle porosity and separate vug porosity; (2) touching-vug porosity that forms preferred, stratiform passageways; and, less commonly, (3) conduit porosity formed by thin solution pipes, bedding-plane vugs, and cavernous vugs. Existing ground-water flow and particle tracking models do not provide adequate information regarding the ability of the aquifer to limit the advective movement of pathogens and other contaminants. Chemical transport and colloidal mobility properties have been delineated using conservative and microsphere-surrogate tracers for Cryptosporidium parvum. Forced-gradient tests were executed by introducing conservative tracers into injection wells located 100 m (328 ft) from a municipal-supply well. Apparent mean advective velocity between the wells is one to two orders of magnitude greater than previously measured. Touching-vug, stratiform flow zones are efficient pathways for tracer movement at the well field. The effective porosity for a continuum model between the point of injection and tracer recovery ranges from 2 to 4 percent and is an order of magnitude smaller than previously assumed. Existing well-field protection zones were established using porosity estimates based on specific yield. The effective, or kinematic, porosity of a Biscayne aquifer continuum model is lower than the total porosity, because high velocities occur along preferential flow paths that result in faster times of travel than can be represented with the ground-water flow equation. Tracer tests indicate that the relative ease of contaminant movement to municipal supply wells is much greater than previously considered.
[1] The Biscayne aquifer is a highly transmissive karst limestone that serves as the sole source of drinking water to over two million residents in south Florida. The aquifer is characterized by eogenetic karst, where the most transmissive void space can be an interconnected, touching-vug, biogenically influenced porosity of biogenic origin. Public supply wells in the aquifer are in close proximity to lakes established by surface mining. The mining of the limestone has occurred to the same depths as the production wells, which has raised concerns about pathogen and chemical transport from these surface water bodies. Hydraulic and forced gradient tracer tests were conducted to augment geologic and geophysical studies and to develop a hydrogeologic conceptual model of groundwater flow and chemical transport in the Biscayne aquifer. Geologic and geophysical data indicate multiple, areally extensive subhorizontal preferential flow zones of vuggy limestone separated by rock with a matrix pore system. The hydraulic response from an aquifer test suggests that the Biscayne aquifer behaves as a dual-porosity medium; however, the results of the tracer test showed rapid transport similar to other types of karst. The tracer test and concurrent temperature logging revealed that only one of the touching-vug flow zones dominates transport near the production wells. On the basis of the rising limb of the breakthrough curve, the dispersivity is estimated to be less than 3% of the tracer travel distance, which suggests that the fastest flow paths in the formation are likely to yield limited dilution of chemical constituents.
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