PATHOGEN TRANSPORT IN A SOLE SOURCE KARST AQUIFER NEAR A PUBLIC WELL FIELD – WILL EVERGLADES LIMESTONE MINE EXPANSION POSE A CLEAR AND PRESENT DANGER TO PUBLIC HEALTH?

A borrow-pit mining district buffer area separates urban-residential areas from the Everglades ecosystem, its wetlands, and water conservation areas. The Miami-Dade County Northwest municipal well field, Florida’s largest, is located on the mining district’s eastern margin which is being transformed from maleuca-infested wetlands into deep quarry lakes. Some lakes are the same depth as the 60-80 ft deep production zone used for water supply. Government, public, environmental stakeholders and the media are closely reviewing industry proposals to expand mining operations within 800 to 3,000 ft of the well field. Increased risk for well-field pathogen contamination from surface water has contributed to cryptosporidiosis public health concerns. Cryptosporidium parvum protozoan contamination could pose serious consequences for a municpal water supply, and would increase treatment costs.

In the well field, the Biscayne aquifer’s dual-porosity system comprises vertically arranged, diffuse-carbonate and preferential conduit flow zones; a highly heterogeneous and anisotropic permeability is largely related to secondary porosity. Large touching-vug solution features form conduit flow zones; small-scale vugs and carbonate matrix comprise diffuse-carbonate flow units. Laboratory and in-situ tracer field experiments have provided insight about microbial transport within a preferential karst flow zone. Static and flow-through column experiments have demonstrated that ~2-, ~3-, and ~5-µm size carboxylated-polystyrene microspheres are a suitable surrogate for the Cryptosporidium pathogen. Aquifer performance and borehole flowmeter test, and convergent tracer experiments were conducted using conservative tracers and microspheres to assess microbial filtration efficacy. Rhodamine WT and deuterium were introduced into a 65-ft deep open borehole 333 ft from a production well; apparent mean advective flow velocity was one order of magnitude or greater than model simulated velocity between the injection point and the production wellhead. Analysis of deuterium, SF6, halon and microsphere tracer breakthrough data from a second test is being conducted; these tracers were injected into an upper flow zone to assess öocyst pathogen removal within a highly permeable 2-ft thick conduit flow zone.