USE OF POLYDISPERSE MICROSPHERES AND SIZE- AND FLUORESCENCE-DISCRIMINATION FLOW CYTOMETRY TO ESTIMATE THE VULNERABILITY TO CRYPTOSPORIDIUM PARVUM OF A SOLE-SOURCE, KARST-LIMESTONE AQUIFER IN SOUTHEASTERN FLORIDA

Municipal well vulnerability in the Northwest Well Field in Miami-Dade County in southeastern Florida to potential contamination by oocysts of the protozoan pathogen, Cryptosporidium parvum, was assessed in a large-scale, forced-gradient (convergent) injection and recovery test. The field study involved a simultaneous pulse introduction of a conservative tracer (SF6, an inert gas) and a polydispersed suspension of oocyst-sized (1.6 ìm, 2.9 ìm, and 4.9 ìm diameter), carboxylated polystyrene microspheres into a section of Biscayne aquifer karstic limestone characterized by complex, triple (matrix, touching-vug, and conduit) porosity. Recoveries 97 m downgradient from the injection borehole were inversely related to diameter and ranged from 2.8 to 5.6%, respectively. The centers of mass for the microspheres arrived at the pumping well ~3-fold earlier than that of the SF6 further underscoring the need for use of colloid tracers and field-scale tracer tests for evaluating time-of-travel well-field protection zones in the Biscayne. In a bench-scale test involving a modified triaxial cell using near in-situ chemical conditions, the microspheres underestimated by several fold the transport potential of the less-electronegative oocysts within interparticle matrix porosity of limestone core samples. Field-scale microsphere test results and the aforementioned bench-scale comparisons of microsphere and oocyst transport behaviors collectively suggest buffer areas much larger than 100 m are required to achieve a ten-fold reduction of oocysts, even though the limestone surfaces exhibit a substantive capability for their sorptive removal. The study further demonstrated the utility of size- and fluorescence-discrimination flow cytometry and the use of large quantities of microspheres in a polydispersed suspension as oocyst surrogates in field-scale assessments of well vulnerability, provided that electrochemical differences between oocysts and microspheres are taken into account.