Abstract
The Florida Department of Health (FDOH) conducts sampling at recreational beaches for Enterococcus and fecal coliform bacteria, and posts advisories recommending against beach swimming when reported concentrations exceed 104 #CFU/100mL for enterococcus and 400#CFU/100mL for fecal coliform). County agencies select beaches to sample, and collect surf-zone samples once weekly. Bacteria concentrations fluctuate rapidly and over short distances, and beach advisories are posted up to four days after high bacteria is first observed (field resampling and laboratory testing consume several days), seriously compromising FDOH's capability to protect beachgoers from exposure to harmful conditions. Precautionary measures could be improved if we could reliably predict conditions under which high bacteria concentrations are likely to occur. The objective of this research is to identify conditions statistically related to high bacterial concentrations using data from ten years of weekly sampling at 16 beaches in Sarasota County, FL. Previous studies in California, Miami FL, and elsewhere, analyzing the immediate vicinity of ocean stormwater outfalls, have identified modest relationships to factors including: magnitude and intensity of storm events that may mobilize bacteria from coastal watersheds; length of antecedent dry period before mobilizing storm, during which bacteria may propagate in conveyances; and tide/surf conditions that may either mobilize bacteria propagating in beach sand, break up bacterial colonies in the surf zone, or generate where bacteria may survive near the shore. The present study attempted multiple regression relating bacteria conditions to explanatory factors including: watershed rainfall in days prior to the sample date; total rainfall over several months, a surrogate for saturation of soils in the tributary watershed as well as for dry periods when bacteria may multiply in conveyances; seasonality, a surrogate for high air and water temperatures that promote propagation in the environment; and tidal stages and levels. Results found no correlation greater than 20%, for Sarasota County beaches in aggregate or for any beach individually, with the magnitude of bacteria measured in samples. Relationships are, however, identified between frequency of high-count bacteria samples and environmental conditions including flood level and stage, one-day antecedent rainfall, and seasonal rainfall (wet vs dry season). Results show the relationships are stronger for beaches influenced by stormwater outfalls, and less evident in beaches where no outfalls are nearby. This demonstrates that different mechanisms affect high-bacteria conditions on different beaches in this region, implying that prediction of potentially unhealthful bacteria conditions may be possible on a site-specific basis but may not be supported for a uniform set of environmental parameters across multiple sites, even for a group of beaches in close proximity with similar precipitation, tidal, and seasonal characteristics