OCCURRENCE OF FECAL INDICATOR BACTERIA IN SURFACE WATERS AND THE SUBSURFACE AQUIFER IN KEY-LARGO, FLORIDA

Sewage waste disposal facilities in the Florida Keys include septic ta nks and individual package plants in place of municipal collection fac ilities in most locations. In Key Large, both facilities discharge int o the extremely porous Key Largo Limestone. To determine whether there was potential contamination of the subsurface aquifer and nearby coas tal surface waters by such waste disposal practices, we examined the p resence Of microbial indicators commonly found in sewage (fecal colifo rms, Clostridium perfringens, and enterococci) and aquatic microbial p arameters (viral din:ct counts, bacterial direct counts, chlorophyll a , and marine vibriophage) in injection well effluent, monitoring wells that followed a transect from onshore to offshore, and surface waters above these wells in two separate locations in Key Large in August 19 93 and March 1994. Effluent and waters from onshore shallow monitoring wells (1.8- to 3,7-m depth) contained two or all three of the fecal i ndicators in all three samples taken, whereas deeper wells (10.7- to 1 2,2-m depth) at these same sites contained few or none. The presence o f fec:al indicators was found in two of five nearshore wells (i.e., th ose that were less than or equal to 1.8 miles [less than or equal to 2 .9 km] from shore), whereas offshore wells (greater than or equal to 2 .1 to 5.7 miles [less than or equal to 3.4 to 9.2 km] from shore) show ed little sign of contamination, Indicators were also found in surface waters in a canal in Key Large and in offshore surface, waters in Mar ch but not in August. Collectively, these results suggest that fecal c ontamination of the shallow onshore aquifer, parts of the nearshore aq uifer, and certain surface waters has occurred. Current sewage;waste d isposal practices may have contributed to this contamination.