Bimodal transport of a waste water plume injected into saline ground waterof the Florida Keys

Two experiments were conducted on Long Key, Florida, United States, to exam ine the fate of waste water following sewage disposal in 10 to 30 m deep in jection wells. This waste disposal practice introduces extraordinary amount s of nutrients into the ground water of the Florida Kegs. In these experime nts, artificial ground water tracers, sulfur hexafluoride (SF,) and radioio dine (I-131) were used to determine transport rates and directions of solub le nonreactive substances injected into the saline ground water underlying the Keys. Toro types of transport were observed: (1) rapid flow (0.20 to 2. 20 m/hr) presumably due to the many conduits present in the limestone; and (2) slower flow (less than 0.003 to 0.14 m/hr) associated with the limeston e's primary porosity, Vertical flow was comparable to horizontal flow due t o either the density-driven buoyancy of the waste mater plume or to prefere ntial flowpaths that allow upward advection or combination of both. These e xperiments showed that conservative artificial tracers injected into the su bsurface reach surface water in a matter of days and can remain in the imme diate vicinity of the injection well for several months.