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

Two experiments were conducted on Long Key, Florida, United States, to examine the fate of waste water following sewage disposal in 10 to 30 m deep injection wells. This waste disposal practice introduces extraordinary amounts of nutrients into the ground water of the Florida Keys. In these experiments, artificial ground water tracers, sulfur hexafluoride (SF₆) and radioiodine (¹³¹I) were used to determine transport rates and directions of soluble nonreactive substances injected into the saline ground water underlying the Keys. Two 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 limestone's primary porosity. Vertical flow was comparable to horizontal flow due to either the density-driven buoyancy of the waste water plume or to preferential flowpaths that allow upward advection or combination of both. These experiments showed that conservative artificial tracers injected into the subsurface reach surface water in a matter of days and can remain in the immediate vicinity of the injection well for several months.