The Floridan aquifer provides potable water for much of the southeastern US, but similar to most karst aquifers is susceptible to contamination because of extensive and rapid mixing between surficial and ground water. Mixing is particularly prevalent at the boundary between confined and unconfined portions of the aquifer. Such a boundary occurs along the Cody Scarp in north-central Florida, where the Santa Fe River flows into a 36-m deep sinkhole called the River Sink. Approximately 5 km south of the River Sink, a first magnitude spring called the River Rise represents the headwaters of the lower Santa Fe River. Large conduits connect the River Sink to the River Rise as well as several intermediate karst windows, as shown by cave diving exploration as well as previous tracer studies using both natural and injected tracers. Major element concentrations and physical models indicate that the source of water at the River Rise depends on discharge into the River Sink, but the location, timing, and volumes of mixing between the surface and ground waters remain unclear. We hypothesize here that ²²²Rn provides a good tracer to determine the volumes of mixing. Radon-222 is a radioactive (t_1/2=3.82 days) daughter of ²²⁶Ra, and thus its activity is elevated in Floridan aquifer water because of abundant Ra in the carbonate rocks of the aquifer and the overlying clay-rich confining unit (Hawthorn Formation). Radon volatizes from surface water to the atmosphere, causing low concentrations in surface water. Its atmospheric concentration is also low because of its short half life. Consequently, ²²²Rn activities in ground water are elevated over those in surface water. We have collected water samples in depth profiles at the River Sink and Rise in an attempt to determine the boundary between surface and ground water in these sinkholes and to determine end-member activities in the water. With these values it should be possible to determine mixing proportions of surface and ground water. An extended drought over the past two years, however, has prevented sampling during floods when surface and ground water mixing should be most extensive. Because of the differences in ²²²Rn activities between surface and ground water, it should provide a robust tracer of mixing between the surface and ground water of most carbonate karst aquifers.