Using multiple geochemical tracers to characterize the hydrogeology of thesubmarine spring off Crescent Beach, Florida

A spectacular submarine spring is located about 4 km east of Crescent Beach , FL, in the Atlantic Ocean. The single vent feature of Crescent Beach Spri ng provides a unique opportunity to examine onshore -offshore hydrogeologic processes, as well as point source submarine ground water discharge. The F loridan aquifer system in northeastern Florida consists of Tertiary intersp ersed limestone and dolomite strata. Impermeable beds confine the water-bea ring zones under artesian pressure. Miocene and younger confining strata ha ve been eroded away at the vent feature, enabling direct hydrologic communi cation of Eocene ground water with coastal bottom waters. The spring water had a salinity of 6.02, which was immediately diluted by a mbient seawater during advection/mixing. The concentration of major solutes in spring water and onshore well waters confirm a generalized easterly flo w direction of artesian ground water. Nutrient concentrations were generall y low in the reducing vent samples. and the majority of the total nitrogen species existed as NH3. The submarine ground water tracers, Rn-222 (1174 dp m l(-1), dpm), methane (232 nM) and barium (294.5 nM) were all highly enric hed in the spring water relative to ambient seawater. The concentrations of the reverse redox elements U, V and Mo were expectedly low in the submarin e waters. The strontium isotope ratio of the vent water (Sr-87/Sr-86 = 0.70 798) suggests that the spring water contain an integrated signature indicat ive of Floridan aquifer system ground water. Additional Sr isotopic ratios from a series of surficial and Lower Floridan well samples suggest dynamic ground water mixing, and do not provide clear evidence for a single hydroge ologic water source at the spring vent. In this karst-dominated aquifer, su ch energetic mixing at the vent feature is expected, and would be facilitat ed by conduit and fractured flow. Radium isotope activities were utilized t o estimate flow-path trajectories and to provide information on potential t ravel times between an onshore well and the spring. Using either Ra-223 and Ra-224 or Ra-228, and qualifying this approach with several key assumption s, estimates of water mass travel times from an upper Floridan well in Cres cent Beach to the submarine vent feature (distance = 4050 m) are in the ord er of similar to 0.01-0.1 m min(-1). (C) 2001 Elsevier Science B.V. All rig hts reserved.