DELINEATING SOURCES OF MOISTURE AND RECHARGE TO THE EVERGLADES ECOSYSTEM OF SOUTH FLORIDA, USA, USING STABLE ISOTOPES OF OXYGEN, HYDROGEN, AND CARBON

Data from a ten-month monitoring study during 2007 in south Florida provide insight into the variation of δ¹⁸O, δD, and δ¹³C in surface water and shallow groundwater of the Everglades ecosystem. Bi-monthly samples were taken from surface water and time-averaged precipitation at Taylor Slough, and shallow groundwater from a well and cave within nearby Palma Vista Hammock.

δ¹⁸O and δD in shallow groundwater from the well and cave remain near the mean of -2.4 ‰ and -12 ‰, respectively (VSMOW scale). ¹⁸O and D are enriched in surface water compared to shallow groundwater. δ¹⁸O and δD in surface water fluctuates in sync with, but of lesser amplitude than, those measured in rainfall. The LMWL for precipitation is in close agreement to the GMWL; however, the LEL for surface water and shallow groundwater is δD = 5.6 δ¹⁸O + 1.5, a sign that these waters have experienced evaporation. The intercept of the LMWL and LEL indicates that the primary recharge to the Everglades occurs primarily from tropical or frontal sources. Local convection merely recycles available water.

Time-series of deuterium excess (d_ex), clearly reveals two moisture sources for precipitation; an evaporation-dominated source with d_ex>10 and a source significantly influenced by transpiration with d_ex<10. Samples with higher d_ex cluster in the fall and winter, and appear to be associated with maritime moisture carried along the Trade Winds. Samples with lower d_ex cluster in the late spring and summer, and could reflect continental moisture carried by the Westerlies or local convection.

δ¹³C_DOC suggests C-3 vegetation as the primary organics source at all sample sites. C:N ratios averaging 20:1 at the cave indicate organics originate from woody material, while an average of 15:1 at the well along with δ¹³C_DOC similar to the cave indicate further decomposition of the organics entering the cave. C:N ratios at the slough averaged 15:1, and with δ¹³C_DOC, suggest a blend of organics not present at the cave and well.