Stable isotopic and geochemical variability within shallow groundwater beneath a hardwood hammock and surface water in an adjoining slough (Everglades National Park, Florida, USA)

Abstract

Data from a 10-month monitoring study during 2007 in the Everglades ecosystem provide insight into the variation of δ18O, δD, and ion chemistry in surface water and shallow groundwater. Surface waters are sensitive to dilution from rainfall and input from external sources. Shallow groundwater, on the other hand, remains geochemically stable during the year. Surface water input from canals derived from draining agricultural areas to the north and east of the Everglades is evident in the ion data. δ18O and δD values in shallow groundwater remain near the mean of−2.4 and−12 ‰, respectively. 18O and D values are enriched in surface water compared with shallow groundwater and fluctuate in sync with those measured in rainfall. The local meteoric water line (LMWL) for precipitation is in close agreement with the global meteoric water line; however, the local evaporation line (LEL) for surface water and shallow groundwater is δ D=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 is tropical cyclones or fronts. δ deuterium to δ18O excess (Dex values) generally reveal two moisture sources for precipitation, a maritime source during the fall and winter (D ex>10 ‰) and a continental-influenced source (D ex<10 ‰) in the spring and summer.