Geochemical indicators of groundwater recharge in the surficial aquifer system, Everglades National Park, Florida, USA

A geochemical investigation (major cations and anions, stable isotopes of oxygen and hydrogen, pH, and salinity) was conducted to identify the sources of groundwater recharge to the surficial aquifer system in Everglades National Park. The weighted mean values of σ ¹⁸ O and σD of rainfall were -2.83% and -10.59%, respectively. A mean deuterium excess value of 12 suggests that evaporation of Everglades surface water contributes between 7% and 12% to the local precipitation. Most shallow groundwater in the surficial aquifer system (<28 m) is recharged throughout the year by Everglades surface water and or canal water exposed to evaporation. Recharge rates between 2 cm/yr and 12 cm/yr were obtained, with the higher rates in areas of little to no standing water. Deep groundwater in the surficial aquifer system (>28 m) is recharged directly from rainfall far upgradient of the northern boundary of Everglades National Park. Groundwater from the underlying Hawthorn Group is geochemically distinct from the surficial aquifer system and recharges the surficial aquifer system from below. There is no geochemical evidence of surface water orshallow groundwater flow between the two major waterways (Shark Slough and Taylor Slough) in Everglades National Park. In this investigation, a combination of stable isotopes (σ ¹⁸ O and σD) and major-ion data was necessary to identify different sources of groundwater recharge to the karst aquifer. The stable isotopes (σ18O and σD) were most useful in deciphering between rainfall and surface-water recharge to the shallow aquifer, whereas the major-ion data were used to identify recharge from deeper aquifers and seawater intrusion.