Browse graphic of the conceptual model of the karst subterranean estuary in an unconfined coastal aquifer setting (top). Mixing between freshwater and marine-derived saline groundwater occurs within the aquifer’s porous geologic setting, including its sinkholes and cave conduits. Mixing zones were accessed through flooded cave networks to obtain high-resolution vertical profiles of chemical parameters (bottom box). Samples were collected with 12-cm spacing in (a) January 2015 and (b) January 2016, as well as with the 2.5-cm spacing (c) in January 2016. The blue line is the sulfate to chloride ratio (0.0515) of the regional seawater.