The case of the Biscayne Bay and aquifer near Miami, Florida: density-driven flow of seawater or gravitationally driven discharge of deep saline groundwater?

Abstract

Coastal groundwater flow investigations at the Cutler site of the Biscayne Bay south of Miami, Florida, gave rise to the dominating concept of density-driven flow of seawater into coastal aquifers indicated as a saltwater wedge. Within that wedge, convection-type return flow of seawater and a dispersion zone were concluded to be the cause of the Biscayne aquifer ‘seawater wedge.’ This conclusion was merely based on the chloride distribution within the aquifer and on an analytical model concept assuming convection flow within a confined aquifer without taking non-chemical field data into consideration. This concept was later labeled the ‘Henry problem,’ which any numerical variable-density flow program has to be able to simulate to be considered acceptable. Revisiting the summarizing publication with its record of piezometric field data (heads) showed that the so-called seawater wedge was actually caused by discharging deep saline groundwater driven by regional gravitational groundwater flow systems. Density-driven flow of seawater into the aquifer was not found reflected in the head measurements for low and high tide which had been taken contemporaneously with the chloride measurements. These head measurements had not been included in the assumption of a seawater wedge and associated dispersion zone and convection cell. The Biscayne situation emphasizes the need for any chemical interpretation of flow pattern to be backed up by head data as energy indicators of flow fields. At the Biscayne site density-driven flow of seawater did not and does not exist. This conclusion was confirmed by five independent methods. The hydrostatic use of vertical buoyancy forces needs, under hydrodynamic boundary conditions, to be replaced with buoyancy forces along the direction of the pressure potential forces [(grad p)/density] which, in the subsurface, can be pointed in any direction in space.