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.

Interaction between sea water, groundwater flow systems, the occurrence and direction of buoyancy forces under hydrostatic conditions [A], the direction of equivalent forces under hydrodynamic conditions [B] in groundwater flow systems, and the occurrence of buoyancy reversal

“…It is remarkable that the rebuttal by Provost et al (2018) refers to groundwater flow systems basically only in one quotation from my paper although, in Florida, flow patterns originating inland and at the coast are dominated by local and far-reaching groundwater flow systems. The above authors' attention, however, is only concentrated on flow of seawater which they propose to essentially migrate inland along equipotential lines (see Weyer 2018, Fig. 2, taken from Langevin 2001also Weyer 2018, Figs.…”

“…To my surprise, they also concern themselves with Hubbert's (1940) derivation of force potentials and not with Muskat's (1937) velocity potential as has been predominantly done in much of engineering sciences in the past. It appears, however, that they may not be familiar with the complex physics of groundwater flow systems and the application of force potentials as summarized and applied in Weyer (1978Weyer ( , 2018.…”

“…Apparently, Provost et al (2018) did not fully consider the physical meaning of Fig. 11 in Weyer (2018) taken from a publication of the USGS (Springfield 1966). Springfield's report documents artesian (flowing) discharge of deep groundwater flow systems to the surface in all of southern Florida including the coastal area in general and the area of Biscayne Bay in particular.…”

“…This interpretation is also reflected in the well-known computer programs they quote, such as SUTRA (Voss and Provost 2002), SEAWAT (Langevin et al 2008), and FEFLOW (Diersch 2014). Weyer (2018) has shown this interpretation to be incorrect from a physics point of view under hydrodynamic conditions.…”

“…In the diagram the upper triangle represents the data range where buoyancy reversal occurs while the lower triangle represents the data range where buoyancy reversal does not occur. For more information on buoyancy reversal consult Weyer (2018). As a consequence, the generally applied concept of Provost et al (2018) of upward-directed buoyancy-equivalent hydraulic forces has been shown to be physically invalid rendering all presently existing and applied computer programs also to be invalid under the conditions of buoyancy reversal.…”