Présentations Colloques

Oral Presentation
Session 8.02: Groundwater Development and Protection in Coastal and Volcanic Environments with Complex Geological Structures
Weyer K. Udo
The case of Biscayne Bay and Aquifer near Miami, Florida- Density driven flow of sea water or gravitationally driven discharge of saline groundwater?
Coastal groundwater flow investigations at the Biscayne Bay south of Miami, Florida (Cooper et al., 1964 in USGS WSP 1613-C) gave rise to the dominating concept of density driven flow of sea water into coastal aquifers indicated as an invading sea water wedge. Within that wedge convection type return flow of seawater and a dispersion zone were concluded to be the cause of the Biscayne ‘sea water wedge’. This conclusion was merely based on the chloride distribution within the aquifer and on an analytical model concept by Henry (1964 in USGS WSP 1613-C) which postulated, for the Biscayne site, convection flow within a confined aquifer without taking non-chemical field data into consideration. The concept was later labelled as the ‘Henry Problem’ which any numerical variable density flow program code has to simulate to be considered acceptable. **Revisiting the summarizing Biscayne publication by Cooper et al. (1964) with its record of piezometric head data showed that the so-called sea water wedge was actually caused by discharging deep saline groundwater driven by gravitational flow and not by dense sea water. Density driven flow of seawater into the aquifer was not found reflected in the head measurements which had been taken contemporaneously with the chloride measurements. These head measurements had been ignored by Cooper et al. (1964). The head measurements also showed that a dispersion zone and a convection cell did not exist at the Biscayne site. Instead there was a sharp dividing line between shallow local freshwater and saline deep groundwater flow. **The Biscayne case re-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 and does not exist. Instead this coast line is the end point of local freshwater and of regional saline groundwater flow systems.**