Note: This information is presented as a working reference to enable the development of a Plan of Study as agreed to in the December 2000 meeting. Its purpose is to provide shared information for use of committee members towards that end. To review the listing of concerns regarding the potential effects on the Floridan Aquifer resulting from a deeper channel, click here. To review the related studies, reviews, and evaluations, click here.
The volume of previous study notwithstanding, gaps in knowledge pertinent to gaining a better understanding of what, if any, impacts to the Floridan aquifer the Savannah Harbor deepening project may have are discussed in the following paragraphs.
The 1998 ACOE study collected core samples from six locations within the immediate study area and submitted twenty-two samples from those cores for laboratory analyses to measure estimates of vertical hydraulic conductivity and to determine grain-size fraction and their statistical distribution. It is noteworthy that the vertical hydraulic conductivity values derived during the ACOE 1998 study were consistent with those reported from previous studies, including in-situ vertical hydraulic conductivity calculations, at locations on Port Royal Island in southeastern South Carolina. However, some members of the Aquifer Committee have expressed opinions regarding the validity of using laboratory derived data as opposed to field derived, in-situ data. Some Committee members have opined that in-situ data are preferable; whereas other Committee members have suggested that reliable in-situ data can not be obtained in the Savannah Harbor area. Nevertheless, the use of laboratory determined geotechnical data is within the standard of professional practice of geologists and engineers and have a precedent of acceptability by the applicable regulatory agencies of the states of Georgia and South Carolina, as well as the U.S. Environmental Protection Agency (U.S. EPA).
Previously reported effective porosity percentages of Hawthorn Group sediments collected from beneath Port Royal Sound had a normally distributed cluster between 40% and 45 % with a maximum peak at 45% (Smith, 1988). Effective porosity is an important input parameter, along with hydraulic conductivity, in calculating estimates of ground-water flow using Darcy's Law. It is recognized that in areas where salt-water is a concern, that Darcy's law does not account for density differences and density gradient. Nevertheless, if a simple calculation of vertical ground-water flow through the Miocene unit were calculated, at least a gross approximation of vertical migration rate could be estimated.
If saline waters have begun to migrate vertically downward through the Hawthorn Group sediments, it is unknown how far they migrated, nor is it known at what rate migration, if any, is occurring. Likewise, if saline water has penetrated vertically downward into the Miocene unit, the location of the salt-water/freshwater interface within the Miocene confining unit is unknown. If saline water has penetrated into the Upper Floridan aquifer, the sodium and chloride concentrations are unknown; nor is it known if water quality standards would be violated as a result.