Concerns for Potential Effects on the Floridan Aquifer 

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 related studies, reviews, and evaluations, click here. To review the  gaps in aquifer knowledge, click here.

Concerns directly relating to GPA’s proposed deepening of the Savannah River Navigation Channel are discussed in the following:

  1. The primary concern is what, if any, impact additional removal of Miocene sediments (Hawthorn Group) may have upon the water quality of the Upper Floridan aquifer in the Savannah area.

Since the existing Savannah Harbor navigation channel cuts into the upper portions of the Hawthorn Group sediments, any additional channel deepening, allowable over-dredging allowances, and advance maintenance would remove additional low permeability confining material, thus causing nominal thinning of the confining unit within the Savannah River Navigation Channel.  The geographic area of primary concern lies over the Tybee high, extending from approximately Fort Pulaski to just off-shore of the Savannah River entrance, inasmuch as the Hawthorn Group sediments are naturally at their thinnest directly over this structural high.  Additional areas of concern are the paleochannels that were identified in the 1998 U.S. Army Corps of Engineers-Savannah District (ACOE) report (Caldwell Smith, personal communication).  The ACOE (1998) report indicated that these paleochannels were identified from seismic reflection profiles and appear to be infilled with primarily fine-grained sediments, albeit relatively few samples were collected from these areas.

  1. Concern has been raised in response to the findings reported in the paper  Preliminary Interpretation of Fracture Sets in Upper Pleistocene and Tertiary Strata of the Lower Coastal Plain in Georgia and South Carolina (Bartholomew et al., 2000). 

The referenced paper describes and discusses the occurrence of fracture sets in Upper Pleistocene and Middle Eocene and Lower Pliocene age strata. The fracture sets are thought to be tectonic in origin and their orientations are reported to be consistent with recent focal mechanism solutions for recent seismic events and with the moderate to large 1886 Charleston earthquake (mb 6.0-6.5). Concern has been expressed that similar fractures may be present within Miocene age sediments underlying the Savannah River and that if present, may represent previously overlooked vertical secondary permeability within the Miocene confining unit.

  1. Should it be demonstrated that near vertical fractures are present in the upper confining unit underlying areas where saline water is present, there is concern that dredging activities may disturb or alter these fractures so as increase their ability to transmit saline water into the Upper Floridan aquifer.

  2.  There is not a scientific consensus regarding a representative range of vertical hydraulic conductivity values of the Miocene confining unit underlying the immediate Savannah River navigation channel

  3. The rate of lateral salt-water migration in the upper Floridan aquifer, where studied, remains uncertain. The estimated rates of lateral salt-water migration, where studied, ranges from 50 to 80 feet per year (Smith, 1988) to 140 to 360 feet per year (Hayes, 1979).

  4. The location of the lateral salt-water/freshwater interface (Gyben-Herzberg position) within the Upper Floridan is unknown.

  5. Although the leakage coefficient through the upper confining unit in the Savannah area has been estimated to be less than 0.01 inches per year per foot from simulations (Bush and Johnston, 1988), no site-specific estimates of leakance have been determined.

  6. The conceptual model of the local hydrogeologic system should be better refined (Cardwell Smith, personal communication, 2000). The conceptual model can be improved by overlying hydrographic surveys over the known stratigraphic column (to scale if possible) and by showing the depths of the existing navigation channel in relation to the known thickness of the upper confining unit.

 

Table of Concerns

Group

Related Information and/or Reference

Hydraulic Characteristics of Confining Unit
  • Lack of site-specific insitu vertical hydraulic conductivity data.
 HydroVision, Inc., (2000), Smith (1987); Randolph and Krause (1984); Aquifer Experts Information Exchange (2000).
  • Lack of site-specific effective porosity data.
 Smith (1988).
  • Lack of site-specific leakance values.
 Bush and Johnston (1988).
  • Vertical secondary permeability.
 Bartholomew et al, (2000); Alexander et al. (2000); Carver (2000); Aquifer Experts Information Exchange (2000).
  • Geographic areas of concern
 ACOE (1998); Foyle et al. (1999).

Hydraulic Characteristics of Upper Floridan Aquifer

  • Transmissivity value ranges
 Hayes (1979); Aucott & Newcome (1986); Crouch et al. (1987); Bush & Johnston (1988); Sharp et al. (1997); ACOE (1998); HydroVision, Inc. (2000).

Salt-Water Movement in Upper Floridan Aquifer
  • Lateral migration of saline waters
 Hayes (1979); Smith (1988); Bush (1988); Hughes et al. (1989); Smith (1994); GAEPD (1997); GAEPD (2000).
  • Vertical migration of saline waters
 Smith (1988); Hughes et al. (1989); ACOE (1998); Alexander et al. (2000); Carver (2000); HydroVision proposal to City of Savannah (2000); Ransom (2000); Aquifer Experts Information Exchange (2000).
  • Saltwater/freshwater interface
 Hayes (1979); Smith (1988); Smith (1988); Bush (1988); HydroVision Inc. proposal to City of Savannah (2000).