Miocene Aquiclude Mapping Project
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\:1~ Nloo .~ i $i r 1 r\o. ?,q Miocene Aquiclude Mapping Project: Phase - I Findings Report by Anthony M. Foyle Georgia Southern University, Applied Coastal Research Laboratory Vernon J. Henry Georgia Southern University, Applied Coastal Research Laboratory Clark R. Alexander Skidaway Institute of Oceanography Georgia Department of Natural Resources Environmental Protection Division Georgia Geologic Survey Prepared in cooperation with Georgia Southern University, Applied Coastal Research Laboratory Skidaway Institute of Oceanography Atlanta 1999 PROJECT REPORT 39 Miocene Aquiclude Mapping Project: Phase - I Findings Report by Anthony M. Foyle Georgia Southern University, Applied Coastal Research Laboratory Vernon J. Henry Georgia Southern University, Applied Coastal Research Laboratory Clark R. Alexander Skidaway Institute of Oceanography Georgia Department of Natural Resources Lonice C. Barrett, Commissioner Environmental Protection Division Harold F. Reheis, Director Georgia Geologic Survey William H. McLemore, State Geologist Prepared in cooperation with Georgia Southern University, Applied Coastal Research Laboratory (GSU Offshore Seismic Studies Contract# 10-21-6-15-120-317) Skidaway Institute of Oceanography Atlanta 1999 PROJECT REPORT 39 CONTENTS EXECUTIVE SUMMARY . iii PART 1: STUDY DESCRIPTION . .. .. ...... .... ..... .. .. ... .. .. .. .. 1 Introduction ..... .. .. .. .. ..... ........ ............ ... .. ...... ... 1 Geologic Framework .... .. .. ... ... ... .. .. .. .. .. .. 1 PART 2: DATA ACQUISITION AND PROCESSING . .. ... ... .. ..... ... .. .. ... 4 Archive Seismic Data . .. .... .. ... ... .. ... .. ....... .. .. .. ... 4 Data Sources .......... .. .. .. .. ... .... .... ... ... .. 4 Data Reduction and Presentation .... ... ...... .. .. .. .... ... .. ... 4 Data Accuracy .. ....... ..... ... ..... .. ... .... ...... .. .. .... 5 Data Resolution ...... .... ... ... .. ... .. ... .... .... .. .. ..... 7 Interpretation Methods ...... .... .. .. .. .. ... ...... .. ·. .... ... ... 7 Seismic Stratigraphy .. ............. .... .... ..... .. .. ...... 11 Published Borehole Data, Maps, and Sections .. .. .... .. ... .. .... 12 PART 3: STUDY FINDINGS . .. ... .... .. .. .... .. ..... ..... .. .. .. .. 13 Seismic-Stratigraphic Relationships .. ........ ..... .. ................ .. .... 13 Structure at the Top of the Floridan Aquifer . .. ... .. .. .. ... ... 13 Thickness of the Miocene Aquiclude ... ...... ... .. .. ... .. .. .... 13 Areas of Concern .... .. ....... .. ..... ... ... .. ... .. .. .. ............ 17 Weighting Factors Used to Rank Areas of Concern . ..... ... .. ... .... .. 17 Relative Ranking of Areas of Concern . .. ... .. .. .. ... .. .. .... ... 19 PART 4: SUMMARY . .. ...... ........ .. .... ........ .. .. ... .... 28 ACKNOWLEDGMENTS .. .. .. .. .... ..... .. ...... ... .. .... ... ... 29 BIBLIOGRAPHY . ... .... .. ....... .. ... ... ... ... .. ... .... 30 FIGURES 1-1 Map showing location of the study area on the Georgia- South Carolina inner shelf and coastal zone . .. .. .. .. ... ...... .. ....... .. .. 2 2-1 Locations of existing seismic-reflection tracklines used in this report .. ....... ... .. 6 3-1 Locations of digitized seismic-reflection tracklines shown in Figures 3-3, 3-4, and 3-6 through 3-11 ... ...... ... .. .... .. .. .. .. .. ....... 14 3-2 Structural contour map showing depths to the top of the Floridan aquifer in northern coastal Georgia and southern coastal South Carolina . ... .. ... 15 3-3 Coast-parallel oriented cross section along Seismic Line GS7-20 between offshore Tybee Island and offshore Hilton Head Island . ..... ........... 16 3-4 Coast-normal oriented cross section along Seism ic Line AMP2-2 between Savannah Light Tower and the TACTS A borehole . .. ... .... .. .. ... 20 3-5 Isopach (thickness) map for the Miocene aq uiclude in northern coastal Georgia and southern coastal South Carolina . ... ..... ... ............. .. .. .. 21 3-6 Coast-parallel oriented cross section from Sku ll Creek between Calibogue Sound and Port Royal Sound .. ...... .... .. .. .. ... .. .. .. 22 3-7 Coast-normal oriented cross section from the Colleton - Chechessee River area . ... 23 3-8 Coast-normal oriented cross section from the Broad River between Haulover Creek and Port Royal Sound . .. .. ..... ..... .... ..... .. .. .. ... .. 24 3-9 Coast-normal oriented cross section from the Beaufort River between Spanish Point and Port Royal Sound . .. ... ... .. .. .. .. .. ... .. .. ... ...... 25 3-10 Coast-parallel oriented cross section along Seismic Line U-57 between Wassaw Sound and Savannah Light Tower .. ..... .. ...... .......... ....... .. 26 3-11 Coast-normal oriented cross section along Seism ic Li ne GS7-19 offshore of Hilton Head Island ... ..... ..... .. ...... .. .... .. .. .. ... .. .... .. 27 TABLES 1-1 Stratigraphic-hydrogeologic units . .. ... .. ..... .. .. .... ... ... .. .. .. .3 2-1 Summary information for archive seismic trackline data . .. ...... ... .. ... .... 8 3-1 Size of potential saltwater-intrusion sites at identified areas of concern .. .... ... 18 3-2 Preliminary saltwater-intrusion relative risk ranking for identified areas of concern . 19 ii EXECUTIVE SUMMARY This Phase-1 Findings Report provides the Georgia Department of Natural Resources, Environmental Protection Division with the results of Phase I, Tasks #1 - #4 of the Miocene Aquiclude Mapping Project under Contract 10-21-6-15-120-317 with the Georgia Southern University Applied Coastal Research Laboratory. · The project concerns the geophysical mapping of Tertiary and Quaternary strata overlying the Upper Floridan aquifer (UFA) to determine potential sites of saltwater intrusion in the estuarine and nearshore region between Wassaw Sound, GA, and St. Helena Sound, SC. The study area includes the intracoastal and inner shelf areas and encompasses the northeastern portion of a regional cone of depression on the potentiometric surface of the UFA that is centered on Savannah, GA. In accordance with Phase I objectives, published and unpublished seismic and geological data were reviewed, evaluated, and synthesized. Eight hundred miles of high-resolution seismic reflection data, collected between 1970 and 1997, were compiled and interpreted using standard methods of analysis. Borehole lithology-log and gamma-log data were used to ground-truth the seismic-stratigraphic interpretations and to provide additional control in areas where seismic coverage was limited. The seismic data set forms the primary basis upon which the observations made in this Phase I report are based. Coverages of past geophysical surveys were summarized, and representative cross-sections show stratigraphic relationships, depths to the top of the UFA, and the thickness of the Miocene aquiclude. Appropriate maps summarize the structure on the top of the UFA, highlight areas where the Miocene aquiclude is thin or absent, and indicate where saltwater intrusio'1 sites are likely to occur. This database will be used to determine the sites and/or areas where new geophysical surveys should be carried out as part of Phase II. Based on the thinness or absence of the Miocene aquiclude (the confining uriit), the potentiometric value within the UFA, and the salinity of the overlying water column, five (5) potential sites of saltwater intrusion were designated and ranked (highest to lowest) as Areas of Concern (AOC): 1. Hilton Head High centered approximately 10 miles southeast of Hilton Head Island. 2. Skull Creek/Caliboque Sound immediately west of Hilton Head Island. 3. Col/eton/Chechessee Rivers, tributaries to the Broad River. 4. Broad River near SC Highway 170 bridge. 5. Beaufort River from its mouth to the City of Beaufort. It is strongly recommended that these AOCs and the region bounded by St. Helena Sound, Port Royal Sound, St. Helena Island, and Beaufort be investigated in detail during Phase II. iii PART 1: STUDY DESCRIPTION INTRODUCTION The purpose of this draft "findings" report is to provide the Georgia Department of Natural Resources, Environmental Protection Division (EPD) with the information described in the SCOPE OF SERVICES, Phase I, Tasks #1- #4, Contract 10-21-6-15-120-317 with the Georgia Southern University (GSU) Applied Coastal Research Laboratory (ACRL). Tasks #1 - #4 contain the following elements: 1. Review, evaluation and synthesis of published and unpublished geological and seismic data . in both the public and private (files/archives) domains. 2. Generation of (1) an isopach map showing the th ickness of aquiclude strata overlying the Floridan aquifer; (2) color-coded maps showing the tracklines of all previous surveys; and (3) stratigraphic cross-sections showing where the overlying strata appear to be breached, thin , or missing. 3. Identification of areas that are susceptible to salt-water intrusion. The study includes the nearshore and estuarine areas between Wassaw Sound, Georgia, and St. Helena Sound, South Carolina (Fig . 1-1 ). It essentially encompasses the northeastern portion of a regional cone-of-depression on the potentiometric surface of the Upper Floridan aquifer (UFA). The cone of depression is centered on Savannah and is caused primarily by the long-term heavy pumpage of groundwater from the UFA in the Savannah area. The principal objectives of the study are to determine where: (1) the strata overlying the UFA may be breached, thin, or missing; and (2) the overlying water column is saline. Such areas may be sites of salt water intrusion into the UFA leading to contamination of drinking water wells and