Special Online Presentation, Dr Jim Reichard Ga Southern U.

Topic: Possible connections between regional tectonic history and structural controls on the groundwater system on St. Catherines Island, Georgia

When: February 21, 2023, 7pm
The meeting will be in via zoom only. Membership required. To Join AGS to to: https://atlantageologicalsociety.org/membership/

Speaker bio: James (Jim) Reichard is a Professor of Geology in the Department of Geology and Geography at Georgia Southern University where he teaches courses in hydrogeology and environmental geology. Jim obtained his Ph.D. in Geology from Purdue University in 1995 and an M.S. and B.S. in Geology from the University of Toledo, Ohio, in 1984 and 1981, respectively. Prior to his dissertation, Jim specialized in structural and petroleum geology and worked as a photogeologist in Denver, Colorado, and then as an environmental consultant in Cleveland, Ohio. His dissertation research at Purdue involved modeling the effects of climate-driven changes in pore pressure on groundwater flow systems and developing computing applications for aquifer tests. Since coming to Georgia Southern, Jim’s attention has shifted towards applied hydrologic research on the Georgia coastal plain. His current project involves identifying permeable pathways within the coastal plain that allow groundwater to flow vertically between aquifer systems. In addition to his hydrogeologic research, Jim has published Environmental Geology with McGraw Hill, an introductory-level textbook that is now in its fifth edition.

Abstract: Hydraulic head and chemistry data from a longitudinal investigation on St. Catherines Island strongly suggest that groundwater moves up from the Lower Floridan aquifer and into the Upper Floridan along vertical faults and or solution collapse features. In addition, head and chemistry data collected from a network of 30 monitoring wells in the surficial aquifer show that unusually large tides cause episodic saltwater intrusion events at discrete locations on the island. Geophysical data collected near the sites of saltwater intrusion reveal the presence of fractures and faults along with a pronounced sag structure.

We hypothesize that prior to modern pumping withdrawals from the Upper Floridan aquifer, artesian groundwater flowed to the surface along regional joint and fault trends in the carbonate system. Solution caverns and collapse features eventually developed along these trends, creating sag structures in the overlying clastic units. Today, large tidal events periodically cause saltwater to move laterally and vertically into the surficial aquifer along these preferred structural pathways. We also hypothesize that the pathways are ultimately related to the recurrent movement of deep-seated Mesozoic basement structures beneath the coastal plain sedimentary sequence.

To test our hypotheses, we are installing shallow wells at a new site north of the original study area on St. Catherines Island. Based on topographic and preliminary geophysical data, this site appears to be associated with a solution collapse feature that falls along a suspected Mesozoic fault trend. To document potential saltwater intrusion events, new wells will be instrumented with data loggers and sampled periodically for chemical analysis. Finally, well cores taken during drilling combined with ground-penetrating radar and electrical resistivity surveys will be used to examine the subsurface stratigraphy and structure at the new field site.