The U.S. Geological Survey (USGS), in cooperation with the National Park Service (NPS), is drilling a deep geologic test well at the Fort Pulaski National Monument in Georgia. When completed, the well will be about 1,500 feet deep. The purpose of this test well is to gain knowledge about the regional-scale Floridan aquifer, an important source of ground water in the area. Also, cores obtained during drilling will enable geologists to study the last 60 million years of Earth his-tory in this area.
The USGS has undertaken a 5-year study of the geology of the greater Savannah region. This endeavor combines mapping of the surface geology with detailed study of the subsurface geology. The Fort Pulaski site was chosen to complete a series of test wells in Georgia that parallel the Savannah River from the Savannah River Nuclear Site to the Atlantic Coast. The data gathered from these wells will be combined with published information from the South Carolina side of the river to enable USGS to model the subsurface geology across the state line.
Ground water is a vital natural resource in the south-eastern Atlantic Coastal Plain. The Floridan aquifer – one of the most productive aquifers in the world – is a regional-scale source of ground water that has been used locally for more than 100 years. Recently, heavy use of this resource has caused some wells along the coast to experience saltwater intrusion, rendering them unsuitable for drink-ing water. As water use increases in response to growing municipal, industrial and agricultural demand, state and local governments will face difficult decisions about future use of the Floridan aquifer. A primary goal of the USGS is to make information on the aquifer avail-able to decision-makers. Defining the internal character and boundary of the aquifer in the subsurface requires detailed knowledge of the rock types, their ages and their physical characteristics. This information will help water resource specialists to model ground water flow and to correlate sub-surface characteristics across state and local boundaries. The results from the test well at Fort Pulaski will be added to the larger database that describes the Floridan aquifer.
The test well is obtained using a technology called wireline core drilling. A hollow drill stem is carefully drilled through the soil into the underlying variably indurated sands, silts, clays and limy sediments, which are collected in an inner core barrel in 10-foot sec-tions. These sections are pulled back to the surface, and the collected core is boxed for study. On site, the freshly cored sedi-ments are described and sampled for laboratory analysis. The inner barrel then is put back down the hole to drill and collect the next 10-foot section. The Fort Pulaski test well will take about 2 months to drill. After the hole is completed, USGS will conduct a variety of tests in the open hole. These are referred to as “downhole geophysics” and involve lowering several instruments on cables in order to measure physical properties of the sand, silt, clay and limy sediment layers such as electrical resistance and natural radiation. Finally, USGS will place a small ground water moni-toring instrument at a specific level in the hole and connect it to a surface graph. The hole then will be permanently closed off. The collected core and sam-ples will be transported back to USGS laboratories for completion of testing and descriptions. Paleontologists at the USGS will exam-ine the core for microfossils that help explain the geologic history.
Following analysis of all the information gathered from the Fort Pulaski test well, the USGS will publish a set of reports and make them available to the public. These reports will include technical descriptions of the entire 1,500-foot corehole, as well as assessments of fossil con-tent, chemical characteristics and ages of the sediments. This information will be correlated with data from other test wells in the Floridan aquifer. Information gathered from the ground water monitoring instrument will be avail-able as those data are accumulated.