Hydrogeology of St. Catherines Island

 

Shallow Groundwater System

The hydrogeology of St. Catherines Island may be considered within the geologic framework of both a shallow and a deep groundwater system. The shallow system within the island core is a water table aquifer consisting of poorly consolidated Pleistocene beach, backbeach and nearshore sands overlain by surficial Holocene eolian sands. Exposures of the sands at Yellow Banks Bluff, vibracore and auger data and Ground Penetrating Radar (GPR) surveys (Bishop et al., 2011; Vance et al., 2011) indicate these sands may be as thick as 6 to 9 meters in the eastern and northern portions of the Pleistocene core, but thin to approximately 2 meters to the west and south. This sequence of permeable sands overlies relatively impermeable Pleistocene muds, most of which represent former salt marsh deposits.

The Pleistocene muds serve as an aquitard that forms the base of the water table aquifer. Water sampling and analyses from a series of shallow wells installed by Reichard and coworkers across the southern end of the Pleistocene core indicate the shallow groundwater is a Na-Cl type water that contrasts sharply with the Ca-HCO3 type water of the deep carbonate aquifer, which is part of the confined Floridan aquifer system (Reichard et al., 2014). The shallow groundwater chemistry also varies with topography in which fewer total dissolved solids (TDS) and less acidic conditions are found in wells at higher elevations compared to water samples from wells in topographically lower areas (Reichard et al., 2014).

The Holocene accretionary terrains constitute an additional shallow hydrologic system consisting of Holocene ridge and swale deposits made up of sandy backbeach and dune deposits that overlie Holocene marsh muds (Meyer 2013, Rich et al., 2014). This combination of sand and mud operates in a similar fashion to the same sediment pair in the shallow groundwater system of the island core; however, the low elevation also makes it more susceptible to the impact of salt spray, marine flooding during storms and lateral infiltration of salt water during peak tides to create local brackish conditions.

The scarp that forms a geomorphic boundary between the higher Pleistocene core and the lower Holocene strata provides an interface for some lateral input of shallow groundwater from the core to the topographically lower Holocene sediments. Scientists have observed of Juncus in the salt marsh at the base of the core scarp south of Yellow Banks Bluff as well as groundwater seeps along the contact of low-permeability humate horizons. Groundwater seeps have also been observed from sands exposed in the cutbanks of tidal creeks eroding into the core support this contention. Joints developed in the Pleistocene strata of the core may offer localized zones of enhanced permeability and influence the movement of shallow groundwater.

 

Deep Groundwater System

The primary groundwater resource on St. Catherines for human and the wildlife associated with three man-made freshwater ponds is the Floridan aquifer. The Floridan aquifer system consists of a thick sequence of Paleocene to Oligocene carbonate sediments in which low permeability units separate the highly permeable zones of the Upper Floridan and Lower Floridan aquifers (Clarke et al., 2011). Miocene sediments overlying the Upper Floridan sequence include low permeability strata that serve as a confining layer for the Floridan aquifer (Falls et al., 2005, Clarke et al., 2011). The Coastal Plain strata that include the Floridan aquifer are exposed in a zone parallel to the fall line and are generally inclined (dip) toward the coast and southeast and extend beneath the barrier islands of Georgia. The southeastern dip of this confined aquifer system generates an artesian system where the potentiometric surface (hydraulic head) was above the land surface prior to major industrial pumping withdrawals (Krause and Randolph, 1989; Clarke et al., 2011). In 1909, W.J. Floyd reported drilling a 432-feet deep well into “marl and rock” on St. Catherines.

Floyd’s well log described strong free-flowing artesian conditions with a static hydraulic head 42 feet above land surface. Locations of St. Catherines artesian wells are shown in Figure 1.8 of Thomas (2011); those with casing tops near sea level continued to flow through 1967 (Hayes et al., 2011, Appendix 2). In addition, colonial journal accounts describe clear springs and spring-fed meadows in the island interior (Hayes and Thomas, 2008), suggesting the existence of artesian springs that fed freshwater wetlands in the island’s Central Depression. Palynological studies of vibracores from ephemeral wetlands of the Central Depression support the former existence of these open fresh water marshes in what is now mostly woodland (Ferguson et al., 2010; Rich et al., 2014 in press).

The occurrence of former artesian springs on St. Catherines Island means either the upper confining unit of the Floridan aquifer is missing in places due to erosion, or the aquifer contains vertical pathways such as joints or faults. GPR profiling on the island revealed sag structures interpreted as the surficial expression of collapsed dissolution cavities at depth (Vance et al., 2011). Dissolution cavities concentrate along pre-existing fluid pathways such as joints or faults; consequently, the presence of these sag structures may mark the dissolution enhanced conduits that penetrated the confining layer of the Upper Floridan, allowing the formation of the old artesian springs. The alignment of former ponds and wetlands on St. Catherines Island with regional Coastal Plain joint trends (Bartholomew et al., 2007, 2009) also supports this hypothesis (Vance et al., 2011, 2014).

Artesian flow no longer occurs from wells or springs on St. Catherines Island as evident by the monitoring of four Upper Floridan wells on the island that reveal a potentiometric surface that is approximately 16 to 30 feet below the land surface and slopes toward a large cone of depression centered on Savannah (Reichard et al., 2014). Sampling and analyses of water from the Upper Floridan wells indicate a Ca-Mg, HCO3-SO4 type water that is slightly alkaline and under reducing conditions (Reichard et al., 2014). Systematic changes in well chemistry from south to north indicate the Upper Floridan is experiencing salt water intrusion with a source toward the southern end of the island. Evaluation of mixing models to explain the chemical trends in the Upper Floridan well samples suggest that the source of the saltwater is not modern seawater, but rather brackish water of the underlying Lower Floridan aquifer (Reichard et al., 2014).

These data support the existence of structurally controlled and dissolution-enhanced vertical fluid pathways as suggested by the sag structures and linear trends of natural pond and wetland sites on the island. Furthermore, the saltwater intrusion that is occurring in the Upper Floridan aquifer to the south at the pumping center in Brunswick, Georgia is interpreted to be due to up-coning along faults and joints (Jones et al., 2002). Consequently, any plans for new well sites or increased withdrawals on St. Catherines Island should consider the impact of these vertical conduits. The vertical pathways that provided access for artesian springs in the past now make it possible for groundwater from the shallow aquifer to move downward and interact with the deep aquifer. Future investigations will explore this possibility.