Abstract St. Catherine’s Island is a 20 km by 2 to 4 km barrier island located between the Savannah and Altamaha Rivers on the

Georgia coast. The Pleistocene core of the island is flanked by Holocene ridge and swale terrain on the north, northeast,

and southeast. LIDAR topographic models of the core show a higher (4.3 – 7.9 meters elevation) eastern portion and

lower (2.4- 5.0 meters elevation) western portion. The models also reveal former ponds, marshes and drainage patterns

along topographic lows that trend parallel to the long axis of the core. It is believed that artesian springs fed the fresh

water ponds and marshes that have since given way to ephemeral wetlands due to post-industrialization loss of artesian

pressure. Ground-penetrating radar profiling has identified sag structures in the Pleistocene core that are compatible with

2 to 5 meter subsidence of sandy surficial strata concomitant with filling of the sag basins. These sag structures are

tentatively interpreted as the uppermost manifestation of solution collapse features that may originate at depth in the

Floridan Aquifer, the principal artesian aquifer for the region.

Recent work has involved installing an east-west transect of four, 4.5 to 7.5 meter deep monitoring wells across the

Pleistocene core. Head data indicate that shallow groundwater moves uniformly away from the topographic high and

towards the adjoining salt marshes during wet periods. As the water table lowers during dry periods, a small groundwater

divide develops, creating a reversal in groundwater flow within the topographic low adjacent to the core. Water chemistry

data show that within the topographic low, the surficial aquifer contains Na-Cl type waters that are acidic (pH 4.5 to 5.0)

and under reducing conditions. On the topographic high portion of the core, the shallow aquifer contains Na-Cl-SO4 type

water that is less acidic (pH 5.5) and under oxidizing conditions. From these data it is hypothesized that any chemical

signature from the previous discharge of alkaline, Ca-HCO3 type water associated with the springs and sag structures

has been erased by the loss of positive artesian pressure within the Floridan Aquifer over the past 40 years.

Geology of St. Catherine’s

Island

Well Installation

St. Catherine’s Island is composed of a

Pleistocene core with Holocene accretionary

beach dune-ridge and swale systems. The

core also has topographic low stands that

trend along the long axis of the island which

is referred to as the Central Depression.

Preliminary Results

Characterizing the Surficial Aquifer Across the Pleistocene Core of St. Catherines Island, Georgia

Brock R. Nelson1; James S. Reichard1; Brian K. Meyer2; R. Kelly Vance1; and Gale A. Bishop3 (1) Department of Geology and Geography, Georgia Southern University, Statesboro, GA 30460

(2) Department of Geosciences, Georgia State University, Atlanta, GA 30302

(3) St. Catherines Island Sea Turtle Program, Georgia Southern University, Statesboro, 30460

Acknowledgements

St. Catherine’s Island Research Foundation Grant and Georgia Southern University COUR Undergraduate Research Grant.

References Falls, W. Fred Harrelson, Larry G., Conlon, Kevin J., and Petkewich, Matthew D. , 2005, Hydrogeology, Water Quality, and

Water-Supply Potential of the Lower Floridan Aquifer, Coastal Georgia, 1999-2002: Scientific Investigations

Report 2005-5124, 98 p.

Meyer, B.K., Keith-Lucas, T., Bishop, G.A., Thomas, D.H., Hayes, R.H., Sanger, M., and Vance, R.K., 2009. Digital Atlas of

St. Catherines Island, Georgia. St. Catherines Research Consortium Publication CD-1

Vance, R. K., Bishop, G. A., Meyer, B. K., Rich, F., and Reichard, J., 2011, St. Catherines Island, Georgia: sag structures,

hydrology and sea level rise: Southeastern Section- 60th Geological Survey of America, n. 25, abstract.

Research Site

GSU Well Transect

Methodology for Monthly Sampling

Download Head &

Temperature,

Specific

Conductivity, and

Barometric

pressure From

Data Loggers

Take Depth to

Water

Purge Monitoring

Wells with pump for

15 minutes

Collect Samples

Field Testing Lab Testing

General Chemistry

(Ion Chromatography)

Alkalinity

Titrations

Physical Parameters:

DO%, Temp., pH,

sp. Conductivity, and

ORP

The east-west transect was strategically placed to

determine the surface hydrology across the

Pleistocene core of the island, and to see if the Central

Depression influences the shallow groundwater

system.

The study area is located on the Georgia coastline approximately ~60 kilometers south of

Savannah.

Possible Geologic Controls on the Surface Hydrology

Ground Penetrating Radar (GPR)

profiles across the Central

Depression show extensive sag

structures within the shallow

subsurface (Vance et. al. 2011) .

These features are interpreted as

surface manifestations of solution

collapse within the Upper Floridan

aquifer, creating fractures and

vertical groundwater pathways

between the surficial and artesian

systems. Under pre-development

conditions, artesian springs could

have been present, but today,

vertical gradients are downwards,

allowing surficial groundwater to

migrate into the Upper Floridan.

Wells were installed using a vibra-core system

combined with water-jetting to drive a 4-inch

aluminum pipe into the ground. Water-jetting

removed sediment and allowed the pipe to be

driven to the desired depth. A permanent 3-inch

PVC casing and 5-foot screen were installed

before removing the aluminum pipe.

.2 .2 .4 .4 .6 (meq/l) .6

Na Cl

Ca HCO3

Mg SO4

S-1

LegendLegend

.6 .6 1.2 1.2 1.8 1.8

Na Cl

Ca HCO3

Mg SO4

S-2

LegendLegend

.5 .5 1 1 1.5 1.5 2 (meq/l) 2

Na Cl

Ca HCO3

Mg SO4

S-3

LegendLegend

.6 .6 1.2 1.2 1.8 1.8

Na Cl

Ca HCO3

Mg SO4

S-4

LegendLegend

1 1 2 2 3 (meq/l) 3

Na Cl

Ca HCO3

Mg SO4

UF-1

LegendLegend

1 1 2 2 3 (meq/l) 3

Na Cl

Ca HCO3

Mg SO4

uf4

LegendLegend

• Continuous data loggers and in-situ

field measurements, have revealed

physical and chemical differences

along the transect.

• Head measurements have shown a

variation in seasonal flow paths.

• Well 1 is the only well undergoing

oxidation; the other 3 wells have

reducing conditions.

• Well 2 is the most responsive to

precipitation events.

• Wells 2-4 interpreted to be screened in

marine deposits

• Upper Floridan does not appear to be

influenced by Na-Cl type waters from

the surficial aquifer

9/10/11 10/20/11 11/29/11 1/8/12 2/17/12 3/28/12

Time (days)

3.0

4.0

5.0

6.0

7.0

8.0

9.0

He

ad

(fe

et

asl)

0.0

1.0

2.0

3.0

Pre

cip

ita

tio

n (

inc

he

s)

Well-1

Well-2

Well-3

Well-4

ppt

50

100

150

200

250

300

Sp

ec

ific

Co

nd

uc

tiv

ity

(

S/c

m)

18.0

18.5

19.0

19.5

20.0

20.5

21.0

Te

mp

era

ture

(oC

)

9/14/11 10/14/11 11/13/11 12/13/11 1/12/12 2/11/12 3/12/12 4/11/12

Date

4

8

12

16

20

24

SO

42- (

mg

/L)

-200

-100

0

100

200

300

OR

P (

rela

tive m

V)

4.4

4.8

5.2

5.6

6

pH

40

60

80

100

120

140

TD

S (

mg

/L)

Legend

MW-1

MW-2

MW-3

MW-4

20

40

60

80

Cl- (

mg

/L)

GeneralFlowpath

6/26/11 8/5/11 9/14/11 10/24/11 12/3/11 1/12/12 2/21/12 4/1/12

Sample Date

2.0

4.0

6.0

8.0

10.0

He

ad

(F

eet

AS

L)

GSU-1

GSU-2

GSU-3

GSU-4

Meyer et. al. 2009

Meyer et. al. 2009

Meyer et. al. 2009

Meyer et. al. 2009