Ground water rechar[g]e potential of fresh water wetlands on Hilton Head Island, South Carolina

[From the U.S. Government Printing Office, www.gpo.gov]

South Carolina
Sea Grant Consortium

COASTAL ZONE
INFORMATION CENTER

Journal
Reprint
Series

GB
625
.S6
M39
1987

GROUND WATER RECHARE POTENTIAL COASTAL ZONE

OF FRESH WATER WETLANDS INFORMATION CENTER

ON HILTON HEAD ISLAND,

SOUTH CAROLINA

JAMES P. MAY, PH.D.

TABLE OF CONTENTS

Page
Abstract .....................................

Introduction and purpose of study ............ 2
Acknowledgements @ ............................. 3

Previous studies ............................. 3
Geographic setting ........................... 4
Geologic setting ............................. 4
Method of study .............................. 7
Discussion of data ........................... 9

Sediment Samples ....................... 9
Water Level Measurements ............... 11
Automatic Water Level Recorder Data .... 17
Precipitation and evapotranspiration effects.. 23
Conclusions .................................. 27

Recommendations .............................. 27

References ................................... 28

LIST OF FIGURES

Page
1. Map of Hilton Head Island showing location
of study sites and geomorphology ...................... 5

2. Yearly distribution of temperature and rainfall
at Hilton Head Island ................................. 6

3. Descriptions of sediment core samples from
Hilton Head wetlands .................................. 10

4. Descriptions of cuttings samples from deep well
at Whooping Crane Pond, gamma log, and DHEC well ...... 12

5. Diagram of piezometer and recorder installation
at Whooping Crane Pond site ........................... 13

6. Horizontal gradients of water level versus time
at Whooping Crane Pond site ........................... 15

7. Horizontal gradients of water level versus time
at Palmetto Pond site ................................. 16

8. Vertical gradients of water level versus time
at Whooping Crane Pond site ........................... 18

9. Vertical gradients of water level versus time
at Palmetto Pond site ................................. 19

10. Plot of water level recorder data on pond surface
(Station #1) level at Whooping Crane Pond ............. 21

11. Plot of water level recorder data on water table
(Station #2) at Whooping Crane Pond ................... 22
1
12. Plot of water level recorder data on deep well
(Station #3) at Whooping Crane Pond ................... 24

13. Rainfall distribution during study period
at Hilton Head (Honey Horn Plantation) ................ 25

THE SOUTH CAROLINA SEA GRANT. CONSORTIUM
221 FORT JOHNSON ROAD
JAMES ISLAND
ABSTFLACT CHARLESTON, S. C_ 29412

Fresh water wetlands on Hilton Head Island have experienced signifi-
cant degradation over the past few decades. Fifty per cent of the origi-
nal fresh water wetlands on the island have been either completely de-
stroyed or significantly altered. This fact,.plus the declining water
levels experienced periodically, have caused much concern over the im-
portance of the wetlands. A major question concerned the role of the
wetlands in the recharge of the local ground water aquifer.

The present study was undertaken in order to evaluate the potential
of the wetlands for water table recharge. The method of study involved
collecting and analyzing geologic samples of the substrate in terms of
their hydrologic characteristics. A deep well was drilled to the Tertiary
Limestone Aquifer (TLA) in order to acquire geologic samples and provide
for sampling and monitoring levels of water in the artesian aquifer. The
study commenced in January, 1983 and continued through June, 1984.

Based on the data collected, several tentative conclusions could be

reached. The slope of the potentiometric surfaces based on piezometer
measurements at 1, 2, and 3 m depths indicated that surface water from
the wetland pond occasionally recharged the ground water system. At
other times, ground water discharged to the wetland. occasional recharge
from the ponds was also indicated by the vertical pressure gradients ob-
served in piezometer clusters at individual stations. The existence of
local recharge to the TLA is not yet evaluated.

INTRODUCTION AND PURPOSE OF STUDY

In recent years there has been increasing interest in the fresh

water wetlands on Hilton Head Island. The South Carolina Coastal

Council in their Special Area Management Plan for Hilton Head Island
(1982) devoted a full chapter to fresh water wetlands. They pointed out
that 33 per cent of the island's original fresh water wetlands have been
eliminated, and that an additional 20 per cent have been significantly
altered. They indicated the generally recognized value of these wetlands
for wildlife habitat, as temporary reserviors for storm drainage, and as
natural pollutant treatment systems. A fourth value, their potential im-
portance for ground water recharge is also mentioned. It is to this last
point that the present study is directed. The ground water recharge poten-
tial of fresh water wetlands is largely unknown.

Related to this question, surface water levels were alarmingly.
low in the wetlands at the time this study was proposed in the winter
of 1981-82. Local interests were concerned about the drying-up of the
wetlands. This is a related question because if the wetlands are, in
fact, recharging the deeper aquifers and if the aquifers are being
drawn down exessively, then there would exist a direct relationship
between ground water pumping and wetland surface water level fluctua-
tion. This is a common occurrence in the limestone terraine of Florida,

but it is not generally recognized in coastal South Carolina. However,
an end to the drought brought wetland surface water levels up to normal
elevations by the time the study was begun in the winter of 1982-83.
This fact notwithstanding, the basic question remains concerning the
relationship of the fresh water wetlands to the water table aquifer,
and, secondarily, to the deep limestone aquifer from which the island's
water supplies are withdrawn.

The present study is primarily concerned with the first aspect of
this question. We concentrated on the surface water levels of the wet-
lands and their relationship to the water table aquifer. We did not
ignore however, the second part of the above-posed question. In this
respect, we have been cooperating with the South Carolina Department
of Health and Environmental Control (DHEC) in their larger study of
general recharge to the limestone aquifer in the region.

ACKNOWLEDGEMENTS

We would like to express our appreciation to the South Carolina
Sea Grant Consortium and the South Carolina Coastal Council for sponsor-
ing the study. Funds were also provided by the Citadel Development
Foundation, The Citadel, and the South Carolina Department of Wildlife
and Marine Resources. Very important support was provided in the way
of encouragement, manpower, and equipment by the South Carolina Water
Resources Commission. We want to thank the Nature Conservancy and
Hilton Head Plantation for permission to work in the Whooping Crane
Pond Preserve. We also thank the Palmetto Dunes Corporation for permis-
sion to work in a wetland on their property, which we have named Pal-
metto Pond. We were'assited in various important ways by Pete Stone
(DHEC), Roger Jones (Nature Conservancy), and Steve Hopkins (S. C. Wild-
life and Marine Resources). Finally, the senior author wishes to thank
his able field assistants, Beti, Coral, and Jimbo, who braved very'cold

water to assist in this study.

PREVIOUS STUDIES-

We discovered that there is a dearth of information concerning
(1) the recharge potential from wetlands to the ground water table and,
(2) the shallow stratigraphy of Hilton Head Island. We found no previ-
ous studies that addressed item (1) directly for Hilton Head Island, or
from any other region in the southeastern United States. There have
been studies in New England that are pertinent. O'Brien (1977) found
that a small wetland underlain by peat acted to recharge the water
table aquifer at certain times of the year in Massachusetts.

Similarly, we found no reference that provided detailed informa-
tion on the shallow (Neogene) stratigraphy of Hilton Head Island. Gen-
eral geologic information on the region is given in Hayes (1979), Spigner
and Ransom (1979), and Glowacz and others (1980). General geologic re-
ports include Cooke (1936), Cooke and MacNeil (1952), Colquhoun (1969),
and DuBar (1971). An important series of papers relating ground water
to geology was published by Siple (1946, 1948, 1956, 1959, 1960, 1967).

GEOGRAPHIC SETTING

Hilton Head Island (Fig. 1) is located at the extreme southwest

end of the South Carolina coast at Latitute 320 101 N. The island is

roughly triangular in shape, elongate parallel to the coast, and has
an area of 120 km2 (46.3 mi2). The island is subequally divided by
Broad Creek into a southern. and a northern portion. Elevations in the
northern portion are generally between 3 and 6 m (10 and 20 ft) msl.
Elevations in the southern portion lie mostly below 3 m (10 ft) msl.
The terrain is generally flat, with subtle coast-parallel beach ridge
traces visible from the air.

The climate is subtropical. Mean annual temperature (Fig. 2) is
18* C (65* F) and ranges from ll* C (52* F) in December to 26* C (79* F)
in July. Rainfall averages about 140 cm (55 in) per year, with about
40 per cent of it falling during the summer months.

GEOLOGIC SETTING

Hilton Head Island is a barrier island-typical of the southern
South Carolina-Georgia section of coast. These barriers tend to be
short and stubby due to the relatively high tidal range (over 2 m).
The general shape of the coastline of South Carolina and Georgia is
concave seaward, producing an embayment referred to as the Georgia
Bight. As a result of the existence of the embayment, the continen-
tal shelf is wider, tidal range greater, and mean wave heights smaller
than those adjacent regions to the north and south.

Hilton Head Island consists of two portions that are geologically
discreet based on time of formation (Fig. 1). The bulk of the island,
as well as unknown parts that may have subsequently been eroded away
on its seaward edge, was formed by beach ridge accretion during the
Sangamon interglacial interval (about 120,000 years ago). That period
of time was characterized by a sea level stand that was slightly higher
than that of the present. There must have been a plentiful sand supply
as the island grew to form an extensive beach ridge plain by the addition
of successive beach ridges on its seaward margin. This period of growth
was ultimately interrupted by a drop in sea level as the Wisconsin glacial
period ensued. Sea level was lowered to a position some 100 m or more

",0 /',nj Cra 7 e

cu.. V

Figure 1. Map of Hilton Head Island showing location of study sites
and geomorphology.

30 TEMPERATURE 86

20 - 68
0c 0 F

10 50

J'F'M'A'M'J'J'A'&'O'N'D'

30 - RAINFALL -12

20 - - 8

cm in

10 - 4

Figure 2. Yearly distribution of temperature and rainfall at Hilton
Isl
d
Hea and.

7
below its present position; hence, the shoreline was approximately 130 km
(70 naut. mi.) southeast of its present location. Stated differently,
the Sangamon aged portion of Hilton Read Island was located 130 km inland
from the coast; hence, it was subject to geomorphic degradation by stream
erosion and slope processes typical of inland regions.

Approximately 15-20,000 years ago (the beginning of the Holocene
interglacial interval), sea level began to rise once more as the Canadian
and other ice sheets began to melt. With the sea level rise, of course,
the shoreline was gradually shifted inland ... what had been coastal plain
became continental shelf. Sea level reached its present position (� 2 m)
about 3-4,000 years ago. During the past few thousand years, island
growth resumed by additional beach ridge accretion on the seaward margin
of the island. These Holocene-aged beach ridges are sharply defined
on air photos (being much younger than the Sangamon ridges inland) and
are easily distinguished.

The surficial, beach ridge-type sediments that occur on Hilton
Head Island are composed of predominantly quartz sand. Fine-grained
organic sediments occur-in the swales (wetlands).between relict beach
ridges. The surficial sands generally extend to a depth of about 10 m.
Beneath them there occur deposits of sand, silt, and clay that are com-
monly heterogeneous both vertically and horizontally. These sediments
compose the undifferentiated Miocene, Pliocene, and remaining Pleisto-
cene section and extend to a depth of about 30 m. Below the sand, silt,
and clay section lie the limestones of older Tertiary (Paleogene) age
which make up the "Tertiary Limestone Aquifer" (TLA). The upper zone
of sand, silt, and'clay is referred to as the Neogene clastics section
and the underlying limestones are referred to as the Paleogene limestone
section (equivalent to the TLA).

METHOD OF STUDY

Two fresh water wetland areas were selected for study. The first,
Whooping Crane Pond (Fig. 1), is located within the Hilton Head Planta-
tion property. It is part of the Whooping Crane Pond Preserve and is
under the management of the South Carolina Nature Conservancy. The
study site proper was located on the eastern side of the northern marsh.
This location provided good accessibility, and was removed from the
bird nesting area located on the western side of the marsh.

A second study site was located in a smaller marsh about 1.6 km (1 mi)
east of Whooping Crane Pond. This fresh water wetland is located on prop-
erty belonging to Palmetto Dunes Corporation; hence, we named it Palmetto
Pond (Fig. 1). Because of extensive real estate development, we could
not locate a suitable study site on the southern part of the island.

At each of the two study sites, three clusters of standpipe piezom-
eters were installed. Each cluster consisted of three open-ended 1.25"
PVC pipes inserted to depths of 1, 2, and 3 m. Stations 1 and 4 (at
Whooping Crane and Palmetto Ponds, respectively) were located well out
into the marsh in standing water. Stations 2 and 5 were located near
the edge of the marsh, which itself is a rather elusive boundary under
changing water level conditions. Stations 3 and 6 were located well
up onto the adjacent upland where the water table was significantly.
below ground surface.

At Whooping Crane Pond, we also drilled a 4" well into the Tertiary
Limestone Aquifer (TLA) to a depth of 35.8 m (118 ft). This deep well
was for the purpose of- monitoring,,wa.ter level changes in the TLA and
comparing them to changes in the shallow aquifer. We installed automa-
tic water level recorders on the deep well, a shallow water table well,
and on the pond surface (at Whooping Crane Pond only). The recorders
measured water level position every 6 minutes; hence, they provided
a detaildd'history of-water level fluctuations. Water levels were mea-
sured periodically in each of the 18 piezometers with an electric circuit-
type device. Daily temperature and rainfall information was obtained
from the Honey Horn Plantation weather station located 3.5 km (2.2 mi)
southwest of Whooping Crane Pond.

All water level measurements and rainfall data were entered into

computer files for subsequent anlysis. Various modes of output and
analysis are presented in the next section.

The basic question to be addressed by this study concerns the po-
tential for recharge of the ground water table aquifer by the fresh
water wetland surface waters. In order to evaluate this potential,
we assumed that water would respond to gravity and pressure; that is,
it would flow downhill and/or from regions of high pressure to regions
of low pressure. By comparing water levels both laterally and verti-
cally among the various piezometers, directions of potential ground

water flow@can be determined.

Sediment samples were collected at each wetland site with a Dutch
Gouge sampler. Also, samples of cuttings were collected at each 10-foot
interval from the deep well drilled. Additional information regarding
the stratigraphy is provided by various geophysical logs of the deep
well. A driller's log was provided by DHEC for the deep well that they
drilled in the same area at the Whooping Crane Pond site.

Geologic and hydrologic date were supplemented by information ob-
tained from various published reports and from personal communications
with several experts in this field. These sources are referenced where
appropriate.

DISCUSSION OF DATA

Sediment Samples
Core samples of the wetlands peat deposits were collected at each
site. They were examined microscopically and the results areillustrated
in Fig. 3. Though a comprehensive study of the sedimentology of t he
wetlands was beyond the scope of the study, and it is not known to what
extent the cores collected are representative of wetlands in general,
some important observations can be made. There is a surprising amount
of quartz sand in the peat samples. At both sites, the percentage of
sand increases rapidly with depth from 1-5 per cent at the upper surface
to over 50 per cent at 30 cm (1 ft.). At 60 cm.(2,ft-@,) the sediment
is an unconsolidated sand with minor amounts of organic and iron cement.
These observations agree with the findings of Otte (unpublished report,
1982), who made stratigraphic cross-sections across Whooping Crane Pond.
He found only 30 cm (1 ft.) or less of root mat or peat, underlain by
"peaty sand".

The significance of the high sand percentage is that it causes
the sediment to possess relatively high permeability. Therefore, water
can flow vertically from substrate to pond and from pond to substrate.
The hydrologic implications are significant to the present study. If
this vertical permeability did not exist, there would be no possibility
of either recharge or discharge through the wetland bottom. The nature
of the bottom permits the flow of water through it and, hence, recharge
and discharge are possible.

Cuttings samples were collected from the deep well that was drilled
to the Tertiary Limestone Aquifer (TLA). These samples were examined

10
Whooping Crane Pond Core
0 0.0 - 0.1 ft: root mat; fibrous peat; dark brown; high
inter- and intragranular porosity; sand,
2%, fine (very fine to fine), subangular.

0.1 - 0.5 ft: peat; dark brown; high inter- and intra-
!lIq granular porosity; sand 10%, fine (very fine
0.
to fine), subangular.
A
0.5 1.0 ft: sand; clear, quartz; high intergranular
porosity; fine (very fine to fine), sub-
angular; poorly indurated, organic cement;
30% organics; plant fragments.
10
1.0 1.2 ft: as above.

1.2 -'1.8 ft: sand; clear, quartz, fine (very fine to
fine), subangular; high intergranular por-
osity; poorly indurated, organic and iron
ox
ide cement; 20% organics.

1.8 2.0 ft: sand; clear, quartz, fine (very fine to
fine), subangular; high intergranu lar por-
osity; poorly indurated, organic and iron
oxide cement; 5% organics.

Palmetto Pond Core

'0
0.0 0.5 ft: root mat and fibrous peat; dark brown;
high inter- and intragranular porosity;
non-indurated; sand, 5%, fine (very fine to
fine), subangular.

0-5 2.0 ft: sand; clear, quartz; fine, subangular;
T
hi h intergranular porosity; non-indurated;
organics.

Note - An extremely tough sand layer was encountered while
installing standpipe piezometers between 1.5 and
2-5 M. Washings were clean, quartz sand, but cement
may have been washed out and may cause decreased
porosity and permeability through that interval.

Figure 3. Descriptions of sediment core samples from Hilton Head wetlands.

microscopically and their sedimentologic attributes described as illus-
trated in Fig. 4. Also shown are the gamma log data and descriptions
from the DHEC well drilled nearby (less than 100 m distant). There
are some areas of agreement and some of disagreement between the logs
of our well and that of DHEC. It is assumed that both are reasonably
accurate and that they represent lateral variation in the nature of
the underlying sediment (i.e. facies change). Lateral variation appears
to be common in the Neogene section on Hilton Head (Ransom, Personal
Communication, 1983). [Some of the minor discrepancies may be due to
poor sample quality rather than representing actual differences.]

The section revealed by our deep well indicates that there are
no significant clay layers that would inhibit the vertical flow of water
between the surface and the TLA. That is, local recharge of the TLA
from the Neogene section above is probably occurring due to the absence
of continuous clay confining layer that would prevent such vertical
movement. In summary,. the geologic conditions at Whooping Crane Pond
are such that there is nothing that should prevent local recharge of
the TIA from occurring.

Water Level Measurements

Water levels were periodically measured manually in 18 standpipe
piezometers, 9 at each wetland site. The 9 at each site were located
as indicated in Fig. 5. Water levels were measured weekly initially.
The sampling interval was increased later in the study. In addition,
automatic recorders were installed at Whooping Crane Pond that recorded
water level every 6 minutes. Recorders monitored water levels on (1)
the pond surface, (2) the water table surface near the edge of the pond,
and (3) the potentiometric head in the TLA. Comparison of the periodic
readings to the continuous records provided a more detailed history
of water level fluctuation in the Whooping Crane Pond wetland and, by
analogy, in the Palmetto Pond wetland. The manual measurements began
January 15 in Whooping Crane Pond and on February 19 in Palmetto Pond.
The recorders were installed March 10 and 11, 1983. Measuring termi-
nated June 30, 1984.

The manually measured water level data provides information on
both horizontal and vertical gradients of potentiometric surfaces.
Figure 6 illustrates horizontal gradients at Whooping Crane Pond.

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POND UPLAND

(21 5)
8 C A R
POND SURFACE -WATER TABLE

SHALLOW (NEOGENEI A@UWER

30 rn rASI

T L A

Figure 5. Diagram of piezometer and recorder installation at TAThooping
Crane Pond site.

Figure 6A shows water level versus time for data from the 1 meter piezo-
meter.. A comparison of the position of the solid line (the pond station)
to the dashed line (the upland station) reveals the slope of the water
table. If the solid line is higher than the dashed line, the water
table slopes away from the pond and recharge is indicated. If the dash-
ed line is higher than the solid line, discharge from the water table
to the pond is indicated. It can be seen that periods of both recharge
and discharge are indicated. The interval of discharge between days
180 and 360 are not as significant as it may appear. Many of these
readings are of a dry bottom, the water level having dropped below the
bottom of the pipe.

Figure 6B and 6C show the data from the 2 and 3 meter piezometers,
respectively. Similarly interpreted, they indicate a dominance of re-
charge conditions throughout the year.

From these interpretations, it can be concluded that the wetlands
'do act as areas of ground water recharge at times. In fact, indica-
.tions are that.recharge is predominant over discharge.

Figure 7A-C illustrates the water level data from Palmetto Pond.
The situation here appears quite different. The data from the 1 meter
piezometer (Fig. 7A) again reflects mostly dry hole conditions. The
first 150 days may be valid, in which case the situation is one of re-
charge being dominant. Figure 7B and X show water levels in the 2
and 3 meter piezometers. These plots look suspect. The upland water
levels are as much as 1 meter higher than the pond edge and the pond
stations. As the stations are only a few tens of meters apart laterally,
the indicated horizontal gradient is abnormally high. It shows up in
two different piezometers; hence it may be assumed that it reflects

a natural condition and not some factor related to the installation.

There may be some type of permeability barrier between the upland station
and the others, though the geologic conditions that would create such
a barrier is difficult to conceive. If, on the other hand, we discard
the data from the upland station, the pond level is consistently higher
than the pond edge, indicating recharge. The latter interpretation
is considered more probable based on the unusual appearance of the upland
data and based on the data from Whooping Crane Pond.

Though the data from Palmetto Pond is somewhat ambiguous, there
is an indication that recharge conditions may occur at times. Coupled

A, WHOOPING CRANE POW DEPTH I METER

4 %%% Sta. #1:
3. 9..
3. a.. Sta. 12:

E11) . .......... L.
Sta. #3: -------
3.4., % *

3.3

3.2

3.1
350 450 548
DAYS

B. i.<WZM OWE POW DEPTH 2 METER

4.3.

4.21
4.1. %
4 VY %

3.5.
UL
EMJ 3.4.
3.3.

3.2.

2.0

2.7.
2.50 iO 184 270 4.5a 540
DAYS

C. WHOOPING CRrkE POND-DEPTH 3 METER

4.3.

4.2..

4A .
4 %%
3.9. "IN Lim)

3.0. 4

3
3. >.
% 3 4
3.6. 3:2
UL U, 3
EM3 3.5. 2.9
2.6
3.4. 2.4
2.2
3.3. 2

2.2.
%%

3 %

2.5.

2. 0
a so ISO 226 3W 430 540
OA*m

Figure 6. Horizontal gradients of water level versus time at Whooping
Crane Pond.

A. PALMETTO PDND DEPT" I MIER

4.3-

4.2..

Sta #1:
4 - - - - - - - - - - - - - - - - - - -
WL 3.0. Sta, J2j see#*@

Sta. #3: ------

3.4
90 ISO 229 3W 45a 540
DAYS

B. PALMETTO POND DEPTH 2 rIETIER

4.4

4.3..

4.2..

4.1.,

4
3.5 %%
3.9

3.7..
%
%

UL
3.4.,
(M)
3.3..

3.2..

2.9

2.9.

2.7.

2.5.

2.3
8 so fee 270 350 450 54a
DAYS

C PALMETTO POND DEPTH 3 METER

4.1.

3.9.

3.8.

3.7.

3.0

3.5. PAIN (IN)

3.4. 4
3
WL 3.3. 31
3.4
3.2. 3.2
3
3.1. 2.6
2.5
3 2.4
2.2
2.9. 2

2.0.
1.4
2.71 1-2
2.6
2.5 *2
2.4 a
a se 168 2" 3GO 459 548
DAYS

Figure 7. Horizontal gradients of water level versus time at Palmetto
'Pond,

with the interpretations at Whooping Crane Pond, we feel that a state-
ment to this effect may be made for wetlands in general on Hilton Head.

Figures 8A-C and 9A-C illustrate the vertical gradients of water
levels at Whooping Crane and Palmetto Ponds, respectively. Figure 8A
and 8B are peculiar in that they indicate that water should flow from
the 1 and 3 meter levels to the 2 meter.level. If this isn't impos-
sible, it is certainly unlikely. In the case of 8A, the 2 meter point
at day 270 is probably erroneous. Unfortunately, there is no such
easy explanation for 8B. Figure 8C indicates that recharge is dominant.
A closer look at the date reveals, however, that the 1 meter piezometer
data is not always valid due to dry hole conditions part of the time.
If the 1 meter data is discounted, conditions appear to be dominantly
one of discharge. Figure 9A-C shows that at Palmetto Pond, conditions
are dominantly recharge, with or without the 1 meter data.

Interpretation based on vertical gradients appear to be more tenuous
than those based on horizontal gradients., The case is not strong, but
the evidence appears to be sufficient to,suggest that, at times, ground
water recharge from the wetlands occurs.

Automatic Water Level Recorder Data
Three automatic water level recorders were provided by the S. C.
Water Resources Commission for use in this study. They were deployed
at Whooping Crane Pond Site as indicated in.Fig. 5. These recorders
were battery-driven, punched tape type devices designed to be machine
read. Sampling was once per six minutes; hence, a voluminous amount
of data was generated. Due to a lack of appropriate facilities, the
tapes were read manually (visually), a most time-consuming and tedious
job of deciphering holes punched in paper.

Recorder #1 was installed at Station #lout in the pond (Fig. 5).
It operated continuously from March to June 8, 1983, at which time it
malfunctioned. This was not discovered until October 1983, because
of the lapse in project funding from July through September. Parts
from recorder #3 were used in an attempt to bring #1 back on line, but
only intermittent results were obtained. The data from #1 is shown
in Fig. 10. The sawtooth shape of the curve results from sharp rises

A. kiMPING CRftC POND STATION I

4.3..

1 meters
3.9.,

2 meter:
3.7.

3. a
3.5. 3 meter: -------

UL 3.4.
EM3 3.3.

3.2

3.1

2.9.,

2.7..

2.0.
a so lea 270 3W 450 548
DAYS

Be WH"ING CRF*C POW STATION 2

4.1

3.9

3.6.,

3.7..

3.a.,
UL
EM3 3.5.,
3.4..

3.3.,

3.2

3.1

3
2.91
a 98 sea 229 3" 459 540
DAYS
Ce WHODPING CRf*C POO STATION 3

4.3.

4.2.

3.9. "IN Elm)

3.9. 4.
3
3. 1 1: '.
3.4
3.0. 3.2
UL 3
3.5. 2
EM 216
2.4
2.2
3.3. 2
3.2. 11

2.1 :2
32. 0, 0
a Be lea 270 3ee 458 540
DAYS

Figure 8. Vertical gradients of water level versus time at Whooping
Crane Pond.

A, PALMETTO POND STATION

%%"%t 1 meter:
3.0
3.5 2 meters

3.4.
iL 3.3. 3 meter: --------
EM3 3.2.

3.2.

2.9.

2.6.

2.7.

2.6.

2.5
a 90 11" 270 350 450 548
DAYS

Be PALMETTO POW STATION 2

4.2.

4.2..

3.9..

3.9.

3.;P..

3.0.
3.5. p

3.4.
UL
CM) 3.3
3.2.

2.9.

2.9.
2.7 If
V1.
M.
2A

2.3

2,4
a Be ISO 270 308 430 540
DAYS

Ce PALMETTO POND STATION 3

4.4. RAIN CINJ

4.3.. 4
4.2.. 3.9
3
4.2. 3: '4
3.2
4 2
2
2: 9a
3.9 2
6JL 2
(M) 3.6. 2:2
3.7.

3.6 2

3.5

2..g
.4
tN \,@-v

0 W 3ea 226 3ce 450 349
DAYS

Figure 9. Vertical gradients of water level versus time at Palmetto Pond.

due to rain followed by slow decline due to evapotranspiration and in-
filtration. It will be seen that the former becomes extremely important
in the spring. Note that mild drought conditions existed during the
spring and early summer (days 100-200).

A more complete record was obtained from Recorder #2 (Fig. 11),
before it failed on April 20, 1984.. The 13 months of continuous record
provide a good illustration of an annual cycle of water table levels.
March, April, and early May, 1983 appear similar to the #1 data. Be-
ginning in mid-May, the curve from #2 begins to show a definite vertical
oscillation. Detailed study of the data reveals a night-day variation
of up to 30 mm in June and tapering off toward zero toward the end of
August. This represents the growing season and results from increased
evapotranspiration during the day. There is a slight recovery at night,
but a net water loss results. The importance of this process, coupled
with deficient rain is dramatically shown by the significant decline
in water level from mid-May through August. During the interval August 25
to November 20, the recorder float rested on the bottom of the stilling
well hole. That is, the-water level declined below the capacity of
the recorder to record it. Comparison to manually measured data (Fig. 6),

it is extimated that the water level declined at least an additional
0.5 m during that interval.

There is another factor that may be significant with respect to
the summertime decline in water levels that causes Whooping Crane Pond
to dry up. The pond is the headwater region from a series of artificial
ponds that border an adjacent golf course. The irrigation water for
the golf course is taken from these ponds or from the water table aquifer,
which amounts to the same thing. Spray irrigation is notoriously inef-
ficient with regard to water consumption. A significant part of the
water level decline at Whooping Crane Pond may be related to pumping
for irrigation.

However, if the above is true, then the water levels at Palmetto
Pond must have been similarly extracted because it declined the same
order of magnitude during the same period. If it can be demonstrated
that Palmetto Pond is essentially a natural system, then this would
argue that Whooping Crane Pond was not signficantly affected by artifi-

cial factors.

"MPTHO CRAPE POND STATION I

4.9

4.7

4.0

4.3

4.4

4.3..

4
.3..

3.9..

3.9

3.7.

3.0.

EL 3.5.
H. 3.4.

3.3.

3.2

3.1

3 4
3.9
2.9. 3.6

2.9. 3.2
3
2.7. 2.9
2.6
2.0. 2.4
2 2
2.5. 2*9
2.4.

2.3.. 2

2.2..
2.1., 2
2 . lid :04
a 30 00 30 122 150 390 230 248 272 300 2m 300
DAYS

Figure 10. Plot of water level recorder data-for pond surface (Station #1)
at Whooping Crane Pond.

AIDWING CRANE POND STATION 2

4.9..

4.6,.

4.5..

4.4

4.3..

4.2..

4.1..

3.9..

2.0

3.7.

3.6.

EL 3.3.

3.4.

3.3.

3.2

4
3.9
2.9. 3.0

2.9.
2.7. 2.4
.2.6
2.6 1@4
2'2
2.3. 2 IN.
J.9
2.4 I.a
2.3 1.2
2.2 . !9
2.3. 1
.2
2 LULLI J Ili hi
30 as so 120 3513 160 218 240 278 300 330 3M 30 80 90 128 Jae 390 2JO 249 2" WO 3m we
DAYS DAYS

Figure 11. Plot of water level recorder data for water table (Station #2)
at Whooping Crane Pond.

Recorder #3 (Fig. 12) was installed on the deep well near Whooping
Crane Pond. Continuous data from the interval from March 10 to October 9,
1983 was recorded; however, due to the complicated nature of the data
as a result of tidal oscillations, only a few months of data were diciph-
ered. These results do not bear directly on the question of recharge
by wetlands. They are of related interest, however, in that they show
similar declines in the spring, and even appear to respond to heavy
rainfalls (a suggestion of local recharge). This data will be analyzed
further at such time that we can manage to get the tapes machine read.

PRECIPITATION AND EVAPOTRANSPIRATION EFFECTS

The casual relationship between water levels and rainfall is obvious
on the data plots. A heavy rainfall generates an immediate rise in water
level in the pond, the water table, and possibly even in the deep lime-
stone aquifer.

From January through April, 1983, rainfall was unusually heavy
(Fig. 13), averaging nearly twice the normal average amount for the
period. The following four months saw less than half the normal average
of rain. The summer months are generally a period of high rainfall,
whereas the summer months of 1983 was one of low rainfall. September
and October, 1983, were average, but the following 6 months (November 1983-
April. 1984) experienced above average rainfall amounts (similar to the
previous year). May and June 1984, are again below normal averages in-
dicating that the summer of 1984 may follow the same rainfall pattern as
1983. This situation is unfortunate because the recent trend toward dry
summers causes a shortage of water at precisely the time when it is
most needed by plants for growth (and, of course, by humans, as well).
As long as these rainfall patterns continue, the wetlands can be expected
to dry up each summer. This appears to be a normal natural process,
though it is aggravated by additional human consumption.

Evaporation is related to temperature; hence, there tends to be
an increased water loss due to this cause in the late spring, summer,
and early fall. In 1983 and 1984, this coincided with periods of light
rainfall, which augmented the decline of water levels. Transpiration
results from water use by growing plants. This is also a late spring
and summer phenomenon and has recently been using water at a time when

WH00PING CRf*C POW STATION 3

1.7

1.4

1.3

1.2

1.3

-.2

-.3

-.4

-.9

30 Go so 120 330 190 230 240 270 330 3W 3W

HILTON HEAD RAINFALL MIA

4 ..

3.6.,
3.4..
3.2..
3 ..
2.9..
2.0.,
2.4..
2.2..
IN. 2

1.4.
1.2.

s 38 ce go 120 :50 10S 2)9 240 2" WO 3W 3"
DAYS

Figure 12. Plot of water level recorder data from deep well (Station
at 'Whooping Crane Pond.

10-

0-
J F M A M J J A S 0 N DI J F M A M J
83 84

Figure 13. Rainfall distribution during study period at Hilton Head
(Honey Horn Plantation). Circles represent average monthly
amounts.

it was scarce. The relative importance of evaporation versus transpira-
tion in water consumption is difficult to evaluate. Since the two gen-
erally occur simultaneously, they are combined as evapotranspiration.
The extent to which the water levels decline at Hilton Head has been
due to evapotranspiration versus below average rainfall cannot be eval-
uated from the data collected by this study. Continuous water table
recordings over several years with varying rainfall trends are needed

for such an evaluation.

A detailed study of January-April, 1983 indicates that local re-
charge may be related to these evapotranspiration/precipitation rela-
tionships. One can compute the average rainfall per day since the pre-
vious set of readings and compare this to the slope of the water table.
In the instances when recharge from the pond are indicated, rainfall
rates for the previous two weeks was on the order of 0.1" per day. A
the other instances when discharge to the pond is indicated, rainfall
rates averaged about 0.3" per day. That is, with higher rates of rain-
fall, percolation into the ground causes the water table to rise pro-
portionately higher than the pond7 surface rises, thus a pondward slope
to the water table surface is developed. Following periods of lower
rainfall rates, the water table declines to a level below that of the

pond surface. As evaporation would not be greater from the water table
surface than from the pond surface, transpiration must be the cause,
even though it is relatively low at this time of year (January - March).
The ground cover vegetation is dominately diciduous and is probably
not significant. The canopy, however, consists of evergreen pines that
are apparently using significant quantities of water even during the

winter.

After April 1, the recharge role of the pond becomes even more
pronounced. Though we might expect an increase in evaporation from
the pond surface, there is apparently an even greater increase in trans-
piration which lowers the water table beneath the upland. That is, due
to the onset of the growing season, the increase in the rate of trans-
piration in the upland in greater than that of the rate of pond evapo-
ration. Total evapotranspiration rate during the growing season is
2 to 3 times that of the dormant season (Linsley, et al, 1949).

CONCLUSIONS

There are strong indications that, at times, the fresh water wet-
lands on Hilton Head Island act to recharge the water table aquifer.
This conclusion is based mainly on the slope of the water table and
on the horizontal gradient of potentiometric heads measured in the va-
rious piezometers. It.is also indicated by vertical pressure gradi-
ents at individual stations, Recharge appears to occur episodically
and other times are characterized by discharge to the pond.

These conclusions are based on data representing 13 months of mea-
surements. The sampling interval was unusually rainy in the winter
and dry in the summer. The results of the study to date are, therefore,
biased towards these weather conditions and will probably be altered
somewhat as additional data representing other weather conditions and
time of year are considered.

RECONNENDATIONS

It is recommended that a limited data collection process be con-
tinued on a permanent basis. This would provide a data base of more
valid statistical value than that which presently exists.

This would consist of automatic recorders being maintained on the
pond (#l) and at near the edge of the pond #2). It is proposed that
these recorders sample hourly and be maintained by monthly visits. The
S. C. Water Resources Commission would appear to be the appropriate

agency to perform this task. The present author would be willing to

Perform the data analysis and develope a more complete model of the
water table aquifer system at Hilton Head.

REFERENCES

Colquhoun, D.J., 1972, Geology and ground water hydrology, in Port
Royal Sound Environmental Study: S.C. Water Resources Commission,
p. 74-84.

Comer, C.D., 1973, Upper Tertiary stratigraphy of the lower coastal
plain of South Carolina: Unpublished M.S. Thesis, Dept. of Geol-
ogy, University of South Carolina, 19 P.

Cooke, C.W., 1936, Geology of the Coastal Plain of South Carolina: U.S.
Geol. Survey Bull. 867, 196 p.

Cooke, C.W., and MacNeil, F.S., 1952, Tertiary stratigraphy of South
Carolina: U.S. Geol. Survey Prof. Paper 243-B, p. 19-29.

DuBar, J.R., 1971, Neogene stratigraphy of the lower coastal plain of
the Carolinas: Atlantic Coastal Plain Geological Association,
12th Annual Field Conference, Columbia, S.C., South Carolina Geo-
logical Survey, FG-10, 128 p.

Glowacz, M.E.,* Livingston, C.M., Gorman, C.L., and Clymer, C.R., 1980,
Economic and environmental impact of land disposal of wastes in the
shallow aquifers.of the lower Coastal Plain_of South Carolina, Vol.
VIII: Summary-Beaufort and Jasper Counties, S.C. Dept. of Health
and Environmental Control, Columbia, S.C., 177 p.

Hayes, L.R., 1979, The ground-water resources of Beaufort, Colleton,
Hampton, and Jasper Counties, South Carolina: S.C. Water Resources
Commission Rept. No. 9, 91 p.

Linsley, R.K., Kohler, M.A., and Paulhus, J.L.H., 1949, Applied Hydrology,
McGraw-Hill Book Co., New York, 689 p.

O'Brien, A.L., 1977, Hydrology of two small wetland basins in eastern
Massachusetts, Water Res. Bul. Vol. 13, p. 325-340.

Siple, George E., 1946, Progress report on ground-water investigations
in South Carolina: Columbia, S.C., South Carolina Geological Survey,
Bulletin 15, 116 p.

1948, Memorandum on ground-water investigations in the Savannah
area, Georgia-South Carolina: U.S. Geological Survey Open-File Re-
port, 11 P.

1956, Memorandum-on the geology and ground-water resources of the
Parris Island area, South Carolina: U.S. Geological Survey Open-File
Report, 29 p.

1959, Guidebook for the South Carolina Coastal Plain field trip of
the Carolina Geological Society, November 16-17: Columbia, S.C.,
South Carolina Geological Survey, Bulletin 24, 27 p.

1960, Geology and ground-water conditions in the Beaufort area,
South Carolina: U.S. Geological Survey Open-File Report, 124 p.

1967, Salt-water encroachment in coastal South Carolina: Columbia,
S.C., South Carolina Geological Survey, Geologic Notes, v. 11,
no. 2, p. 21-36.

South Carolina Coastal Council, 1982, Special Area Management Plan for
Hilton Head Island, 31 p.

Spigner, B.C. and Ranson, Camille, 1979, Report on ground-water conditions
in the Low Country Area, South Carolina, SCWRC, Rpt. No. 132, 149 p.

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