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GEOLOGICAL CONTROLS ON SEASONAL-POOL HYDROPERIOD IN AKARST SETTING
Michael A. O’Driscoll1 and Richard R. Parizek2
1Department of Geological Sciences
East Carolina University
Greenville, North Carolina, USA 27858
E-mail: [email protected]
2Department of Geosciences
The Pennsylvania State University
University Park, Pennsylvania, USA 16802
Abstract: Shallow depressions found in karst terrains may contain temporary (vernal) pools that are
inundated seasonally in response to changes in meteorological conditions. The hydrogeology of 16 pools
(0.06–0.4 ha) was studied in an Appalachian karst valley in central Pennsylvania, USA. The objective was to
determine the effect of the geologic substrate on pool hydroperiod. Meteorological, geophysical, and
hydrogeological data collected from November 1997–August 1999 and from January 2002–January 2004
suggested that hydroperiod was primarily controlled by meteorological conditions (total annual
precipitation) and surficial aquifer geology. Multiple regression models were found to predict most of the
spatial variability of pool hydroperiod with the following variables: thickness of the surficial sandy aquifer;
sediment electrical resistivity; and annual precipitation. It might be expected that hydroperiod would be
longer for clay pools than sandy pools because clay sediments can act as a seal to perch shallow ground-
water and surface-water. Our data revealed the opposite to be true. Sandy residual sediments helped capture
infiltration and direct this water along perched ground-water lenses or sheets to seasonal pools. This resulted
in annual hydroperiods that were 115 days longer for sandy pools when compared to clay pools. The results
suggest that the geologic substrate can be a major control on the duration of hydroperiod.
Key Words: isolated wetlands, karst pans, perched ground-water, vernal pools, wetland/ground-water
interactions
INTRODUCTION
The recent US Supreme Court ruling (Rapanos vs
United States, June 2006) and the current legal
status of federal protection of isolated wetlands calls
for an improved scientific understanding of isolated
wetland hydrology (Leibowitz et al. 2008). Seasonal
or ‘‘vernal’’ pools are ‘‘…temporary or semi-
permanent pools occurring in shallow depressions
that typically fill in the spring or fall and may dry
during summer or in drought years’’ (Calhoun and
DeMaynadier 2008). Seasonal pools are generally
isolated but intermittent surface-water connections
may occur.
Pool hydroperiod is the number of days per year
that the aquatic phase occurs. Hydroperiod is an
important control on pool ecology because it
influences invertebrate/ vertebrate species richness,
community composition, predator abundance, and
fire occurrence (Brooks 2000, Mitsch and Gosselink
2000). Specialized organisms that utilize seasonal
pools are able to survive the extreme changes in local
hydrology (Zedler 2003, Colburn 2004) and for this
reason pool habitats support numerous federally
listed threatened and endangered plant and animal
species (Tiner 2003). Pool hydroperiod has a major
influence on amphibian populations. In Rhode
Island, USA it was found that successful reproduc-
tion of most pond-breeding amphibians required
hydroperiods of 4–9 months with pool inundation
occurring from March–August (Paton and Crouch
2002).
Precipitation and evapotranspiration are the
major controls on seasonal-pool hydroperiod, but
ground-water inputs and losses may also be
important (Leibowitz and Brooks 2008). Ground-
water fluxes are controlled by the permeability of the
surrounding aquifer, geomorphology/topography,
and meteorological conditions (Winter and La-
Baugh 2003, Rheinhardt and Hollands 2008). Local,
intermediate, and regional scale ground-water flow-
paths can feed seasonal pools. Pools nourished by
larger-scale flowpaths are less likely to be affected by
recent weather patterns and are more likely to have
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WETLANDS, Vol. 28, No. 4, December 2008, pp. 1004–1017’ 2008, The Society of Wetland Scientists
1004
longer hydroperiods (Winter and LaBaugh 2003).
Geomorphic variables found to be related to
hydroperiod include pool depth and volume, and
possibly pool area. Larger pools tend to remain wet
longer (Brooks and Hayashi 2002, Skidds and Golet
2005).
Several studies have documented geologic con-
trols on seasonal-pool hydrology, mainly focused in
the Central Valley, CA and the glaciated northeast-
ern US. In the Central Valley, CA vernal pools were
found to be fed by a shallow ground-water system
perched on top of low-permeability sediments
(Rains et al. 2006). In these settings, Rains et al.
(2008) found that although vernal pools may appear
similar, differences in their underlying geology(hardpan vs. clay-rich soils) can result in large
differences in ground-water inputs and surface-water
geochemistry. In contrast, in central Massachusetts,
Grant (2005) used a GIS approach to map the
presence of potential seasonal pools and found that
pools were more likely to be found in more
permeable glacial deposits than in till/bedrock
settings. Rheinhardt and Hollands (2008) recently
reviewed the importance of glacial geology and
topography on seasonal-pool occurrence in the
Northeast. In a recent review of seasonal pool
hydrology, Brooks (2005) suggested that the distri-
bution of pools is controlled by: 1) an impermeable
sediment or bedrock layer near the surface, 2) the
presence of small topographic depressions, and 3)
meteorological conditions.
In a previous study, we documented a large range
of annual hydroperiod (33–192 days) for a series of17 seasonal pools in central Pennsylvania located
within several hundred meters of each other in a
karst setting (O’Driscoll and Parizek 2003). Meteo-
rological conditions, vegetative cover, and topogra-
phy were very similar across the chain of wetlands. It
was hypothesized that subsurface geology controlled
the variability in hydroperiod across the site. The
current study was designed to determine the effect of
the geologic substrate on seasonal-pool hydroper-
iod.
STUDY AREA
The study site occupied 22 hectares at thePennsylvania State Gamelands 176 in Spring Creek
Watershed, Centre County, Pennsylvania (77u549000
N, 40u509000 W) in a karst-underdrained valley in
the Appalachian region of central Pennsylvania,
USA (Figure 1a). The research site at the Game-
lands was located several hundred meters down-
gradient of a Pennsylvania State University waste-
water irrigation site and is comprised of crop fields
(60%), woodlands (35%), and wetlands (5%).
Wetlands were typically surrounded by mixed
deciduous forest. Previous studies revealed that
irrigation wastewater does not migrate to the
wetlands (O’Driscoll 2000, O’Driscoll and Parizek
2003).
The predominant geological formation underlying
the research area is the Gatesburg Formation
(Cambrian), which consists of dolomites, sandy
dolomites, and thin sandstones (Carrucio 1963).
Wetlands wetl-28-04-12.3d 9/10/08 16:06:56 1005 Cust # 08-88
Figure 1. a) Site map including geology and location ;of
study site in the Spring Creek Watershed, Centre County,
PA. b) The topography of the southwest corner of State
Gamelands 176 site, including the location of piezometer
nests, staff gages, vernal pools, and the transect A-A’. U
indicates an uncased piezometer nest. Topographic
contour elevations are in meters above sea level. Contour
interval is 1 meter. The soil contact from the Centre
County Soil Survey (Braker 1981) is overlain.
O’Driscoll & Parizek, GEOLOGIC CONTROLS ON SEASONAL-POOL HYDROPERIOD 1005
The Upper Sandy Member of the Gatesburg
Formation underlying the southern portion of the
site is approximately 150 m thick and consists of
alternating sandstones and dolomites that weather
to sandy loam residual soils (Butts and Moore 1936,
Braker 1981). The Mines Dolomite Member of the
Gatesburg Formation occurs stratigraphically above
the Upper Sandy Member, is approximately 70 m
thick, and underlies the area to the north of the
chain of wetlands. The Mines Dolomite Member
consists of coarse-grained dolomite, weathering to
clay-rich residual soils. Overall, the bedrock in the
area is covered by a thick mantle of residual soils,
ranging in depth from , 10 to 50 m. In a previous
study of this chain of seasonal pools, the perched
and regional ground-water system was mapped and
the catchment draining to seasonal-pool wetlands
was delineated. Direct precipitation and shallowperched ground-water fed these pools and the
perched ground-water system was influenced by
geologic controls including the presence of a tear
fault, permeability contrasts of residual soils, and
organic clay seals underlying pools (O’Driscoll 2000,
O’Driscoll and Parizek 2003).
METHODS
To determine the influence of the meteorological
and geological controls on hydroperiod, a field and
modeling approach was used. Hydrological moni-
toring was conducted to determine hydroperiod,
hydraulic head, and to characterize the ground-
water system. Geological and geophysical analyseswere performed to characterize the soil/aquifer
properties. Statistical models were developed to
evaluate the effects of meteorological and geological
conditions on pool hydroperiod.
Hydrologic Monitoring
In the fall of 1997, 124 nested piezometers were
installed at 31 locations throughout the site, with
each location containing a cluster of four piezom-
eters, one each at depths of 1.5, 3.0, 6.1, and 9.1 m
(Figure 1b). Piezometers consisted of 2.54-cm-diam-
eter PVC pipe with a 61-cm-long screened section
with a slot width of 1 mm located at the bottom of
the pipe. Boreholes for the piezometers were drilledwith a hollow-stem auger mounted on an all-terrain
vehicle. A sand pack was added to cover the
screened interval, and then a bentonite sealing layer
was added to prevent construction-related, vertical
hydraulic connection among piezometers in the
same nest. Piezometers were vented to the atmo-
sphere and developed by purging with a peristaltic
pump. Seventeen staff gages were installed in
wetland depressions to measure surface-water ele-
vations (Figure 1b). Pool 16 extends outside of the
study property and straddles the Gamelands prop-
erty line. The largest pond, Toftrees Pond (Pool 15),
is impounded and used to store irrigation water for a
neighboring golf course. In the current study, we
focused on the 16 remaining undisturbed pools.
Staff gage and piezometer location and elevation
were surveyed with a laser theodolite and added to
the topographic base map. Topographic data were
surveyed to create a 0.3048 m contour interval map
to allow direct comparison between surface-water
and ground-water head data.
Surface-water and ground-water levels were mea-
sured at the site during two intervals from Novem-
ber 1997 to August 1999 and from January 2002–
January 2004 at a bi-weekly interval during periods
when pools were inundated (monthly interval when
pools were dry from August 1998–January 1999)
with a one-time sampling during April 2004 during
an extreme wet period. Surface-water levels were
measured at staff gages installed in pools and
ground-water levels were measured in piezometers
using an electrical water-level meter. The first year of
complete surface and ground-water level sampling
was from August 1998–1999. Pools were all dry
from August 1998 until January 1999 during
drought conditions and completely dried up again
in August of 1999. The remaining two periods were
from January 2002–2003 and January 2003–2004.
Over the 3 year-long study periods, pools were
inundated for a total of 65 sampling visits. During
these visits 646 surface-water level measurements
were collected. Hydroperiod was estimated from
surface-water data collected during field visits by
considering the initial inundation to be the first
sampling date when water was present and the end
of the hydroperiod was considered to be the last
sampling date when water was present. The hydro-
period estimation error should be less than a month
for each year. Based on more frequent site visits
early in the study bi-weekly sampling was deemed
adequate because it generally took pools more than
several weeks to dry up after filling.
Total annual precipitation data from 1899–2006,
daily air temperature, and daily precipitation data
for the period of study were obtained from the
Pennsylvania State University weather station,
located approximately 4 km from the site (Pennsyl-
vania State Climatologist 2007; http://climate.met.
psu.edu/). Potential evapotranspiration (PET) esti-
mates for the period of study were based on the
Blaney-Criddle equation, PET (mm) 5 kp (0.46T +8.13). The consumptive coefficient is k, T is air
Wetlands wetl-28-04-12.3d 9/10/08 16:07:00 1006 Cust # 08-88
1006 WETLANDS, Volume 28, No. 4, 2008
temperature (degrees-C), and p is the percentage of
total daytime hours for the period of interest of total
daytime hours of the year (4,380 daytime hours/yr).
Daily air temperature and sunlight hours were
averaged for monthly periods to estimate the
monthly PET. The consumptive coefficients were
0.85 for the growing season (April–August), 0.45 for
the dormant season, and 0.65 for the transition
months, March and September (Xu and Singh
2002). In a previous study, Xu and Singh (2001)
found the Blaney-Criddle method to be more
accurate than six other temperature–based potential
evapotranspiration estimation techniques with a
maximum deviation from monthly pan evaporation
of , 25%.
However, this approach may overestimate actual
evapotranspiration if plants are water stressed. At our
site, the location of the wetlands in a topographic low,
the presence of a perched ground-water system, and
the presence of surface-water would tend to reduce
the likelihood of water limitation. A comparison
between PET estimates for this study (mean estimate
of 82% of annual precipitation) and past studies of the
Spring Creek Watershed based on water budgets
(43%–70% of annual precipitation) suggests that PET
values may overestimate actual evapotranspiration on
an annual basis (Giddings 1974, O’Driscoll 2004). For
the purposes of this study, we used PET data in a
limited way, to show time periods throughout the year
when water deficits were likely.
Geologic and Geophysical Characterization of Soils
and Residual Sediment
Continuous split-spoon sediment samples
(3.81 cm diameter) were collected at 30 of the 31
drilling locations to depths of 10 m. These sediment
borings were logged and classified in the lab.
Detailed logs are provided in O’Driscoll (2000).
Sediments were correlated based on texture, distinct
stratigraphic layers, abundance of rock fragments,
type of rock fragments, Munsell color, gleying, and
other criteria. Recovery of sediment cores was not
100%; 250 m of residual sediment core was logged
out of a potential 300 m.
Electrical resistivity surveys were conducted at the
site during the late fall–early winter of 1998 andrepeated during the late spring–early summer of 1999
to determine the subsurface sediment distribution
and the depth and extent of clay or sand lenses. Direct
current was injected into the subsurface by electrodes
that contacted the surficial sediments and additional
electrodes were used to measure the voltage between
two points. The greater the resistance to electrical
current flow at depth, the higher the apparent
resistivity (apparent resistivity refers to the composite
resistivity of numerous geologic layers to a certain
depth in the subsurface) of the underlying materials
(Burger 1992). Computer simulations are used to
create a model of the resistivity of subsurface units
that matches the measured surficial voltage measure-
ments. This method can help differentiate between
buried clay and sand sediments because they conduct
electricity differently. Clay and silt-rich sediments
have low apparent resistivities (10 s–100 s) (good
conductors), whereas sandy sediments have greater
apparent resistivity values (100 s–1000 s) (poor
conductors) (Burger 1992).
A common survey approach is to locate two
potential electrodes (to measure voltage) centered
above the point of interest at a fixed distance apart
(known as a-spacing) and locate two current
electrodes (one on each side of the potential
electrodes to inject current) the same distance from
the potential electrodes. This electrode geometry is
known as the Wenner array, and when a-spacing is
increased the depth of current penetration, and
hence the depth of subsurface imaging, increases. A
‘‘depth sounding’’ is performed when a-spacing is
systematically increased, the apparent resistivity
measurements represent a greater slice of the
subsurface and an electrical sample of earth
materials at depth is collected (Burger 1992).
Resistivity data were collected using an IP-Plus
resistivity meter (EDA Instruments, Inc.). Twenty-
five depth soundings were performed at the site, with
surface measurement points spaced approximately
95 m apart. In some areas, it was necessary to
deviate from this spacing to avoid surface-water or
hedgerows. Apparent resistivity was estimated at
each surface point for a series of 10 logarithmically
spaced a-spacings of 1, 1.47, 2.15, 3.16, 4.64, 6.81,
10.00, 14.68, 21.54, and 31.62 m (Burger 1992). Each
a-spacing represented a different depth of sediment
(10 resistivity measurements were collected at each
surface point and 250 total resistivity measurements
for the entire study site).
Computations performed using field data and
ERModel (Burger 1992) indicated that sediments at
depths of up to 10 m were characterized during
surveys with a-spacings of 31.62 m and for a-
spacings of 6.81 m earth resistivity values would
image depths of approximately 2.2 m in the
subsurface. The resistivity surveys imaged sediment
electrical resistivity at sufficient depths to character-
ize the perched aquifer system feeding the pools at
the site. Hydraulic head data from this and previous
studies showed that ground-water feeding pools was
always coming from depths of less than 9 m
(O’Driscoll 2000, O’Driscoll and Parizek 2003).
Wetlands wetl-28-04-12.3d 9/10/08 16:07:00 1007 Cust # 08-88
O’Driscoll & Parizek, GEOLOGIC CONTROLS ON SEASONAL-POOL HYDROPERIOD 1007
Resistivity data for the 6.81 m and 31.62 m a-
spacing surveys were used to generate resistivity
maps and contoured using the kriging approach with
Surfer contouring software (Golden Software; http://
www.goldensoftware.com/). Resistivity measure-
ments collected at these a-spacings were chosen for
contour maps because they provide insight into the
framework of the shallow aquifer system adjacent to
and underlying the pools. The 6.81 m and 31.62 m a-
spacings represent the shallow (2 m deep) sediments
adjacent to pools and the deeper sediments (10 m)
underlying the entire site, respectively.
Depth soundings were used to image a vertical
section of the subsurface. Greater a-spacing resulted
in a deeper subsurface investigation. The voltage
and current were measured 100 times at each survey
point and then an average voltage and current
reading was given for each surface measurement
point, specific to the a-spacing. These measurements
were then used to calculate the apparent resistivity
of subsurface materials below that surface point for
each particular a-spacing using the following equa-
tion for the Wenner array, r 5 2paV/I, where r is
apparent resistivity (ohm-m), p is 3.14159, V is
voltage measured (volts), I is current injected into
the subsurface (amps), and a is the a-spacing (m), or
distance between electrodes.
Statistical Analysis of Variables Affecting
Pool Hydroperiod
Eight geological/geophysical variables were eval-
uated to determine their relationship to pool
hydroperiod. These variables included sandy aquifer
thickness, pool depth, pool area, pool volume, soil
infiltration rate (obtained from the Centre County
Soil Survey), electrical resistivity at 6.81 m a-
spacing, electrical resistivity at 31.62 m a-spacing,
and pool topographic catchment area (m2). All
variables were log transformed and compared to
pool hydroperiod (mean of 3 year-long study
periods). The least squares estimation method was
performed to obtain Pearson correlation coefficients
to measure the strength of linear relationships
between the variables and hydroperiod. Following
these analyses, best subsets regression was per-
formed on the variables and hydroperiod to find
the best-fitting regression models for the specified
predictor variables (Minitab, Version 15). To
develop a general model that used the geologic
variables (Geologic Model) to predict hydroperiod
across the site, the variables were used to predict
mean hydroperiod at the 16 pool locations (n 5 16).
To develop a general model that used the geologic
variables and took into account the changing
meteorological conditions for the three separate
years of study (Geologic and Meteorologic Model)
the geologic variables for each pool and the total
precipitation and hydroperiod for each given year-
long study period were used (n 5 48). The frequency
distribution of annual rainfall (% exceedance) over
the period of 1899–2006 was estimated using an
empirical cumulative distribution function.
RESULTS
Temporal Variability in Seasonal-pool Hydroperiod
and Water Level
The three year-long periods studied had a range of
81–131 cm annual precipitation. The long-term
mean annual precipitation for State College is
98.3 cm (1899–2006) with a standard deviation of
14.7 cm. The 1998–1999 study year occurred during
drought conditions (81 cm) and the remaining two
study years experienced average to above average
precipitation (2002-104 cm and 2003-131 cm). Dur-
ing the driest sampling year (1998–1999) the mean
hydroperiod for the 16 pools was 112 days. At the
other extreme, during 2003, the mean hydroperiod
was 248 days. The mean annual pool hydroperiod
had a positive relationship with the magnitude of
annual rainfall (Figure 2a). Pool water levels peaked
in spring (April) and during late fall-early winter
(December). Precipitation excess (precipitation-po-
tential evapotranspiration (PET) affected pool water
levels, but in complex ways related to seasonal
variability in precipitation-PET and the timing of
the shallow ground-water response (Figure 2b).
Precipitation generally was greatest during summer
months, but summer was the period of peak PET so
water deficits were common. The drawdown of pool
levels during the growing season corresponded with
the period when there was a water deficit, resulting
in lowest pool levels commonly occurring in August
and September. On average, the month with the
lowest water levels and the occurrence of the greatest
number of dry pools was September; during this
month over 80% of the pools were dry for the three
one-year periods (Figure 2c). Conversely, April was
the month when pool levels were greatest and pools
had the lowest likelihood of being dry. During April
sampling dates only 9% of the pools sampled were
dry. There was a large degree in variability of drying
across the pool sites. The wettest pool (16) was dry
for 4.6% of the sampling dates. The driest pool (17)
was dry for 74% of the sampling dates (n 5 65).
These differences can be attributed to the contrast-
ing geological conditions across the site that will be
discussed next.
Wetlands wetl-28-04-12.3d 9/10/08 16:07:01 1008 Cust # 08-88
1008 WETLANDS, Volume 28, No. 4, 2008
Geological and Meteorological Controls on Pool
Water Level and Hydroperiod
Electrical Earth Resistivity and Sediment Distribu-
tion. Electrical resistivity depth soundings, verified
by sediment cores, suggested three general types of
sediment distribution at the site: 1) thick sand
deposits (. 5 m); 2) thick clay and/or silt-rich
deposits (. 5 m); and 3) a thin surficial sand aquifer
underlain by clay at 2–4 m depth. Thick sand
deposits are common in the southern portion of
the site; depth sounding #1 (Figure 3a) shows an
apparent resistivity curve in this region indicative of
sandy soils to depths of 10 m. Thick clay and/or silt
deposits are common in the northern portion of the
site; depth sounding #10 (Figure 3a) shows a clay-
type apparent resistivity curve in this region.
Sediments in this northern portion of the site
consisted of silt, clay, and clay loam sediments from
the surface to at least 9 m depth. The thin surficial
sand aquifer underlain by clay at 2–4 m depth in
proximity of the ponds is evident in depth sounding
#5 located near pool 14 (Figure 3a). Here the
resistivity values for the a-spacings up to 3.16 m are
representative of sandy sediments, but as the survey
includes deeper sediments at greater a-spacings the
apparent resistivity declines to values of approxi-
mately 200 ohm-m, suggesting a clay-layer at depths
of 3 m or more. Sand layers exist beneath many of
the ponds and these aquifers may act as preferential
pathways for shallow perched ground-water to
nourish wetlands. In addition, cores taken in pools
revealed a 1–2 m thick organic clay layer underlying
pool depressions.
Apparent resistivity values at depth sounding
locations varied from 120–1425 ohm-m across the
site and were related to sediment distribution. Sandy
sediments had resistivity values greater than
500 ohm-m and silt or clay-rich sediments had
resistivity values less than 500 ohm-m. For the
6.81 m a-spacing survey a general pattern existed
across the site; higher apparent resistivity values
Wetlands wetl-28-04-12.3d 9/10/08 16:07:01 1009 Cust # 08-88
Figure 3. a) Depth soundings for three representative
sites. Apparent resistivity versus a-spacing curves for
settings with sand (depth sounding 1: DS-1), clay (depth
sounding 10: DS-10) and sand underlain by clay (depth
sounding 5: DS-5). Greater a-spacing indicates greater
sediment depth. b) Apparent resistivity map for the
6.81 m a-spacing survey. Units are in ohm-m. Wetland
pools, locations of three representative depth =soundings,
and the soil contact from the Centre County Soil Survey
(Braker 1981) are overlain.Figure 2. a) Annual rainfall vs. hydroperiod for the three
year-long sampling periods. b) Mean monthly variations
in mean pool depth (m- on right y-axis), PET, precipita-
tion, and precipitation excess (cm) for the entire study
period. c) Mean percentage of pools (n 5 16) that were
dry for a< given month for all of the three year-long
sampling periods.
O’Driscoll & Parizek, GEOLOGIC CONTROLS ON SEASONAL-POOL HYDROPERIOD 1009
were confined to the sandy southern corner of the
site and a low resistivity area (clay/silt rich
sediments) extended from the southern edge of the
wetlands area to the entire northern limits of the site
(Figure 3b). A lobe of higher resistivity materialswas apparent in the wetland area around pools 8, 9,
13, and 14. This trend represents a shallow sandy
aquifer that is present in the wetland region. The
sandy surficial aquifers are approximately 1–7 m
thick and extend from 1–7 m below grade and are
nearly always underlain by silt or clay-rich sediments
or at least have inclusions of silt or clay-rich
sediments. Resistivity data from 31.62 m a-spacingsurveys revealed similar patterns to those collected
at 6.81 m a-spacing.
Pool Classification by Sediment Type. On Figur-
es 1b, 3b, and 4 the contact between the Huble-rsburg silt loam and Morrison sandy loam has been
overlain from the Centre County Soil Survey
(Braker 1981). The resistivity surveys and sediment
cores collected during the current study suggest that
the sandy loam-silt/clay loam contact in this area
differs from the soil survey map, likely due to the
spacing of soil sampling locations used to generate
the original soils map. The 6.81 m a-spacingresistivity map and the sediment core data (upper
2 m) from 30 logs were used to more accurately map
the contact between silt/clay loam and sandy loam
soils (Figure 4). This approach is based on 55 data
points (30 cores at piezometer nests and 25 depth
soundings) per 22 ha or 2.5 data points/ha. The
contact line is a manual approximation based on
shallow sediment core data and on resistivity data inareas where no cores were taken. In most cases the
contact followed the resistivity contours. In a few
locations the contact slightly deviated from the
resistivity data because sediment cores nearby
indicated a local change in soil type.
This approach resulted in a classification of pools
into two groups: 1) sandy and 2) clay/silt pools (for
simplicity the latter will be referred to as clay pools).
Clay pools typically had adjacent apparent resistiv-
ity values of less than 500 ohm-m and were located
on the northern side of the site. Sandy pools had
apparent resistivity values of . 500 ohm-m and
were located in the southern portion of the site.
Seven sandy pools (pools 2, 8, 9, 10, 13, 14, and 16)
and nine clay pools (pools 1, 3, 4, 5, 6, 7, 11, 12, and
17) were mapped.
Controls on Pool Hydroperiod. Pool hydroperiod
was related to pool type (Figures 4 and 5). Sandy
pools remained wet longer than clay pools as
indicated by mean hydroperiod for the duration of
Wetlands wetl-28-04-12.3d 9/10/08 16:07:06 1010 Cust # 08-88
Figure 4. Updated soils map and mean annual hydro-
period for the three years of study. The dashed line shows
the sand-silt/clay loam contact overlain from the Centre
County Soil Survey (Braker 1981) map and the solid line
shows the sand-silt/clay loam contact inferred from
sediment logs and resistivity data. Sandy loam soils are
shaded gray and silt/clay loam soils are in white. Numbers
are mean hydroperiod (days).
Figure 5. a) Box and whisker plot of mean clay pool
versus mean sandy pool hydroperiod. Mean is for >the 3
year-long study periods. Box indicates the range from 25th
to 75th percentile and mid line is the median. b) Percentage
of sampling dates when a given pool was wet over the 3–
year long sampling periods versus pool depth (n 5 65).
Black circles indicate pools that were present in clay-rich
sediments and white circles indicate pools that were
present in sandy sediments.
1010 WETLANDS, Volume 28, No. 4, 2008
the study (Figure 5a) and the percentage of sam-
pling dates when pools were wet (Figure 5b). In
addition, deeper pools tended to be inundated more
frequently. A Mann-Whitney comparison of hydro-
period for sandy pools versus clay pools revealed
that hydroperiods were significantly (p , 0.05)
shorter for clay pools by approximately 115 days.
Of the variables analyzed, all but catchment area
were significantly (p , 0.05) correlated with hydro-
period based on Pearson correlation tests (Table 1).
The strongest relationships existed between log
hydroperiod and log sand thickness, log depth,
and log resistivity (6.81 m a-spacing) (Table 1).
Pools adjacent to thick sandy surficial aquifers
tended to have longer hydroperiods than those that
were not connected to sandy aquifers. Pools
surrounded by greater resistivity sediments (sandier)
tended to have longer hydroperiods. The infiltration
rate of soils surrounding the pools also was related
to the duration of hydroperiod. Sandy soils with
greater infiltration rates correlated with pools with
longer hydroperiods. In addition, pool morphology
had a relationship with hydroperiod; deeper and
larger pools tended to remain inundated for longer
periods.
Models for Predicting Hydroperiod. A best subsets
regression analysis revealed several regression equa-
tions that could predict pool hydroperiod (Table 1).
A multiple regression model (Geologic Model)
including sand thickness and earth resistivity values
(31.62 m a-spacing) could predict 86% of the
variability in mean annual (log) hydroperiod for
the pools (Table 1 and Figure 6) suggesting the
Wetlands wetl-28-04-12.3d 9/10/08 16:07:11 1011 Cust # 08-88
Table 1. Correlations and regression equations for (log) hydroperiod. SE indicates the standard error. Bold correlation
coefficients and p-values indicate significant results at p , 0.05.
Parameter Pearson’s Corr. Coefficient p-value
log sand thickness (m) 0.906 0.000
log depth (m) 0.796 0.000
log resistivity (6.81 m) 0.734 0.001
log volume (m3) 0.702 0.002
log infiltration rate (cm/hr) 0.651 0.006
log resistivity (31.62 m) 0.637 0.008
log area (m2) 0.531 0.034
log catchment area (m2) 0.337 0.202
Equation Adjusted R2 SE (Predictor) p-value
log hydroperiod 5 2.30 + 0.142 log sand thickness (m) 0.81 0.018 0.000
log hydroperiod 5 2.19 + 0.912 log depth (m) 0.61 0.19 0.000
log hydroperiod 5 0.663 + 0.581 log resistivity ( 6.81 m) (ohm-m) 0.51 0.14 0.001
log hydroperiod 5 1.44 + 0.302 log volume (m3) 0.46 0.082 0.002
log hydroperiod 5 1.89 + 0.503 log infiltration (cm/hr) 0.38 0.16 0.001
log hydroperiod 5 0.807 + 0.524 log resistivity (31.62 m) (ohm-m) 0.36 0.17 0.008
log hydroperiod 5 1.35 + 0.302 log area (m2) 0.23 0.13 0.034
log hydroperiod 5 1.71 + 0.123 log sand thickness (m) + 0.216 log
resistivity (31.62 m) (ohm-m)
0.86 0.018 0.000
0.09 0.036
hydroperiod 5 2355 + 43.1 log sand thickness (m) + 102 log resistivity
(31.62 m) (ohm-m) + 2.73 precipitation (cm)
0.80 6.04 0.000
31.67 0.002
0.302 0.000
Figure 6. A comparison of predicted and measured
mean annual hydroperiod for each specific pool. The
Geologic multivariate model used log sand aquifer
thickness and log apparent resistivity (at 31.62 m a-
spacing) to predict mean annual log hydroperiod for each
site (n 5 16). The Geologic and Meteorologic multivariate
model used log sand aquifer thickness, log apparent
resistivity (at 31.62 m a-spacing), and precipitation to
predict mean annual hydroperiod for each site for each
given year (1999, 2002, and 2003) (n 5 48).
O’Driscoll & Parizek, GEOLOGIC CONTROLS ON SEASONAL-POOL HYDROPERIOD 1011
importance of geologic controls on pool hydroper-
iod. With this model, the greatest discrepancy
between predicted and measured mean annual
hydroperiod at a specific pool was 58 days (pool
5) and the mean difference between predicted and
actual hydroperiod (absolute value) was 21 days.
An additional multivariate model (Geologic and
Meteorologic Model) was developed that accounted
for the annual variability in hydroperiod due to
variations in annual precipitation for the three years
of study. This multivariate model could predict 80%
of the variability in annual hydroperiod for a
specific pool (Table 1 and Figure 6). For this model
the mean difference between predicted and actual
hydroperiod (absolute value) was 32 days. Two
outliers occurred in 2002 (Figure 6). At pool 5, the
model predicted 191 days but the actual hydroperiod
was 289 days. At pool 8, the model predicted 198
days for 2002 but the actual hydroperiod was
measured at 48 days. For the remaining years the
model predicted the hydroperiod at these pools
relatively well (between 3%–17% for pool 5 and
17%–29% for pool 8). Overall, these data suggest
that geologic substrate and precipitation are major
controls on pool hydroperiod in this setting.
To evaluate hydroperiod response across a range
of meteorological conditions the Geologic and
Meteorologic Model, including annual precipitation,
sand thickness, and resistivity, was used to approx-
imate hydroperiod for sandy and clay pools for the
long-term record of annual precipitation for the
State College area (1899–2006). The probability
distribution for precipitation was used to predict the
occurrence of mean hydroperiod over the last
century for all pools, clay pools, and sandy pools
(Figure 7). The model revealed that for similar
meteorological conditions clay pools were much
more likely to remain dry. For mean pool hydro-
period to be at least four months (conditions
favorable for amphibian reproduction; Paton and
Crouch 2002) at least 86 cm of precipitation is
needed. This is likely to occur for at least 80% of
years. However, clay pools as a group would only
have hydroperiods greater than four months for the
wettest 40% of years, requiring at least 102 cm of
annual precipitation.
The spatial distribution of pools with hydroper-
iods greater than four months for extreme wet (5%
exceedance 2123 cm annual precipitation), median
(50% exceedance 298 cm annual precipitation), and
extreme dry (95% exceedance 274 cm annual
precipitation) conditions are presented in Figure 8.
It is only during extreme wet conditions, which
occur infrequently, that clay pools would have
hydroperiods greater than 4 months. For example,
all clay pools have hydroperiods greater than 120
days for the 5% exceedance, which requires 123 cm
of annual precipitation (Figure 8a). For 50% of the
years all sandy pools should have hydroperiods
suitable for amphibian reproduction (. 4 months),
but most clay pools would have hydroperiods
shorter than 4 months and presumably less than
ideal conditions for amphibian reproduction (Fig-
ure 8b). For 95% of the years at least 6 pools should
have hydroperiods longer than 4 months, 5 of these
pools are in the sandy region (Figure 8c). These data
suggest that over time, sandy pools would be more
likely sites for amphibian reproduction.
Ground-water Response of Clay versus Sandy
Pools. Nested ground-water head and surface-
water elevation data were compared between a
sandy pool (MW 7, pool 10) and a clay pool (MW
15, pool 11) (Figure 9) for the period of September
2002–2003. These pools were selected because they
had both staff gages and piezometer nests located
within the pools and sediment cores showed that
they were screened in predominantly clay-rich (pool
11) and sandy (pool 10) sediments. Over this period,
the clay pool had a hydroperiod of 111 days and the
sandy pool had a hydroperiod of 234 days. During
the same period, the clay pool was inundated during
three periods, whereas the sandy pool remained
inundated for most of the year. The 9 m deep
ground-water head measurements at both pool sites
Wetlands wetl-28-04-12.3d 9/10/08 16:07:11 1012 Cust # 08-88
Figure 7. The probability of exceedance for precipitation
and the corresponding modeled mean hydroperiod for a
given year indicated for clay pools, sand pools, and all
pools (mean) based on the long-term (1899–2006)
distribution of annual precipitation. Four months is
shown to represent the minimum duration of hydroperiod
suitable for amphibian reproduction (Paton and Crouch
2002). For mean pool hydroperiod to be at least four
months at least 86 cm of precipitation is needed.
1012 WETLANDS, Volume 28, No. 4, 2008
were always below the pool bottom elevation,
indicating that ground-water feeding these pools
must come from depths shallower than 9 m (Figur-
es 9a and 9b). For the sandy pool, there was a
period from March–September 2003 when ground-
water heads at 6 m (and shallower depths) were
sufficient to provide enough energy to transport
ground-water from these depths to the pool bottom
(Figure 9a). The deeper sandy aquifer surrounding
this pool (Figure 10) provided ground-water to
nourish the pool during periods of low rainfall and
high evapotranspiration. For the clay pool, the 6 m
deep ground-water head was never great enough to
supply ground-water to the elevation of the pool
bottom and 3 m deep ground-water only had
sufficient head to feed the pool on two sampling
dates for the period (Figure 9b). This pool was
rarely inundated because it was fed by very shallow
perched ground-water (, 3 m deep) and direct
precipitation.
These data suggest that the presence or absence of
the surficial sandy aquifer is a major control on
ground-water inputs and hence hydroperiod of these
seasonal pools. However, site observations suggest
that if sandy layers are very thick (. 7 m) and do
not contain clay aquitards, then closed depressions
will drain too quickly to form seasonal pools.
Evidence for this exists in the sandy southern
portion of the site. In this area, piezometer nest 8
was completely dry at all depths (up to 9 m deep) for
Wetlands wetl-28-04-12.3d 9/10/08 16:07:13 1013 Cust # 08-88
Figure 9. a) Ground-water and surface-water head
distribution for September 2002–2003 for Pool 10 (sand).
b) Ground-water and surface-water head distribution @for
September 2002–2003 for Pool 11 (clay). c) Daily
precipitation for the period of September 2002–2003.
Figure 8. Three different scenarios of hydroperiod
distribution based on corresponding precipitation distri-
bution: a) the 5% exceedance? (123 cm annual precipita-
tion); b) the 50% exceedance (98 cm of annual precipita-
tion); and c) the 95% exceedance (74 cm of annual
precipitation). In-filled (black) pools indicate pools that
were inundated for more than 4-months/year (pool areas
are not scaled to the actual volume of water predicted).
The numerical values adjacent to the specific pools
indicate the duration (days) that the pool would
contain water.
O’Driscoll & Parizek, GEOLOGIC CONTROLS ON SEASONAL-POOL HYDROPERIOD 1013
91% of the sampling dates. Sand was approximately
9 m thick at this coring location and major clay
units were absent from the core. Depressions and
swales in this sandy area did not typically hold water
for any extended period.
DISCUSSION
Differences in hydroperiod across the site were
attributed to the underlying geology and morphol-
ogy of wetland depressions. The hydroperiod
duration was positively correlated with thickness of
the surficial sandy aquifer. Two model outliers,
pools 5 (underpredicted) and 8 (overpredicted), in
2002 suggest that pool 5 may have a greater
connection to ground-water and pool 8 may have
less of a connection to the adjacent aquifer than
indicated by the model. Discrepancies between
model hydroperiod predictions and actual hydro-
period estimates may be related to local variability
in runoff generation, ground-water inputs, and/or
surface-water losses.
When the sand aquifer was thick and located
adjacent to a pool, the hydroperiod was longer when
compared to pools located in silt/clay-rich sedi-
ments. However, it is important to mention that
where the sandy aquifer was not underlain by a clay
or silt confining unit (in the extreme southern
portion of the site) the permeability of the sandy
residual sediments was too large to allow for the
formation of pools in depressions and ground-water
wells remained dry for most sampling dates even at
depths of 9.1 m. These data suggest that shallow
confining units are important controls on the
presence of seasonal pools.
Shallow sandy aquifer thickness was related to
hydroperiod because of its influence on the magni-
tude of ground-water inputs to the pools. Sandy
perched aquifers provide perched ground-water to
pools from deeper sediments and also from larger
catchment areas. Sandy aquifers adjacent to pools
were of limited extent and were typically perched on
top of clay/silt-rich sediment confining layers at
depths of 1–7 m at the site. There was limited
storage in these aquifers and variable degrees of
connection to the pools. The ground-water flux to
the pools was from shallow depths and responded to
seasonal variations in meteorological conditions.
Our results are in agreement with the conclusions of
Brooks (2005) and Rains et al. (2008) that seasonal/
vernal pools are generally not fed by deep ground-
water. In this setting, ground-water source was 6 m
or less.
Geophysical surveys and core collection for
subsurface geological characterization are costly,
time consuming, labor intensive, and may be
prohibited in sensitive wetland areas. Hence, de-
tailed geological and geophysical data are not
broadly available for seasonal pool sites. It would
be advantageous to be able to predict hydroperiod
using readily available, mapped data, such as soils
and topographic data. Infiltration rate data suggest
this may be possible; there was a positive relation-
ship between mean hydroperiod and soil infiltration
rate/soil type. For this study, the infiltration rates
were based on literature values for the appropriate
soil type from the Centre County Soil Survey and
may not adequately represent the local variability in
soil characteristics adjacent to the pools. Using
surficial soil data to infer hydroperiod requires
caution in these settings because the formation of
residual soils reflects the variability in the compo-
sition of the parent material and in some cases the
sandy residual sediments are buried by several
meters of clay or silt-rich sediments. Therefore, the
surficial sediments mask the underlying aquifer
materials. Conversely, soils data do not typically
provide information on the presence or absence of
confining units below the soil profile; a key factor
influencing the formation of perched ground-water
systems important to seasonal-pool formation. For
these reasons, sediment sampling or geophysical
surveying may be needed to characterize the
distribution of shallow, sandy, aquifer systems. This
is evident when looking at the mean hydroperiod for
pool 6 (Figures 1b and 4). Based on surficial
sediment data this pool is classified as a clay/silt-
rich pool, however adjacent to the pool there is a
buried sandy aquifer greater than 1 m thick and this
is the likely reason for this pool having a mean
hydroperiod of 260 days, one of the longest mean
hydroperiods observed.
Wetlands wetl-28-04-12.3d 9/10/08 16:07:18 1014 Cust # 08-88
Figure 10. Cross-section A-A’ (location presented in
Figure 1b) showing residual sediment distribution below
and adjacent to pools 10 and 11. Note the thick sandy
loam aquifer adjacent to Pool 10 that is absent from Pool
11. Numbers at the land surface represent piezometer nests.
1014 WETLANDS, Volume 28, No. 4, 2008
Other surficial indicators of hydroperiod included
pool depth and pool area which were positively
related to hydroperiod duration. Many of the
variables that were compared with hydroperiod are
interrelated and interact in complex ways. In
addition, subsurface sediments are highly variable
in folded and faulted karst settings. These factors
make it difficult to completely isolate the importance
of one particular variable. For example, electrical
resistivity measurements are influenced to some
extent by every other variable. The relationship
between hydroperiod, sediment texture, and thick-
ness is complicated by the depth-hydroperiod
relationship. Sandy pools tended to be deeper and
larger, which can also influence the length of
hydroperiod. The occurrence of deeper pools in
areas of thicker sands suggests that the Upper Sandy
Member of the Gatesburg Formation may be more
prone to dissolution and collapse than the adjacent
Mines Dolomite Member. Differential subsidence
through time during the weathering process together
with variable abundance of insoluble minerals
within the carbonate source rock adds to the
complexity of soil sequences at our site and very
likely in other carbonate terrains.
In addition to the surficial aquifer distribution,
another important geologic control on the presence
and duration of seasonal pools in this setting is the
presence of a tear fault underlying the site (Fig-
ure 1a). Uneven westward movement of bedrock
units during development of the Birmingham thrust
fault (located due west of the site) has resulted in the
development of a tear fault that trends NW-SE
through the site and underlying the area that
contains the wetland pools. This fault was mapped
by Carrucio (1963) and extends for more than 16 km
to the southeast of the Birmingham thrust fault
(Clark 1965, Parizek et al. 1967). The tear fault
mapped through this site is likely responsible for the
topographic low region, or trough, trending NW–SE
through the site and underlying the pools. Residual
soils in this general area exceed 16 m in thickness
based upon a number of test borings used to
characterize the region. The slopes on both sides of
this trough collect water and distribute it to the pond
area; hence, the tear fault can be considered a major
control on the location of these wetland ponds. The
series of pools and their trend and location in a linear
series in the trough suggest a geologic control on
their origin. Uneven dissolution of carbonate rocks is
also enhanced by systematic joint sets that cross the
tear fault zone at an oblique angle. A number of
pools tend to be elongate parallel to this joint trend.
In previous studies at the site, recharge areas for
the individual pools were delineated based on
hydraulic head data and the entire recharge area of
the pools was found to span 2–8 ha, with typical
ground-water flowpath lengths to pools of less than
150 m. Water chemistry, hydraulic head, and
surface-water level data all suggested ground-water
inputs were derived from local shallow perched
aquifer systems. A deep well at the site (. 30 m
deep), approximately 200 m northwest of piezome-
ter nest 28, had ground-water levels greater than
30 m below the surface. These data confirm the
pools are part of a perched local ground-water flow
system (O’Driscoll 2000, O’Driscoll and Parizek
2003). Subsurface geology affects the degree of
perching and the nature of local ground-water
flowpaths to pools. Pools that were better connected
to the sandy perched aquifer system remained wetfor longer periods.
Karst watersheds exhibit a wide range of surface-
water-ground-water interactions related to hetero-
geneous hydraulic properties and bedrock dissolu-
tion processes (White 1988). In karst watersheds
with thin sediment deposits overlying bedrock,
seasonal pool wetlands are less likely to occur
because vertical seepage through permeable bedrock
is common. In temperate karst areas, with thick
residual sediment deposits and the presence of closed
depressions, seasonal pools are more likely, partic-
ularly if permeable sediments are underlain by
shallow low permeability sediments that can support
perched ground-water systems. The results of this
study should be applicable in karst and other
settings where local perched ground-water flow
systems are the predominant ground-water source
to pools, but not in settings where regional ground-water inputs have a substantial influence on the
hydroperiod.
CONCLUSIONS
The relationship between surficial and/or shallow
sand aquifer thickness, electrical resistivity, and
hydroperiod suggests that geophysical surveys may
help improve the understanding of geologic controls
on seasonal-pool hydrology in this and other
settings. It may be possible to utilize remote sensing
approaches (sensu Meng et al. 2006) to gather
similar data that would help better explain the
spatial variability in hydroperiod and the geologiccontrols on seasonal-pool location, occurrence, and
water level variability.
At first glance, one might have expected the
hydroperiod to be longer for clay pools than sand
pools in this karst setting, i.e., the need for a seal to
perch shallow ground-water and surface-water. The
opposite was shown to be true. Sandy residual soil
Wetlands wetl-28-04-12.3d 9/10/08 16:07:21 1015 Cust # 08-88
O’Driscoll & Parizek, GEOLOGIC CONTROLS ON SEASONAL-POOL HYDROPERIOD 1015
helped capture infiltration and directed this water
along perched ground-water lenses or sheets to
nearby seasonal pools. When sandy sediments were
not underlain by any type of confining unit in the
southern portion of the site, the sediments were too
well drained to allow for the formation of seasonal
pools. These results suggest that sandy surficial
aquifers can help prolong seasonal-pool hydroper-
iods but that they require a subsurface confining
layer to slow vertical seepage from the pools.
The observed differences in hydroperiod based
on subsurface geological conditions suggests that
geologic controls and the importance of perched
ground-water inputs will have an important effect
on a seasonal pool’s response to climate and/or
land-use change. In this karst setting, clay pools
may be more vulnerable to climate change due to
their shorter hydroperiods and greater sensitivity to
recent weather conditions. Sandy pools will show
more subdued responses due to the buffering effect
of shallow perched ground-water inputs. However,
land-use changes that affect recharge to or dis-
charge from surficial aquifer systems may have
greater effects on sandy pools and their hydroper-
iods than on clay pools. Overall, the observed
influence of subsurface geology on pool hydroper-
iod suggests that spatial variations in subsurface
geology may relate to spatial variations in pool
ecology.
ACKNOWLEDGMENTS
The authors greatly appreciate a helpful review by
Rick Rheinhardt and the comments provided by
several anonymous reviewers and the Associate
Editor. This paper is a result of research performed
at Pennsylvania State Gameland # 176; the authors
thank the PA Game Commission for access to the
site. We owe special appreciation to the Pennsylva-
nia State University Office of Physical Plant who
provided financial support for drilling, construction,
materials, and manpower to set up the monitoring
network. They also contracted for the detailed
topographic survey of a 1 ft. contour interval and
accurate survey of piezometer and staff gage
elevation critical to this investigation. In particular,
Bill Shaw and Lou Brown provided technical
assistance and water chemistry analyses; John
Gaudlip provided wage support, materials, maps
and technical information; and Frank Raymond
drafted the site topographic map. The authors also
appreciate the assistance of graduate students who
assisted with fieldwork: Tyrone Rooney and Garth
Lewellyn.
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O’Driscoll & Parizek, GEOLOGIC CONTROLS ON SEASONAL-POOL HYDROPERIOD 1017