2. Materials and methods
Sumter county, located in southwestern Georgia, lies within the Coastal Plain physiographic province. Deep geologic materials in this area were derived from
fluvio-marine deposited gravel, sand, silt and clay. The research site, near Plains Ž
. Ž
. Ž
X Y
Y X
Y Y
. Sumter , GA Fig. 1
31859 0 to 45 N; 84824 0 to 30 W , is located in the Ž .
Ž southern portion of the Fall Line Hills region, and is underlain by: 1 the 12
. Ž
. Ž .
m below the surface Tuscahoma formation
Paleocene age ; and 2
the Ž
. Ž . Ž
shallower Tallahatta formation f 3 to 12 m Eocene age
Hicks et al., 1991b; .
Stewart and Hicks, 1996 . The Claiborne aquifer is contained dominantly in the lower Tallahatta formation and is relatively unconfined and generally reflective
Ž .
of surface topography Hicks et al., 1991b . Since sediment deposition, fluvial events and dissection of terraces have altered the landscapes. The research site is
situated on an interfluverfluvial terrace landscape position, and soils have Ž
. mostly developed in fluvial parent materials Fig. 1 . The west portion of the
site is bounded by the third order Ty Ty creek, and the east side is bordered by a first order tributary. The uplands range in elevation from 125 to 145 m, and the
Ž .
surface slopes to the south 1 at the crest.
2
Ž .
Two 10 = 10 m subplots NW and SE were established on this interfluve in areas that have soils with the extremes of argillic horizon clay content and sandy
Ž .
Fig. 1. Location of site near Plains, GA USA , and the location of the subplots.
epipedon thickness found in upland positions of this region. Adjacent to each subplot, pits were excavated, and pedons were described, sampled, and classi-
Ž .
fied according to standard Soil Survey Staff 1996 techniques. An additional pedon was sampled in the NE corner, but no subplot was established as this soil
possessed properties intermediate of the NW and SE pedons. Soils examined classified into coarse–loamy, kaolinitic, thermic Typic Kandiudults in the SE;
loamy, kaolinitic, thermic Grossarenic Kandiudults in the NW; and loamy, kaolinitic, thermic Grossarenic Kandiudults in the NE. The SE soil possessed
the most clayey argillic horizon, the NW soil had the sandiest, and the NE was
Ž .
Ž .
intermediate Table 1 . Saturated hydraulic conductivity K
measurements
s
Table 1 K data and selected physical properties for pedons
s
Ž .
Hor Depth cm
K
s
Field mean Lab
Clay Bulk
y1
Ž .
cm h horizontal
density
y1 y3
Ž .
Ž .
cm h g cm
SE: coarse – loamy, kaolinitic, thermic Typic Kandiudult
d
Ž .
Ap 21
11.5 0.5 14.8
2.7 1.71
d
Ž .
Bt1 21
52 17.1 5.6
19.9 11.6
1.67
d
Ž .
Bt2 52
92 12.1 2.7
19.9 13.2
1.61
d
Ž .
Bt3 92
126 11.2 9.5
31.0 16.3
1.65
d
Ž .
Bt4 126
164 3.3 3.3
5.6 28.1
1.69
d
Ž .
Bt5 164
250 6.3 11.4
5.2 27.1
1.73
d
Ž .
BC1 250
293 3.0 2.3
– 27.9
–
c,d
Ž .
BC2 293
340 42.2 56.6
– 21.5
–
c,d
Ž .
BC3 340
393 28.4 31.5
– 14.7
–
a
Ž .
CB1 393
445 431.0 19.2
– 2.2
– NW: loamy, kaolinitic, thermic Grossarenic Kandiudult
c,d
Ž .
Ap 26
30.2 17.0 23.0
3.4 1.77
b,c,d
Ž .
E1 26
75 52.4 33.3
– 4.1
1.70
b,c
Ž .
E2 75
109 77.7 5.2
33.1 4.4
1.62
b
Ž .
E3 109
138 108.4 85.4
– 5.8
1.65
d
Ž .
Bt1 138
191 14.4 6.9
22.1 15.2
1.70
c,d
Ž .
Bt2 191
246 32.0 1.2
– 14.1
–
c,d
Ž .
BC 246
300 31.9 0.4
25.1 9.7
–
c,d
Ž .
CB 300
366 39.7 12.1
– 7.8
–
c,d
Ž .
C1 366
434 48.6 37.2
– 6.4
– NE: loamy, kaolinitic, thermic Grossarenic Kandiudult
d
Ž .
Ap 26
16.4 1.3 20.9
3.7 1.69
c,d
Ž .
E 26
110 45.3 48.7
27.3 9.0
1.63
d
Ž .
Bt 110
200 12.7 1.6
10.5 21.0
1.71 Numbers in parentheses are standard deviations. Means with superscripts of the same letter
are not significantly different at the ps 0.05 level.
were conducted within each horizon in the field using the Compact Constant Ž
. Ž
. Head Permeameter CCHP borehole technique Amoozegar, 1989 .
2.1. Laboratory Bulk samples collected as above were air-dried and weighed, and coarse
fragments were removed by crushing samples and dry sieving through a 2-mm Ž
. sieve. Particle size analyses PSA was conducted with the pipette method
Ž .
Kilmer and Alexander, 1949 . Bulk density was measured by the clod method Ž
. Ž
Blake and Hartge, 1986 . Horizontal and vertically oriented small cores 8.5 = 6
2
. cm
were collected from each horizon for measurement of hydraulic properties. Ž
. Saturated hydraulic conductivities K
on small horizontal cores were measured
s
Ž .
with the constant head method Klute and Dirksen, 1986 . Water release curve Ž .
measurements were conducted on vertical cores over pressures h from 10 to 15 000 cm H O. Water release data were fit to a modified van Genuchten
2
Ž .
expression van Genuchten, 1980; Lappala et al., 1993 for comparison and for Ž
simulation model input using the Variably Saturated Two-Dimensional Solute .
Transport, VS2DT; Lappala et al., 1993 :
1 y1
N N
u s 1 q ya h u y u q u
1
Ž . Ž
. Ž .
Ž .
s r
r
where: a s scaling length parameter related to the air entry potential; reciprocal Ž
y1
. of a is an estimate of the suction where the first pores empty cm
. N s pore Ž
3 y3
. size distribution parameter. u s residual volumetric water content cm cm
.
r
Ž
3 y3
. Ž
u s saturated volumetric water content cm cm . h s matric potential cm
s
. H O .
2
Ž Curve fitting was conducted using a least squares algorithm Marquardt–
. Levenburg which minimized sums of squares. Although u is sometimes fixed
s
Ž . in this equation as equal to porosity
w , w was utilized only as an upper constraint since entrapped air often causes u to be slightly lower than w. Using
s
w Ž .x
these parameters, unsaturated hydraulic conductivity K h curves were devel-
Ž .
oped van Genuchten, 1980 :
yM 2
M y1
2 N
N
K h s K 1 q a yh 1 y 1 y 1 q a yh
2
Ž . Ž
. Ž
. Ž .
s
Ž
y1
. where M s 1 y 1rN, and K s saturated hydraulic conductivity cm h
.
s
2.2. Field methods During a 2-year period, tensiometers installed at 60, 90, 120, 160, 200 and
Ž . 350 cm monitored soil matric potentials h at 1-h intervals in NW and SE
subplots. Measurements were collected utilizing calibrated pressure transducers attached to tensiometers and connected to a Campbell
w 1
CR 10 data logger with a multiplexer. Triplicate tensiometers were installed in each subplot at each
Ž .
depth 2 m apart . Some of these data exhibited a diurnal fluctuation as
displayed by rapidly ascending and descending data, most likely associated with the heating and cooling of the column above the water in the tensiometers
Ž .
Cassel and Klute, 1986; Butters and Cardon, 1998 . This was differentiated from changes in moisture status.
2.3. Rainfall simulations Ž
2
. Two simulated rainfall events were applied to portions 10 = 5 m
of each of the subplots. Subplot surfaces were roto-tilled prior to applying rainfall, and
strips of furnace filter were placed on the surface to minimize surface crusting and maintain infiltration. The first simulation was conducted in January, 1995.
Ž .
Experiments were conducted by raining water obtained from adjacent well for 1 h, taking measurements, then raining for an additional hour. Rainfall intensi-
Ž .
Ž .
y1
ties were 4.74 1st hour and 4.46 2nd hour cm h for the NW subplot, and
Ž .
Ž .
y1
4.25 1st hour and 3.86 2nd hour cm h for the SE subplot. During rainfall
events, h was monitored in both plots with tensiometers nests as described above Ž
. Ž
logged at 5-min intervals . Soil moisture was monitored 15 cm depth incre- .
w
ments at select times with a Troxler Capacitance Probe.
Ž .
During the second simulation August, 1995 , rainfall intensities were 4.64 Ž
. Ž
.
y1
Ž .
1st hour , and 4.82 2nd hour cm h for the NW site, and 3.08 1st hour , and
Ž .
y1
4.03 2nd hour cm h for the SE site. At each site, the simulation was
conducted for 1 h, halted for 0.5 h during sample collection, then resumed for 1 h. Soil matric potential heads were monitored as above.
During the August simulation, 5 kg of KBr were placed at a 5 cm depth Ž
. approximately 30 cm in diameter at one end of each plot under the simulators.
Ž
y1
. KBr was chosen due to it is high water solubility 53.5 g 100 ml
and relatively conservative behavior. At 1, 3, 6, 11, 17 and 45 h, soil samples were
collected. Samples were taken from the center, and at 40, 65 and 90 cm from the Ž
. center in four directions N, S, E, W , at depths of 15, 30, 60, 90, and 120 cm
for early sample times, with additional depths of 160 and 200 cm for later sample times. Soils were air dried, crushed, and 10 g of soil were added to 30
ml of water, and shaken for 0.5 h. Suspensions were analyzed for Br with an ion
Ž .
Ž .
specific electrode ISE using a 5 M NaNO 1 vrv ionic strength adjustor
3
with a double junction reference electrode. Due to ISE detection limits and
1
Use of a commercial product does not indicate endorsement by the authors or their agencies.
y1
Ž
y5
background levels, only concentrations above 0.2 mg kg 1 = 10
M for .
10.0 g in 30 ml were considered. Bromide data was compiled as a function of x, y, and z coordinates.
2.4. Modeling procedure In order to assess the probability of lateral flow and to permit a more holistic
Ž .
view, we used VS2DT to simulate landscape water flow Lappala et al., 1993 . VS2DT is a finite-difference model that uses numerical techniques with a fully
implicit scheme to simulate two-dimensional water flow at subsequent time Ž
. Ž
. steps. Measured anisotropy, K , a , N, u , u
and estimated specific storage
s r
s
hydraulic parameters for E1 and E2 horizons and Bt1 and Bt2 horizons were combined for the NW soils. Parameters for Bt2 and Bt3 horizons and Bt4 and
Bt5 horizons were combined for the SE soils. Ž .
VS2DT was utilized to simulate total head H in a landscape cross-section Ž
. 50 m length set at 1 slope to mimic the actual landscape of the site
consisting of equal proportions of the NW, NE, and the SE soil, similar to soil distribution relationships at our site. NE pedon properties were placed between
Ž the other two soil types for it possessed properties morphological and hy-
. draulic
intermediate of the NW and SE soils, which is typical on these Ž
. landscapes Bosch and West, 1998 . The upper boundary conditions were set at
Ž
y1
. Ž
. a constant infiltration rate 40 mm h
for the first recharge period 2 h , then Ž
. changed to a no flow boundary remaining 70 h . Left, right, and bottom
boundaries were set to h s y100 cm H 0 during the first recharge period, then
2
changed to y50 cm H 0 during the rest of the simulation. The distance between
2
10 and 40 m was evaluated on the transect to minimize any edge affects caused by the boundary conditions. Initial condition was set to a constant matric
Ž .
potential head h s y75 cm H 0 . The simulation was conducted for 72 h.
2
3. Results and discussion
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