According to USDA (1981), the average annual precipitation ranges from
925 to 1,400 mm, the average annual temperature ranges between 13 to 16°C,
and the average freeze-free period is 170 to 210 days.
Physiography and Geology
The Ridge and Valley MLRA 128 consists of sediments from the Paleozoic
Era. These rocks include limestone, sandstone, siltstone, shale, and dolomite
from the Ordovician and Cambrian periods. Mineral resources of this area
include zinc, iron ore, bauxite, fluorite, and barite as well as crushed
stone, marble, cement, sand, and gravel.
The occurrence of soil series and associated properties for MLRA 128
are summarized in Table 1. STATSGO soils
are depicted in Fig. 1. Soil morphology, classification, and parent material
for typical pedons are presented in
2, 3, and 4. Total elemental concentrations for typical pedons are
presented in Tables 5, 6, and 7.
MLRA 128 lies in the Valleys and Ridges subregion of the Appalachian
Plateaus and Valley and Ridge ground water region of North America. The
topography is generally a sequence of ridges and valleys that reflect the
structure of the underlying rock which has been intensely deformed with
many folds and thrust faults. This subregion is one of the major karstlands
of the eastern United States (Back et al., 1988). Altitudes within the
subregion range from 200 to 500 m above mean sea level and are generally
lower than the Blue Ridge to the east and the Appalachian Plateaus to the
west. The ground water occurrence in the subregion is characterized by
adjacent, but isolated, shallow flow. These flow systems have been developed
because the lithology and dip-oriented streams effectively compartmentalize
flow (Back et al., 1988). Exceptions to the shallow flow systems are the
warm springs that are part of deep circulation systems and that are prevalent
through out the MLRA.
Most of the groundwater flow is from ridge to valley, either discharging
into streams, or being intercepted by permeable layers or zones. These
highly permeable layers and zones may be coarse-grained carbonate rock
with secondary permeability, or fracture zones. Flow from springs is usually
concentrated at a few large outlets. The regolith is often thick and stores
a significant amount of water for recharge of the deeper aquifers. Although
most of the rocks have low primary porosity and permeability, the secondary
porosity and permeability from fracturing and dissolution are significant
for water storage and transmission.
There are numerous regional aquifers within MLRA 128, including the
Knox-Beekmantown carbonate sequence that is the most transmissive aquifer
in the Ridges and Valleys subregion (Back et al., 1988). This sequence
reaches from Pennsylvania south through the region to Alabama. The shallow
ground water systems typically have dissolved-solids concentrations that
range from 50 to 500 mg L-1, while water in the deeper systems
generally has twice those concentrations (Brahana et al., 1986).
Water and Solute Transport
Extensive hydrologic transport investigations have been conducted on
selected soils of this MLRA, particularly on the U.S. Department of Energy’s
Oak Ridge Reservation. This reservation has many forested soils and soils
that have converted back to forest from agricultural use since the federal
government took over the land during the early 1940s. Selected sites within
the reservation have been used for waste disposal and waste storage operations,
and some intensive water and solute transport research has been undertaken
at some of these sites. We discuss the issues of water and solute transport
at sites in the Oak Ridge Reservation by first reviewing investigations
of soil hydraulic properties and of infiltration characteristics under
saturation and tension flow conditions. Water flow and water budget characteristics
are reviewed, and finally issues of solute transport are summarized.
Two distinct geological parent materials are common in MLRA 128 and
these weather to produce soils with contrasting depths to bedrock. Shale
parent materials have shallow weathering and generate soil profiles up
to a few meters deep. In contrast, soils developed on dolomite parent material
are deeply weathered down to about 30 m on ridge tops. These differing
soils, however, can have similar responses to precipitation events due
to restrictive subsoil layers that induce lateral subsurface flow on sloping
terrain. A comparison of field and laboratory determinations of soil water
characteristics for a Fullerton (from dolomite) and a Sequoia (from shale)
soil (Luxmoore, 1982) showed lower water content at a given matric pressure
from in situ field determination than from laboratory determination
with soil cores. This difference was attributed to entrapped air in the
Soil Hydraulic Properties
Shallow Soils Formed on Shale Parent Material
Wilson et al. (1992) used a Fermi function to represent the macropore
(0 to 10 cm suction head) contribution combined with the van Genuchten
model for mesopore (10 to 300 cm suction head) and micropore (>300 cm suction
head) contributions to the hydraulic properties of a shale derived soil
on Melton Branch watershed on the Oak Ridge Reservation. The water content
was 0.38 m3 m-3 at saturation declining to 0.2 m3
m-3 at a suction head of 10,000 cm.
A summary of hydraulic measurements from several sources for soils developed
on Conasauga shale was assembled for an assessment of radionuclide transport
from the Solid Waste Storage Area 6 area on the Oak Ridge Reservation.
These water retention data (Table 8, adapted
from Lee et al., 1997) show high total porosity (0.39 to 0.47 m3
m-3) that is typical of forest soils in the area. Available
soil water, the difference in soil water content between field capacity
(60 to 100 cm suction head) and wilting point (15,000 cm suction head)
provides significant stored water for plant uptake. The saturated hydraulic
conductivity generally declines (exponentially) with depth as weathering
decreases, and this conductivity decline results in lateral flow on sloping
terrain during some precipitation events.
Deep Soils Formed on Dolomite Parent Material
Soil hydraulic properties have been determined for the dominant soils
of Walker Branch watershed in both laboratory and field determinations.
These soils, developed on Knox dolomite parent material, have decreasing
depth to bedrock from ridges to valleys. Perennial streamflow occurs over
bedrock. Soil cores taken from 13 pits across a range of sites in the watershed
show wide variability in water retention and saturated hydraulic conductivity
(Peters et al., 1970). Water content of the argillic B horizon ranges from
0.15 to 0.30 g g-1 at a pressure head of -100 cm, showing significant
spatial variability of water content at field capacity.
Field determinations of soil hydraulic properties have been estimated
by the instantaneous profile method (Luxmoore et al. 1981a, Luxmoore 1982)
and from soil water monitoring (Luxmoore 1983, Hanson et al. 1998). Bypass
flow along the boundary wall of the isolated pedon used in the instantaneous
profile method probably resulted in excessively high estimates of hydraulic
conductivity for subsurface horizons. Hydraulic conductivity decreases
significantly in the argillic B horizon. This horizon has fine cracks between
aggregates that provide flow paths around the aggregate matrix during high
Comparison of Infiltration
Watson and Luxmoore (1986) and Wilson and Luxmoore (1988) report infiltration
measurements by the ponded double-ring method and at selected tensions
with tension infiltrometers. Over 35 locations were characterized in two
subwatersheds with either dolomite or shale derived soils. The frequency
distribution of infiltration rates was lognormal in both subwatersheds
with mean ponded infiltration rates in excess of 13 x 10-5 m
s-1. They estimated that at an infiltration rate of 2 x 10-5
m s-1 that essentially all rainfall events could infiltrate
and not generate any overland flow. At this flow rate water moves through
mesopores that are less than about 0.2 mm in equivalent diameter according
to capillarity (Wilson and Luxmoore 1988). Forest soils typically have
high infiltration rates that are not generally exceeded by the rainfall
rates occurring during storm events.
Subsurface infiltration characteristics of saprolite material (derived
on shale parent material) were measured at approximately 1 to 2 m depth
at 48 locations on a 2 x 2 m grid on an excavated site in a Litz-Sequoia
association (Typic Hapludult, Luxmoore et al. 1981b). Infiltration was
determined by the ponded double ring method and rates were shown to be
lognormally distributed with no spatial correlation at separation distances
of 2 m or greater. The geometric mean infiltration of this subsoil material
was 2.3 x 10-7 m s-1 (2 cm day-1) with
a coefficient of variation of 130%. This subsoil infiltration rate is over
100 times less than for the surface horizon.
The Guelph permeameter was used by Wilson et al. (1989) to characterize
the subsurface flow rates on subwatersheds of Walker Branch (dolomite parent
material) and Melton Branch (shale parent material) watersheds. At Walker
Branch no spatial correlation was shown from geostatistical analysis of
measurements from 14 locations taken at an average measurement depth of
95 cm. The saturated permeameter flux at Walker Branch was about double
that for the Melton Branch site. Estimated hydraulic conductivity was significantly
higher than the infiltration rates measured by Luxmoore et al. (1981) for
the shale derived subsoil. Conversion of permeameter fluxes to saturated
conductivity is problematic due to preferential flow violating the assumptions
used in calculation of hydraulic conductivity. At the Melton Branch site,
measurements were made at 25 locations at an average depth of 1.2 m. Spatial
correlation of these flux measurements was significant for separation distances
up to 20 to 30 m between measurement sites.
Water Budget and Water
Monthly precipitation is approximately uniform through the year with
an annual total of 1200 to 1400 mm. The annual precipitation on Walker
Branch watershed for the 15-year period, 1969-1983, was 1368 mm (Luxmoore
and Huff 1989). Precipitation exceeds evapotranspiration resulting in significant
drainage and subsurface flow in the soils of the Ridge and Valley MLRA.
The strong seasonality of evapotranspiration during the growing season
results in high drainage and subsurface flow during the non growing season
from late autumn to early spring (Luxmoore 1983, Wilson et al. 1993). During
other times of the year soil water increases by precipitation and depletes
Luxmoore and Huff (1989) summarized water budget data from 1969 to 1983
for Walker Branch watershed and calculated a mean net gain (precipitation
- streamflow) of 655 mm yr-1, which on an annual basis is an
estimate of evapotranspiration. More recent analysis of the water budget
for Walker Branch suggests a decline in mean net gain to about 620 mm yr-1
(Dr. P. Mulholland, personal communication, Oak Ridge National Laboratory,
Oak Ridge, Tennessee). This decline in evapotranspiration may reflect aging
effects in the forest community. Streamflow has been shown to increase
from a watershed as forest communities age (Vertessey et al., 1994). Recent
eddy covariance measurements over a deciduous forest on Walker Branch watershed
estimate annual evapotranspiration of 600 to 620 mm (D. Baldocchi, personal
communication, Atmospheric Turbulence and Diffusion Division, NOAA, Oak
Ridge, Tennessee). This estimate of evapotranspiration is lower than previous
estimates derived from water budgets and modeling possibly due to continuing
decline in forest water use with stand age.
Wilson et al. (1990) described the hillslope hydrology of a forested
subwatershed on Walker Branch watershed and showed that subsurface flow
generated during precipitation events occurred within the 1.0- to 2.5-m
depth interval of the Bt2 horizons as a perched water table developed during
storm events. Preferential flow through macro- and mesopores was considered
to be the predominant stormflow mechanism and this was at a sufficiently
high rate to contribute to peak streamflow and not just the recession limb
of the hydrograph.
Chemical transport investigations have been conducted at a wide range
of scales from soil column, pedon, subwatershed to the watershed scale.
These experiments have been conducted on soils from Walker Branch and Melton
Dr. P. M. Jardine and colleagues have conducted numerous column experiments
and have determined several important features of chemical transport of
inorganic and organic contaminants.
At flow rates less than the saturation flow rate chemical adsorption
coefficients (Kd) determined by the batch equilibrium method are suitable
for prediction of transport. During saturated flow, however, the effective
Kd is less than the batch equilibrium value due to preferential flow bypassing
some soil matrix. In this case Kd can be determined by a transient method
(Jardine et al., 1993).
Flow interruption in solute breakthrough experiments results in outflow
solute concentration being lower or higher for the rising and descending
limb, respectively, due to diffusion of solutes into and out of matrix
pores into flow paths (Reedy et al., 1996). The same behavior is expected
during wetting and drying cycles in the field.
Solutes with differing Kds (Br, NO3 , NH4 , dissolved
organic carbon) may be transported to similar depths by preferential flow
(Jardine et al., 1989). Precipitation events cause a rise in solute (Br)
concentration in macropores (Jardine et al., 1990a). During preferential
flow solutes diffuse into or out of the matrix according to concentration
gradients (Jardine et al., 1990a).
and Hillslope Experiments
Several solutes (Na, K, Mg, Ca, and S) show rising concentration with
rising subsurface flow rate (Luxmoore et al., 1990). Some chemicals (Al,
Fe, and Mn) show decreasing concentration with rising subsurface flow rate
(Luxmoore et al., 1990). Bromide tracer released from a subsurface source
traveled more than 65 m during 3.2 hours following a storm event. About
half of the tracer diffused into the soil matrix (Wilson et al., 1993).
Several solutes (K, S, and P) show rising concentration with rising
streamflow rate (Elwood and Turner, 1989). Several solutes (Na, Ca, and
N) show decreasing concentration with rising streamflow rate (Elwood and
and Pathlength-supply Hypotheses
Similar chemical transport dynamics have been observed for the migration
of chemicals within soils driven by precipitation at the pedon (4 m2
area, Jardine et al., 1990), subwatershed (6,000 m2 , Luxmoore
et al., 1990), and watershed (350,000 m2 , Elwood and Turner,
1989) scales. In each case, the concentration of transported solute increased
with the rising limb of the flow hydrograph. At first glance this may seem
contrary to the expectation that added precipitation would cause solute
dilution. A pathlength-supply hypothesis was first offered as an explanation
for these observations (Luxmoore et al., 1990); however, this was later
discounted in favor of the disconnect-reconnect hypothesis (Luxmoore and
Ferrand, 1992). Nevertheless, the pathlength-supply mechanism may provide
an enhancing effect to a disconnect-reconnect mechanism of solute transport
under field conditions.
According to the disconnect-reconnect hypothesis, flow paths through
soil drain unevenly after a precipitation event leaving some flow-path
water stranded in "patches" without pore continuity for complete drainage
to field capacity. The non-draining pocket phenomenon was demonstrated
with percolation modeling presented by Luxmoore and Ferrand (1992); this
is the disconnect phase of the hypothesis. Solute diffusion causes water
in flow-path patches to gain the solute signature of the water in the soil
matrix (e.g., within soil aggregates) during the intervening period ending
with the next drainage event. Thus, soil water stranded in flow paths gains
the old water signature. The next precipitation event causes patches of
stranded flow-path water to reconnect and drain. The hypothesis considers
the extent of reconnection to increase with flow rate. As flow rate increases,
a greater proportion of reconnected soil water discharges with the old
water solute signature giving a rise in solute concentration with rise
in flow rate. In addition, new water from precipitation can also gain some
to the old water signature by matrix diffusion during the flow event; the
pathlength-supply hypothesis. According to this latter hypothesis, increasing
flow rates have contributions from longer pathlengths draining through
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