Climate 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.
Soils 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 Tables 2, 3, and 4. Total elemental concentrations for typical pedons are presented in Tables 5, 6, and 7.
Hydrogeology 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 field soil.
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 drainage periods.
Comparison of Infiltration Behavior 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 Flow 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 by evapotranspiration.
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.
Solute Transport 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 Branch watersheds.
Column Experiments 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.
Pedon Experiments 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).
Subwatershed 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).
Watershed Observations 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 Turner, 1989).
Disconnect-reconnect 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 the landscape.
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