Field Facilities for Storm Driven Subsurface Transport Research

Two field installations on contrasting watersheds have been developed on the Oak Ridge Reservation, in Oak Ridge, TN (USA) to investigate one-, two-, and three-dimensional flow and transport processes in unsaturated subsurface environments. The facilities are ideal for quantifying nutrient and contaminant fluxes in soils with spatially variable physical and chemical characteristics. One site is located on the Walker Branch Watershed and is a deep, highly weathered Ultisol with Chepultepec dolomite as the parent material. The experimental subwatershed drains an area of 0.47 ha from an elevation of 334 to 322 m. The second site is located on the Melton Branch Watershed and is underlain by Maryville limestone, which is a limy shale formation. The weathered parent material of interbedded shale and limestone is an Inceptisol and is near the ground surface ranging from 0.5 to 3 m in depth. The experimental subwatershed drains an area of 0.63 ha from an elevation of 275 to 258 m (Fig. 1).

These facilities are unique in that subsurface drainage can be collected and monitored from 2.5 m depth by 16 m long trenches that have been excavated across the outflow regions of the subwatersheds. At both sites, six stainless steel pans are pressed into the face of the trench to intercept the subsurface outflows (Fig. 3a). The various pans are designed to capture water from different regions of the watershed. The flows from the six pans, a surface collector, and water below the pans, are routed into tipping-bucket rain gauges positioned in the back of the approach section of each of two H-flumes (Fig. 3b). The H-flumes are each equipped with a Manning ultrasonic level recorder to give an integrated flow rate for the four rain gauges, which is needed when the gauge capacities are exceeded. ISCO automatic groundwater samplers are used to acquire real time water samples for tracer and solute analysis.

A buried line source is located at the top of each site and they are used to disseminate tracers into the subsurface hillslope. Extensive instrumentation has been installed downslope of the line source for monitoring the spatial and temporal distribution of water and solutes. Transects of solution samplers, tensiometers, and piezometers equipped with pressure transducers for assessing perched water table dynamics, are located at numerous depths throughout the watershed. All well tranducers, surface rain gauges, subsurface flow rain gauges, and ultrasonic flow level recorders are interfaced for computer-based data acquistion.

The research sites have been intensively characterized for spatial variability in chemical and physical properties. Surface and subsurface hydraulic conductivity have been measured in over 40 locations, and moisture retention functions are known for all soil types with depth. Site hydrodynamics have been quantified with nonreactive tracer studies, and these investigations have demonstated the importance of rapid preferential flow coupled with slow matrix diffusion during solute transport (Fig. 6). The spatial variability in chemical properties such as cation and anion exchange capacities, indigenous cations and anions, organic matter, pH, ion exchange thermodynamic equilibrium constants, and soil clay minerals have also been assessed. Further, each site contains a 2m × 2m × 3m deep isolated, undisturbed pedon equipped to monitor water and solute movement with depth through multiple pore regions during 1-dimensional flow (Fig. 7).

Scientists interested in subsurface transport processes are invited to develop cooperative projects using these watershed facilities. Experimental and simulation modeling methods may be used to test hypothese or evaluate concepts.

For more detailed information concerning the field facilities, contact Philip Jardine (jardinepm@ornl.gov) or Robert Luxmoore (luxmoorerj@ornl.gov) at the Oak Ridge National Laboratory or Glenn Wilson (gwilson@utkvx.utk.edu) at the University of Tennessee, Knoxville. Additional reading material concerning site activities can be found in References.