Effects of trench design and infiltration fluxes on soil-moisture migration into burial trenches at Barnwell, South Carolina

Numerical analysis of soil-moisture migration in the unsaturated zone at two experimental burial trenches at the Barnwell, South Carolina low-level radioactive-waste burial site was used to examine how water movement is affected by trench design and by precipitation and evapotranspiration fluxes. The unsaturated zone consists of 1 to 2 meters of surface sand underlain by 10 to 12 meters of clayey sand. Both trenches were excavated in the unsaturated zone to a depth of 7 meters, backfilled with 5 meters of stockpiled surface sand, and capped with 2 meters of compacted clayey sand. The surface sand around trench 1 replaced with a 3-meter wide compacted clayey-sand barrier. Modeling results show that during periods of infiltration, soil moisture moved downward through the trench cap of both trenches and into the trench backfill sand. In the undisturbed sediments adjacent to the trenches, water moved downward through the surface sand and either continued downward into clayey sand or moved laterally along the surface-sand/clayey-sand interface into the trench backfill sand, depending on trench design. The use of the compacted clayey-sand barrier at trench 1 greatly inhibited soil-moisture that moved along the interface from entering the trench, whereas the absence of the barrier at trench 2 allowed the soil moisture to enter the trench. Water ponded on the bottom of trench 1 to a maximum depth of 0.3 meter. At trench 2, water ponded to a maximum depth of 2.3 meters. The depth to which evapotranspiration and precipitation influenced soil-moisture migration in the unsaturated zone was dependent on the duration and magnitude of each flux. 4 references, 8 figures