Assessment of the effect of a bentonite seal on groundwater storage in underlying waste disposal trenches at Oak Ridge National Laboratory

For a 49-trench section that was sealed in Solid Waste Disposal Area 6, the water-table elevation was periodically measured to assess the effectiveness of the seal for controlling the intrustion of water. Water was found in a majority of trenches throughout the study period (May 1978-December 1979). Mean seasonal water-table fluctuations indicated an average rise of 131 +- 56 cm in waste trenches between October 30, 1978 (minimum level) and February 15, 1979 (maximum level). This mean difference corresponds to an estimated gain of 1000 m3 of groundwater storage in the trench area. The measured depths to water in the trenches showed a general correspondence with those in monitoring wells (augered during September 1979) outside the trenches in the undisturbed zone in the downslope southern half of the study area. In contrast, the measured depths to water were relatively greater in wells in the undisturbed zone in comparison with those in trenches in the upslope northern half of the study area. This suggests that perched water tables developed within the trenches in the upslope area. The groundwater table (potentiometric surface) contours essentially followed the topographic contours and indicated a predominant gradient toward the south-southwest, which is in the direction of a backfilled topographic draw with a known seep at its base. The reported hydrologic characteristics of the Oak Ridge area, laboratory studies in the development of bentonite application rates, and simulation studies using a hydrologic computer code suggested that a significant part of the groundwater observed in the study area was associated with groundwater intrusion from the surrounding upslope area