Interpreting INEEL vadose zone water movement on the basis of large-scale field tests and long-term vadose zone monitoring results

ties. To date, most descriptions of these properties are based on vertical borehole information that is inherently A review of the results from two large-scale field tests and vadose limited because of its point-source nature. Downhole zone monitoring data at two facilities at the Idaho National Engiinstrumentation and long-term monitoring data have neering and Environmental Laboratory (INEEL) has provided information to support development of a conceptual model of water move- provided some small-scale properties measurements. ment in the highly heterogeneous vadose zone beneath two INEEL Large-scale measurements are more difficult to obtain facilities. Both of the large-scale field tests were ponded water infiltra- as well as logistically cumbersome, costly, and often tion tests that approached the size of the two facilities. An assembly difficult to interpret. of perched water wells, neutron access tubes, and tensiometers have Of the facilities on the INEEL currently being investibeen used to monitor water movement beneath these sites. Once gated to assess their potential environmental impacts, water has percolated through the surficial sediment at these facilities, two have significant potential vadose zone contaminathe presence of the surficial sediment–basalt interface results in some tion sources. These are the Subsurface Disposal Area perching of the percolated water with limited lateral movement. Water (SDA) at the Radioactive Waste Management Complex movement in basalt formations is mainly gravity-dominated in prefer- (RWMC) and the Idaho Nuclear Technology and Engiential fracture pathways and rubble zones and results in rapid vertical neering Center (INTEC). Both facilities have undergone flow especially frequently under conditions of positive hydrostatic extensive vadose zone characterization efforts and have head. Detection of wetting front advance is typically not sequential monitoring instrumentation systems in the vadose zone. with depth and has been shown to vary between episodic infiltration events. At the basalt–sediment interface of the interbeds, perched The characterization and monitoring activities have water is often detected and has resulted in more lateral spreading of been conducted with the goal of understanding the mechrecharge than at the surficial sediment–basalt interface. Gaps in the anisms controlling water and contaminant movement in sedimentary interbeds will allow some of the recharge water to bypass the lithologically complex subsurface at the sites. This the interbeds and rapidly continue toward the aquifer. This complex conceptual understanding is embedded in simulation water movement in the vadose zone beneath the INEEL supports models that assist in the selection between possible rethe need to control surface infiltration, identify gaps and the lateral medial actions and in improving long-term monitoring continuity of the interbeds, and investigate the influence of nearby systems in the vadose zone. The conceptual model of surface water sources on perched water movement beneath these water and contaminant movement is developed from two facilities. information gained from the facility characterization and monitoring programs, and from infiltration experiments conducted on the INEEL to improve understand