A review of the results from two large-scale field tests and vadose zone monitoring data at two facilities at the Idaho National Engineering and Environmental Laboratory (INEEL) has provided information to support development of a conceptual model of water movement in the highly heterogeneous vadose zone beneath two INEEL facilities. Both of the large-scale field tests were ponded water infiltration tests that approached the size of the two facilities. An assembly of perched water wells, neutron access tubes, and tensiometers have been used to monitor water movement beneath these sites. Once water has percolated through the surficial sediment at these facilities, the presence of the surficial sediment–basalt interface results in some perching of the percolated water with limited lateral movement. Water movement in basalt formations is mainly gravity-dominated in preferential fracture pathways and rubble zones and results in rapid vertical flow especially frequently under conditions of positive hydrostatic head. Detection of wetting front advance is typically not sequential with depth and has been shown to vary between episodic infiltration events. At the basalt–sediment interface of the interbeds, perched water is often detected and has resulted in more lateral spreading of recharge than at the surficial sediment–basalt interface. Gaps in the sedimentary interbeds will allow some of the recharge water to bypass the interbeds and rapidly continue toward the aquifer. This complex water movement in the vadose zone beneath the INEEL supports the need to control surface infiltration, identify gaps and the lateral continuity of the interbeds, and investigate the influence of nearby surface water sources on perched water movement beneath these two facilities.

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