FLUID TRANSPORT MONITORING DURING PERCOLATION POND INFILTRATION AND RECHARGE TO A COMPLEX, DEEP, HETEROGENEOUS VADOSE ZONE AT THE IDAHO NATIONAL ENGINEERING AND ENVIRONMENTAL LABORATORY

Predicting fluid and contaminant transport in the vadose zone at the Idaho National Engineering and Environmental Laboratory (INEEL) has been problematic due to the complex geology underlying the site. In an attempt to better understand the controlling mechanisms of subsurface fluid transport, a system of monitoring instruments was installed in boreholes around the perimeter of newly constructed percolation ponds and along an ephemeral river approximately one kilometer to the north. This instrumented region has been designated as the Vadose Zone Research Park (VZRP). INTEC facility-water discharge to the new percolation ponds began in October 2002 at a rate of approximately a million gallons per day. Continuous monitoring of subsurface hydraulic properties at the VZRP since May 2002 coupled with periodic tracer injections suggest that subsurface flow is controlled by the presence of preferential flow paths and extreme contrasts in hydraulic conductivity between sedimentary interbeds and fractured basalt flows. In addition, the observations show that perched water does not dissipate rapidly in the absence of local recharge and that it may extend tens of meters both laterally and vertically from the source area. In contrast to these observations, the most prevalent current conceptual model for fluid transport in the vadose zone at the INEEL adopts diffuse type flow and assumes that perched water dissipates rapidly in the absence of local recharge. Continued monitoring and transport studies at the VZRP can contribute significantly to the understanding of fluid transport at the INEEL and similar sites throughout the country. Efforts are currently underway to establish the VZRP as a collaborative, multidisciplinary field research center that can be utilized to develop an improved conceptual model for fluid and contaminant transport in heterogeneous, complex vadose zones.