FLOW AND TRANSPORT IN A HETEROGENEOUS, LAYERED VADOSE ZONE AT THE VADOSE ZONE RESEARCH PARK, IDAHO NATIONAL ENGINEERING AND ENVIRONMENTAL LABORATORY: INITIAL RESULTS OF TRACER TESTS

The Vadose Zone Research Park (VZRP) at the Idaho National Engineering and Environmental Laboratory provides an opportunity to explore flow and transport in a heterogeneous vadose zone. The geology at the VZRP consists of fractured basalt, interbedded with sedimentary material and overlain by some 20 m of alluvium. The vadose zone is approximately 150 m thick at the VZRP, with perched zones at the base of the alluvium, and at the basalt / interbed contacts. A percolation pond consisting of two cells, each approximately 5776 m², provides a means of introducing water into the system. Twenty-three vadose zone wells and instrumented boreholes are in place within the VZRP. The vadose zone wells and boreholes are distributed in nested sets that each consist of a shallow monitoring well completed at the alluvium / basalt contact, a deeper monitoring well completed at an interbed at approximately 37 m depth, and an instrument package containing tensiometers, lysimeters, gas-sampling ports, water-content sensors, and thermocouples inserted at depths ranging from 12 to 146 m.

In July 2003, we began directing water at 4100 m³/day into the northern cell of the pond that had not been used for the previous nine months. For the first 140 minutes of infiltration a tracer mixture containing two non-reactive tracers (bromide ion and 2,4-difluorobenzoic acid) was added to the discharge, thus initiating a long-term tracer test. We collected water samples from monitoring wells and suction lysimeters every two to four hours for roughly one month, and at longer intervals for another two months. Transport distances from the pond to the sampling locations are on the order of 100-200 m. Based on the bromide breakthrough curves and preliminary modeling we estimate seepage velocities of 5-10 m/day. Multiple peaks in the breakthrough curves, together with fast arrival times and variable mass recovery indicate that the water followed a number of different flow paths.