Transport Studies at the Vadose Zone Research Park, Idaho National Engineering and Environmental Laboratory: Results of Initial Tests

The Vadose Zone Research Park (VZRP) provides a unique opportunity to investigate flow and transport in a thick, fractured and layered vadose zone. The VZRP includes two newly constructed percolation ponds each approximately 160000 square ft in area, which receive roughly 1.0 to 1.5 million gallons/day of uncontaminated process water. Monitoring wells and instrumented boreholes surround the percolation ponds. These are distributed in nested sets that allow continuous monitoring and sample collection along two important hydrologic contacts; one located at roughly 60' bls along a contact between alluvium and basalt and the other at 125' bls, along a sedimentary interbed in basalt. Both of these contacts support perched water zones. Hydraulic data have been collected nearly continuously since the first use of the percolation ponds in August 2002. Samples for geochemical studies were also collected during the first few weeks of discharge to the south pond to observe geochemical trends during initial wetting of the subsurface. During the summer of 2003, two tracer tests were performed. The first test consisted of injecting a conservative tracer (2,4,5-trifluorobenzoic acid) into the south pond, which had been receiving water for almost 10 months prior and for which hydraulic data indicated a steady state hydraulic system. The second tracer test was conducted in the north pond and consisted of simultaneous injection of two conservative tracers with different diffusion coefficients (2,4-difluorobenzoic acid, and Br- ion). Tracer injection coincided with the switching of water from the south to the north pond, which had been dry for 10 months prior. Thus, this test afforded us the opportunity to evaluate transport behavior in a relatively dry vadose zone, and to compare this to observed transport behavior under the earlier steady state, more saturated flow condition. Results from the first tracer test show tracer breakthrough in a shallow well, close to the south pond within approximately 30 hours with the peak at approximately 70 hours. In an adjacent, though deeper well located in a perched water zone at the 125' interbed, two tracer peaks were observed, one at approximately 50 hours and the other at approximately 200 hours, indicating multiple flow pathways and different travel times. Flow velocities calculated from this test are on the order of 100 m/day, in good agreement with velocities determined through hydraulic data. Initial results from the second tracer test show tracer recovery in at least four of the sampled wells. During this test, the discharge and four wells were also sampled for colloid concentration and particle size distribution. Colloid concentrations in the wells are roughly equivalent to, or larger than, those from the discharge and show sharp peaks up to an order of magnitude above background values. Comparison of colloid concentration data from the discharge, shallow wells located in the alluvium, and deeper wells in fractured basalt suggest that colloids are liberated in the alluvium and that advection through the fractured basalt does not affect the stability of the colloids. Preliminary tracer data show that tracer breakthrough in the monitoring wells occurred at similar times to colloid peaks. Further analytical work will yield breakthrough curves for the 2,4-tFBA that will be quantitatively compared with the colloid peaks.