HR: 1330h
AN: H32A-0520    [PDF]
TI: Field Observations of Fluid Transport in a Complex Heterogeneous Vadose Zone at the Idaho National Engineering and Environmental Laboratory (INEEL)
AU: * Baker, K
EM: bakeke@inel.gov
AF: Idaho National Engineering and Environmental Laboratory, P.O. Box 1625, Idaho Falls, ID 83415-2107 United States
AU: Hull, L
EM: hulllc@inel.gov
AF: Idaho National Engineering and Environmental Laboratory, P.O. Box 1625, Idaho Falls, ID 83415-2107 United States
AU: Mattson, E
EM: matted@inel.gov
AF: Idaho National Engineering and Environmental Laboratory, P.O. Box 1625, Idaho Falls, ID 83415-2107 United States
AU: McLing, T
EM: tm1@inel.gov
AF: Idaho National Engineering and Environmental Laboratory, P.O. Box 1625, Idaho Falls, ID 83415-2107 United States
AB: Predicting fluid and contaminant transport in the vadose zone near the Idaho Nuclear Technology and Engineering Center (INTEC) at the INEEL has been problematic due to the complex geology underlying the site. In an attempt to better understand the controlling mechanism of subsurface fluid transport, a system of monitoring instruments were installed in boreholes around the perimeter of newly constructed percolation ponds, consisting of 2 cells each approximately 160,000 ft$^{2}$ in area. The instrumented region surrounding the ponds has been designated as the Vadose Zone Research Park (VZRP). Continuous discharge to the south cell began in October 2002 at an average flux rate of 1.5 million gallons per day and continued until July 2003 at which time the discharge was switched to the north cell. Hydraulic data were collected nearly continuously, monitoring hydraulic responses to discharge events to both cells. Discharge to the south cell resulted in rapid vertical percolation until reaching the surficial gravel/basalt interface (at about 60 ft below ground surface) at which time rapid lateral transport was observed in a southern direction. A near steady state of water levels was reached during this 10-month period. Switching discharge location only 100 feet to the north cell drastically altered hydrological conditions and flow paths within the subsurface. Recharge was observed in several new locations, while some locations ceased receiving water from the ponds entirely. Other locations temporarily drained, then received "new water" from alternate flow paths a few days later. Prior to switching discharge locations, 3 wells were installed in the north cell at depths of 7 ft, 12 ft, and 19 ft below ground surface. The wells were instrumented with electrical conductivity probes to monitor discharge from the INTEC facility, which receives a high conductivity spike every 12 hours from water softener regeneration. Field observations show that water reached the 7 ft well within a couple hours after the switch, while the other 2 wells remain dry even after one month. Based on these data, it appears that discharge to the north cell percolates vertically to a depth of approximately 10 ft before reaching a low permeability zone, which diverts flow laterally in a northern direction. Field observations of hydrological data indicate that initial water arrival times and locations appear to be controlled by low permeability zones and fast pathways. Arrival progression was neither vertically nor laterally sequential. Importantly, field observations indicate that minor transients in discharge flux and/or location resulted in extreme changes in fluid transport behavior. Based on these observations, it is apparent that our original steady-state conceptual model needs to be modified to incorporate not only lithologic complexities, but also temporal changes in discharge location and flux. Continued field monitoring combined with ongoing tracer testing at the VZRP is aimed at providing the information needed to improve predictive models designed specifically for complex heterogeneous subsurface environments.
DE: 1829 Groundwater hydrology
DE: 1832 Groundwater transport
DE: 1866 Soil moisture
DE: 1875 Unsaturated zone
DE: 1894 Instruments and techniques
SC: Hydrology [H]
MN: 2003 Fall Meeting