Tuesday, 11 July 2006

Fluxmeters for Monitoring Recharge in Desert Settings.

Glendon W. Gee1, Jason Keller1, Fred Zhang1, Andy Ward1, and Brian Andraski2. (1) Battelle, 3200 Q Ave., Richland, WA 99354, (2) Water Resouces Division, U.S. Geological Survey, Carson City Office, Carson City, NV 89706

Water fluxes must be known to assess contaminant migration in the vadose zone. In desert settings, water fluxes are generally very low and often sporadic, responding to extreme events such as rapid snowmelt or heavy but infrequent rainstorms during winter periods, when evaporation is low. A series of water fluxmeters with divergent controls were placed in simulated barren waste-burial grounds located at the U. S. Department of Energy's Hanford Site near Richland, Washington and at a U. S. Geological Survey test site south of Beatty, Nevada, to measure drainage and estimate recharge. The fluxmeters were monitored for over 3 years at each site. Results indicated that at the cold desert Hanford site, as much as 60 mm (~1/3 of the annual precipitation) occurs as drainage through bare coarse. The water fluxmeter data are consistent with observed water potential profiles and also with measured flux rates obtained from on-site deep-drainage lysimeters. For Beatty, a warm desert site, there is annually less precipitation and significantly less drainage than at the Hanford Site. During the first three years of operation there was less than 1 mm of drainage, as measured with the water fluxmeters. While one of the two fluxmeters subsequently failed to operate, the other unit began draining after nearly 4 years of operation. This occurred in response to excess winter rains and also to additional water added to the lysimeter surface. The finer soil at the Beatty site in most winters is likely responsible for wicking winter rain to the surface where it evaporates. However, with bare surfaces, the water potential in the simulated waste site at Beatty remains high and the potential for drainage remains correspondingly high. Additional years of flux measurements will be required to fully document recharge at Beatty. This study was supported by the Hanford Site Remediation and Closure Project sponsored by the U.S. Department of Energy under contract DE AC06 76RL01830 and at the Amargosa Desert Research Site by the U. S. Geological Survey's Toxic Program.

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