HR: 1340h
AN: NG33B-0181    [Abstracts]
TI: Monitoring Contamination of the subsurface with Quasi-Static Deformation
AU: * Spetzler, H
EM: hartmut.spetzler@colorado.edu
AF: Dept. of Geological Sciences and CIRES, University of Colorado, Boulder, CO 80309 United States
AU: Snieder, R
EM: rsnieder@mines.edu
AF: Center for Wave Phenomena and Dept. of Geophysics, Colorado School of Mines 1500 Illinois Str., Golden, CO 80401 United States
AU: Zhang, J
NG33B-0181 AF: Knowledge Systems Inc., One Sugar Creek Center Blvd. Suite 1100, Sugar Land, TX 77478 United States
AB: Data from a field experiment suggest that we can detect the infiltration of contaminated water (contaminated with 150 ppm of a biosurfactant, Rhamnolipid) into the vadose zone with tilt meters. Three sets of instruments were installed in the vicinity of a 50m x 50m field, which was instrumented and could be irrigated in a controlled manner. Each set consisted of one tiltmeter at the bottom of a 10m cased borehole and one seismometer buried to a depth of about 1m. The instruments were installed in late summer and early fall of 2002. The drift in tilt caused by their installation decayed to near background levels in about one year. The site was irrigated with plain water at 50,000 l/day for 40 days during the fall of 2003. The irrigation was repeated in the fall of 2004 for 50 days, again with 50,000 l/day. This time the irrigation water contained 150ppm of a biosurfactant. This surfactant was chosen to simulate a contaminant through its effect on the modification of surface tension and because it is environmentally benign. It was developed for bioremediation. We expected subtle changes in the relative responses of the instruments as the elastic properties of the vadose zone were altered by the contaminant. This expectation is based on a long series of laboratory measurements, e.g. W. Brunner and H.A. Spetzler 2002. We used natural sources for the excitation of the instruments, solid Earth tides for the tiltmeters and microseisms, i.e. ocean generated noise at about 6s periods, for the seismometers. In the case of the tilt meters we used theoretical site specific tilt and compared that with our measured tilt. We found no significant change in the correlation coefficient between theoretical and measured tilt for the water-only irrigation. The correlation coefficient was generally above 0.9. The correlation coefficient dropped precipitously about ten days after we had started irrigating with the surfactant. It recovered again about ten days after irrigation was stopped. A preliminary analysis of the seismic data shows similar trends, but with much more noise. Using a Born approximation we calculate that a slight perturbation of the complex moduli near the surface does indeed yield changes in tidally-induced tilt near the observed values. Brunner, W. M. and H. A. Spetzler, Contaminant-Induced Mechanical Damping in Partially Saturated Berea Sandstone, Geophys. Res. Lett., Vol.29, No.16, 10.1029/2002GL015455 ,2002
DE: 0418 Bioremediation
DE: 1842 Irrigation
DE: 1865 Soils (0486)
DE: 1866 Soil moisture
DE: 1875 Vadose zone
SC: Nonlinear Geophysics [NG]
MN: Fall Meeting 2005