HR: 1340h
AN: H23F-1502    [Abstracts]
TI: Calibration of a Ground-Water Model for an Underground Nuclear Test Site to Magnetotelluric Results
AU: * Forsstrom, A M
EM: ftamf@uaf.edu
AF: University of Alaska Fairbanks, Water and Environmental Research Center Department of Civil and Environmental Engineering, 306 Tanana Drive, Fairbanks, AK 99775-5860 United States
AU: Barnes, D L
EM: ffdlb@uaf.edu
AF: University of Alaska Fairbanks, Water and Environmental Research Center Department of Civil and Environmental Engineering, 306 Tanana Drive, Fairbanks, AK 99775-5860 United States
AU: Unsworth, M J
EM: unsworth@phys.ualberta.ca
AF: University of Alberta, Department of Physics, P-412, Avadh Bhatia Physics Laboratory, Edmonton, T6G2J Canada
AU: Kosson, D S
EM: David.Kosson@Vanderbilt.Edu
AF: Vanderbilt University, Department of Civil and Environmental Engineering, VU Station B 351831, Nashville, TN 37232 United States
AU: Soyer, W
EM: wsoyer@gmail.com
AF: University of Alberta, Department of Physics, P-412, Avadh Bhatia Physics Laboratory, Edmonton, T6G2J Canada
AB: Between 1965 and 1971 three underground nuclear tests were conducted below the water table on Amchitka Island, Alaska, USA. A consequence of this testing has been the deposition of large amounts of radioactive material in the subsurface. This poster presentation will summarize results from ground-water modeling performed in the vicinity of the first underground nuclear test Long Shot that was detonated with an approximate yield of 80 kt at a depth of 700 meter below sea level (mbsl). The formation of the southeastern part of Amchitka Island is mainly Banjo Point Formation of Oligocene or Miocene age. The main rock types encountered in the vicinity of Long Shot are pyroclastic rocks of andesitic and basaltic composition. Subsurface mapping of Amchitka Island is limited due to the remoteness of the island. At the Long Shot site five exploratory drilling holes were performed in the 60's. One notable stratigraphic feature found was a fractured andesite sill layer encountered at a depth between 678 and 771 mbsl having higher hydraulic conductivity compared to the rock matrix. The lateral extension and width of this layer is not known. Amchitka Island consists of a ground-water system typical for island hydrology with a freshwater lens overlaying denser saltwater. The location of the transition zone between relatively freshwater and underlying saltwater transition zone affects the ground-water travel time from ground zero to the marine environment. In 2004, measurements of the depth to the transition zone in the regions of the three tests were made in collaboration with the University of Alberta using magnetotellurics (MT). It was found that the transition zone at Long Shot is located approximately between a depth of 700 and 1,600 mbsl. A ground-water flow model has been developed to determine the ground-water travel time from the working point of Long Shot to the marine environment. Hydraulic conductivities and recharge values were obtained from literature and incorporated into the model. The model was calibrated by changing either the hydraulic conductivity or the recharge until the location of the freshwater saltwater transition zone matched the location determined in the MT survey. The best comparison between the transition zones obtained from modeling to the transition zone measured by MT is obtained when major geologic features, specifically andesite sills, are included in the model. These results indicate that the sills do influence the ground-water flow significantly. The shortest resulting transport time for ground water flowing from the working point to the sea floor is approximately 400 years when the sills are accounted for in the model. If the sills are not included in the model the travel time for a likely scenario is approximately 1,000 years. Future modeling efforts include adding the third dimension, the cavity, chimney and convective heat created from the detonation.
DE: 1829 Groundwater hydrology
DE: 1832 Groundwater transport
DE: 1847 Modeling
SC: Hydrology [H]
MN: Fall Meeting 2005