Electrical Resistivity Correlation to Vadose Zone Sediment and Pore-Water Composition for the BC Cribs and Trenches Area

Serne, R. Jeffrey ; Ward, Anderson L. ; Um, Wooyong ; Bjornstad, Bruce N. ; Rucker, Dale F. ; Lanigan, David C. ; Benecke, Mark W.

Pacific Northwest National Laboratory (U.S.)

关键词: Bc Cribs And Trenches;    Hydraulics;    Geophysics;    Water Soil Electrical Resisitivity;    Focusing;   

DOI  :  10.2172/962849
RP-ID  :  PNNL-17821
RP-ID  :  AC05-76RL01830
RP-ID  :  962849

美国|英语

来源: UNT Digital Library

【 摘 要 】

This technical report documents the results of geochemical and soil resistivity characterization of sediment obtained from four boreholes drilled in the BC Cribs and Trench area. Vadose zone sediment samples were obtained at a frequency of about every 2.5 ft from approximately 5 ft bgs to borehole total depth. In total, 505 grab samples and 39 six-inch long cores were obtained for characterization. The pore-water chemical composition data, laboratory-scale soil resistivity and other ancillary physical and hydrologic measurements and analyses described in this report are designed to provide a crucial link between direct measurements on sediments and the surface-based electrical-resistivity information obtained via field surveys. A second goal of the sediment characterization was to measure the total and water-leachable concentrations of key contaminants of concern as a function of depth and distance from the footprints of inactive disposal facilities. The total and water-leachable concentrations of key contaminants will be used to update contaminant distribution conceptual models and to provide more data for improving base-line risk predictions and remedial alternative selections. The ERC “ground truthing” exercise for the individual boreholes showed mixed results. In general, the high concentrations of dissolved salts in the pore waters of sediments from C5923, C5924 and C4191 produced a low resistivity “target” in the processed resistivity field surveys, and variability could be seen in the resistivity data that could relate to the variability in pore- water concentrations but the correlations (regression R2 were mediocre ranging from 0.2 to 0.7 at best; where perfect correlation is 1.0). The field-based geophysical data also seemed to suffer from a sort of vertigo, where looking down from the ground surface, the target (e.g., maximum pore-water salt concentration) depth was difficult to resolve. The best correlations between the field electrical resistivity surveys and borehole pore water data sets were obtained when focusing on areal extent of the salt plume. Lateral resolution of the geophysical field data is best conducted by comparing an aggregated set of geophysical data on all boreholes together. When assembling the pore-water data for all four boreholes in an aerial view, the field ERC data produce a reasonable aerial picture of where high salt plumes exist below the BC Cribs and Trenches area. Future work that relies on more laboratory soil resistivity and incorporation of other field data (spectral gamma, neutron moisture and soil density logs) and physical and hydraulic measurements on samples obtained from the boreholes will used develop a more detailed petrophysical model of the sediments below BC Cribs and Trenches. This more detailed model can be used as a more realistic “earth model” in the inversion process to better manipulate the raw field survey data. It is also recommended that one more borehole be drilled after a thorough vetting of the current data with geophysics experts and other Hanford stakeholder to optimize where to place the borehole, what electrical and other geophysical surveys should be conducted , where to take sediment samples and what parameters should be measured on the sediments to attempt one more “ground truthing” exercise.