Geophysical Investigations For Environmental Risk Assessments

Geophysical techniques and surveys are useful for realistic site characterization, site monitoring<br>strategies, and/or detection and mapping of contaminant plumes for verification of environmental<br>risk assessments for nuclear waste facilities. The NRC staff has evaluated the role and<br>applicability of geophysical techniques at three research field locations involving unsaturated,<br>heterogeneous soils and fractured rock. The techniques evaluated included: (1) electroresistive<br>tomography (ERT) to assess water flow through unsaturated, heterogeneous soils and fractured<br>rock, to identify fracture aperture sizes that contribute to water migration and rapid fluxes, and to<br>image three-dimensional resistivity within the zone bounded by the boreholes; (2) surface<br>electromagnetic induction (EM) to detect changes in the apparent electrical conductivities from<br>which water content changes can be inferred; and (3) multi-sensor capacitance probes (MCP) to<br>determine temporal and spatial resolution of infiltration, deep percolation, and ground-water<br>recharge in shallow heterogeneous soils. Specifics of the geophysical techniques and surveys<br>used at the three sites are discussed and evaluated in the context of environmental risk<br>assessments involving radionuclide transport.<br>Key words: downhole geophysical methods, electromagnetic induction, electroresistive<br>borehole tomography, environmental risk assessment, geophysical investigations,<br>multi-sensor capacitance probe.