Petrophysical properties of saprolites from the Oak Ridge Integrated Field Research Challenge site, Tennessee

摘要

At the Oak Ridge Integrated Field Research Challenge site, near Oak Ridge, Tennessee, the shallow saprolitic aquifer is contaminated by nitric acid, uranium, and metals originating from the former S3 settling ponds. To interpret low-frequency geophysical methods used to image contaminant plumes, we have characterized the petrophysical properties of three representative saprolite core samples. Their hydraulic conductivity ranges from 10-~7 to 10-~6 ms-~1 in agreement with field data. Complex conductivity measurements, in the frequency range of 1 mHz to 45 kHz, were performed with NaCl solutions with electrical conductivities in the range 5 × 10-~3 to 2.35 Sm-~1, a range representative of field conditions. The electrical conductivity data were well reproduced with a simple linear conductivity model between the saprolite conductivity and the pore water conductivity. The conductivity plots were used to estimate the formation factor (the cementation exponent was about 2.2 + 0.3) and the surface conductivity (0.007-0.040 Sm-~1). The magnitude of the surface conductivity depended on the degree of weathering and therefore on the amount of smectite and mixed layer (illite-smectite) clays present in the saprolite. The chargeability of the core samples was in the range of 20 to 800 mVV-1 and is strongly dependent on the salinity. We also performed streaming potential measurements with the same pore fluid composition as that used for the complex conductivity measurements. We found an excess of movable electrical charges on the order of 100 to 500 Cm-~3 in agreement with previous investigations connecting the movable excess charge density to permeability. The zeta potential was in the range of-10 to-20mV independent on the salinity. The electrical measurements were consistent with an average cation exchange capacity in the range of 1.4 to 11 cmol kg-~1 and a specific surface area on the order of 4000 to about 30,000 m2 kg-~1.