| Abstract: | Nuclear magnetic resonance (NMR) geophysical tools have been widely used in the petroleum exploration industry since the 1960s and have improved significantly in the last two decades. These tools can provide estimates of bulk porosity and fluid content, quantification of bound versus mobile fluids, and estimates of hydraulic conductivity (K). Although the size and cost of oil‐field tools historically limited their use for near‐surface applications, smaller and more economical downhole NMR logging tools are now available for detecting and characterizing the formation water content and K to support environmental and groundwater resource investigations. These tools can be deployed using direct‐push drilling techniques or they can be lowered into existing open borings or wells with nonconductive polyvinyl chloride casings and screens. In many cases, using the tool in existing wells offers a safer and more cost‐effective alternative compared to drilling new boreholes. For environmental investigations, NMR can provide useful high‐resolution quantitative hydrostratigraphic information that can provide additional valuable data to further inform and refine the conceptual site model. This paper highlights several NMR field investigations that demonstrate the viability of this technology as a site characterization tool for near‐surface investigations. NMR measurements were compared to data from lithologic logs, cone penetrometer testing data, and prior field hydraulic tests. Use of NMR to detect vadose zone water, including previously unidentified perched groundwater zones, provided hydrostratigraphic details that could not be gleaned from historical well drilling logs and were used to evaluate drainable pore water versus pore water bound in small pores by capillary forces or electrochemically clay‐bond water. NMR also produced K estimates similar to those from conventional hydraulic tests, but the improved vertical resolution from NMR provided additional information regarding the vertical heterogeneity of the formation along the entire length of the well or borehole. Additionally, bench‐scale tests are presented that confirm the capability for NMR to reliably detect and quantify light nonaqueous phase liquid saturation (specifically diesel fuel and weathered gasoline) in situ. The field tests combined with bench‐scale testing results affirm the applicability and potential for NMR as a practical characterization tool that should increasingly be utilized in environmental investigations. |
