In a field study conducted in Pine Ridge, South Dakota, nuclear magnetic resonance (NMR) logging measurements were used to investigate an area of hydrocarbon contamination from leaking underground storage tanks. The NMR logging measurements are directly sensitive to hydrogen-bearing fluids in the sediments surrounding a well and can be used to estimate in situ fluid volumes. The relaxation time T2 and diffusion coefficient D of the fluids may be used to differentiate between signal from water and signal from contaminant, enabling the estimation of the hydrocarbon volume. In this study, NMR measurements were collected in two PVC-cased monitoring wells, with D and T2 measurements used together to detect a contaminant smear zone at both the wells. Although the contrast in D between the fluids was found to be inadequate for fluid typing, the T2 contrast between the contaminant and water in silt enabled the estimation of contaminant volumes. Using this technique, the vertical extent of the smear zone was found to be more than 3 m with up to 5 vol% contaminant in the sediments at one well and up to 9.5 vol% at the other well. Our work reveals that NMR logging can, under certain circumstances, be used to detect and quantify in situ contamination, but it also highlights the significant limitation that sediment and contaminant properties at many sites may result in insufficient contrast in T2 and D.

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