Detecting and Quantifying Organic Contaminants in Sediments with NMR

Abstract:

Nuclear magnetic resonance (NMR) methods have the potential to detect and monitor free-phase organic contaminants in sediments, both in the laboratory and in the field. NMR directly detects signal from hydrogen-bearing fluids; the signal amplitude is proportional to the total amount of hydrogen present, while the signal decay rate provides information about fluid properties and interactions with the surrounding sediments. Contrasting relaxation times (T₂) or diffusion coefficients (D) allow the separation of water signal from contaminant signal.

In this work, we conduct a laboratory study to assess the use of NMR measurements to detect and quantify diesel, gasoline, crude oil, and tri-chloroethylene in sediments. We compare the T₂ distributions for sediments containing only water, only contaminant, and both water and contaminant, confirming that the identification and quantification of contaminants using T₂ data alone is limited by overlapping water and contaminant T₂ distributions in some sediments. We leverage the contrast between the diffusion coefficient of water and that of diesel and crude oil to separate contaminant signal from water signal in D-T₂ maps. D-T₂ distributions are measured both using a pulsed gradient method and a static gradient method similar to methods used with logging tools, allowing us to compare the ability of each method to quantify diesel and crude oil when water is also present. There is the potential to apply these methods to characterize and monitor contaminated sites using commercially available NMR logging tools.