ABSTRACT

We applied laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) techniques to make spatial measurements of chemical tracer concentrations in three natural rock types (tuff, sandstone, greywacke) in order to characterize important processes affecting chemical fate and transport (e.g., diffusion, sorption, imbibition, and advective transport). Specifically, we (1) evaluated LA-ICP-MS factors (e.g., spot size, laser pulse, primary grain size, and surface roughness) affecting quantitative laser sampling in both surface and depth mapping; and (2) applied LA-ICP-MS to investigate the fate and transport processes of a suite of chemicals (both nonsorbing and sorbing) in natural rock through laboratory studies of imbibition, diffusion under both saturated and unsaturated conditions, and transport in a rock core traversed by a saw-cut fracture. The results indicate that micro-scale mapping using LA-ICP-MS is a robust and sensitive technique for evaluating fate and transport of chemical species in natural rock. Direct mapping of chemical distributions using LA-ICP-MS techniques in natural rock offers distinct advantages over other methods of study, such as batch sorption experiments using crushed rock samples, and through-diffusion studies involving lengthy experiments during which test conditions may be difficult to maintain. Values of sorption distribution coefficient as determined using LA-ICP-MS techniques with centimeter-sized tuff samples, are consistently smaller, in both imbibition and diffusion experiments, than those obtained from batch-sorption tests using crushed samples with grain sizes in the 0.5-2.0 mm range.