Simultaneous Water Flow and Contaminant Transport in Unsaturated Rock

Interest in contaminant transport in fractured rock has grown rapidly in the past few decades. Conventional approaches to evaluating contaminant sorption to rock, adapted from techniques developed on soils, generally involve batch experiments using crushed rock samples. There are concerns about the applicability of this batch sorption approach, because crushing the samples creates new sorption surfaces, and the well-mixed saturated conditions are not representative of the transient conditions in unsaturated rock. A less common method is to run column experiments with saturated rock cores that are subsequently sliced for sampling, chemical extraction and analysis. By interfacing laser ablation with inductively coupled plasma-mass spectrometry (LA-ICP-MS) for direct micro-scale chemical analysis of rock samples, we have developed a new and powerful capability to investigate contaminant (e.g., metal and radionuclide) transport and sorption in unsaturated solid rock. An unsaturated centimeter-scale rock sample is placed in contact with a tracer solution, and capillary-driven imbibition transports chemicals into the sample through the imbibing face. Following imbibition, LA-ICP-MS is used to measure tracer concentration as a function of distance from the imbibing face. Measurements are rapid, and tracer concentrations are obtained at high spatial resolution. The numerical code HYDRUS is then used to describe the transient processes, yielding transport and sorption parameters by inverse modeling of the variably-saturated flow experiments. Results from this method generate sorption data under realistic unsaturated transport conditions, addressing flaws inherent to batch-sorption methods.