Soluble contaminants or tracers migrate through natural systems defining concentration versus time curves that vary between sample locations. Often, these curves show long tails that are not described by the processes of advection and dispersion. Whether we need to predict evolution of natural systems or the quality of our water supply, we must be able to quantitatively predict such tailing behavior. The reliability of contaminant transport predictions depends on a good understanding of in-situ flow and transport processes; however, large contrasts, complex connectivity, and extreme spatial localization of hydraulic properties combine with sparse data to limit our understanding of fundamental processes and properties controlling field-scale hydrologic behavior. This research looks to help answer the following question: Under what scenarios can long tailing of concentration histories be described by commonly used advective-dispersive models given hydraulic conductivity heterogeneity, versus the need for bicontinuum models with local diffusion between mobile and less mobile porosity domains?
Bicontinuum mass transfer has been used to explain complex transport behavior in some environments, but experimental verification of this process is problematic because geochemical samples only represent the mobile component of the pore space, making determination of processes in the immobile domain impossible. Recently, the PI and colleagues have demonstrated that electrical geophysical techniques provide a means of verifying the occurrence of bicontinuum transport and estimating the parameters which control these processes by sampling the total pore space, rather than just the mobile domain. While these results show promise for using remotely sensed data for directly estimating hydrologic parameters controlling mass transfer?immobile porosity and mass-transfer rate?in situ, determining when dual-domain systems control transport is unclear. In particular, it is not clear what scales of mass transfer rates (or diffusion lengths) and immobile porosity fractions can be practically investigated with this approach, or heterogeneity controls on transport. The objectives of this research are to determine 1) whether solutes locally diffuse between mobile and less mobile zones at three field sites of differing geology, and 2) how mass transfer processes within and between heterogeneous zones affect our macroscopic view of solute transport in the field.
Tied to this research is the development of an integrated hydrogeophysics summer course, where undergraduate researchers will combine field experimentation, in-class instruction, and numerical modeling to develop and test hypotheses regarding the processes controlling transport under different regimes. This field camp will be run in collaboration with three HBCUs (Historically Black Colleges and Universities) partnered with Penn State and the Summer Research Opportunity Program (SROP), a summer-long internship that engages students from minority groups and institutions in cutting-edge research at majority institutions of the Committee on Institutional Cooperation.
