The fate and transport of reactive contaminants in variably-saturated subsurface environments is complex, involving linked hydrologic, geochemical, and microbiological processes. As water moves through the layers/lenses/fractures, it may differentially "pick-up" organic matter and inorganic ions from contact with minerals and soil structures and experiences various reduction/oxidation (redox) conditions. Although these processes are evident, measurements and conceptual modeling to quantify the importance/role of these processes have not been undertaken largely due to the inability to measure pore-scale water content/matric potential and complete geochemical suites on the small volumes of fluid available in pore water systems. In addition, unquantified evolutionary redox processes occurring across various hydrologic interfaces including atmosphere-vadose zone, soil layers/lenses, ground water-vadose zone, soil minerals-organics, and soil matrix-fractures confound the ability to predict and evaluate the success of using monitored natural attenuation to remediate a contaminated site. Using emerging pore water sampling and monitoring technologies and novel experimental designs, we propose to conduct several controlled soil column experiments with (1) soil textural layering with different hydraulic properties, (2) staggered geological lenses with different mineralogy, (3) fractures with preferential flow and transport, and (4) groundwater capillary fringe with known chemistry. Benchmark data sets from these experiments will be used to isolate and understand the contribution of various physical and chemical factors governing evolutionary transport processes of major elements including linked C, N, S, and Fe cycles. New conceptual understanding of the flow-induced redox processes will be made by corroborating data from designed experiments and loosely-coupled soil hydrologic (HYDRUS_1D) and geochemical (PHREEQC) process models. Subsequently, our new and improved conceptual and numerical models along with upscaled (bio) geochemical constitutive parameters will be applied and tested at Norman Landfill site, Oklahoma, where several previous and ongoing complementary environmental studies have been conducted. Our experiment-modeling study will provide improved knowledge necessary to more accurately predict rates of natural attenuation in any reduced site useful in petroleum and mixed contaminant systems, as in Norman landfill site in Oklahoma, and may allow for this remediation strategy to be implemented at a greater number of sites resulting in significant cost savings. Undergraduate and graduate students will be trained in the laboratory, field, and modeling studies. Concerted efforts will be made to recruit students from under-represented groups. This interdisciplinary project will enhance interaction between basic science and engineering education and will help develop improved hydrologic and biogeoscience curriculum with research emphasis. Important research findings will be disseminated to high school teachers through an ongoing NSF-supported geosciences education program Professional Learning Community Model for Alternative Pathways in Teaching Science and Mathematics (PLC-MAP) at TAMU.
Understanding and predicting chemical fate and transport in subsurface systems is the key to protect drinking water reserves and ecosystem health. In the past, primary focus has been understanding transport in the (saturated) groundwater aquifer, often using rainfall chemistry as an input on the upper boundary. This assumption neglects any chemical changes to the rainwater that occur in (unsaturated) soil due to processes such as mineral-water interactions, or biogeochemical cycling. Reduction and oxidation (Redox) state in saturated and unsaturated systems is complex involving linked hydrologic, geochemical, and microbiological processes. Physical characteristics (i.e., pore size and shape, geometry, fractures), mineralogy (e.g., depositional patterns, and textural layering), and chemical composition (e.g., species, sorbed and aqueous phase concentrations) are responsible for the rate of geochemical and microbial processes which influence contaminant transport by reduction, stabilization, or enhancement of chemical mobility. These coupled processes are complex and non-linear, posing uncertainties in measuring them accurately, and developing predictive process-based models. In addition to preferential flow, another manifestation of vadose zone heterogeneity includes retarded flow due to soil layering with varying hydraulic properties lending higher residence time for chemicals in less permeable medium. Improved understanding of the biogeochemical processes operating in the macroporous-fractured and layered unsaturated zone is essential to the design and evaluation of remediation strategies including bioremediation, bioaugmentation, and monitored natural attenuation. Our experiment-modeling research effort brought unprecedented insight in to the biogeochemical process understanding across various hydrologic interfaces (land surface-vadose zone, soil layers/lenses, groundwater-vadose zone, soil minerals-organics, and soil matrix-fracture) of various industrial/mixed chemicals in unsaturated geological media in general and Norman landfill site in particular. Important findings of our research include: (1) hydrologic interfaces are the hot spots of geochemical cycling, (2) iron- and sulfate-reducing bacteria showed 1 to 2 orders of magnitude greater community numbers in the layered soil than homogeneous soil, (3) existence of aqueous phase FeS near the soil layers reflects the acceleration of biogeogeochemical activity, (4) redox signals and its temporal dynamics at the landfill site is guided by site-specific hydrologic events such as seasonality of vegetation and surface-groundwater dynamics, and (5) adaptive modeling and upscaling schemes helped transfer pore-scale process understanding to plume-scale behavior. The outcomes of this basic inter-disciplinary research among hydrologists and biogeochemists provide a new platform not only to scientists and engineers interested in migration of fluids and chemicals through the unsaturated geological media, but also to members of industry, regulatory agencies, and environmental planners. We expect the new fundamental knowledge obtained from this work will further advance our ability to predict rates of natural attenuation, and design/evaluate remediation and management plans for redox sensitive systems such as groundwater contaminant plumes. Better understanding of biogeochemical processes with newly developed modeling tools will help predict, and prevent or remediate contamination of natural resources (water and soil) at landfill, oil spill, and other anaerobic subsurface sites. Successful implementation of bioremediation strategies by natural attenuation will generate great cost savings and enhance social ecology. Several inter-disciplinary Ph.D. graduates were produced from this effort cross-cutting the disciplines of hydrology and biogeochemistry. A number of open house, seminars, presentations, K-12 educational material, and journal publications were produced during the course of this study to disseminate our findings to a broad spectrum of our population.
