Flow-Induced Redox Geochemistry Within Fractured/Macroporous and Layered Vadose Zone

Texas A&M Engineering Experiment Station, College Station TX

Investigators

Abstract

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.