PRISMS - Profile Resolving In-Situ Soil Moisture Sensor

We are developing a subsurface monitoring technique for providing spatially resolved profiles of soil moisture in the vadose zone that can be easily deployed using either cone penetration testing (CPT) technology, conventional drilling, or in some applications, direct burial. The technology has application to a wide variety of earth and environmental science and infrastructure monitoring challenges. The ability to field an in situ monitoring device that can spatially resolve soil moisture (as well as water table elevation) in profile will substantially assist with understanding specific sites, as well as help improve the state-of-the-art in model development, verification, and long-term monitoring that are crucial to effective remediation and understanding coupled biological, geochemical, and physical (hydrologic) processes in the unsaturated zone. Our technical approach extends the state of the art in time domain reflectometry (TDR) for soil moisture determination to enable quantitative and robust decision support at vadose zone environmental management sites. The sensor we will develop combines advances in the processing of TDR data with innovations in the physical sensing apparatus. These advancements enable high temporal (and thus spatial) resolution of reflectance continuously along the inside surface of a borehole using a flexible parallel waveguide that is permanently emplaced against the geologic formation. The spatially resolved reflectance is directly related to impedance changes along the waveguide that are dominated by electrical permittivity contrast due to variations in soil moisture. PRISMS will thus be able to monitor soil moisture in profile (as well as water table elevation) with high spatial and temporal resolution, to enable substantial improvements in the state-of-the-art of model development, verification, and long-term monitoring crucial to effective site management and remediation. In contrast, most existing commercial TDR-based moisture sensors perform a bulk characterization of the soil volume surrounding a short, un-insulated waveguide, and therefore cannot resolve spatial variability in the soil condition. Phase I included laboratory to research performance of the method in a range of soil types and at varying pore water salinities, and also researched and identified commercial manufacturing techniques to produce waveguides of the physical characteristics required to maximize precision, accuracy, durability, and achievable length of continuous measurement profile. Phase I will also assessed issues of deployability and produced prototype PRISMS that were emplaced at a DOE IFRC site, where time series data were acquired from PRISMS. The goals of Phase II are to continue to increase the technical effectiveness of PRISMS and to advance its readiness for commercial production. Specific tasks include: improving sensitivity and noise immunity through waveguide design optimization; refining and extending multi-reflection deconvolution and other algorithms to improve response to steep moisture gradients; addressing complications due to elevated pore water ionic strength; determining requirements for integration into third party electronic reflectometry equipment; and perfecting various methods of emplacing long vertical PRISMS waveguides in situ. Field demonstrations are also included in the proposed work.