Real-time 4D ERT monitoring of river water intrusion into a former nuclear disposal site using a transient warping-mesh water table boundary (Invited)

The Hanford 300 Area, located adjacent to the Columbia River in south-central Washington, USA, is the site of former research and uranium fuel rod fabrication facilities. Waste disposal practices at site included discharging between 33 and 59 metric tons of uranium over a 40 year period into shallow infiltration galleries, resulting in persistent uranium contamination within the vadose and saturated zones. Uranium transport from the vadose zone to the saturated zone is intimately linked with water table fluctuations and river water intrusion driven by upstream dam operations. As river stage increases, the water table rises into the vadose zone and mobilizes contaminated pore water. At the same time, river water moves inland into the aquifer, and river water chemistry facilitates further mobilization by enabling uranium desorption from contaminated sediments. As river stage decreases, flow moves toward the river, ultimately discharging contaminated water at the river bed. River water specific conductance at the 300 Area varies around 0.018 S/m whereas groundwater specific conductance varies around 0.043 S/m. This contrast provides the opportunity to monitor groundwater/river water interaction by imaging changes in bulk conductivity within the saturated zone using time-lapse electrical resistivity tomography. Previous efforts have demonstrated this capability, but have also shown that disconnecting regularization constraints at the water table is critical for obtaining meaningful time-lapse images. Because the water table moves with time, the regularization constraints must also be transient to accommodate the water table boundary. This was previously accomplished with 2D time-lapse ERT imaging by using a finely discretized computational mesh within the water table interval, enabling a relatively smooth water table to be defined without modifying the mesh. However, in 3D this approach requires a computational mesh with an untenable number of elements. In order to accommodate the water table boundary in 3D, we propose a time-lapse warping mesh inversion, whereby mesh elements that traverse the water table are modified to generate a smooth boundary at the known water table position, enabling regularization constraints to be accurately disconnected across the water table boundary at a given time. We demonstrate the approach using a surface ERT array installed adjacent to the Columbia River at the 300 Area, consisting of 352 electrodes and covering an area of approximately 350 m x 350 m. Using autonomous data collection, transmission, and filtering tools coupled with high performance computing resources, the 4D imaging process is automated and executed in real time. Each time lapse survey consists of approximately 40,000 measurements and 4 surveys are collected and processed per day from April 1st , 2013 to September 30th, 2013. The data are inverted on an unstructured tetrahedral mesh that honors LiDAR-based surface topography and is comprised of approximately 905,000 elements. Imaging results show the dynamic 4D extent of river water intrusion, and are validated with well-based fluid conductivity measurements at each monitoring well within the imaging domain.