RICHLAND, Wash. – At the southern tip of the sprawling Hanford Site, the soil beneath the 300 Area contains residual uranium from a handful of now-removed settling ponds and trenches that stored liquid waste from the processing of spent nuclear fuel rods.
Located about a quarter-mile west of the Columbia River shoreline, underground uranium concentrations remain high after years of plutonium production decades ago.
EM’s Richland Operations Office and its site contractor CH2M HILL Plateau Remediation Company (CHPRC) recently finished injecting a polyphosphate solution into the ground to bind with uranium through a process called sequestration, preventing the uranium from reaching the groundwater and Columbia River. Polyphosphate is like a time-release medicine — it breaks up over time. As the phosphate reacts with the uranium, it makes a rock, which is naturally occurring, and stabilizes the uranium in the soil.
Researchers with DOE’s Pacific Northwest National Laboratory (PNNL) worked with CHPRC to successfully implement a state-of-the-art approach for monitoring the delivery of the polyphosphate remediation.
The approach uses PNNL’s Real-time Four-Dimensional Subsurface Imaging Software (E4D) to take images of the vertical and lateral movement of the polyphosphate solution. E4D was developed with support from DOE and the U.S. Department of Defense and is freely available to anyone.
E4D uses electrical resistivity tomography (ERT) measurements to reconstruct time-lapse images of the electrical conductivity of the soil. As the polyphosphate solution permeates the soil and the ground’s electrical conductivity increases, an array of ERT sensors continuously measure the change in conductivity. E4D uses the measurements to produce images of the polyphosphate remedy distribution over time.
“It’s sort of like using infrared goggles to see heat signatures in the dark, except this is underground — there is no direct line of sight,” said Tim Johnson, senior geophysicist at PNNL and lead developer of the E4D software. “With E4D, data collected by remote sensors are processed by a computer tomography algorithm to produce an image that reflects the environment.”
The polyphosphate remedy, delivered at two depths from a patchwork of 48 total injection wells, spreads through the soil. To help CHPRC employees view the spread of the phosphate solution underground, PNNL placed its ERT sensors in a unique cross-hole pattern within clusters of injection and monitoring wells.
During the phosphate solution injections, the ERT system injected electrical current into the subsurface. Sensors running the length of each well measured the corresponding changes in soil voltage.
Those measurements instantly traveled via wireless internet to Constance, a supercomputer at PNNL’s Institutional Computing Center. There, Constance processed the data, combining geology, physics, mathematics, and chemistry with E4D’s modeling software to create time-lapse 3-D images of the solution and its location within minutes.
As the E4D software executed its program, color-coded pools quickly began appearing on the display. Through a custom web interface, the science team, operations staff, and key stakeholders at multiple locations watched the mobility of the phosphate solution in near real-time the fourth dimension in E4D.
Within minutes of data acquisition, the E4D modeling software translated that information into images for onscreen display.
“We really appreciate the collaboration with PNNL on being able to use this technology,” said Marty Doornbos, CHPRC’s director of groundwater remedy selection and implementation. “It allowed us to monitor and verify the progress in real time to help ensure we reduce this risk to the Columbia River.”
Over the course of three weeks, the compiled images revealed a noticeable “breathing” pattern. The colored pools appeared as the injections took place during the day shift, then tapered off overnight, then reappeared as injections started again the next day.
“Right off the bat it looked like the new monitoring technique was working as well as or better than expected,” said Rob Mackley, a hydrogeologist at PNNL and manager of PNNL’s technical support on the project. “After two weeks of data, we knew it was a home run.”