Evaluation of zirconium-89 complexes for spatio-temporal imaging of soil-water flow via positron emission tomography

Abstract

Understanding how water, nutrients and toxic pollutants flow through soil is critical in many geotechnical and geoenvironmental applications. However, this flow is difficult to observe directly. Positron emission tomography (PET) and computed tomography (CT) are increasingly used for real-time imaging to study soil-water interactions non-destructively. While past PET studies have utilised non-reactive radiotracers to monitor water flow, the limited duration of these experiments is constrained by the tracers' short decay period. This is particularly challenging in materials like clays, where water infiltration occurs slowly. This research aimed to develop a suitable radiotracer based on the 89Zr radionuclide (with a 78 h half-life compared to 2 h for 18F, which is widely used for PET imaging of water flow) to increase both the duration of infiltration experiments and the resolution of PET images. For this purpose, the metal ion must be sequestered by a ligand to form a stable complex before being used as a radiotracer in PET imaging of water flow. The assessment of the retardation factor of obtained [89Zr]Zr-complexes, namely [89Zr]Zr-DFO and [89Zr]Zr-DOTA, revealed the following: [89Zr]ZrCl4 ˃ [89Zr]Zr-DFO ˃ [89Zr]Zr-DOTA. No chemical degradation products of the complexes over three days of interaction with sand and kaolin were observed. The initial infiltration tests revealed that only approximately 0.5% of [89Zr]Zr-DOTA was retained by sand and 5 % by kaolin. The tracer’s velocity was calculated using several breakthrough curve metrics (first arrival, centroid, peak, and Gaussian peak time) extracted from PET/CT scans. These velocities were comparable to those estimated from the volumetric flow rate suggesting low tracer adsorptivity. Analysis of space-time activity patterns in the columns revealed preferential flow and captured the change in hydraulic pressure in unsaturated sand, insights that could not have been gained from conventional techniques. Taken together, these results demonstrate: 1) the suitability of [89Zr]Zr-DOTA as a radiotracer in PET/CT imaging for sand and sand/kaolin columns, and 2) the potential that the tracer offers to study water flow in a wide range of geomaterials. This innovation opens doors for researching various geological and hydrological processes, including groundwater flow, contaminant transport, and subsurface fluid dynamics.