Reactive solute transport driven by coupled stress fields affected by injection pressure and permeability during in-situ chemical oxidation remediation of contaminated groundwater.

Groundwater contamination at the site is worsening, and in-situ chemical oxidation (ISCO) has been widely adopted for contaminated groundwater remediation due to its low environmental risk and low cost. Through laboratory experiments and field simulations, we optimized the system for TCE degradation using activated sodium persulfate and explored how injection pressure and formation permeability affect TCE remediation. The results show that both sodium citrate (TCD) and citrate (CA) chelating agents can increase TCE oxidation by persulfate, with optimal molar ratios of Na2S2O8:TCD:Fe2+:TCE of 20:10:10:1, and Na2S2O8:CA:Fe2+:TCE of 20:5:10:1, achieving high TCE removal rates of 97% and 98%, respectively. Pressurized injection increases the hydraulic gradient and flow velocity, thereby enhancing advection and mechanical dispersion, especially in high-permeability media. In low-permeability media, pressurization also modifies porosity and tortuosity and increases the effective diffusion coefficient, which is relatively more important for solute transport. Pressure-induced permeability changes were limited under the tested 0.1-1 MPa range and were secondary to advection, mechanical dispersion, and effective diffusion. In layered subsurface systems, placing horizontal wells at the interface between high- and low-permeability layers was more effective in delivering oxidants to contaminated regions than vertical wells. In summary, higher-pressure oxidant injection improved remediation in the simulated low-permeability formations by increasing advection, mechanical dispersion, and effective diffusion. The relative contributions were site-specific, and diffusion enhancement was not the sole controlling mechanism. The numerical model can effectively simulate ISCO of TCE by accounting for the multi-field-coupled stress field. It can provide valuable insights for designing groundwater remediation projects.