Enhanced delivery of remedial reagents in low-permeability aquifers through coupling with groundwater circulation well

Limited dispersion and mass transfer because of hydrogeological heterogeneity make it difficult to deliver remedial reagents to desired locations. Remediation alternatives that overcome mass transfer limitations are required urgently to improve the efficiency of in situ remediation techniques. In this study, a combination of two-dimensional sandbox experiments and numerical simulations was used to explore the migration process of remedial reagents driven by groundwater circulating well (GCW). The influences of the circulating flow rate, concentration of the remedial reagent (H2O2 as a model) and cycle operation mode on the migration of GCW enhanced remedial reagents were investigated. The flow rate of the pumped water can enhance the concentration gradient at the interface of different permeation zones. The redistribution of reagents by increased Fick diffusion significantly increased the dispersion range. The results of parametric sensitivity analysis showed that the aggregation effect of remedial reagents in the low permeability area (∼10-7 m/s) and related to the injection site. Considering remedial reagents attenuation, a reactive transport model can better simulate the concentration changes in low-permeability regions (R2 > 0.960). The numerical simulation results showed that the principal component of the hydrodynamic field was tangential to the main axis of the target area. The diffusion coefficient of the low-permeability region was one order of magnitude lower than that of the high-permeability region (∼10-3 m/s). Thus, the low-permeability region was less affected by the hydraulic action of the GCW and had a longer retention time for remedial reagents. Specific aquifer parameters and operating modes are key engineering design factors of a GCW that should be considered in conjunction with other in situ chemical technologies. This study provides some new insights for in situ remediation of contanminated aquifers with a GCW, and suggests that (i) the hydraulic circulation driven by GCW can promote the accumulation of injected remedial agents in low permeability areas, especially injection point is located near wellbore; and (ii) residence time of reagents in contaminated areas should be comprehensively considered when chemical remediation technology is coupled with GCW. The results of this study can be used to improve the delivery efficiency of remedial reagents in contaminated aquifers.