## 5 Discussion  
The feasibility of migrating bacterial and cementing solutions to a confined aquifer via a recharge well had been preliminatively proved by simulation. The uneven distribution of the generated calcium carbonate was a problem that affected the integrability of MICP curing [13]. The reason was that bacterial cells had negative charges on their surfaces. Electrostatic forces between the cells repelled each other, but this negative charge attracted Ca\({}^{2+}\) from the solution, which neutralized the charges and clumps together. The aggregated aggregates blocked the pores of the sand particles, which was not conducive to subsequent liquid transport, resulting in uneven calcium carbonate production throughout the sand column. Most MICP curing laboratory tests had noted the uneven distribution of carbonate deposition, mainly due to the plugging of perfusion points [48]. Through different grouting methods, the uniformity and distribution of calcium carbonate precipitation after MICP treatment could be changed, so as to change the strength and permeability of the soil after MICP treatment. The simplest grouting method was to inject the mixture of bacterial and cementation solution into the soil. However, this method easily caused the clogging of precipitated calcium carbonate crystals around the injection point and their non-uniform distributions over time, which blocked the channel, preventing the subsequent slurry from being injected into the soil. Thus, the spatial distribution of soil strength after MICP treatment was also highly non-uniform [49]. [PERSON] et al. proposed a step-based grouting method [50]. After injecting a bacterial solution into the soil, it stood for a period of time so that bacteria could be absorbed on the surface of the soil particles before injecting the cementing solution. Compared with the mixed perfusion method, the sand porosity was obviously reduced, so as to improve the effect of improving permeability. On this basis, [PERSON] et al. improved the two-step grouting method [51]. After instillating a bacteria solution into soil for a period of time, a stationary solution (0.05 mol/L, CaCl\({}_{2}\) solution) was injected at a low speed to pretreat sand samples. On the basis of the two-step grouting method, a three-step grouting method was proposed. The stationary solution injection was added after bacterial solution injection and before cementation solution injection, which played a role in the uniform distribution of bacteria. This method increased the cementing length and significantly improved the plugging situation at the grouting mouth. However, when the MICP grouting technology was used to form the HSRB, the permeability of the sand layer under the action of MICP was gradually reduced, with the pores between the sand particles gradually filled by calcium carbonate precipitation. The sand layer needed to be injected several times over several days to achieve the designed hydraulic conductivity. Figure 24: Migration diagram of cementation solution: (**a**) Horizontal; (**b**) Vertical; (**c**) Legend. With the MICP reaction of each solution in the previous MICP technology, calcium carbonate precipitation was generated, and the pores were filled gradually. Subsequently, the injection of bacteria solution, stationary solution, and cementation solution was hindered. In this simulation, the permeability and porosity of the model were changed to simulate the hindrance of solute migration in the later filling solution. In accordance with the test results of MICP reducing the permeability of sand, the hydraulic conductivity of the involved area was reduced in a corresponding proportion in the model, as shown in Table 7.  
In accordance with the laboratory test results, MICP technology to reduce the permeability of sand was effective after 7 days, that is, the hydraulic conductivity of sand was reduced to the lowest after 7 days of grouting, and the pores between sand particles were filled with calcium carbonate to the maximum.