## 3 Results and Discussion
### Hydraulic Conductivity Results
When these additives were used together, the dense structure formed by the gel effect of SA became more stable due to the cementing effect of MICP, leading to further enhancement in low hydraulic conductivity. In the S2 soil, the lowest hydraulic conductivity coefficient, 1.2 \(\times\) 10\({}^{-5}\) cm/s, was measured in the mixture containing 1% SA, 15% pellet, and MICP. However, in some specimens, the addition of MICP did not result in the expected reduction in hydraulic conductivity. This was likely due to factors such as insufficient microbial activity, a short precipitation period, or the non-uniform distribution of the additives. Overall, SA + MICP combinations proved to be highly effective in reducing hydraulic conductivity. When comparing the untreated and treated soils, it is clearly observed that the treatment process causes a significant reduction in the hydraulic conductivity of the sand. While the untreated soil has a hydraulic conductivity of approximately 10\({}^{-1}\) m/s, this value decreases to around 10\({}^{-5}\) m/s in the treated soils. Through different mechanisms, both injection and mixing procedures provide effective results in research aimed at reducing soil hydraulic conductivity. The injection method, particularly when combined with SA and MICP, enables the controlled delivery of additives into the soil, thereby providing more consistent and effective low hydraulic conductivity outcomes. The synergistic effect of the gel-like structure of SA and the calcite precipitation induced by MICP effectively fills soil voids, narrowing water flow paths. For instance, it has been reported that in situ-formed Ca-alginate gel reduced the hydraulic conductivity coefficient of sandy soil from 5.0 \(\times\) 10\({}^{-4}\) m/s to 2.2 \(\times\) 10\({}^{-9}\) m/s, demonstrating the potential of this method to significantly enhance low hydraulic conductivity [28]. On the other hand, the mixing method has the potential to improve low hydraulic conductivity by ensuring a more homogeneous distribution of additives within the soil. However, the success of this method strongly depends on the uniform dispersion of the additives and their application at optimal ratios. While high pellet contents can enhance soil stability, the non-uniform distribution of the additives may result in irregularities in hydraulic conductivity reduction. Moreover, the biological processes involved in MICP may not always proceed with the desired efficiency, leading in some cases to lower hydraulic conductivity values in mixtures containing only SA. It has been emphasized that high SA concentrations can increase viscosity, hinder the homogeneous distribution of additives, and adversely affect low hydraulic conductivity [37]. Although the combination of SA and MICP generally reduces hydraulic conductivity, the effectiveness of this combination is directly influenced by the distribution of the additives within the soil and the duration of precipitation [38]. In conclusion, both methods possess significant potential for enhancing soil low hydraulic conductivity. However, the sustainability of their effectiveness depends on the appropriate dosage of additives, application under method-specific conditions, soil properties, and environmental factors. In particular, for SA + MICP combinations, the careful control of microbial activity levels, precipitation time, and ion balance is critical to ensuring long-term low hydraulic conductivity performance.