In this study, the bacterial mixtures containing all strains showed a reduced CV migration length compared with that of single cultures (Fig. 3). The advantages of the impermeability test are its simplicity of operation, lower cost compared with other processes, and the absence of calcite formation (Kang et al., 2014). The reduction in the retention time can be attributed to the calcium carbonate crystals that were deposited between sand particles, which resulted in plugging. Improvement in the strength of sand columns due to bacterially induced calcium carbonate has also been previously reported (De Jong et al., 2010; Whiffin et al., 2007). Recent research initiatives have shown that calcite crystals form cohesive “bridges” between existing sand grains, increasing the stiffness of the sand while causing only a limited decrease in permeability (De Jong et al., 2010; van Paassen et al., 2010).