3.3. Impermeability test
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).