Determining the minimum sticking efficiency of six environmental Escherichia coli isolates - Transport of Multiple Escherichia Coli Transport in Saturated Porous Media - page 57

grains are likely to possess the so called minimum sticking efficiency. Such variability in bio-
colloid surface properties can result in an interaction potential distribution within the bio-colloid
population (Li et al., 2004) leading to differences in cell-collector grain interactions, finally
resulting in a distribution of

Α-values.

Geochemical heterogeneity on collector grain surfaces has been implicated as one of the factors
leading to distributions in Α and deviation of deposition patterns from the CFT (Johnson et al.,
1996; Bolster et al., 2001; Loveland et al., 2003; Foppen and Schijven, 2005), but the 99.1%
pure quartz grains we used in the experiments enabled us to exclude geochemical heterogeneity
as a candidate for the observed differences in Α-values. In addition, the possibility of straining
as a contributing factor was ruled out, since the ratios of bacteria equivalent diameter to the grain
diameter in the experiments were well below 0.007, as observed for the occurrence of straining
(Bradford et al., 2007). The pulse application of bacteria solution also allowed us to eliminate
blocking as the possible source of comparatively higher breakthrough at segments where Α was
negative. It should be noted that all of the above explanations treat bacteria as 'static'
biocolloids, unable to adapt to their environment. This 'unable to adapt'-concept, however, may
not be true: subsurface transport and sticking efficiencies of chemotactic Pseudomonas putida
G7 have, for instance, been found to heavily depend on the substrate availability and location in
column experiments (Velasco-Casal et al., 2008), thereby demonstrating the effect of
aut(ecological) adaptations. To our knowledge, information on relatively fast aut(ecological)
adaptations of E. coli strains during transport in columns is not available in the literature, and the
same is true for the relation between sticking efficiency variations and aut(ecological)
adaptations. This could be an interesting topic for future research.

Results of the curve fitting exercise indicated that power-law and exponential distributions are
very important in describing the probability distributions of the cells affinity for the quartz grains
surfaces for the two solutions. Our results are consistent with observations made by Tufenkji et
al. (2003) and Redman et al. (2001a,b) who observed power-law deposition patterns from the
analysis of experimental results from other researchers and their experiments respectively. As
explained earlier in this section the variation in cell surface properties of the strains results in
differences in sticking efficiency and thus gives rise to the observed power-law probability
distributions. Results obtained indicated that 64-99 % and 80-100 % respectively in DI and
AGW of the cells affinity for quartz grain surfaces could be explained by a power-law
distribution.

4.4.2 Minimum sticking efficiencies

Have we found a set of realistic values for the minimum sticking efficiencies, which are so
important in quantifying health impacts of biocolloids traveling in aquifers? For ionic strengths
comparable to groundwater conditions, including monovalent and divalent ions, and for
biocolloid concentrations below 10 5 cells/mL, which we consider to be the maximum
concentrations of pathogenic or fecal indicator organisms traveling in plumes of wastewater in
aquifers or, more in general, saturated porous media, the minimum sticking efficiency for most
of the strains we used was in the order of 10 -2 or more ( Table 4.3 ) , while removal was complete