Transport of Escherichia coli strains isolated from spring water - Transport of Multiple Escherichia Coli Transport in Saturated Porous Media - page 87

6.5.2 Distributions in sticking efficiencies and minimum sticking efficiencies
The good power law type of relation between F and Α for all strains (with the exception of
FR05) is also consistent with results of others (Lutterodt et al., 2009b, 2011; Brown and
Abramson, 2006; Tufenkji et al., 2003; Redman et al., 2001a, 2001b). The power-law
distribution was used to calculate the so-called minimum sticking efficiency, mi Α , of the
strains, which is defined as the sticking efficiency belonging to a bacteria fraction of 0.001%
of initial bacteria mass flowing into a column, after removal of 99.999% (5 log reduction) of
the original bacteria mass has taken place. The minimum sticking efficiency practically
represents the sticking efficiency of a minor fraction of bacteria cells. However, within this
minor fraction of bacteria cells, the sticking efficiencies are again not constant, but they are
distributed, and therefore, within this sub-fraction, the minimum sticking efficiency is the
highest possible sticking efficiency (Lutterodt et al., 2011). The low values of mi Α in the
order 10 -4 and 10 -5 obtained from this study were compared with those obtained from
Lutterodt et al. (2009b, 2011; Table 6.2 ) . The UCFL strains were obtained from the soil of a
dairy farm, where cows were grazing, and as such have not been subject to transport in
aquifers. At the beginning of this study, we hypothesized that if the segment sticking
efficiency reduced with transport distance, then the minimum sticking efficiency of the E.
coli strains harvested from springs was also expected to be low to very low. Looking at Table
6.2 , we conclude that, because the mi Α of FR05 was very high (around 0.1), and because the
mi Α -values of UCFL-94 and UCFL-131 were much lower (in the order of 10 -7 ) than the
values determined in this study; we had to reject our hypothesis. However, for most of the
strains in our laboratory column set-ups, the segment sticking efficiency indeed reduced with
transport distance. Possibly, the aquifer is heterogeneous, there are preferential flowpaths, the
spring protection structure is not optimally designed and functioning or a combination of
these factors was true, which all confounded the relationship between transport and minimum
sticking efficiency of strains harvested from the environment.
If we eliminate our hypothesis formulated at the introductory section of this study, and if we
consider strain FR05 to be an outlier, then we are left with a well defined and narrow range of
minimum sticking efficiencies. We would like to argue that the importance of our work is in
this set of values: for worse case scenarios in predicting pathogen transport in aquifers,
sticking efficiency values of 10 -4 to 10 -5 should not be considered unrealistically low. Instead,
we demonstrated that these values are likely characteristic for most of the E. coli strains that
have undergone transport through an aquifer.
6.7 Conclusions
The segment sticking efficiency, Α, of the six E. coli strains harvested from various springs
in Kampala,Uganda, was not a constant, but reduced with increasing transport distance.
Power-law distribution functions were capable of adequately describing the relation between
F and Α, and minimum sticking efficiencies ( mi Α ), calculated from these power-law
distribution functions usually ranged from 10 -5 to 10 -4 . For worse case scenarios in predicting