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5.5 Discussion
Low values of total sticking efficiencies in the order 10 -4 and 10 -3 were measured for UCFL-131
and UCFL-94, respectively, while values of segment sticking efficiencies were as low as 10 -6 to
10 -5 . Both Α L , the total sticking efficiencies and Α, the sticking efficiencies determined for
column segments, decreased with increasing transport distance. Minimum sticking efficiency
values were extrapolated to 10 -8 and 10 -7 for UCFL-131 and UCFL-94, respectively. Power-law
distributions could be used to describe the distribution of sticking efficiencies of bacteria strains
at large travel-distances (> 6m), whereby substantial fractions (>10%) of bacteria mass were
transported over distances in excess of 25 m.
5.5.1 Tracer and bacteria breakthrough
Though tracer breakthrough curves looked symmetrical, a test for skewness of the curves
revealed low positive and low negative at the first two sampling ports and the two most distant
sampling ports, respectively. The observed low positive skewness indicated possible tailing of
the breakthrough and could be attributed to the possible settling of sand particles, which might
have caused differential porosities across the vertical face of the column and promoted the
possible occurrence of transverse dispersion, in spite of the uniform porosities measured at the
column segments. The measured low negative skewness is mainly due to the incomplete nature
of tracer breakthrough curves; this assertion is supported by the less than 100 % mass recoveries
computed for all sampling distances and even for extrapolated breakthrough, which confirms the
possibility of tailing. In spite of these limitations, there was negligible effect of shear flow in the
set up as indicated by the high performance of the HYDRUS-1D and can be attributed to the
large diameter of the helix formed, which reduced the curvature of the entire set up and
minimized the effect of shear flow. It is known that higher curvature helix columns induce
variable flow across the cross section thus increasing the axial velocity in one half compared to
the other (Nield and Kuznetsov, 2004, Benekos et al., 2006). In addition to the possible
contribution of differential porosities to high bacteria breakthrough, the long duration of the
experiments might have caused starvation of the cells with possible changes in important surface
properties causing low adhesiveness of the cells to the quartz grains (Haznedaroglu et al., 2008).
Although there are limitations associated with the plating method of quantifying bacteria, such as
the development of clumps and chain of cells into a single colony, and also the growth of only
colonies for which cultural conditions are suitable for, results of inactivation tests indicated
insignificant reduction in influent concentration over the duration of the experiments for both
strains. We were therefore convinced that breakthrough concentrations measured indirectly via
the spectrophotometer were indeed for culturable cells.
5.5.2 Low sticking efficiencies, inter-strain and intra-strain heterogeneities
Contrary to the classical colloid filtration theory, the common finding of variable E. coli
attachment efficiency during transport in saturated porous media (Schinner et al., 2010, Bolster
et al., 2009, Bolster et al., 2006) was also observed in our experiments. Sticking efficiencies
reduced with increasing transport distances for the two bacterial strains we used, as was also
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