Transport of Escherichia coli in 25 m columns - Transport of Multiple Escherichia Coli Transport in Saturated Porous Media - page 63

diameter (2.33 m) and a negligible pitch is to reduce the curvature of the helix in order to reduce
the effect of shear flow (Nield and Kuznetsov, 2004) associated with high curvature helical
columns. In helical columns, the ratio of the radius to the sum of the square of the radius and the
square of the pitch defines the curvature (Nield and Kuznetsov, 2004).
Sampling ports were placed on one side of the column, at distances of 6, 12.15, 19 and 25.65 m
from the influent end of the column and each extended from the outer circumference to the axis
of symmetry (1.6 cm). This was to ensure that samples were representative of breakthrough at
the respective distances. The column was connected at the influent end with a Master flex pump
(Console Drive Barnant Company Barrington, Illinois, USA) via a teflon tube. The pump
discharge was adjusted to a fluid approach velocity of 1.14x10 -4 m/s to coincide with a flushing
regime of 1 pore volume (PV) in 24 hours.
An important advantage of using a helical column is the ability to conduct bacterial transport
experiments over long distances within a relatively small laboratory space. Since attachment
efficiencies are expected to reduce with increasing transport distances, the arrangement is
applied to measure very low values which are very important in determining transport distances
relevant for 'real case' situations in the environment. The disadvantage of a helical column for
bacteria transport experiments is the variable flow velocity across the cross-section of the
column with faster flow in the inner part, which results in a dispersion and stretched
breakthrough of an injected solute (Benekos et al., 2006, Cirpka and Kitanidis, 2001). In order to
assess the longitudinal dispersivity and the possible occurrence of shear flow within the system,
tracer experiments were conducted. Thereto, the column was continuously flushed with de-
mineralized (DI) water until the EC was equal to the EC of DI water. Then, the column was
flushed with 0.8 L of 1.5 g/L NaCl solution with an EC of 2.87 mS/cm, while 2 mL samples
were taken at the four sampling distances at 30 minutes intervals. Each sample was diluted 10
times in 18 mL DI water and the EC were measured using an EC meter.