Biology Reference
In-Depth Information
5.1 Introduction
In protecting groundwater sources (springs, wells, boreholes, etc.), used for consumption
purposes, from contamination by pathogenic microorganisms, distances are set between potential
sources of contamination and wells or springs (e.g. Taylor et al., 2004). Mostly, the objective is
to obtain target concentrations or a specified degree of removal of the pathogenic
microorganisms or indicator organisms over a given distance (Tufenkji et al., 2003).
Traditionally, the classical colloid filtration theory (Yao et al., 1971, Tufenkji and Elimelech,
2004a) is applied to predict particle transport distances. The theory predicts a first-order
reduction in both fluid-phase and retained colloid concentration with transport distance. First
order deposition of colloids with distance is based on the assumption that colloid affinities for
surfaces of collectors are invariable, resulting in a constant colloid sticking efficiency. However,
often, the required reductions in bacterial concentrations at estimated target distances are not
achieved. The failure of the filtration theory to accurately predict bacteria transport distances has
been ascribed to variations in bacteria sticking efficiencies (Albinger et al., 1994, Tong and
Johnson, 2007, Foppen et al., 2007a, b, Lutterodt et al., 2009a, b, Dong et al., 2006, Li et al.,
2004, Redman et al., 2001, 2001a Albinger et al., 1994). Recent research has indicated that the
cell affinity for collector surfaces varies within (Lutterodt et al., 2009a, b,
chapters, 2
&
3
) and
among (Lutterodt et al., 2009a,b,
chapters 2
and
3
,Bolster et al., 2009, Foppen et al., 2010)
bacteria strains. Recently, Foppen et al. (2010) reported 2 log unit variation in the maximum
relative peak breakthrough concentrations of 54
Escherichia coli
(
E. coli
) strains over 7 cm
transport distance under similar physical and aqueous chemical conditions, confirming the inter-
strain differences observed by many researchers. The intra-strain and inter-strain heterogeneities
have necessitated the need for cautious extrapolation of experimental results obtained from a
single strain to predict transport distances (Bolster et al., 2009, Lutterodt et al., 2009a) and the
inappropriateness of using a single
E. coli
test strain as a fecal indicator for removal of other
enteric pathogens (Yang et al., 2008).
An important question is how low the sticking efficiency of fractions of cells within a population
can be?. Note that lower values may enable longer transport distances. Most experiments that
revealed sticking efficiency distributions of biocolloid populations have been conducted over
relatively short distances of 0.05 to 1 m (e.g. Foppen et al., 2007a,b, Brown and Abramson,
2006, Walker et al., 2004, Tufenkji and Elimelech, 2005b, Tufenkji, et al. 2003, Bolster et al.
2000
,
Martin et al., 1996). As a result, high sticking efficiency values in the order 10
-2
to 1 were
estimated, which indicate complete removal within very short distances. A review by Foppen
and Schijven (2006) showed that the sticking efficiencies of
E. coli
determined from field
experiments were lower (0.002-0.2) than those determined under laboratory conditions (0.02-
0.9), possibly due to the presence of preferential flow paths and dissolved organic matter known
to enhance particle transport in aquifers. Thus the application of laboratory measured values to
predict real-world transport distances likely results in the underestimation of actual bacteria
transport distances in the subsurface with consequences of polluting drinking water sources
(springs, boreholes, and wells). Until now, the longest reported transport distances of any form of
laboratory controlled colloid transport experiment has been conducted over 8 m (Close et al.,
2006) for microspheres and 4.83 m (Lutterodt et al., 2009a,b) for
E. coli
with gravels and quartz
sand as porous media, respectively.
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