Biology Reference
In-Depth Information
6.1 Introduction
Globally, groundwater systems provide 25-40% of the world's drinking water (Morris et al.,
2003). The importance of the resource is often attributed to the assumption that it is free of
pathogenic microorganisms (e.g. Bhattacharjee et al., 2002). However, many water borne
disease outbreaks are caused by the consumption of groundwater contaminated by pathogenic
microorganisms (Close at al., 2006; Bhattacharjee et al., 2002; Macler and Merkle, 2000;
Powell et al., 2003).
Traditionally, strategies employed to protect groundwater sources from contamination rely
upon effective natural attenuation of sewage-derived microorganisms by soils (and rocks)
over set back distances (Taylor et al., 2004). The prediction of transport distances of
microorganisms in aquifers has usually been determined with the classical colloid filtration
theory (CFT; Yao et al., 1971; Tufenkji and Elimelech, 2004a, b). The theory is based on the
assumption that colloid retention follows an invariable rate deposition on collector surfaces,
while fluid phase colloid concentrations reduce log-linearly with increasing distance of
transport. However, recent research results indicate that the sticking efficiency of a biocolloid
population varies due to variable surface properties of individual members of the population,
resulting in differences in affinity for collector surfaces (Albinger et al., 1994; Baygents et
al., 1998; Simoni et al., 1998; Li et al., 2004; Tufenkji and Elimelech, 2005a; Tong and
Johnson, 2007; Foppen et al., 2007, Lutterodt et al., 2009a). Like other workers (Redman et
al., 2001a, b; Tufenkji et al. 2003), we demonstrated in our previous works (Lutterodt et al.,
2009b, 2011) that a power-law best describes the distribution of relative bacteria mass
fraction retained in the saturated porous medium and their corresponding so called segment
sticking efficiencies (Lutterodt et al., 2009b, 2011) when transported through columns of
saturated quartz sand. Others found a log-normal distribution (Tufenkji et al., 2003; Tong and
Johnson, 2007) or a dual distribution (Tufenkji and Elimelech, 2004b, 2005a,b; Foppen et al.,
2007). Based on these power-law distribution functions mentioned above, the segment
sticking efficiency of 0.001% of the initial bacteria mass applied to a column was defined as
the minimum sticking efficiency (Lutterodt et al., 2009b, 2011). This parameter quantifies the
rate of interaction of a lower end non-attaching fraction of a bacteria population with clean,
saturated, quartz sand. In any bacteria population, the fraction of cells that possesses the
minimum sticking efficiency is the fraction that consists of cells with surface characteristics
that promote transport or reduce the efficiency of attachment (Lutterodt et al., 2009b, 2011).
Escherichia coli (E. coli) , a gram-negative, facultative non-spore forming, rod shaped
bacterium is commonly used as indicator of fecal contamination of drinking water supplies,
because E. coli is a consistent, predominantly facultative inhabitant of the human
gastrointestinal tract. In addition, E. coli is easy to detect and quantify. Furthermore, the net
negative surface charge and low inactivation rates of E. coli ensure that they may travel long
distances in the subsurface and these characteristics make E. coli a useful indicator for fecal
contamination of groundwater (Foppen and Schijven, 2006). Due to the importance of E. coli ,
considerable attention has been given to understanding their transport and fate in saturated
porous media (e.g. Foppen et al, 2006, Schinner et al., 2010, Bolster et al., 2010). In most of
these studies, bacteria strains isolated from different sources, for example from zoo animals
(Foppen et al., 2010), a swine lagoon (Bolster et al., 2010), a dairy cow manure and sewerage
(Haznedaroglu et al., 2008), or a soil of a pasture used for cattle grazing (Foppen et al., 2010,
Lutterodt et al., 2009a, b, Yang et al., 2008) were used. To our knowledge, there is no study
reported in the literature that focuses on the transport of E. coli strains isolated after they
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