Introduction - Transport of Multiple Escherichia Coli Transport in Saturated Porous Media - page 2

1.1. Groundwater contamination by faecal matter

According to the WHO, an estimated one billion people lack access to an improved water supply
and two million deaths per year are attributable to unsafe drinking water, sanitation and hygiene.
In addition, many countries still report cholera to the WHO (WHO, 2004). Groundwater may be
an important source of water for safe drinking and industrial water supplies, however, many
water borne disease outbreaks are known to have been caused by the consumption of
groundwater contaminated by pathogenic microorganisms (Goss et al., 1998; Macler and Merkle,
2000; Bhattacharjee et al., 2002; Close et al., 2006; Powell et al., 2003). Pathogenic
microorganisms find their way into the sub-surface through the spreading of sewerage sludge on
fields, leakage from waste disposal sites and landfills (Taylor et al., 2004), infiltration from
cesspits, septic tank infiltration beds, and pit latrines (Foppen and Schijven, 2006), the
application of human and animal excreta to land as crop manure (Gagliardi and Karns, 2002,
Bolster et al., 2009, 2010) and pasturing of livestock, animal feeding operations (Gerba and
Smith, 2005), thereby posing a threat to public health.
One of the explicit goals set by the United Nations and the international water community is
environmental sustainability, with a target to halve the proportion of people without sustainable
access to safe drinking water and basic sanitation by the year 2015. To achieve this Millenium
Development Goal (MDG) of environmental sustainability, effective management and protection
of water supply sources need to be practiced. One effective way of protecting groundwater
sources from contamination by pathogenic microorganisms leaked into aquifer systems is by
delineating well head protection areas around a drinking source. This strategy relies upon
effective natural attenuation of sewage-derived microorganisms by soils (and rocks) over set
back distances (Taylor et al., 2004). While natural processes may assist in reducing pollution,
most biological contaminants can travel through soils and aquifers until they either enter
someone's water well or are discharged into streams (Corapcioglu and Haridas, 1985). Although
efforts have been made to understand the transport behaviour of bio-colloids in saturated porous
media, still, much understanding is needed to improve prediction of interaction between bacterial
cells and aquifer media.

1.2. Colloid filtration theory
To understand and predict microbial transport in the subsurface, studies are often performed in
the laboratory and results obtained are applied to natural conditions. The retention and transport
of microorganisms when passed through sand has commonly been determined with the classical
colloid filtration theory (CFT; Yao et al., 1971; Tufenkji and Elimelech, 2004a). The theory is
based on the assumption that colloids are retained at an invariable rate resulting in a log-linear
reduction in deposition rate with increasing transport distance. The one dimensional
(macroscopic) mass balance equation for mobile bacteria suspended in the aqueous phase
excluding bacteria growth and decay is normally expressed as (Corapcioglu and Haridas, 1985.;
Foppen et al., 2007a,b)

∂ C ∂

2

C ∂ C Ρ

Θ

∂ S

= D − v −

bulk

(1.1)

2

∂ t ∂ x ∂ x ∂ t