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Groundwater quality and contaminant hydrogeology 243

Table 6.11 Analytical results of Chalk core profiling, Granta surface-derived contamination. A full judicial inquiry
catchment, Cambridgeshire. After Clark and Gomme (1992). was set up by the Ontario Provincial Government
into the circumstances surrounding the outbreak
Depth of sample Isoproturon Chlortoluron Linuron
(m below concentration concentration concentration and it also acted to introduce a new drinking water
−1
−1
−1
ground) (mgkg ) (mgkg ) (mgkg ) regulation to protect water supplies (Holme 2003).
Cryptosporidiosis is a significant cause of gastro-
0.0–0.5 0.11 0.53 – enteritis in the United Kingdom with an estimated
0.5–1.0 – – – 42,000 cases in England and Wales in 1995 (Adak
1.0–1.5 <0.03 0.21 –
1.5–2.0 – 3.04 – et al. 2002). Cryptosporidiosis is caused by the pro-
2.5–3.0 – – n.r. tozoan pathogen Cryptosporidium parvum which is
4.5–5.0 – – n.r. widespread in the environment and is found in the
6.5–7.0 – – n.r. intestinal regions of most humans and animals. It
9.5–10.0 – – n.r.
is excreted from infected individuals as an oocyst
which can survive for long periods in the environ-
−1
n.r., not recorded; – below detection limit (0.03 mgL ).
ment and is resistant to disinfection by conventional
water treatment. Outbreaks of cryptosporidiosis have
Livestock farming produces waste containing occurred due to oocyst-contaminated groundwater
many pathogenic micro-organisms associated with supplied by wells, mainly in hydrogeological settings
serious gastrointestinal disease, including bacteria characterized by fractured material. These outbreaks
such as Escherichia coli and Streptococcus, viruses such have occurred in karst limestone aquifers, for ex-
as enterovirus, and protozoa such as Cryptosporidium ample the Edwards Aquifer in Texas (Bergmire-Sweat
and Giardia. The presence of faecal coliform bacteria et al. 1999) and the Chalk aquifer in the north London
indicates that other disease-causing organisms may Basin (Willocks et al. 1999). Cryptosporidium contam-
be present. In Ontario, 17% and 20% of farm wells in ination hazard assessment and risk management for
coarse- and fine-textured sediments were contam- British groundwater sources are discussed by Morris
inated with faecal coliform and E. coli, respectively and Foster (2000).
(Goss et al. 1998). Coliform bacteria were also present
in 17% of domestic water wells in loess and till
deposits in eastern Nebraska (Gosselin et al. 1997). 6.4.4 Saline water intrusion in coastal aquifers
Not all strains of E. coli are harmful but some
strains, such as O157:H7, are serious pathogens. A Intrusion of saltwater into an aquifer occurs where
stark example illustrating that pathogen occurrence seawater displaces or mixes with fresh groundwater.
is not only restricted to developing countries, is The intrusion of saltwater is one of the most com-
the case of Walkerton, Ontario, when in May 2000, mon pollutants of fresh groundwater (Todd 1980;
E. coli O157:H7 and Campylobacter jejuni contamin- Custodio 1987) and often results from human activ-
ated the drinking water supply leading to the death ities which reduce groundwater flow towards the sea.
of seven individuals and illness in over 2000 others. In an aquifer where freshwater is flowing towards the
Escherichia coli bacteria were found to have entered sea, the Ghyben–Herzberg relation predicts, for
the Walkerton drinking water supply through a well freshwater and seawater densities (ρ and ρ ) of 1000
f s
−3
which had been contaminated by cattle manure and 1025 kg m , respectively, that the depth below
spread on a nearby farm. Normally, water can be sea level to the saline water interface, z , is approxim-
s
treated using chlorine which acts to kill E. coli bac- ately 40 times the height of the freshwater table
teria, but in the case of the Walkerton outbreak above sea level, z . This can be shown with reference
f
chlorine levels had not been sufficiently maintained. to Fig. 6.26a and assuming simple hydrostatic condi-
Also, exceptional environmental factors contributed tions in a homogeneous, unconfined coastal aquifer
to the outbreak with heavy rainfall in early May that in which:
assisted transport of the contaminants to Well 5,
located in a shallow fractured aquifer vulnerable to ρ gz = ρ g(z + z ) eq. 6.24
s s f f s