Agriculture Reference
In-Depth Information
used for drinking is required to be fi ltered and disinfected. Filtration ensures the
removal of protozoan parasites ( Giardia and Cryptosporidium ), which are very resis-
tant to chlorine. Unless special conditions are met (e.g., absence of coliform bacteria
in the water and no potential sources nearby), all groundwater used as drinking water
must be disinfected. The minimum treatment in both cases must be capable of reduc-
ing the protozoa by at least 99.9% and the enteric viruses by 99.99% (Regli and others
1991). Because enteric bacterial pathogens are more sensitive to disinfectants it is felt
that these treatment requirements would be suffi cient to eliminate waterborne bacteria
below levels of concern.
In practice, water used for pesticide and herbicide application is collected from
irrigation channels, ponds, or other sources, which are not treated. Because application
often takes place by direct spraying, the contamination of produce occurs. Water for
washing may also come from untreated sources, but chlorine is often added to the
water to reduce possible cross-contamination and numbers of organisms on the surface
of the produce. Because pathogens attached to the surface of produce are signifi cantly
more resistant to inactivation, much greater concentrations of chlorine are used than
to treat drinking water (200 mg/l vs. 1 - 3 mg/l).
Occurrence of Foodborne Pathogens in Water
Groundwater
Under normal conditions, coliform bacteria are absent from drinking-water supplies.
In most countries, monitoring of treated water supplies for coliform bacteria ensures
the absence of E. coli , which is a member of the coliform group. However, small
systems and private wells are monitored less frequently or not at all, depending on
local regulations. In general, enteric pathogens are far less common in groundwater
because of the natural fi ltering mechanism of soil. However, every groundwater source
is potentially susceptible to contamination. The construction of wells (or location of
a spring), nature of the substrata, depth to groundwater, and rainfall can affect the
microbiological quality of the well water. Pathogens enter groundwater from latrines,
septic tanks leach fi elds, land application of wastewater for irrigation, oxidation ponds,
leaking sewer lines, and unlined landfi lls. Unprotected wells may allow for surface
water to run into the well during storm events. In well-structured soils, protozoa and
bacteria are easily fi ltered out during transport through the soil. However, in fractured
limestone and clay soils and gravel/sandy soils, long-distance transport is possible.
Viruses are more likely to contaminate groundwater and travel long distances (hun-
dreds of meters) because of their small size (Keswick and Gerba 1980).
A survey of 144 private water supplies in the Netherlands found that 11% were
positive for fecal indicators and 2.7% for E. coli O157:H7. All were located in agri-
cultural areas with grazing animals (Schets and others 2005). In India 79% of the rural
water (both surface and groundwater) drinking water supplies contained pathogenic
serotypes of E. coli including E. coli O157:H7 (Ramteke and Tewari 2007). Although
waterborne outbreaks of E. coli O157:H7 are primarily associated with surface waters,
large outbreaks have been associated with contaminated groundwater (Hunter 2003).
Rainfall and irrigation provide a mechanism for leaching into groundwater. E. coli
O157:H7 has been shown to travel the subsurface for 2 months after initial application
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