The Role of Water and Water Testing in Produce Safety - Microbial Safety of Fresh Produce - page 134

example, frequency of sampling may be determined by the size (number of customers)
of the utility providing the water. Sampling must proceed in some random fashion so
that the entire system is characterized. Because of the wide variability in numbers of
indicators in water, some positive samples or tolerance levels or averages may be
allowed. Usually, geometric averages are used in a standard setting because of the
often skewed distribution of bacterial numbers. This prevents one or two high values
from giving overestimates of high levels of contamination, which would appear to be
the case with arithmetic averages.
Irrigation Waters
Irrigation using surface water and groundwater mirrors similar water quality and
pathogen concentrations as discussed in the previous two sections (see above). There
are a number of sources that can contribute to fecal contamination of irrigation systems
even if there are no sources of sewage discharge:
￿ Recreational use of source waters. Bathers can contribute signifi cant levels of patho-
gens to reservoirs or that water may be used to store or collect water for use in
irrigation systems (Gerba 2000 ).
￿ Return fl ows. During fl ood irrigation non-infi ltrated water may be discharged back
into the irrigation canal system or be used to irrigate other fi elds. If these fi elds have
manure applied they may contribute pathogens to the irrigation water.
￿ Wildlife. Migrating birds such as ducks or geese may rest on the canal waters. Deer,
rabbits, and other animals may also have access to the channeled water.
￿ Storm water. In some arid areas irrigation channels may be used as storm drains.
Rainfall may also allow collected animal feces along the banks of the canal to wash
into the channel. In some areas of the world animals are allowed to graze along the
canal banks. In some urban areas they are used as linear parks where pet feces may
collect to be later washed into the canal during a storm event.
Studies are limited on the occurrence of pathogens in irrigation water impacted by
nondirect or purposeful reuse of sewage in both developed and developing countries.
In a study of irrigation waters in several Central American countries and the United
States, the protozoan parasites Giardia , Cryptosporidium , and microsporidia were
detected (Thurston-Enriquez and others 2002). Giardia concentrations were similar
in almost all countries (60% of the samples), whereas Cryptosporidium concentrations
were much greater in Central America. In a study in western Mexico, 48% of the
surface irrigation waters were positive for Cryptosporidium oocysts and 50% for
Giardia cysts (Chaidez and others 2005). Both of these parasites have also been
detected in irrigation water used for bean sprout irrigation in Norway (Robertson and
Gjerde 2001 ). Salmonella has been reported in 2-14% of the irrigation waters used
for produce production in Nigeria (Okafo and others 2003) and 23.5% of irrigation
water used for cantaloupe production in Brazil (Espinoza-Medina and others 2006).
Both E. coli O157:H7 and Salmonella have been reported in irrigation waters in
western Canada (Gannon and others 2004). Izumi and colleagues (2008) reported the
detection of both Salmonella and E. coli O157:H7 in irrigation water used to irrigate