Agriculture Reference
In-Depth Information
(Gagliardi and Karns 2000). In a survey of 448 drinking water supply wells of 35
states, human enteric viruses were found in 31% before disinfection (Abbaszadegan
and others 2003). Another study, also in the United States, reported the detection of
Cryptosporidium
,
Giardia
, or both in 11% of 166 sites using groundwater (Moulton-
Hancock and others 2000). Many of the positive sites were from infi ltration galleries
located under or near streams to supply drinking water. Finally, the majority of drink-
ing water disease outbreaks documented every year in the United States are caused
by fecally contaminated wells (Reynolds others 2008). Thus, it should never be
assumed that groundwater is free of pathogens.
Surface Water
Surface water is always more susceptible to contamination than groundwater because
of the direct discharge of sewage and impact of runoff from rainfall events. Any
freshwater surface source is likely to contain enteric pathogens at one time or another.
The greatest levels of pathogens occur in water sources receiving discharges of
untreated sewage and in watersheds with intensive levels of animal production. In the
United States, sewage discharges are usually disinfected and storm water runoff from
agricultural land and septic tanks (referred to as
nonpoint sources
) are the largest
contributors of pathogens to surface water. However, in Europe and many other coun-
tries of the world sewage discharges are not disinfected. For these reasons fi nding
enteric pathogens in surface waters in these countries is not uncommon (Bosch and
others 2006 ).
The greatest loading of pathogens occurs in surface waters after rainfall events,
although concentrations may be elevated during low fl ow when the proportion of
wastewater to natural runoff is highest. During runoff events accumulated feces are
washed into nearby streams or collection systems forcing release of partially or
untreated sewage from wastewater treatment plants. Studies of waterborne disease
outbreaks associated with drinking water in the United States and Canada have been
correlated with above average rainfall events with drinking water- associated disease
outbreaks (Curriero and others 2001; Thomas and others 2006).
Recent data on the occurrence of enteric pathogens in the United States and Europe
are fairly limited, probably because of the diffi culty in isolating pathogens directly
from water. At this time, the prevalence of
E. coli
O157:H7 in surface waters in the
United States is unknown, but sporadic illnesses and outbreak data suggests that
surface waters play a role in
E. coli O157:H7
disease transmission via recreational
water exposures and drinking water sources. Waterborne disease outbreaks caused by
E. coli
O157:H7 are primarily associated with recreational lake waters (Bruce and
others 2003). This is due to the ingestion of untreated water during swimming.
Domestic and wild animals are believed to be the source of contamination in most
recreation waters.
Kurokawa and others (1999) detected 10
2
to 10
5
per ml
E. coli
0157:H7 in river
water heavily polluted by industrial and agricultural wastes using polymerase chain
reaction (PCR). However, this method may detect both viable and dead bacteria. In
surface water supplies in southern Alberta,
E. coli
O157:H7 was isolated in 1.7% of
1,608 samples over a 2-year period (Gannon and others 2004). Most isolations of the
organism occurred during the summer.
