Food Protection Trends - May/June 2024 - 240

TABLE 3. Pathogen prevalence in irrigation water, 2013 to 2023 (cont.)
Pathogen
Salmonella
Salmonella
Sampling location
Surface water
(streams, rivers, creeks, ponds)
Source water upstream of district control,
point of diversion where the district takes
control, irrigation delivery points within
the district
Salmonella
Salmonella
(invA gene)
aMMS, micromembrane system.
at each sampling location (standing water furrow, furrow
ditch, drainage ditch, pond, standing water, and flowing
waterbodies) (10). Weller et al. (140) suggested that stream
sediments may " act as an in-channel store of bacteria, " which
are released back into the water column when rain events
cause sediment disturbances. Based on their flume model of
an irrigation canal system, Sassi et al. (100) concluded that
although resuspension of E. coli increased as the velocity of
the overlaying water increased, the bacteria were also resuspended
from sediment at low flow rates and not just when
the sediment was disturbed.
However, not every study has revealed higher bacterial levels
in sediment than in the overlaying water. In a 2-yr study of
Salmonella in 10 irrigation ponds in the southeastern United
States, Salmonella levels and prevalence in all 10 ponds were
significantly higher in the water (0.29 most probable number
[MPN]/liter, 37.4% of samples) than in pond sediment (0.22
MPN/liter, 17.0% of samples) (72). E. coli levels also were
higher in the overlaying water (6.26 MPN/100 ml) than in
the sediment (4.44 MPN/100 ml). However, individually
some ponds had significantly higher levels of Salmonella and
E. coli in sediment than in water (e.g., in one pond, E. coli
was 9.34 MPN/100 ml in water and 32.48 MPN/100 ml in
sediment). Variability in study results underscores the importance
of understanding the spatial and temporal dynamics of
sediment-water interactions of each individual water source
for effectively managing agricultural water.
Various researchers have focused their work on gaining a
greater understanding of bacteria in sediment and, more specifically,
the relationship between bacterial levels in sediment
240 Food Protection Trends May/June
and in the overlaying water column. In a study modeling the
effect of sediment on E. coli levels in water, water samples
exceeding the standard of 126 MPN/100 ml had higher levels
of E. coli in sediment than did water samples with E. coli levels
below the standard (121). In their flume model of an irrigation
canal system, Sassi et al. (100) found no difference in the
resuspension rate between E. coli and the much smaller MS2
coliphage, but they did find differences in E. coli resuspension
related to the sediment's soil types. E. coli resuspension rates
in clay sediments were significantly higher than those in sand.
The authors suggested that this finding was likely due to the
dynamics of sediment resuspension, transport, and resettlement
rates rather than the properties (e.g., type and size)
of the microorganisms themselves. Other researchers have
corroborated the impact of sediment properties on bacterial
resuspension in agricultural water. Garzio-Hadzick et al. (38)
reported that in sediment with identical granulometric (sand,
clay, and silt) composition, E. coli survived better in sediment
with higher portions of fine particles and organic carbon.
Perkins et al. (96) also found significant positive correlations
between the abundance of pathogens and fecal indicators and
the sediments that contained higher proportions of silt and/
or clay and associated organic matter.
Although a microorganism's size may not have much impact
on its resuspension into the water column, other properties
of microorganisms affect their resuspension in water.
In their study of Squaw Creek in Ames, IA, Liang et al. (67)
found differences between 44 E. coli strains isolated from
sediment and 33 strains isolated from the overlaying water.
The E. coli strains isolated from stream sediment had signifiSource
water upstream of district control,
point of diversion where the district takes
control, irrigation delivery points within
the district
Farm ponds
State, region
VA, eastern
shore and
mainland
WA, irrigation
districts
WA, irrigation
districts
FL, westcentral
Total
no. of
samples
(% positive)
120 (21.7)
330 (16.4)
104 (34.6)
540 (4.8)
Sample
volume,
type
1-liter grab
1-liter
Reference(s)
82
94
10 liters
500-ml grab
94
119
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