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| Escherichia
coli contamination |
Background
E. coli is a bacterium commonly found
in the digestive tract of animals and humans. According to Health
Canada and the U.S.
Environmental Protection Agency, the presence of E. coli in
water is a strong
indication of recent sewage or animal waste contamination. Sampling of
the
Portneuf River in September 2002 revealed elevated levels of E. coli
at
locations within the Pocatello city limits. Samples collected at Rainey
Park,
just above the concrete channel, and at Carson Street, immediately
below the
concrete channel, show a dramatic increase in E. coli density
between those locations (Figure 1). Four sites immediately below the
concrete
channel (Carson Street, North City Park, Simplot, Batiste Road) had
individual
samples that exceeded 500 colony forming units per 100 ml water. |
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The Idaho standard for primary contact (e.g., swimming) is no more than
406 E. coli organisms per 100 ml, and the standard for
secondary contact
(e.g., wading, fishing) is no more than 576 organisms per 100 ml.
Average
values for E. coli concentrations exceeded the standard for
primary
contact at two sites (Carson Street, North City Park), and the average
value at North City Park exceeded the standard for secondary contact.
At
Carson Street 6 of 8 individual samples exceeded the primary standard,
and two of those samples exceeded the secondary standard. At North City
Park 5 of 7 individual samples exceeded the primary standard, and two
exceeded
the secondary standard. At Simplot 3 of 8 individual samples exceeded
the
primary standard, and one exceeded the secondary standard. At Batiste 1
of 8 samples exceeded the primary standard. |
Sampling
We collected water samples at 12 locations from the Greenway footbridge
at the upstream end of the concrete channel, to the Main Street
Extension
bridge. Water samples were taken from the downstream side of
bridges.
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| Table
1. Sample Locations and
distances
from Rainey Park site. The concrete channel begins shortly below Rainey
Park and ends shortly after Carson Street. Based on some of the
initial
sampling we discontinued sampling at some of the sites that were
identified
in the initial proposal. |
| Site |
Distance (km)
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Site |
Distance (km)
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| Greenway bridge |
0.00
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W Fremont |
0.99
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| W Benton |
0.19 |
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W Wyeth |
1.10
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| W Whitman |
0.30
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W Custer |
1.91
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| W Lewis |
0.53
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W Carson |
2.17
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| W Center |
0.65
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|
Carson Gauge |
2.45
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| W Clark |
0.77
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N Main Extension |
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Results
None of the samples collected from January through June 2003 had
concentrations
of E. coli that were above the secondary contact limit
established
by the Idaho DEQ. There was a small increase in the concentration of E.
coli in samples taken closer to the end of the concrete channel
(Figure
2). Samples collected in July 2003 showed significantly elevated
concentrations of E. coli (Figure 2). Samples at all sites had
higher
concentrations of E. coli than samples collected earlier in the
year, and samples from Fremont to the downstream end of the channel all
averaged above both the primary and secondary contact standards.
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Where
is this E. coli coming from?
One
possible source of the E. coli
that we recorded is pigeons that roost under the bridges spanning the
concrete
channel and in the mouths of the many drain outlets that enter the
walls
of the channel. |
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Why is the
concentration of E. coli so much higher in July?
It may be that the concentration of E.
coli is higher in July because the amount of water flowing in the
channel
is so low (see discharge). Given a
constant
input of fecal material into the channel, the concentration of E.
coli
will be inversely proportional to the volume of water flowing in the
channel. |
What can be
done to reduce the concentration of E. coli in the Portneuf?
There are two
possible approaches to this
problem, reducing inputs of fecal material into the river and
increasing
the volume of water in the river. Fecal inputs into the river could be
reduced by removing pigeon roosting sites over the channel. This might
be accomplished by covering the ends of drains and by installing
netting
of some kind to prevent pigeons from roosting under the bridges. The
volume
of water in the river could be increased by reducing the amount of
water
that’s withdrawn from the river upstream (i.e., for irrigation), and by
reducing the amount of groundwater that’s pumped out of the Portneuf
aquifer
for industrial and domestic consumption.
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In response to a
request from
a resident of Oakwood Manor we also collected samples from the North
Main
Extension bridge. We took these samples to see if there was any
evidence
of elevated E. coli concentrations that might reflect leaking
sewer
lines within Oakwood Manor. These samples provided no evidence of
additional
inputs of E. coli
or other fecal coliform bacteria between
Carson
Street and the North Main Extension bridge. |
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