Environmental Engineering Reference
In-Depth Information
inflows. A higher survival has been reported for coliphages compared to faecal
coliforms and E. coli , both in water [17] and soil [18] , and coliphages survived the
best of the organism groups in the sediment [6] . In contrast, pathogens as Giardia
survived best in the water column and least in the sediment, but Giardia was
together with Cryptosporidium observed in concentrations one to three orders of
magnitude higher in sediment compared to the water phase [6] . The increase
observed for the somatic and F-specific RNA coliphages in the present study may
result from the high survival characteristics combined with the resuspension of
settled microorganisms, or by a point source in the pond vicinity, for example birds
swimming in the pond or other animals residing at the waterside.
Results from the current study suggest that stormwater ponds may represent an
ineffective barrier for protecting raw water sources from microbial contaminants.
After entering the pond, microbial levels are reduced over time, governed by the
survival properties of each pathogen or indicator organism. Microbial survival rates
in stormwater sediments has been reported in another study [7] , and T 50 -values
(time for a 50% reduction) for E. coli , faecal enterococci, Clostridium and
coliphages of 27, 27, 252 and 370 days, respectively, were reported. In addition,
F-specific RNA coliphages has been reported to represent a model for norovirus
survival [17] . Bird drops and domestic animals may potentially represent additional
sources for faecal contamination in the pond. F-specific RNA coliphages may origi-
nate from water fowls such as gulls and goose [19] , and at the Järnbrott pond these
birds together with wild ducks have been observed, roughly between 20-50 speci-
mens simultaneously. If we assume the increased F-specific RNA coliphage con-
centration in the pond originates from bird droppings we can calculate an estimate
of the number of specimens needed to yield that concentration increase. In a study
made on gull faeces [19] the observed F-specific RNA coliphage concentrations
were in average 7.5·× 10 4 PFU/g of faeces. Since the concentration increase were
about 300 CFU/100 mL in the Järnbrott pond, with a dry weather volume of 6,000 m 3 ,
the total counts increased with 18·× 10 9 . Using the faecal concentration reported on
the gull faeces [16] , and assuming an average faeces production of 30 g/day per
gull and 10 days dry period (in average), this would imply a total production of
2.25·× 10 5 coliphages per gull (during 10 days). Using this value with the coliphage
increase in the pond, the number of gulls needed to verify this is calculated to
80,000 (18·× 10 9 /2.25·× 10 5 ) gulls in the pond all day long. This is of course impos-
sible and indicates that the concentration increase in the pond must have another
source than droppings of waterfowls.
A high release of coliphages into raw water sources from stormwater ponds
therefore represents a human health risk in relation to virus that needs to be further
assessed. The composite sampling technique applied in the present investigation
gives a representative description for parameters which concentration in water and
sediment is well described by a normal distribution. Environmental parameters may
however rather be log-normal or poisson-distributed, which may typically be the
case for several pathogens [20] . The composite sampling technique may bias the
analyzed mean concentration for the sample, representing the entire event, towards
the peak concentration in one subsample. As the microbial health risk due to
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