Abstract:
The purpose of this study was to evaluate the impact sediment has on the survival of
bacteria in a typical rangeland stream. This information is an important component in
determining a time integrated prediction of bacterial numbers in the stream sediment and
overlying water. Bacterial survival in stream is a crucial factor in the complex
relationship between stream quality and range management. Once this relationship has
been defined, it will lead to a scientifically-based, rational process for rangeland
management decisions.
Bear Creek flows through a semi-arid rangeland in Central Oregon. Two sediment
samples (clay loam and sandy loam) were collected from Bear Creek. Sediment samples
were combined by weight with fresh bovine feces at 250:1, 20:1, and 6.67:1 ratios
(sediment:feces). The inoculated sediment samples and a contaminated water sample
were stored a 8°C and monitored to determine the change in concentrations of fecal
coliform (FC) and fecal streptococci (FS) with time.
Stream sediments were found to increase the survival of fecal coliform and fecal
streptococci in an aquatic environment. FC was found to exhibit a significantly lower
die-off rate than FS in sediments inoculated with bovine feces. No significant difference
was observed in the die-off rate of FS between sediment types or inoculation levels with
a half-life ranging from 9 to 12 days. Die-off rates for FC were found (half-life ranging
from 13 days to 31.5 days) to be significantly different between inoculation levels and
sediment types. No relationships were observed between FC and FS die-off rates to
particle size or inoculation level.
This study concluded that stream sediment allows enteric bacteria to survive,
possibly for several months, in an aquatic environment. Resuspension of these bacteria
may account for the erratic FC and FS levels often encountered in water monitoring
programs since grab samples of water give only an immediate measure of bacterial levels.
If enteric pathogens behave similarly, significant public health hazards could arise.
Existing state bacteriological standards and monitoring procedures fail to address these
problems. Therefore, a more meaningful and accurate indication of water-quality
conditions would be obtained by also monitoring indicator bacteria levels in surface
sediments.
