Fecal Indicator Bacteria Entrainment from Streambed to Water Column: Transport by Unsteady Flow over a Sand Bed

Area of Science:

• Environmental Science
• Microbiology
• Hydrology

Background:

• Storms elevate fecal indicator bacteria (FIB) in streams via runoff and sediment release.
• Previous research linked FIB to fine sediments, but sand-bed stream contributions were less understood.

Purpose of the Study:

• Investigate how water velocity affects the release of coliforms from sand-bed sediments.
• Improve methods for quantifying sandy streambeds as FIB sources.

Main Methods:

• Simulated storm hydrographs in a flume.
• Collected water and sediment samples upstream and downstream of sand beds.
• Analyzed coliform and suspended sediment concentrations at varying velocities.

Main Results:

• Highest total coliform and suspended sediment concentrations occurred downstream during rising hydrograph limbs.
• A first flush effect was observed at velocities exceeding ~0.2 m/s.
• Coliforms increased rapidly past this velocity threshold, then declined, suggesting sediment layer depletion.

Conclusions:

• Sand-bed streams can harbor significant FIB populations.
• Sandy streambeds act as a source of FIB during storm events.
• Total Maximum Daily Loads (TMDLs) should consider sand-bed sediments as a FIB source.