Effects of overlying velocity, particle size, and biofilm growth on stream-subsurface exchange of particles

Stream-subsurface exchange strongly influences the transport of contaminants, fine particles, and other ecologically relevant substances in streams. We used a recirculating laboratory flume (220 cm long and 20 cm wide) to study the effects of particle size, overlying velocity, and biofilm formation on stream-subsurface exchange of particles. Sodium chloride was used as a non-reactive dissolved tracer and 1- and 5-μm fluorescent microspheres were used as particulate tracers. Surface-subsurface exchange was observed with a clean sand bed and a bed colonized by an autotrophic-heterotrophic biofilm under two different overlying velocities, 0·9 and 5cm s^-1. Hydrodynamic interactions between the overlying flow and sand bed resulted in a reduction of solute and particle concentrations in the water column, and a corresponding accumulation of particles in both the sediments and in the biofilm. Increasing overlying velocity and particle size resulted in faster removal from the overlying water due to enhanced mass transfer to the bed. The presence of the biofilm did not affect solute exchange under any flow condition tested. The presence of the biofilm significantly increased the deposition of particles under an overlying velocity of 5 cm s^-1, and produced a small but statistically insignificant increase at 0·9 cm^-1. The particles preferentially deposited within the biofilm matrix relative to the underlying sand. These results demonstrate that hydrodynamic transport conditions, particle size, and biofilm formation play a key role in the transport of suspended particles, such as inorganic sediments, particulate organic matter, and pathogenic microorganisms in freshwater ecosystems, and should be taken into consideration when predicting the fate and transport of particles and contaminants in the environment.