Colloids are often in forms of inorganic and organic particles whose
particle sizes (dp) are ranging from 1.0 nm to
10.0 μm. On the one hand, their transport processes in the hyporheic
zone such as deposition, resuspension, clogging and release are
substantially affected by hyporheic exchange. On the other hand, the
existence of colloids can modify the hydraulic structure of the
hyporheic zone due to clogging. Despite the general acknowledgement that
particle size plays an important role in contaminant adsorption and
clogging, it remains unclear how the particle size of colloids changes
during their transport processes in the hyporheic zone. This study aims
to investigate the variation of the particle size of colloids in the
overlying water and the effects of settlement and convection-diffusion
via laboratory experiment and numerical simulation. The results show
that both settlement and convection-diffusion play roles in the exchange
of colloids between the stream and the streambed. More specifically,
settlement is the dominant factor affecting the exchange process of
large-sized particles (dp > 3.06 μm)
in the hyporheic zone as the high settling velocity dominates the
outflux of colloids in the overlying water; the exchange process of
small-sized particles (dp < 1.10 μm) is
more affected by convection-diffusion and some of them can be released
from the streambed to the overlying water; while the exchange process of
middle-sized particles (1.10 μm < dp
< 3.06 μm) is affected by both convection-diffusion and
settlement. Thus, this study may provide important insights into the
variation of the particle size of colloids in the overlying water and
the effects of settlement and convection-diffusion.