Effects of background water composition on stream-subsurface exchange of submicron colloids

While very fine sediments (colloids) are normally assumed to be readily transported downstream without deposition, recent evidence suggests that these particles will often deposit into streambeds due to a combination of physical and chemical mechanisms. This study investigates a regime of particle deposition where settling is unimportant and thus where particle deposition can only result from advective stream-subsurface exchange followed by deep-bed filtration. Laboratory flume experiments were conducted to examine the deposition of 0.45 mum diameter silica colloids into a silica sand bed. This system was selected for study because submicron sized colloids will not settle and silica colloid filtration by silica sand is generally quite low. Despite the lack of settling and the weak particle-particle interactions, the ongoing interfacial flux of colloids to the subsurface still produced significant filtration of silica colloids over the course of the experiments. Variation of the background ionic strength caused significant modification of filtration behavior and silica colloid deposition. In addition, cleaning the sand surface with mild acid and base washes reduced both filtration and net colloid exchange. These experimental results are interpreted in terms of a fundamentally based physicochemical model which predicts net particle deposition based on stream and subsurface hydrodynamic conditions and subsurface filtration. These results show that both particle surface chemical conditions and background water chemistry play a critical role in controlling the net transport and deposition of fine sediments. It is important to recognize the effects of physicochemical processes both when designing laboratory experiments and when analyzing environmental particle transport.