| Fine particles are necessary for biogeochemical cycling and stream metabolic processes, but they pose a risk to water quality if they are present in excess or carry sorbed contaminants. According to recent United States Environmental Protection Agency estimates, suspended particles, and contaminants that they carry, are responsible for approximately half of the streams not meeting national water quality standards. The objective of this work is to elucidate the effects of streamflow on the in-stream transport and streambed retention of fine suspended particles within a mountain stream. Suspended particle transport processes are compared to those of a conservative solute tracer. Field experiments illustrate the increased retention of titanium dioxide (TiO2) suspended particle tracer within a stream reach as streamflow decreased. Under high streamflow, typical of spring, the majority of the suspended particle tracer exited the reach. Under low streamflow, typical of a late summer drought, the majority of the suspended particle tracer remained within the reach. Laboratory studies demonstrated that flocculation and settling was responsible for only a portion of the fine particles retained under the lower flow conditions. Numerical modeling simulations of stream-streambed exchange indicated that advective pumping, or the flow of water from the stream into the streambed based on a pressure gradient, could be responsible for remainder of the fine particle retention in the stream reach under drought flow conditions. According to laboratory experiments and numerical simulations, when streamwater with suspended particles flowed through the streambed, the majority of the suspended particles were filtered out by the streambed sands. Through a unique combination of field measurements, laboratory column experiments, and modeling of stream-streambed exchange, I have demonstrated the dependence of streambed particle filtration on pore water flow conditions, which, in turn, depend on surface water flow. The results of my study indicate that under typical summer baseflow and lower flow conditions, many suspended particles become filtered and retained just under the streambed surface, (e.g. in the hyporheic zone). This finding has many implications for water quality and stream ecosystem management because many biological and chemical transformations, such as stream metabolism and the methylation of mercury, take place in the hyporheic zone. |
