Segment-Scale Patterns and Hydraulics of Trace Metal Concentrations in Fine Grain Sediments of a Cobble and Boulder Bed Mountain Stream, Southeast Alaska

Abstract

This study documents and analyzes spatial patterns of variability in 12 trace metals in sand-size and finer sediments over a 2 km segment of a boulder and cobble bed mountain stream. The analysis focuses on concentrations rather than the total metal mass budget, because it is typically variations in metal concentrations that are important for assessing potential environmental impacts. There are no consistent downstream variations in metal concentrations, which indicates that there are no significant geochemical changes or external sources of metals in the study segment. In fact, much of the spatial variability within the stream segment appears to be random, although a dimensionless shear stress model predicts the majority of locations where peak concentrations of metals contained in heavy minerals occur. The dimensionless shear stress model, however, only predicts the potential for enrichment and many sites where potential enrichment is predicted contain low concentrations of metals. Peak concentrations do not appear to be associated with metal-rich sediment waves moving through the system, but instead are probably the result of local hydraulic variability. Sampling of boulder and cobble streams should be stratified by hydraulic environments in order to capture the full range of minimum and maximum metal concentrations within the stream system.