Evaluating sediment transport and metal sorption in the San Juan River watershed

The physical and chemical characteristics of sediment influence the transport of metals in rivers. The San Juan River and its tributaries are located in the Four Corners Region in the southwestern United States and the watershed contains a wide variety of potential metal sources. Comparisons of past and present sediment data provide insight into the effects of seasonality and storm events on the transport of sediment within a watershed. Comparisons suggest finer (&lt;0.63 µm) sediment particles increase at a greater rate during intense storm events than coarser sediment particles. These fine, clay-sized sediments have a greater potential for metal sorption. Statistical analyses compared upper and lower portions of the San Juan River. Results show that total elemental concentration decreases in sediments (1785.15 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>μ</mml:mi> </mml:math> g l −1 ) and that concentrations increase in the aqueous phase (2063.08 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>μ</mml:mi> </mml:math> g l −1 ) downstream in the San Juan River. Analyses using geochemical and scanning electron microscopy with energy dispersive spectroscopy suggest that sediment clay particles are the most likely constituent to transport metals in the San Juan River. Metal transport is further aided by metal oxide coatings that develop on the surfaces of larger particles. Increases in aggregation of fine-grained particles in the downstream portions of the San Juan River are also likely to bind elements within sediments, which can act as both a source and sink for metals in the lower watershed.