Modelling retention of sorbing solutes in streams based on tracer experiment using 51Cr

The transport and retention of a sorbing solute in streams are affected by the transversal uptake in various hyporheic zones, each characterized by its individual turnover time. The uptake is included in a one-dimensional model for the along-stream transport on the form of first-order transfer functions by parameterizing physically derived relationships. A 51Cr(III) tracer experiment, conducted in a brook in an agricultural watershed in Sweden, provided a basis for deducing constitutive equations for three different exchange rates governing the retention of the sorbing solute in the brook. The solute exchange that occurs between the main stream channel and slowly recirculating zones, like side pockets along the stream, eddy zones behind boulders, or side pockets, is practically instantaneous and is manifested as a marked retardation of the pulse propagation in relation to the main stream velocity. The considerably slower exchange with permanent substratum, including armored bed sections, dense vegetation, or adjacent wetland, causes a reduction of the peak concentration and prolonged tails of the breakthroughs. Uptake in the alluvial sediment occurs both in particulate and dissolved phases of the solute, and is affected by the bed dynamics. The uptake in the alluvium was evaluated separately by fitting of an analytical solution of the vertical concentration profile to measured concentration profiles in the sediment. In the experimentally investigated reach, the exchange with the fine bed sediment was dominated by the dissolved phase of Cr, due to advection driven by pressure variabilities along the bed surface.