Transport of nonlinearly adsorbing compounds between stream water and sediment bed in a laboratory flume

Abstract

The exchange of nonlinearly adsorbing compounds between stream water and sediment beds covered with stationary bedforms was investigated in laboratory experiments. The dominant physical exchange process is advective pumping caused by dynamic pressure variations over dunes on the bed. Observations of net mass exchange of cationic surfactants in a 5-meter long recirculating flume were used to validate the exchange model, which is based on the hydraulics of advective pumping and nonlinear adsorption isotherms derived from batch experiments. The flume experiments were conducted under steady, uniform flow conditions. The pH and ionic strength of the flume water was controlled by adding sodium chloride and sodium bicarbonate to deionized water. The sand was washed prior to every experiment. The mass exchange of cationic surfactants and bromide was determined by measuring the depletion of these compounds in the overlying water column as it mixed with the clean porewater from the bed. Porewater concentration profiles were acquired to monitor the penetration depth of the compounds in the bed. Bromide was used as a conservative tracer to observe the hydraulics of water exchange between the bed and the overlying water. Garnet sand was used as the model sediment because it had heterogeneous properties similar to natural sediments. The net mass exchange with a bed covered with stationary bedforms was greater than the exchange with a flat bed. The mass exchange of the cationic surfactants versus time observed in the flume experiments could not be modeled using linear adsorption; however, linear approximations provided upper and lower limits on the exchange. The total mass transfer of the cationic surfactants to the bed increased with their hydrocarbon chain lengths. The model for the exchange of nonlinearly adsorbing compounds solves the advection equation to track the transport of the compounds within the bed and computes the net mass flux through the bed surface. Nonlinear adsorption was modeled by the means of four different isotherm equations fitted to the batch adsorption data. The effect of the choice of isotherm on the exchange models for the flume experiments was found to be very small. The model generally predicted the flume results well without calibration. Additional model simulations were performed to provide a sensitivity analysis for the model inputs.

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