A field study of nonequilibrium and facilitated transport of Cd in an alluvial gravel aquifer.
A natural-gradient tracer experiment was conducted to study Cd transport in an alluvial gravel aquifer. Both a conservative tracer and Cd exhibited tailing in their breakthrough curves (BTCs), indicating the presence of nonequilibrium transport. Solute transport was evaluated using a three-dimensional nonequilibrium analytical model, and the results were compared with those obtained from a previous laboratory study. At similar flow velocities, the field Cd data gave significantly lower retardation factors (R=7 to 30, median 22), higher fractions of instantaneous sorption sites (beta), and greater mass transfer coefficients (omega) than the laboratory data because of a high degree of aquifer heterogeneity, the presence of preferential flow, and the larger transport scale in the field conditions. Multiple peaks in the Cd BTCs were observed due to bacteria-facilitated and perhaps also colloid-facilitated transport. The early peaks showed narrower, more symmetric shapes with higher concentrations than the later peaks, and compared well to those of the bacterial BTCs. The multiple peaks of the Cd BTCs imply that a significant fraction of Cd could travel with little, if any, retardation over a 20 to 40 m travel distance when Cd and bacteria coexist in a contamination event.
Pang
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Institute of Environmental Sciences & Research, P.O. Box 29181, Christchurch, New Zealand. lpang@esr.cri.nz
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