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 ta iling in their breakthrough curves (BTCs), indicating the presence of noneq uilibrium transport. Solute transport was evaluated using a three-dimension al nonequilibrium analytical model, and the results were compared with thos e obtained from a previous laboratory study. At similar flow velocities, th e field Cd data gave significantly lower retardation factors (R=7 to 30, me dian 22), higher fractions of instantaneous sorption sites (beta), and grea ter mass transfer coefficients (omega) than the laboratory data because of a high degree of aquifer heterogeneity, the presence of preferential flow a nd the larger transport scale in the field conditions. Multiple peaks in th e Cd BTCs were observed due to bacteria-facilitated and perhaps also colloi d-facilitated transport. The early peaks showed narrower, more symmetric sh apes 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 o ver a 20 to 40 m travel distance when Cd and bacteria coexist in a contamin ation event.