Field-scale physical non-equilibrium transport in an alluvial gravel aquifer

The effects of pore-water velocity (v), transport distance (L), and hydraulic residence time (L/vm, in which subscript m refers to mobile region) on the fraction of mobile water (β), mass transfer rate (α), and longitudinal dispersivity (λ) were evaluated from 53 breakthrough curves of a conservative tracer (rhodamine WT), observed from tracer experiments conducted at a field site. The individual effects of hydraulic conductivity (K) and hydraulic gradient (I), which determine v, were also evaluated. A three-dimensional non-equilibrium analytical model, N3DADE, was used to analyze the data, combined with a modified method of moments for estimation of pore-water velocity. At v values of 5–104 m/day in an alluvial gravel aquifer, the following significant tendencies were found: (1) β and α increased while λ decreased with increasing v and K; (2) α decreased while λ increased with increasing L and L/vm; (3) there was no significant relationship between β and α and between β and λ, but α had a strong inverse correlation with λ. The results of this study suggest that both aquifer properties (K) and fluid dynamics (I and L/vm), as well as transport scale (L), were controlling factors in physical non-equilibrium transport and parameter interrelationships. β was largely a property of the aquifer medium, while α and λ appeared to be properties of both aquifer medium and fluid. There was significant ‘immobile waterʹ in the alluvial gravel aquifer system, probably resulting from aquifer heterogeneity, in which slow mixing between zones of contrasting permeability was important.