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

The effects of pore-water velocity (v), transport distance (L), and hydraul ic residence time (L/v(m), in which subscript m refers to mobile region) on the fraction of mobile water (beta), mass transfer rate (alpha), and longi tudinal dispersivity (lambda) were evaluated from 53 breakthrough curves of a conservative tracer (rhodamine WT), observed from tracer experiments con ducted at a field site. The individual effects of hydraulic conductivity (K ) and hydraulic gradient (I), which determine v, were also evaluated. A thr ee-dimensional non-equilibrium analytical model, N3DADE, was used to analyz e the data, combined with a modified method of moments for estimation of po re-water velocity. At v values of 5-104 m/day in an alluvial gravel aquifer , the following significant tendencies were found: (1) beta and alpha incre ased while lambda decreased with increasing v and K; (2) alpha decreased wh ile lambda increased with increasing L and L/v(m); (3) there was no signifi cant relationship between beta and alpha and between beta and lambda, bur a lpha had a strong inverse correlation with lambda. The results of this stud y suggest that both aquifer properties (K) and fluid dynamics (I and L/v(m) ), as well as transport scale (L), were controlling factors in physical non -equilibrium transport and parameter interrelationships. beta was largely a property of the aquifer medium, while alpha and lambda appeared to be prop erties of both aquifer medium and fluid. There was significant 'immobile wa ter' in the alluvial gravel aquifer system, probably resulting from aquifer heterogeneity, in which slow mixing between zones of contrasting permeabil ity was important. (C) 1999 Elsevier Science B.V. All rights reserved.