Effect of transverse dispersion on solute transport in a vertical dipole flow test with a tracer

Vertical dipole flow test with a tracer (DFTT) have been proposed as an in situ method to determine the longitudinal dispersivity from analysis of the breakthrough curves (BTCs) in the extracted screened interval by assuming that only the longitudinal dispersion has an effect on the BTCs. However, the tracer must undergo transverse dispersion before it enters the extracted screened interval and will certainly influence BTCs in the extracted screened interval. The purpose of this study is to investigate the effect of the transverse dispersion on the BTCs in the extracted screened interval during a vertical DFTT and to examine the applicable condition of a DFTT in determining the longitudinal dispersivity. A previously developed mathematical model for reagent transport in a vertical circulation flow field is modified and used for the investigation. Simulation results demonstrate that the transverse dispersion exerts significant effects on the BTCs in the extracted screened interval in an aquifer with a large hydraulic conductivity anisotropy ratio and a large longitudinal dispersivity. The selection of operational parameters including the interval location, lengths of the screened interval and interval distances does not significantly promote the applying condition of the DFTT for determining the longitudinal dispersivity. It suggests that longitudinal dispersivity cannot be solely determined by analysis of the BTCs, simply because the BTCs are simultaneously affected by both the longitudinal and transverse dispersions. One should note that the DFTT can only be applied to evaluate the longitudinal dispersivity in an aquifer with a small hydraulic conductivity anisotropy ratio or a small longitudinal dispersivity. (C) 2011 Elsevier B.V. All rights reserved.