Non-equilibrium solute transport in dipole flow tracer test (DFTT) considering the mixing effect in the wellbore

Dipole flow tracer test (DFTT) has been proposed to estimate the vertical distribution of hydrogeological parameters by fitting the simulated and observed breakthrough curves (BTCs) in the extraction chamber of a vertical circulation well (VCW). Previous studies of DFTT models neglected the non-equilibrium sorption of tracer and the mixing process in the wellbore, which have been proved to strongly influence the estimation accuracy of the DFTT. In this study, we developed an improved DFTT model considering a two-site non-equilibrium sorption of the tracer as well as the impact of the mixing effects on the BTC behavior in both the extraction and injection chambers. The results indicate that when interpreting the DFTT with commonly-used organic tracers, an overestimation and underestimation occurs, especially when the models do not account for sorption and consider equilibrium sorption, respectively. In addition, the mixing effect was observed to affect the transport process as well as the BTC behaviors in the DFTT, leading to an overestimation of the dispersivity. The improved model proposed in this work was used to interpret the DFTT test conducted with Rhodamine WT (RWT) at the Lizzie Field Site, North Carolina. Results showed that the improved model had better performance in interpreting the DFTT compared to the stream tube model proposed by Sutton et al. (2000). This is especially when fluorescent dyes such as RWT or Rhodamine B are used as tracers. Furthermore, unlike previous models, the improved DFTT model is not limited as it is fit for tracers with various types of sorption properties. Meanwhile, the consideration for mixing effect in the wellbore, characterized by an ordinary differential equation, promotes the DFTT interpretation accuracy and also has great potential in the simulation of in situ remediation of subsurface contaminants through VCW, especially for in-well bioreactor technology.