Dipole Flow Tracer Test Models for Identifying Skin Properties and Ambient Groundwater Flow Effects.

As a special single-well test with dual-screened configuration, dipole flow test with a tracer (DFTT) has been an efficient tool for aquifer parameter interpretation. The tool is excellent for characterizing the vertical distribution of hydraulic conductivity, dispersivity and in situ reaction rate in aquifers. It is noteworthy that the effects of skin and ambient groundwater flow (AGF) on the solute transport, which have been widely investigated in terms of conventional tracer tests, have not been comprehensively studied with respect to DFTT. Meanwhile, it is important to elucidate the potentials of DFTT in identifying the primary skin properties as well as the methods to avoid the AGF effects on DFTT. Consequently, we developed a mathematical model for DFTT accounting for skin and AGF. The results show that a negative skin acts as a preferential pathway for tracer transport and the AGF effects on breakthrough curves (BTCs) can be easily mistaken for reaction. Through extensive mathematical model-based experiments, the BTCs calculated from the DFTT models with skin effects were categorized into four types, each of which is able to imply a specific kind of skin. Additionally, a new dimensionless parameter , composed of injection/extraction rate, hydraulic conductivity, hydraulic gradient of AGF and screen parameters, was defined to quantitatively indicate the AGF effects on the overestimation degree of the in situ first-order reaction, implying that one can adjust the field parameters of DFTT to keep less than 3.7% to achieve an overestimation error of reaction rate below 50%.