Impact of skin effect on single-well push-pull tests with the presence of regional groundwater flow

Abstract. Single-well push-pull (SWPP) test is one of the most important ways to estimate aquifer transport parameters, e.g. porosity, dispersivity, rate of biogeochemical reaction, but its application for determining the regional groundwater velocity has rarely been discussed in previous studies. In this study, a new numerical model of SWPP test considering regional groundwater flow and skin effects was established using the finite-element COMSOL Multiphysics. The effects of regional groundwater flow velocity and skin properties on breakthrough curves (BTCs) were thoroughly analyzed. Several important results were obtained in this study. Firstly, the regional groundwater velocity affects the types of BTCs through changing the pattern and location of the dividing streamline. Secondly, a positive (or negative) skin leads to a slower (or faster) tracer transport process. That is, a positive skin results in a higher concentrations at early stage at a given time. Thirdly, a smaller hydraulic conductivity ratio δ of the positive skin to the formation results in greater solute plume retardation in the skin zone. Besides, a larger thickness of the positive skin leads to a higher tracer concentration around the well. The opposite is true if the skin is negative. The general conclusion is that the skin effects on SWPP test are significant and should be considered.