Biased Estimation of Groundwater Velocity from a Push-Pull Tracer Test Due to Plume Density and Pumping Rate

Abstract

The single-well push-pull tracer test is a convenient and cost-effective tool to estimate hydrogeological properties of a subsurface aquifer system. However, it has a limitation that test results can be affected by various experimental designs. In this study, a series of laboratory-scale push-pull tracer tests were conducted under various conditions controlling input tracer density, pumping rate, drift time, and hydraulic gradient. Based on the laboratory test results, numerical simulations were performed to evaluate the effects of density-induced plume sinking and pumping rate on the proper estimation of groundwater background linear velocity. Laboratory tests and numerical simulations indicated that the actual linear velocity was underestimated for the higher concentration of the input tracer because solute travel distance and direction during drift time were dominantly affected by the plume density. During the pulling phase, reasonable pumping rates were needed to extract the majority of injected tracer mass to obtain a genuine center of mass time (t com ). This study presents a graph showing reasonable pumping rates for different combinations of plume density and background groundwater velocity. The results indicate that careful consideration must be given to the design and interpretation of push-pull tracer tests.Although the push-pull tracer test has an advantage of being easy to perform by using only one tested borehole, hydraulic conditions of the tested field or test design of the push-pull test could cause misinterpretation of the aquifer properties. Hwang [16] performed single-well push-pull tests under different experimental conditions by changing extraction rate, drift time, hydraulic conductivity, and hydraulic gradient. Based on the test results, Hwang [16] concluded that the mass recovery rate was inversely proportional to the drift time and the hydraulic gradient. Hebig et al. [17] conducted single-well push-pull tests with different chaser volumes and found that the chaser volume could affect the main peak concentration of the breakthrough curve. Wang et al. [18] reported that single-well push-pull test results can be different in steady-state and transient flow conditions and such differences increased with the decreasing specific storage. Paradis et al. [12] considered the displacement during the injection phase, which resulted in more accurate calculation of effective porosity.Under some hydrogeological conditions, density differences between the background fluid and solute or thermal plume can appear because of changes in solute concentration or temperature. Plumes related to seawater intrusion, high-level radioactive waste disposal, groundwater contamination, and geothermal energy production can be examples [19]. In such cases, a density-induced sinking or rising effect can appear during transport and the density-induced sinking has been discussed in transport-related studies [20][21][22][23][24][25].The method of Hall et al. [15] was theoretically developed for a field-scale. However, it w...

Noble gas as a proxy to understand the evolutionary path of migrated CO2 in a shallow aquifer system

“…As salt does not undergo phase-partitioning in a groundwater system, entrapment of salt mass into low permeability zones was a possible reason for the mass loss in the SWTT (Kim et al, 2018(Kim et al, , 2019. Y-axis displays the recovery ratio of tracers recorded above the background level.…”