Laboratory Investigation of Well Storage‐Mixing Effect on High‐Stress Low‐Flow Groundwater Sampling Approach

Conventional constant‐rate sampling methods are inefficient in low‐permeability, low‐storativity formations, where water from casing storage persistently biases collected samples. The High‐Stress Low‐Flow (HSLF) approach addresses this limitation by switching from a high to a low pumping rate, using the drawdown transition to create a sampling window in which downward movement and mixing of casing storage are minimized. In this study, we investigated the purging process of the HSLF approach through controlled experiments in a physical well model, measuring transient drawdown and solute mixing with pressure transducers and the light‐attenuation (LA) technique. A well storage‐mixing model for bounded confined aquifers was developed to interpret the laboratory results and scale them to field conditions. Results show that when the switch occurs later and the pumping‐rate ratio is sufficiently high, a sampling window forms. In bounded aquifers within the tank, the window length approaches a steady‐state limit, whereas in large field‐scale aquifers it increases linearly with later switch times. Comparisons between the constant‐rate and two‐stage purging methods confirm that, with an optimal switch time and pumping‐rate ratio, the HSLF approach effectively minimizes casing storage effect, accelerates mixing, and substantially reduces purge volume and time across all sampling scenarios. Observed drawdown and mixing behaviors are generally consistent with model predictions, and analytical extension of the aquifer domain suggests the potential applicability of the HSLF approach at the field scale. Nonetheless, a comprehensive field study is required to fully assess and validate its performance under complex hydrogeologic conditions.