On the ideal groundwater sampling window by utilizing transition pumping period

•Critical issue addressed for applying the HSLF method, when to switch pumping rate.•Ideal sampling window is approximately linearly related to the switch time.•Developed and verified a practical guideline for field applications. A new method of groundwater sampling has recently been proposed, which involves two-phase well purging – a high-flow rate pumping phase followed by low flow rate purging and sampling. The high pumping rate yields a rapid well drawdown and enhances hydraulic gradients within the aquifer. After switching to a low pumping rate, the stressed flow field is maintained to prevent the water in the well casing from moving downwards to bias the collected samples. This study addresses two practical issues that are critical for applying such a sampling method: the determination of the pumping switch time and the transition time interval available for groundwater sampling. We use Theis and Papadopulos solutions to study the impact of wellbore storage on the correlation between switch time and transition time interval. By imposing critical conditions to the derived solutions, we suggest that the transition time interval is solely associated with the switch time. Results show that a later switch time yields a longer transition time interval. Theis solution provides an approximate linear relationship between the switch time and the transition time interval, which can be used as a simple approximate practical guideline. Results from Papadopulos solution confirm the linear relationship at late times and provide a more accurate relationship for early times by accounting for the well storage impact. A sensitivity analysis indicates that aquifers with smaller storativity under a higher pumping ratio will generate a longer transition time interval given the same switch time. Similarly, an earlier switch time is allowed given a predefined transition time interval. This study presents a guideline to determine reasonable switch time under various aquifer hydrogeologic conditions and estimate the ideal groundwater sampling window.