Proof‐of‐Concept Modeling of a New Groundwater Sampling Approach

Source abstract

Abstract This note provides a proof‐of‐concept modeling study to examine the theoretical feasibility of a new groundwater sampling approach, the high‐stress low‐flow approach, for improving the representativeness of groundwater samples and shortening the sampling duration. The approach consists of two‐phase pumping: an initial high pumping rate followed by low‐flow purging and sampling. We conceptualize the sampling process by pumping in a fully penetrating, large‐diameter well in a confined aquifer and mixing within the well screened zone, which are characterized by the aquifer‐responding time scale for aquifer water dominating the inflow ratio and the well‐mixing time scale, respectively. We derive an analytical solution to evaluate the sample concentration and representativeness. Results show that higher pumping rates in phase 1 create larger drawdowns and overstressed flow fields when switching to lower pumping rates in phase 2, thereby slowing down the drawdown within the well and preventing downward movement of the well casing water. As a result, the ratio of aquifer water into the well screened zone and in water pumped can rise sharply at the pumping‐switch moment, and the aquifer‐responding time scale and groundwater sampling duration can be significantly shortened. The proposed high‐stress low‐flow approach is particularly effective in systems limited by long aquifer‐responding time scales, which are typically encountered in low‐permeability aquifers. Based on the analytical solution, we also provide preliminary guidelines and graphic tools to select the pumping rate ratio and the switching time. Our proof‐of‐concept modeling study demonstrates great potential of the new groundwater sampling approach in field applications.