Low-Flow Sampling: practical guidelines, modelling and new approaches for boosting the representativeness of groundwater samples and aquifer knowledge.

Groundwater contamination is considered worldwide an emerging challenge due to the industrial and human activities. It has been demonstrated that a cost-effective remediation is increasingly dependent on high-resolution site characterization (HRSC), which is supposed to be necessary prior to the interventions. In this framework, groundwater sampling and monitoring strategy have become key factors in interpreting results and subsequently modelling contamination phenomena. Nowadays, low-flow purging and sampling (LFPS) is a consolidated methodology in groundwater monitoring, consisting of pumping water at low flowrates (in fine-grained soils from 0.1 to 1 L/min)  prior to the sampling, until the stabilization of measured chemical physical parameters has been obtained. This is mainly due to minimize the induced stabilized drawdown in the well and, consequently, the aquifer stress too.

Recent outcomes focused on the great potential of a new low flow sampling method (the high-stress low-flow sampling) and the use of collected water level data to estimate hydraulic conductivity (He et al., 2022; De Filippi et al., 2023). The high-stress low-flow (HSLF) approach is characterized by an initial high pumping rate followed by low-flow and it is particularly effective in systems limited by long aquifer-responding time scales, typically low-yield aquifers. Preventing downward movement of the well casing water is the main goal and groundwater sampling duration can be significantly shortened. The second one, taking advantage of monitoring operations on groundwater quality, allows to provide to the stakeholders a very large amount of quantitative data on the aquifer over time, reducing time and costs for site characterization in case of future contamination. This new quali-quantitative approach can provide much more information and knowledge about the site, reducing time and costs of further activities. In addition to that, as the monitoring continues and new quantitative values are estimated, this approach allows also to track changes in aquifer hydrodynamic properties after the application of remediation techniques due to a possible contaminant release. Modelling the groundwater flow to the intake helps to get some precautions and be prepared to local hydrogeological conditions prior to the field work. In this way, the LFPS procedure could lead to obtain more representative groundwater samples in a shorter time and provide hydrogeological parameters.