Validation of the dual pumping technique for level-determined groundwater sampling in a contaminated aquifer

The dual pumping technique (DPT) was introduced recently as a new and inexpensive technique to measure the level-determined solute concentrations in groundwater. Using two pumps, one placed near the groundwater table and one placed near the bottom of a fully screened well, this technique allows to determine vertical concentration profiles for solutes in groundwater given that additional information about the influx distribution into the well is known. Until now, however, the DPT was applied only in an aquifer with a thickness of about 40 m and the validation was weakened by the lack of a reliable reference system. The present study aims to investigate the applicability of the DPT for shallow (thickness <10 m) and unconfined aquifers and to validate the results of the DPT with a better reference system. For this purpose the DPT was applied in Menziken, Switzerland to a gravel aquifer contaminated with petroleum hydrocarbons. Solute concentrations measured at that site with an establish- ed drive point sampling technique (Ram technique) were taken as a reference to demonstrate the applicability of the DPT. Results for eight different solutes showed a reasonable agreement between both techniques. An algorithm was developed that allowed the computation of a single solute concentration profile incorporating measured data from both pumps. It was possible to demonstrate that this alternative algorithm can improve the quality of the solute concentration profiles obtained by the DPT. This indicates that the DPT is a useful tool for determining the vertical concentration profiles in groundwater for conditions similar to those in Menziken (unconfined gravel aquifer, screen length less than 10 m).