The dual pumping technique (DPT) for level-determined sampling in fully screened groundwater wells

A new and inexpensive technique to obtain vertical hydrochemical profiles in aquifers is presented. The Dual Pumping Technique (DPT) is designed for use in fully-screened groundwater wells and represents an alternative to packer installations or similar sampling devices. Two pumps are placed at either end of the well screen. The ratio of their pumping rates is varied from 1 to 0. Depending on this ratio, the two pumps abstract variable portions of the influx distribution. A water divide develops in the well, separating the flow upward from the flow downward. A set of samples from the upper and lower pump are taken for different pumping rate ratios. Measured solute concentrations in these samples, together with the influx distribution determined by flow logging, are used to reconstruct vertical concentration profiles. The measured data are evaluated with a simple, robust algorithm, which is derived in the text and exemplified in the Appendix. The DPT was used to determine vertical concentration distributions of chlorinated hydrocarbon solvents (CHS) and tritium in a contaminated aquifer near Heidelberg, Germany. Comparison of the concentration profile obtained with the DPT with CHS and tritium data from a 50-m distant multilevel well showed the principal applicability of the new technique. The pattern of the vertical concentration distributions was successfully identified with the DPT, but the absolute CHS concentration values were one order of magnitude higher in the multilevel well, due to the fact that the multilevel well is probably closer to the centre of the plume than the well used for the DPT. The 3H values could be compared directly and showed an excellent agreement. Further evaluation of the DPT in a situation where a multilevel reference well is in closer vicinity to the test well is planned.