Direct-push-installed, gas-driven mini-pumps for discrete-point groundwater sampling: A new in-situ approach to long-term monitoring

Discrete-point sampling is important for a variety of hydrogeological investigations. A new approach to vertical chemical profiling has been developed in which low-volume mini-pump samplers (MPS) are installed in a single borehole using direct-push methods. The new, positive-displacement, gas-driven mini-pumps overcome sampling depth limitations of conventional suction pumps. Up to ten pumps can be simultaneously operated using a multi-channel pneumatic controller that drives water to the surface through alternating pressurization and depressurization pulses. By combining direct-push chemical profiling with MPS installation, the pumps may be placed at the most appropriate depths for a particular investigation. This study assessed the potential of the new approach in an alluvial aquifer that has been the site of a great deal of previous work. Two sets of mini-pump samplers, comprised of four pumps each, were installed in an interval characterized by a steep chemical gradient. The MPS installations were placed within one meter of conventional multilevel samplers with similar intake depths. Chemical field parameters (DO, pH, ORP, conductivity and temperature) and dissolved constituent concentrations (NO3, SO4, Cl, Fe and Mn) were measured in the two sets of paired samplers. Although the vertical chemical trends observed in the multilevel samplers were also observed in MPS installed using direct-push rods composed of nitrided steel, redox sensitive measurements from the MPS were affected by installation with standard steel rods. The combination of MPS installation and direct-push characterization allows for repeat sampling of intervals of interest without the need for permanent wells. Ongoing work addresses the long-term performance of the MPS.