Micro Push–Pull Tests for Investigation of Small‐Scale Processes in Unsaturated Porous Media

Abstract

The micro push–pull test (μPPT) was recently developed for the in situ study of chemical and microbiological reactions in water‐saturated porous media at the millimeter scale. In a μPPT, a test solution containing reactants and nonreactive tracers is injected into a porous medium using suction cups and then re‐extracted from the same location. We adapted the μPPT method to unsaturated conditions and tested it against numerical simulations. Experiments were performed using sand packs in thin‐slab chambers as a model porous medium. We injected ~250 μL of a test solution containing either Br− or Acid Red 1 as the nonreactive tracer into the sand pack, followed by slow extraction of ~750 μL of solution from the same location. Extraction was feasible for applied capillary pressure heads in the range of 0 to 24 cm H2O (corresponding water saturation of 1 to 0.34). To obtain spatially resolved information on water saturation and migration of injected test solutions in sand packs, visible light transmission combined with image analysis was used. Relative mass recoveries of injected tracers ranged from 56 to 99%. Extraction breakthrough curves of the tracers were successfully simulated considering advection, dispersion, and molecular diffusion without fitting any parameters. Experimental and numerical results showed that the μPPT method is a promising technique to study small‐scale behavior of solutes (e.g., ligands, biodegradable solutes, etc.) in unsaturated soils under in situ conditions.