Development and evaluation of micro push-pull tests to investigate micro-scale processes in porous media

Soils and sediments are porous media characterized by heterogeneities across a wide range of spatial scales. Physical, chemical, and biological properties have been found to show great variation even at subcentimeter scales. Here we present a new micro technique for the in situ study of chemical and microbiological reactions in water-saturated porous media at the mm-scale. This technique combines micro suction cups with the principle of single-well injection-withdrawal tests ("push-pull" tests). Push-pull tests have been used extensively on larger scales in groundwater research to obtain quantitative information of physical, chemical, and microbiological characteristics of an aquifer. The micro push-pull technique presented here was developed and validated using a thin-slab chamber filled with sand. A porous micro cup was used to inject about 250 μL of a test solution into the water-saturated sand pack and then to slowly extract about 850 μL water from the same point. The extraction-phase breakthrough curves of the solutes were modeled considering advection, dispersion, and molecular diffusion without fitting any parameters. As an example we quantified the degradation of citrate injected into the water-saturated sand pack inoculated with denitrifying bacteria. The results show that the new technique can be used to assess local microbial degradation processes under in situ conditions on the micro scale.