In situ experiment to determine advective-diffusive controls on solute transport in a clay-rich aquitard

Solute transport in clay-rich aquitards is characterized as molecular diffusion- or advection-dominated based on the Péclet number (Pe). However, few field-based measurements of the coefficient of molecular diffusion (De) exist, and none with a range of advection- or diffusion-dominated conditions in the same aquitard. In this long-term field experiment, standing water in a recovering well was spiked with deuterium (2H), then water-level recovery and δ2H values were monitored as the well returned to static conditions over 1054 days. After a second 2H spike, water levels and δ2H values were monitored to day 1644 while under near static conditions. Modeling of the second spike was used to define the De of 2H as (3–4) × 10− 10 m2 s− 1 for an accessible porosity of 0.31. Reservoir concentrations from the initial spike were modeled to define the transition from advection- to diffusion-dominated transport. This occurred after 200 days, consistent with a transition in Pe from < 1 to > 1 when the length term is taken as the radial extent of the tracer plume (normalized concentration < 0.05). This study verifies plume extent as the characteristic length term in the calculation of Pe and demonstrates the transition from advection- to diffusion-dominated transport as the value of Pe decreases below unity.

► Solute transport in aquitards can be due to molecular diffusion or advection.
► Long-term in situ tracer (2H) tests were conducted in a low K, clay-rich, aquitard.
► Field data show the transition from advection- to diffusion-dominated transport.
► Péclet number (Pe) predicts this transition when length term equals plume extent.