Solute Transport Across an Angled Interface: Assessments of Spreading and Mixing

Mixing in aquifers is an important process for the proper characterization of various subsurface chemical reactions, however its quantification is difficult in practice. A commonly used method in the assessment of mixing relies on the evaluation of concentration breakthrough data at a well over time. For homogeneous systems this is perfectly acceptable, however natural systems are generally heterogeneous. To study the specific relationship between spreading and mixing in a heterogeneous system, solute transport is simulated across an angled interface separating two homogeneous porous media. Discernible differences in the concentration breakthrough at the outlet are examined and measures of mixing are evaluated as dimensionless parameters are varied. It is shown that the inherent transverse dispersivity and direction of flow can lead to variations in mixing despite breakthrough curves remaining nearly identical. This implies that for the defined system, limited information is obtained regarding mixing from the calculated concentration breakthrough curves.