Preferential solute transport in layered homogeneous sands as a consequence of wetting front instability

Abstract

Laboratory solute transport experiments were carried out in layered homogeneous sand with a fine textured layer overlying a coarse layer. Pulses of blue dye are used to characterize the solute movement. Unlike the traditionally expected one-dimensional homogeneous flow, the solute moves in preferred paths or “fingers” induced by infiltration flow instability starting at the textural interface between the fine and the coarse layer. The effect of repeated long term ponded infiltration cycles, intermittent ponding events and of uniform initial moisture content at field capacity on the flow field structure and solute breakthrough curves is studied. The possibility of using a simple lumped dispersion coefficient that includes additional mixing processes due to instability is explored and implications for field solute transport monitoring practices are discussed.