Solute Transport In Solution Conduits and the Problems of Persistent Breakthrough-Curve Skewness and Multiple Peaks: Experiments and Analyses

Solute transport in karstic aquifers is primarily constrained to solution conduits where transport is rapid, turbulent, and relatively unrestrictive. Breakthrough curves generated from tracer tests are typically positively-skewed and often exhibit multiple peaks. The persistent skewness is commonly attributed to immobile-flow regions that have been confirmed by numerous studies but the cause of multipeaked breakthrough curves remains elusive. Multiple peaks are sometimes ascribed to tracer entry into ramiform auxiliary channels that become confluent downstream with main channel flow but supporting evidence for such diverging-converging flow is not strong.

In order to understand the circumstances under which multipeaked positively skewed breakthrough curves occur, physical experiments utilizing single- and multiple-flow channels were conducted. Simulation also included waterfalls, short-term solute detention in pools, and flow obstructions.

Results of the experiments demonstrated that breakthrough curve skewness always occurs but is magnified as immobile-flow regions are encountered. Multiple breakthrough curve peaks occurred when main channel blockages forced divergence of solute into auxiliary channels but also when waterfalls and detention in pools occurred. These results suggest that a greater degree of insight into the shape of breakthrough curves is necessary to better assess the type of mathematical model to apply. Various solute-transport models have been explored to see which best represents the data. These models mainly focused on double-porosity formulations with differing mass-transfer functions. A triple-porosity formulation is currently being investigated.