Effect of multiple lateral cavities on stream solute transport under non-Fickian conditions and at the Fickian asymptote

- We use CFD to model the effect of lateral cavities on stream solute transport.
- Subsequent tracer entrainment is greater for cavities in series than in parallel.
- Solute entrainment impacted more by cavity configuration than number of cavities.
- 1-D transport model parameters do not adequately describe fundamental flow physics.

SummaryIn field studies of solute transport, transient storage within lateral cavities and other stream features generates breakthrough curves (BTCs) with pronounced and persistent skewness. Current solute transport theory requires that the coefficient of skewness (CSK) decrease over time because the system eventually reaches Fickian conditions. However, published data show that CSK is constant in time. To aid development of solute transport theory that explains field observations, we quantify the effect of lateral cavities on solute transport under non-Fickian and Fickian conditions. Six hydrodynamics models were developed: one with no lateral cavities, three with lateral cavities in series, and two with lateral cavities in parallel. Results reveal that lateral cavities in series have longer tails and smaller peak concentrations compared to lateral cavities in parallel. Lateral cavities in series cause greater dispersion and require larger distances to reach Fickian conditions (xFick) compared to lateral cavities in parallel. Cavity configuration has a greater influence on longitudinal dispersion and xFick than the number of cavities present. CSK changes with monitoring location and maximum CSK (= 10-20) near lateral cavities is higher than empirical estimates (≈1.18). We postulate that adding more transient storage zones would increase channel complexity and yield closer results between simulated and empirical CSK, and testing this hypothesis warrants future research. Finally, while current models can obtain good fits to measured BTCs by parameterizing mass exchange rates and volume ratios, these parameters do not adequately describe the fundamental fluid mechanics driving exchange.