Abstract
Measurements of the upward advection and pore‐water profiles of salinity were made at three coastal settings (the Venice Lagoon, Italy; Jamaica Bay, New York; and Mattituck, New York). Coincident measurements of seepage rates using vented benthic chambers and profiles of salinity in pore water collected with a piezometer are used to quantify the coefficient of dispersive mixing. Three mathematical methods were used to quantify this coefficient: a simple, linearized salt balance and two analytical solutions to the steady‐state, one‐dimensional, advection‐diffusion equation in either a slab or a semi‐infinite medium. The majority of dispersive mixing coefficients in fine‐ to medium‐grained silty sand were calculated to be between 0.04 and 0.8 m 2 d ‐1 . In some cases, the dispersive mixing seemed to involve small‐scale preferred pathways for vertical transport, such as would be the case for bioirrigation, gravitational convection, or salt fingering.