The effect of groundwater advection on salinity in pore waters of permeable sediments

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.