Salt marsh sediment and metal fluxes in response to rainfall

We applied simulated rainfall to a salt marsh surface to mimic the effects of a low-tide summer storm, and we collected runoff and sediment transported to the downslope end of the 1- ×  2-m plot. Sediment concentration (SC), discharge, and rainfall rate were assessed in the field, and sediment metal contents were analyzed in the lab (Cr, Mn, Fe, Ni, Cu, Zn, As, Se, Ag, Cd, Al, Sn, Hg, Tl, and Pb). SC peaked at 6100 mg L^–1 after 3 min and remained within a quasi-steady range for 16 min. Time series of metal contents were highly variable, but most were elevated relative to the background content by a factor of 4–4000. Up-scaling the plot results indicates that a single storm may mobilize 4.8–8.4 tonnes km^–2 min^–1 of sediment, or up to 96–168 tonnes yr^–1. We used the observed metal contents on the order of 0.1–200 μg g^–1 to estimate the annual loading of heavy metals that occurs in the larger subtidal channels, fed by intertidal creeks draining the salt marsh. This study highlights the importance of low-tide salt marsh rainfall-runoff processes as an intraestuarine material cycling process and shows that rainfall-entrained sediment is a potentially large nonpoint source of metals to benthic and aquatic ecosystems.