A mechanism for the accumulation and retention of heavy metals in tidal freshwater marshes of the upper Delaware River estuary

Abstract

A mechanism for the accumulation and retention of heavy metals in tidal freshwater marshes is proposed. Although metals are incorporated into tidal freshwater marsh substrates through the year by accumulations into live stem and leaf material, absorption by root systems, and transportation on suspended sediments, the bulk of metal accumulations occurs through the incorporation and burial of plant litter. Plant litter acts as a sponge, absorbing metals and holding them as a temporary sink. Due to high sediment accumulation rates and high rates of decomposition, the litter and its associated metals are rapidly buried and incorporated into the marsh substrate. Since litter is most abundant late in the growing season, metal accumulations are greatest during the fall and subsequently dominates the long-term accumulation of metals preserved in the substrate. To utilize preserved metal concentrations in quantitatively assessing historic regional metal budgets, research would be required to determine the relationship between fall metal accumulations and the annual exposure of metals these marshes are subjected to. Differences between salt and freshwater tidal marshes in their ability to assimilate metals from amended soils are due to differences in the accumulation of organic material into the substrates. In tidal salt marshes there is a well developed belowground mat of roots and rhizomes that absorbs metals directly into the substrate while in tidal freshwater marshes metals accumulate primarily in plant litter lying on top of the marsh surface and rely on its burial for incorporation into the substrate. The sequence presented here demonstrates a mechanism for the building of long-term metal sinks in marshes of the upper Delaware River estuary.