Early Sediment Changes and Element Mobilization in a Man-made Estuarine Marsh

Abstract

Extensive monitoring of a marsh created from fine-grained sediment dredged from a nearby channel of the James River estuary, in Virginia, revealed that most mobilization of heavy metal pollutants occurred during the dredging operation. Despite its rapid formation by hydraulic dredging, this marsh was sedimentologically very similar to a nearby natural marsh. Root growth resulted in increased sediment cohesion, but compaction was less than 15%; sediment water content even increased slightly during the first 2 years. Volatile solids were higher, especially in the upper 10 cm of the man-made marsh, as a result of plant litter accumulation. The measured elements (Ca, Fe, Mn, and Zn) in the pore water of both marshes showed no correlation with sediment-exchangeable phase concentrations. Rather, pore water concentrations were consistently correlated with concentrations from a 0.1 M hydroxylamine-hydrochloride and 0.01 M nitric acid extract, which represented easily reducible sediment phase constituents. This suggests that the slow mobilization of Mn and Ca, and possibly Fe and Zn, from the sediments of both marshes is not a simple exchange reaction process. Reducing conditions in the sediments, plant uptake, and partial recycling of these elements to the surface of the marsh via plant litter are important processes in their mobilization from marsh sediment.