Metal contamination pathways of a restored marshland in an industrial estuary

Abstract

Contaminated surface sediments were removed from a coastal marsh and replaced with clean engineered sediments before marsh vegetation was replanted and the site was reopened to the tide. Differences in trace metal concentrations (Cd, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Zn) were monitored for 11 years at different sediment depths, under two types of engineered sediments (with and without compost), and at three marsh surface elevations. Except for Hg and Cd, all others followed the patterns of Fe and Mn of decreasing concentration during drought years and increasing concentration during wet years. Metals in deep sediments (> 25 cm) are always higher than on the surface. Surfaces higher in the terrain had fewer metal concentrations compared to lower elevations. The compost treatment had no insignificant effect on metal concentration. The main pathways involved in remobilization from the buried legacy sediments are organic metal complexation, reductive dissolution, and oxidation of metal sulfides. Hg showed an enrichment pattern throughout the study period suggesting atmospheric deposition as the main pathway. Similarly, sources of Cd are most likely from atmospheric wet deposition since most of the Cd variation is explained by changes in the amounts of rain. Metal concentration in the sediments decreased up to 50% from pre-restoration conditions; the only exceptions were Hg and Cd which continue to accumulate. Increasing dry periods and storm frequencies from changing global weather patterns have the potential to create conditions for greater bioavailability of metals from this and similar world estuaries.