Effects of Static vs. Tidal Hydraulic Conditions on Biogeochemical Processes in Mesocosms: Degradation of Aromatic Hydrocarbons (AHs)

Abstract

This study addressed the feasibility and practical considerations related to the possible use of contaminated sediments in coastal wetland restoration and creation efforts. Dredge and other sediments are frequently contaminated with chemical pollutants, and large volumes of this potentially recoverable resource are disposed of in dumpsites rather than utilized. This work addresses the possible deployment of sediments contaminated with aromatic hydrocarbons (AHs) in salt marsh creation efforts. The experimental system was a hydrodynamic mesocosm developed to study well-drained, permanently saturated-flooded and alternately flooded-drained (tidal) hydraulic effects on biogeochemical and plant processes. The target AHs for this work were naphthalene, phenanthrene, 1-methylnaphthalene, 1,3-dimethylnaphthalene, 3-methylphenanthrene, and 3,6-dimethylphenanthrene. The results of this work confirmed that different hydraulic conditions and the resultant biogeochemical properties of salt marshes influence the rate, transformation-time profile, and completeness of transformation of AHs in sediments. In general, the relative transformation rates for the AHs in the different systems examined here were drained/oxidized Eh ≥ tidal/oscillating Eh ≫ flood/reduced Eh, as observed previously with other classes of organic chemicals. The generation of hydroxylated AHs coincided with observed compound transformation profiles: decreases in target compounds on sediments were correlated with increases in hydroxylated AHs in the water. While well-drained aerobic conditions seem to be optimal for the transformation/depuration of AHs in salt marsh sediments, in natural settings this kind of system would not be a “wetland” either geohydrologically or with respect to vegetation. So, when evaluating the use of contaminated sediments in “wetland” construction or expansion, the relevant zones are streamside and marsh interior. This work indicates that AH contaminated sediments can be used, alone or mixed with clean sediments or other industrial materials (e.g., phosphogypsum and bauxite “red mud”), for wetland creation or enhancement. It would seem that either or both marsh locations (interior vs. streamside) are appropriate, at least with respect to rapid degradation and elimination of these AHs from the sediments. The tradeoff involves time and potential offsite risks: at streamside, the removal rates were twice as fast as and somewhat more complete than in the interior, but the action of the tides could mobilize contaminants to the estuary and beyond. The interior sediments will allow for less exchange of the pollutants with the estuarine water, but will take longer to depurate, particularly if the sediments become compacted.