Physicochemical changes to soil and sediment in managed realignment sites following tidal inundation

Abstract

The recognition of the value of salt marshes and concerns over salt marsh loss has led to the adoption of managed realignment in coastal areas. Managed realignment involves the landward relocation of the seawall, allowing an area of agricultural land to be tidally inundated. It is believed that managed realignment sites can act as a sink for contaminants. However, these sites may also act as a contaminant source and pose a risk to estuarine biota. In this thesis, the potential for metal and herbicide release from agricultural soil and dredged sediment in managed realignment sites was investigated by laboratory microcosm experiments. The agricultural soil and dredged sediment were subjected to two different salinities and drying-rewetting treatments. Results indicate the release of metals (Cu, Ni and Zn) and herbicides (simazine, atrazine and diuron) was dependent on their strength of binding to the soil and sediment, and complexation and competition reactions between seawater anions, cations and the sorbed metals. The release patterns indicated that metal and herbicide release into overlying water may continue for extended periods of time after an initial rapid release. The total metal and herbicide loads released into the overlying water followed the order: Cu < Zn < Ni and diuron < atrazine < simazine with a greater release from the soil than sediment. The increase in CO2 release, mineralisation rates, total metal and herbicide loads after drying and rewetting the soil suggested an increase in the mineralisation of organic matter and the release of organic matter associated metals and herbicides. Results of linear regression analyses provided evidence that the release of the metals and herbicides as DOC-complexes was important for soil but not for sediment. These findings indicate that there is a lower potential for contaminant release from managed realignment sites where dredged sediments are beneficially re-used.