SPATIAL AND DEPTH CONSISTENCY OF RESIDUAL ORGANIC MATTER IN DREDGED SEDIMENTS USED AS TIDALFLAT SUBSTARATE MATERIAL

Abstract

 Dredged sediment can serve as organic matter storage when it is incorporated as a base material for construction of tidal flats. This carbon storing effect contributes to the reduction of greenhouse gas emissions. The field survey was conducted in July 2022 on an artificial tidal flat in Hannan District 2 in Osaka Bay, Japan. A total of four sites in the intertidal and subtidal zones within the tidal flat were surveyed. Vertical core samples (3 m long, 10 cm diameter) were collected using a vibrating core sampler. Laboratory analyses were conducted to measure grain-size composition, water content, soil particle density, loss on ignition (combustion temperature 300°C: IL₃₀₀; combustion temperature 600°C: IL₆₀₀), total organic carbon (TOC), total carbon, and corrosive substances. There were no significant trends in the lateral distribution of IL₆₀₀ in the dredged sediment layer (20 of 22 sites) sampled with the soil test cane. IL₆₀₀ in the samples from 20 sites at different locations were highly correlated with water content as well as with sampling depth in the mud; however, (IL₆₀₀−IL₃₀₀)/IL₆₀₀ varied little. We infer that the degree of degradation of organic matter was consistent across the study area. The spatial distribution of organic residuals on tidal flats constructed using dredged material was found to be unaffected by depth within the sediment, position in the intertidal or subtidal zones, or distance from the mid-breakwater. The estimated residual organic carbon in the dredged sediment utilized in this tidal flat was 82.5 ± 11.6 %.