RATE OF SEQUESTRATION OF ORGANIC CARBON IN DREDGED SEDIMEMTS USED AS BACKFILL FOR A DEPRESSION-TYPE, SUBAQUEOUS BORROW PIT

<p> Sediments dredged from navigation channels have been used as a foundation for artificial tidal flats and as backfill for depression-type, subaqueous borrow pits. The goal of this study was to determine the rate of sequestration of organic carbon and production of methane in the dredged sediments used as backfill for a subaqueous borrow pit. A field survey was conducted in September 2023 at the restoration site of a depression-type, subaqueous borrow pit in Mikawa Port, Japan at two sites: St. 1, where no sand capping was applied, and St. 2, where sand capping was applied. Vertical core samples were collected using a vibrating core sampler. Laboratory analyses were conducted to measure, the particle size distribution, water content, loss on ignition (600°C; IL), total organic carbon, and methane in the sediment. The IL was used as an indicator of organic matter to calculate the rate of sequestration of organic carbon in the dredged sediments. The initial values used to calculate the sequestration rate of organic carbon were taken from an earlier survey conducted in 2008. The sequestration rates calculated for St. 1 and St. 2, 117% and 106%, respectively, exceeded 100%. The initial IL value for the dredged sediments in the 2023 survey (estimated from the relationship between the silt/clay ratio and the IL of the dredged sediments at the time of the 2008 survey) was higher at St. 1 (96.9%) without sand capping than at St. 2 (83.3%) with sand capping. Depending on construction conditions, the rate of sequestration of organic carbon may be higher when dredged sediments are used as backfill for a subaqueous borrow pit than when they are used as substrate for tidal flats. The amount of methane produced in the dredged sediments was small compared to the carbon sequestered in the dredged sediments used as backfill for the depression-type, subaqueous borrow pit.</p>