Functional properties of soils formed from biochemical ripening of dredged sediments: subsidence mitigation in delta areas

Purpose: In delta areas, dense networks of canals have been developed through time and have to be periodically dredged. Lowering the groundwater level in delta areas deepens the aerobic zone, leading to the oxidation of organic matter and possibly to land subsidence. The use of the dredged sediments on land can be a solution to mitigate land subsidence in delta areas. Materials and methods: Five types of dredged sediments with different organic matter content and particle size distribution were dewatered for 7 days and then submitted to biochemical ripening during 141 days on a laboratorial scale with constant temperature and relative humidity. The functional properties analysed were the type and content of organic matter, pH, total C, N, P and S, dry bulk density, water retention capacity, aggregate stability and load-bearing capacity. Results and discussion: After biochemical ripening, there was no significant loss in the mass of organic matter but there was an increase in the fraction of stable organic compounds, observed by an increase in oxygen-bearing compounds and a decrease in hydrocarbons during biochemical ripening. The pH was not affected by biochemical ripening, and the total C, N, P and S concentrations are high and therefore the dredged sediments can improve the quality of the land. Most volume lost during dewatering and biochemical ripening can be attributed to the loss of water. The water retention capacity of the dredged sediments changed with biochemical ripening. The soils formed from biochemical ripening have very stable aggregates, and its load-bearing capacity is enough to sustain cattle and tractors. Conclusions: Most volume lost during dewatering and biochemical ripening can be attributed to the loss of water and not organic matter. Therefore, the studied dredged sediments have potential to mitigate land subsidence in delta areas when spread on land.