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Abstract
PurposeIn 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 methodsFive 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 discussionAfter 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.ConclusionsMost 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.
Highlights
In many delta areas, dense networks of ditches and canals have been developed through time to regulate the water level, to avoid flooding and to facilitate navigation, transport, agriculture and infra-structure construction
The studied dredged sediments have potential to mitigate land subsidence in delta areas when spread on land
The results of O2 consumption and CO2 production have significant error bars which might be justified by the natural heterogeneity between biological systems and by the possible leakage of some of the batch bottles (BBs), since no differences were observed in the organic matter content between replicates
Summary
Dense networks of ditches and canals have been developed through time to regulate the water level, to avoid flooding and to facilitate navigation, transport, agriculture and infra-structure construction. Lowering the groundwater level in these areas deepens the aerobic zone of the soil, leading (especially in peatlands) to the oxidation of organic matter and further land subsidence (Querner et al 2012). Some studies report that the high subsidence rates observed in the layer above the groundwater level in low moor peat soils are 15 % ascribed to shrinkage and 85 % to oxidation of organic matter (Schothorst 1977), whereas others relate land subsidence mainly with consolidation (Teatini et al 2011). Land subsidence as a result of water discharging is occurring in many low-lying areas and delta areas worldwide (Kolker et al 2011; Teatini et al 2011; van Asselen 2011; Wöppelmann et al 2013), and many attempts are made to attenuate or reverse this process (Querner et al 2012). The practice of spreading dredged sediments on land has been carried out for many years, and all reported studies about this practice refer to the behaviour of contaminants (Vandecasteele et al 2002; Bedell et al 2003; Ohimain et al 2004; Perrodin et al 2006), without detailed characterization of the properties and behaviour of the sediments
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