Anaerobic sediment organic matter decay generates methane, delays sediment consolidation, reduces sediment density, viscosity and shear strength, all impacting the sediment rheological parameters and the navigable depth. This study quantifies the share of anaerobically and aerobically degradable sediment organic matter (SOM) in a depth profile and along a transect through the tidal river Elbe in the section of the Port of Hamburg. From exponential organic matter decay functions, organic matter decay rates (mg C g_TOC⁻¹ d⁻¹) were derived and clustered with a k-Means Cluster Analysis. The reactivity of different (kinetic) organic matter pools along the river transect were characterized based on their biodegradation rates. A fast, medium, slowly and non-degradable pool (pools 1–4) were identified based on the measured organic matter lability. SOM lability decreased from upstream to downstream, evidenced by the decreasing amount of the easily degradable pool 1 material from upstream to downstream. The size of the slowly degradable pool 3, assumed to be associated with SOM bound to the mineral particles, did not show any spatial gradient and is therefore suggested to represent a baseline share of hardly accessible SOM in the investigation area (about 12%−16% of TOC). Total degradability thus appears to be governed by the amount of SOM present in addition to this basis (pool 3), which in turn follows a source gradient and an age gradient from upstream to downstream. The recalcitrant pool 4 was the largest at any part of the harbour, for any depth, and for both, anaerobic and aerobic conditions (about 75%−85% of TOC). This indicates that the sediment in the investigation area, including the uppermost fluidic and freshly settled layers, mostly comprises stabilised organic matter and contributes largely to storage of organic carbon. Differently sized anaerobic SOM pools with depth were observed as well as seasonal changes of the easily degradable SOM pool 1. The degradability was larger in upper sediment layers, it was also larger under aerobic conditions (by about 10% of TOC) but the differences between aerobic and anaerobic decay decreased from upstream to downstream.

@en

The anaerobic degradation of sediment organic matter leads to considerable gas production in constructions made from sediments and in landfills where contaminated sediments are disposed of, inducing problems with the mechanical stability of constructions or necessitating extraction and treatment of gas. However, little is known about the magnitude of gas generation from dredged sediment, hence validated input parameters for gas production modelling are missing. On the occasion of drillings performed for the installation of inclinometers on a mono-landfill for contaminated dredged sediment, eleven waste layers of known were sampled. Samples were analysed for gas generation in a long-term laboratory incubation experiment carried out for 757 days. It was found that the residual gas potential of the deposited dredged material ranged between 2 and 12 m3 MgDW −1, relating to 3–11% of the organic matter being degraded. Correlation analyses with material properties suggest a strong role of nitrogen, either directly or as indicative parameter, with the gas potential increasing with total nitrogen content and the share of degradable carbon decreasing with increasing TOC/TN ratio. The by far greatest share of organic matter was bound in the heavy density fraction >1.4 g cm−3, suggesting that the readily available light organic matter pool had already been depleted during pre-treatment of the dredged sediment in dewatering fields and the subsequent years of landfilling. Consequently, the correlation of the remaining gas potential with heavy fraction nitrogen was even stronger than for bulk nitrogen. The gas potential as revealed from the long-term test correlated well with short-term values, but outreached the commonly applied potential measured for 21 days by the factor of four. The data improve the state of knowledge on gas production from the large mineral waste stream of dredged material and serve to improve gas production modelling for these types of wastes. The strong correlation of gas potential to TN suggests that TN may serve as a proxy to estimate total gas potential.@en

The water balance and effectiveness of a landfill cover containing a liner from fine-grained dredged material (‘METHA-material’) and a drainage layer from sandy dredged material (‘METHA-sand’) from the Port of Hamburg (Northern Germany) was investigated for 20 years using two test fields (lysimeters), each 500 m² in size. The layering within the standard design test field (FS) is identical to the layering of the cover on the landfill, whereas the alternative design test field (FA) was designed to provoke desiccation of the liner. The setup abstained from continuous side walls cutting through the liners in order to allow for material properties representative for the landfill cover, at the cost of possible minor lateral inflows or outflows, which were, however, considered negligible. By example of the lysimeter FA the risk of desiccation-induced failure of a liner from METHA-material under the given climate could be shown. In contrast, the discharges and hydro-chemical parameters measured for the lysimeter FS indicate continuous high efficiency of the liner until today. The average discharge below the liner was 14.9 mm/yr. The entire cover contributes to the good liner performance. Reason are (1) the low hydraulic conductivity of the liner (5 × 10⁻¹⁰ m/s) and its large thickness (1.5 m); (2) the increased stresses on the liner due to the 2.5 m thick overlaying cover; (3) the 1.0 m thick drainage layer from slowly-draining METHA-sand, allowing for a continuous nearly water-saturated fringe above the liner; and (4) the sufficient amount of plant available water in the recultivation layer.

@en

Purpose: Climate change necessitates heightening and strengthening of dikes, requiring large volumes of suitable soil. This study investigated soil ripening and long-term development of hydraulic behaviour of a dike constructed from slightly contaminated, partially dewatered dredged material from the Port of Hamburg under realistic conditions of construction and operation. 

Materials and methods: Two test fields of 126 m² each were constructed in 2004 for long-term analysis of hydraulic behaviour in combination with the possible mobilisation of contaminants. In test field 1 (MS), the 1-m-thick cover above the sand core consisted of traditionally used alluvial marsh sediment (reference), while in test field 2, the lower 0.7 m of the cover was substituted with dredged material (DM + MS). An HDPE tray below each field served the collection of bottom fluxes and hence the quantification of discharges and analysis of their chemical composition. In 2012, an excavation was carried out to inspect the development of the soil structure. 

Results and discussion: The discharge pattern revealed three distinct phases related to the post-construction structure development and therefore physical ripening of the soils. Within a single year, bottom fluxes typically started in mid-winter and ended in late spring. The dike cover containing DM had a high water retention capacity; however, the vertically continuous primary shrinkage cracks led to higher total bottom fluxes compared to the dike constructed from MS only. System hydraulic conductivities increased by up to six orders of magnitude compared to the as-built condition before soil ripening and structuring. The dredged material maintained a reduced geochemical status for about two years after construction. After first shrinkage and intrusion of oxygen, the material changed to an oxidised state, earmarking the onset of the chemical ripening process. Oxidising conditions were sustained in the long term, seen from the absence of previously elevated ammonium concentrations in the dike seepage. 

Conclusions: System hydraulics of the test field with dredged material were mainly determined by the covering layer of marsh sediment. After construction, the dredged material underwent physical, chemical and biological soil ripening processes, coupled to a respective change in discharge patterns, which, however, did not impair dike stability. It is recommended that soil ripening processes are induced and completed as part of the material’s pre-treatment, precluding the formation of irreversible shrinkage cracks and changeover of redox conditions after construction. The findings contribute to assessing the feasibility of the beneficial use of dredged material.

@en