The ratio of nitrogen (N2) to argon (Ar) in landfill gas was compared to the atmospheric gas ratio to quantify the balance between N2 generating (anaerobic ammonium oxidation, denitrification) and N2 consuming (nitrogen fixation) processes on three landfills undergoing in-situ stabilization. In the aerated landfills, as much as 22% of the extracted N2 could be explained by net denitrification, with coexisting aerobic and anaerobic domains fostering nitrification-dependent denitrification. Nitrogen fixation was also occasionally observed. Removal of nitrogen via the gas phase exceeded nitrogen removed via the leachate by up to a factor of 33. Contrastingly, the anaerobic landfill under leachate recirculation showed a net reduction of N2 in relation to Ar, indicating nitrogen fixation as the dominant mechanism, equivalent up to 28% of the nitrogen in the extracted landfill gas. The balance between denitrification and nitrogen fixation in the aerated sites varied seasonally, likely caused by increased evapotranspiration in the summer, allowing greater air intrusion through the cover soil, resulting in higher NO3– and NO2– availability for denitrification and anammox. No such variability was observed for the landfill under liquid recirculation. The nitrogen transforming microbial community comprised of species responsible for nitrification, ammonification, denitrification, and anammox, indicating all processes may coexist. The findings show aeration supports nitrogen removal through the gas phase, but also suggest that nitrogen fixation adds nitrogen to the waste body in anaerobic domains. This could delay reaching environmental compliance criteria for leachate nitrogen, both for in-situ treatment by aeration and by leachate recirculation.@en

Purpose
Recirculation dredging is a port maintenance concept developed in the Port of Emden, Germany to create a navigable fluid mud layer. This study investigates the effects of recirculation on key sediment properties, including density, yield stress, and oxygen concentration.

Methods
Six field monitoring surveys were carried out at two locations at different times of the year to assess changes before and after recirculation. Bathymetry, bulk density, yield stress, and oxygen concentration profiles were measured in situ. The settling properties and oxygen concentration levels on collected fluid mud samples were analyzed in the laboratory.

Results
The investigation reveals minimal changes in the density of recirculated fluid mud. However, the post-recirculation measurements showed a decrease in yield stress, ranging from 18 to 51% at Große Seeschleuse (GS) and 36% to 52% at Industriehafen (IH). The yield stress and density vary depending on the frequency of dredging. After structural density (1166 kg m−3 in GS and 1173 kg m−3 in IH), the yield stress of fluid mud increased exponentially. Therefore, monitoring of the yield stress is important for recirculation. A slight increase in oxygen concentration was observed post-recirculation, especially during winter. Yet, the rapid decline in oxygen levels post-mixing in the laboratory showed that sustaining long-term elevated oxygenation levels is not feasible by recirculation dredging alone.

Conclusions
The findings highlight the effectiveness of the recirculation on the yield stress, density, and oxygen concentration of fluid mud and illustrate the importance of considering both density and yield stress in sediment management practices. Future research should address the temporal evolution of density, yield stress, and oxygen levels following a dredging intervention and the influence of extracellular polymeric substances (EPS) and organic matter decay on sediment behavior.@en

This study quantifies the field hydraulic performance of a dual-functionality landfill cover, combining microbial methane oxidation with water diversion using a capillary barrier. The investigated 500 m2 test field, constructed on a landfill in the Netherlands, consisted of a cover soil optimised for methane oxidation, underlain by a sandy capillary layer and a gravelly capillary block. Outflows from these layers were measured between 2009 and 2023. Average precipitation was 848 mm/a, evapotranspiration, diverted infiltration and breakthrough amounted to 504 (59.4 %), 282 (33.3 %) and 62 (7.3 %) mm/a, respectively. On average, the capillary barrier diverted 82 % of the inflow into the capillary layer. Breakthrough occurred mainly from October to March when evapotranspiration was low and the maximum water storage capacity of the cover soil was reached. During this period, inflow into the capillary barrier exceeded its diversion capacity, caused by the relatively high hydraulic conductivity of the cover soil due to its optimisation for gas transport. The diversion capacity declined drastically in the year after construction and increased again afterwards. This was attributed to suffusion of sand from the capillary layer into the capillary block and subsequent washout to greater depths or the influence of iron precipitates at the bottom of the capillary layer. The effect of a more finely grained methane oxidation layer on the hydraulic and methane oxidation performance should be investigated further. These measures could further improve the combined performance of the dual functionality landfill cover system under the given conditions of a temperate climate.@en

Purpose: The share of microbially degradable sediment organic matter (SOM) and the degradation rate depend, among others, on the intrinsic properties of SOM as well as on the type and concentration of terminal electron acceptors (TEA). Next to its role as TEA, molecular oxygen enhances SOM decay by oxygenase-mediated breakdown of complex organic molecules. This research investigated long-term SOM decay (> 250 days) under aerobic and anaerobic conditions to (1) provide a basis for sediment carbon flux estimates from the River Elbe estuary and (2) assess the potential for carbon burial in relation to redox conditions and dredging interventions. 

Methods: Long-term aerobic and anaerobic SOM decay in fluid mud, pre-consolidated and consolidated sediment layers was investigated over three years along a transect of ca. 20 km through the Port of Hamburg, starting at the first hydrodynamically determined hotspot of sedimentation after the weir in Geesthacht. Absolute differences between aerobic and anaerobic cumulative carbon mineralization were calculated, as well as their ratio. Findings were correlated to a suite of solids and pore water properties. 

Results: SOM decay followed first order multi-phase exponential decay kinetics. The ratio between C release under aerobic and anaerobic conditions ranged around 4 in the short-term, converging to a value of 2 in the long term. Strong gradients in absolute C release along the upstream–downstream transect did not reflect in a corresponding gradient of the aerobic-anaerobic ratio. C release was most strongly correlated to the water-soluble organic matter, in particular humic acids. Contact of anaerobically stabilized sediment with the oxygenated water phase induced significant release of carbon. 

Conclusion: SOM degradability in the study area exhibited strong spatial gradients in relation to the organic matter source gradient but was mainly limited by the high extent of organic matter stabilization. Under these conditions, molecular oxygen as TEA provides little thermodynamic advantage. Carbon-sensitive sediment management, considering SOM reactivity patterns in stratified depositional areas, is a powerful strategy to reduce environmental impacts of dredging measures.

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Degradability of organic matter in river sediments differs in relation to origin and age. In order to explain previously observed spatial patterns of organic matter degradability and stabilization, this study investigated sediment organic matter (SOM) properties along a tidal Elbe river transect using dissolved organic matter (DOM) fractions, density fractions, carbon stable isotopes and thermometric pyrolysis (Rock-Eval 6). These properties were linked to SOM decay rates and biological indicators such as chlorophyll a and silicic acid in the water phase, and sediment-bound extracellular polymeric substances (EPS), microbial biomass and oxygen consumption. Sediment source gradients were established using the concentration of Zn in the fraction < 20 μm as proxy. The specific Zn concentration showed that the most upstream location was nourished primarily by upstream fluviatile sediments while the other locations carried a downstream signature. The upstream location was also characterised by the highest concentrations of chlorophyll a, microbial biomass, silicic acid, EPS, humic acids and hydrophilic DOM, the most negative δ13C signature and by the highest oxygen consumption rate, with decreasing trends towards downstream locations. This trend was also evident in the decreasing SOM lability from upstream to downstream, an increasing share of total SOM found in the acid-base-extractable fractions and a
decreasing share of carbon in the light density fractions. Thermometric pyrolysis revealed the highest H-index(easily degradable SOM) for the most upstream location and the ratio of the I-index (immature SOM) to the R-
index (refractory SOM) to correlate positively with measured SOM decay rates. This study suggests that spatial patterns of SOM degradability can be explained by a source gradient, with young organic matter entering the system from upstream from predominantly biogenic sources, while down-stream sources (North Sea sediment) deliver more refractory SOM that is stabilized in organo-mineral associations to a higher extent. In the investigated sediments, dissolved organic matter represented 0.23–1.20% of the total organic carbon (TOC) from anaerobically degradable SOM, while 4.10–11.46% TOC was liberated as CO2 and CH4 after long-term incubation (250 days). Thermometric pyrolysis is shown to serve as a useful proxy for SOM degradability in river sediments, with the Hydrogen-Index (HI) correlating well with degradability and the relationship between the I-index and R-index changing consistently towards lower I-indices and higher R-indices with an increasing degree of SOM stabilization.

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The recirculation and infiltration of leachate in landfills may be carried out to facilitate the flushing of contaminants and accelerate the stabilisation of waste. Flushing contaminants through recirculation relies on the movement of fluids through the landfill body to basal drains, which will predominantly be driven by gravity. Leachate recirculation and infiltration measures commenced at de Kragge II landfill (Bergen op Zoom, The Netherlands) in March 2018. Up to 90 m3/day of treated leachate is recirculated into the top of a 20 m deep, 5 ha landfill cell through 14 horizontal drains installed at the surface. Poor connectivity between the waste and the basal drainage system has resulted in saturated conditions forming in the lower 7-8 m of the landfill. Knowledge about the leachate flow within the waste body is essential for evaluating the success of the stabilisation measures. To investigate the flow regime within the saturated waste, 22 Single Borehole Dilution tests were carried out in 13 piezometers at different depths, between 8.4 and 18.1 m below ground level, and locations across the landfill cell. Tests were repeated in a number of the piezometers to demonstrate repeatability. Flow was measured in all piezometers. Calculated Darcy flow velocities ranged between 0.01 and 1.02 m/day, with the highest velocities measured in the deepest piezometers. Four tests were carried out in one nest of piezometers installed at different depths, with the leachate recirculation system switched off for two days prior to and during the test. Although flows were somewhat higher in two of the piezometers, it was not possible to conclude whether the infiltration of leachate significantly influences flow.@en

This paper presents the preliminary results of field trials conducted to investigate the air permeability of waste at the Braambergen landfill located near the city of Almere, the Netherlands. Pressure variations were monitored in surrounding wells during air extraction tests using differential pressure transducers. The magnitude of the pressure response to gas abstraction indicates suitability of the method to investigate waste permeability and the swiftness of the pressure response indicated good connectivity within the investigated well field. The obtained air permeability values showed a trend where permeability decreased as the distance between two wells increased, suggesting higher permeability in closer proximity to a well. Although the values are comparable to those reported in other landfills, the differences can be explained by the influence of site-specific factors on permeability.@en

In the Netherlands, three full-scale pilots have been in operation for approximately five years to understand the effects of leachate recirculation or aeration on waste stabilisation. This study employs the ratio of N2 to Ar in the landfill gas in comparison to the ratio in atmospheric air to derive the share of N2 that originates from denitrification. We collected samples from the three pilot landfills from different gas wells, gas collector systems and from the total bulk extracted gas and measured its composition using gas chromatography. We estimated the aeration efficiency of the two landfills under in situ aeration based on the CO2/CH4 ratio as an indicator of aerobic processes. Denitrification dominated in the aerated landfills, with as much as 13% of N2 being explained by the net effect of denitrification, whereas the landfill under leachate recirculation showed a net ‘loss’ of N2, indicating N2 fixation to be a dominant mechanism. There was a seasonal variability of the balance between denitrification and N2 fixation in the aerated sites, likely caused by increased aeration efficiency and hence increased availability of NO3- for denitrification under summer conditions with lower moisture content in the cover soil, allowing for increased air ingress. No such variability was observed for the landfill under liquid recirculation. Future evaluation of the microbial community composition will further elucidate N transformation pathways in landfills under different in-situ stabilisation treatments.@en

Nitrogen undergoes multiple biogeochemical transformations during waste degradation, which depend on speciation, prevailing geochemical boundary conditions, and waste surface properties. This study developed a waste biodegradation model with high flexibility in accommodating reaction pathways to assess different process dynamics. The model was applied to landfill simulator reactors operating anaerobically. Model results show that dilution with adsorption matches the experimental dissolved NH4+ concentration (C/N=25) at the early experimental stages. Also, NH4+ binding decreases due to competition with Ca2+, and the model better captures the dissolved NH4+ behavior when CaSO4 is present in solution. Mass removal due to sampling and posterior dilution are the main mechanisms to reduce NH4+ concentration in the leachate. The model highlights the role of nitrogen sorption as the main mechanism for nitrogen accumulation in the solid phase of municipal solid waste.@en

This study quantifies the share of aerobically produced carbon (aeration efficiency) during six years of a full scale landfill aeration project using the balance between methane and carbon dioxide in the bulk extracted gas. Aeration was realized by overextraction. Aeration enhanced carbon release in comparison to the anaerobic ‘base case’, as predicted by the Afvalzorg multiphase model, by a factor of 3.7. Aeration efficiency, averaging around 44%, varied seasonally, and was lower in periods of low or no evapotranspiration and hence higher moisture content in the landfill cover soil (winter). Higher aeration efficiencies were observed when evapotranspiration enables increased cover soil permeability (summer). Correspondingly, aeration efficiency was linearly related to the concentration of N2 in the bulk extracted gas. To a lesser extent, condensate and its removal also affected flow and hence the aeration efficiency. Except for the modulation by seasonal effects, the cumulative amount of extracted ‘aerobic carbon’ increased linearly over time, independent of changes in the blower pressure and flow. This suggests that below the cover soil, within the waste body, flow is chanelled in preferential pathways, limiting the intrusion of oxygen into the bulk waste. Aeration can hence only be enhanced by reducing well spacing. The blower efficiency, assessed by the ratio of flow to pressure, decreased markedly over time, likely indicating diminishing waste permeability as a result of waste consolidation.@en

In-situ stabilization of waste bodies can be achieved by the infiltration of water or recirculation of leachate into the landfill, which is thought to enhance the microbial degradation of waste organics by (re-)moisturizing dry zones and flushing out metabolic products of organic matter decay. The success of in-situ stabilization should reflect in initially accelerated and thereafter reduced rates of anaerobic waste organic matter decay rates. This paper compares the methane generation that was modelled using the Afvalzorg multiphase model without the added effect of leachate recirculation with actually extracted methane in the landfill and gas generation on sampled wastes following five years of leachate recirculation on a Dutch landfill. Laboratory incubations revealed a methane potential between 0.03 kg CH4/t dw and 15.8 kg CH4/t dw for 365 days. Clear trends with respect to depth, moisture content, total organic carbon or share in hard plastics did not emerge as overall waste heterogeneity was high and likely obfuscated the correlation analysis. The results showed a recovery efficiency of 30.4% for 2021, with 0.07 kg CH4/t dw for the recovered methane and 0.23 kg CH4/t dw for the predicted methane in compartment 3. The average methane potential measured in the laboratory was almost twice as high as the remaining methane potential predicted for the period of 2021-2093. The discrepancy could be due to (i) enhanced waste degradability as a result of five years of recirculation, (ii) enhancing effects of material perturbation during sampling and/or (iii) impeded on-site methane generation and gas and water transport limitations due to presence of plastics. Overall, the laboratory incubations demonstrate a significant potential for waste biodegradation still residing in the waste.@en

Application of biochar to landfill cover soils can purportedly improve methane (CH4) oxidation rates, but understanding the combined effects of soil texture, compaction, and biochar on the activity and composition of the methanotrophs is limited. The amendment of wood biochar on two differently textured landfill cover soils at three compaction levels of the Proctor density was explored by analyzing changes in soil physical properties relevant to methane oxidation, the effects on CH4 oxidation rates, and the composition of the methanotrophic community. Loose soils with and without biochar were pre-incubated to equally elevate the CH4 oxidation rates. Hereafter, soils were compacted and re-incubated. Methane oxidation rates, gas diffusivity, water retention characteristics, and pore size distribution were analyzed on the compacted soils. The relative abundance of methanotrophic bacteria (MOB) was determined at the end of both the pre-incubation and incubation tests of the packed samples. Biochar significantly increased porosity at all compaction levels, enhancing diffusion coefficients. Also, a re-distribution in pore sizes was observed. Increased gas diffusivity from low compaction and amendment of biochar, though, did not reflect higher methane oxidation rates due to high diffusive oxygen fluxes over the limited height of the compacted soil specimens. All soils, with and without biochar, were strongly dominated by Type II methanotrophs. In the sandy soil, biochar amendment strongly increased MOB abundance, which could be attributed to a corresponding increase in the relative abundance of Methylocystis species, while no such response was observed in the clayey soil. Compaction did not change the community composition in either soil. Fir-wood biochar addition to landfill cover soils may not always enhance methanotrophic activity and hence reduce fugitive methane emissions, with the effect being soil-specific. However, especially in finer and more compacted soils, biochar amendment can maintain soil diffusivity above a critical level, preventing the collapse of methanotrophy.@en

Within the framework of the Dutch sustainable landfill project iDS, four compartments of the Dutch landfill Braambergen have been treated by in-situ aeration since 2017. The aeration infrastructure comprises 230 wells with a spacing of 15 to 20 m, distrib-uted over an area of around 10 ha, intercepting a waste body of 1.2 × 10⁶ t of contam-inated soils, soil treatment residues, bottom ashes and construction and demolition waste. The wells, used in an alternating fashion for air injection and gas extraction, can also be used to monitor water tables within the waste body. In order to describe the spatial variability of waste hydraulics, design a larger scale leachate pumping test and, eventually, support model predictions of the site’s water balance and emission potential, analyses of leachate composition and pumping tests on individual wells have been conducted. The spatial variability of leachate quality and water tables is very high with no geospatial relationship between the sampling points. Each sampling point is representative of itself only. Large differences prevail not only between and across the compartments, but also between directly neighbouring wells. Both the small scale differences in leachate tables as well as in leachate quality indicate a spatial pattern of zones with low horizontal connectivity within the waste body. Recovery rates of drawdown in the wells yielded preliminary estimates of horizontal waste hydraulic conductivity in the order of 1×10^-7 to 6×10^-4 m/s.

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Urbanization influences soil carbon (C) stocks and flows, which, in turn, affect soil-derived ecosystem services. This paper explores soil C storage in urban greenspaces in the Dutch city of The Hague along a transect from the suburban seaside towards the city centre, reflecting a toposequence from dune to peaty inland soils. C storage and C mineralisation potential were evaluated in relation to soil type and greenspace categories. Several soil-quality characteristics were measured, including dissolved organic C, pH, electrical conductivity, nitrogen, phosphorus, sulphur, calcium carbonate, and the water-holding capacity of the soil to evaluate what drives soil C storage in the urban context. The total SOC storage of the upper 30 cm of the greenspaces in The Hague (20.8 km2 with 37% greenspace) was estimated at 78.4 kt, which was significantly higher than assumed given their soil types. Degradability of soil organic matter in laboratory batch tests varied between 0.2 and 3 mg C gSOC−1 day−1. Degradability was highest in the seaside dune soils; however, extrapolated to the topsoil using the bulk density, topsoil C mineralization was higher in the urban forest. Soils beneath shrubs appeared to be hotspots for C storage, accounting for only 13% of the aerial cover but reflecting 24% of the total C storage. Land ownership, land use, greenspaces size, litter management and soil type did not result in significantly different C stocks, suggesting that processes driving urban soil C storage are controlled by different factors, namely land cover and the urbanization extent.@en

Purpose
Sediment organic matter (SOM) influences settling and thus the rheological behavior of suspended particles by enhancing flocculation or reducing surface charges by forming organo-mineral complexes that facilitate particle–particle interactions in consolidating sediments. It was, therefore, assumed that the microbial degradation of SOM and its spatio-temporal variability would affect sediment rheological properties and enhance port maintenance dredging and navigability of ports and waterways.

Methods
To investigate this effect, samples were taken at six locations along a transect of 30 river kilometers through the Port of Hamburg, Germany, during nine sampling campaigns within two years. The collected samples were divided into different layers based on the differences in visual consistency and strength. For analysis of SOM degradability, the samples were incubated in the laboratory for 250 days in glass bottles under aerobic and anaerobic conditions following the evolution of gas composition (CH4, CO2) and pressure in the bottle headspace over time. Yield stress was analyzed before and after the dissolved organic matter (DOM) decay using a rheometer with Couette geometry. Standard properties of solids and pore water were also analyzed.

Results
Shear strength decreased upon SOM decay under both anaerobic and aerobic conditions. Under anaerobic conditions, organic matter decay reduced static and fluidic yield stresses to an average of 74% and 79% of the fresh sample values. Consolidated layers at lower depths showed the highest absolute decrease in fluidic yield stress of up to –110 Pa due to a larger absolute amount of degradable organic matter in these layers in connection to higher bulk density. Pronounced spatial trends with higher changes in yield stress at upstream locations and lower yield stress changes at downstream locations coincided with a decreasing gradient of SOM degradability from upstream to downstream. Seasonal trends indicated that the investigation area is impacted by temporally changing factors.

Conclusion
The availability of easily degradable organic matter significantly affects sediment strength, especially under the anaerobic conditions, even when the mass loss of organic matter mass loss is small. Seasonal variability in yield stress changes upon SOM decay indicate that the site-specific responses were modulated by overarching seasonal effects impacting the entire investigation area. It was assumed that during an anaerobic decay, the formation of gas bubbles added an additional physical component to the effect of biological SOM decay.@en

Maintenance of the nautical depth in the seaport of Emden, Germany, is achieved by re-circulation of fluid mud using a trailing suction hopper dredger. Continued re-circulation has proven to maintain low settling rates and to keep yield stresses of re-circulated fluid mud below 50-100 Pa, in combination with densities of 1.15- 1.2 t/m³ enabling safe navigation through these layers. It is assumed that low-density extracellular polymeric substances (EPS), produced by a highly adapted microbial community, are responsible for the desired effect of keeping fine-grained sediment in suspension and that the regular contact with the oxygenated water phase during recirculation dredging supports thriving of this community. Climate change models have indicated increased future discharge of fresh water from the hinterland into the saline port of Emden. It is hypothesized that increased freshwater discharge alters the composition and activity of the microbial community and hence changes the prerequisites for maintaining the currently favourable properties of fluid mud. In this paper we evaluate this hypothesis by elucidating the main factors governing the success of the current practice of re-circulation dredging in the Port of Emden by means of field and laboratory investigations. Initial results obtained in this ongoing research suggest that the low yield stresses of fluid mud in the Port of Emden are not the result of a specialized microbial community producing high concentrations of EPS, but can be solely attributed to a reduction in density as achieved by re-circulation dredging. Secondly, it appears that future possible increases in the share of freshwater do not affect the sediment’s rheological response, settling behaviour and concentration of extracellular polymeric substances and will hence not adversely impact the maintenance of the nautical depth. @en