Single-well injection-withdrawal tests as a contaminant transport characterisation tool for landfilled waste.

A new single well injection withdrawal (SWIW) test was trialled at four landfills using the tracers lithium and deuterium, and by injecting clean water and measuring electrical conductivity. The aim of the research was to develop a practical test for measuring lateral contaminant transport to aid in the design of landfill flushing. Borehole dilution tests using dyes were undertaken prior to each SWIW test to determine background flow velocities. SWIW tests were performed at different scales by varying the volume of tracer injected (1 to 5,800 m3) and the test duration (2 to 266 days). Tracers were used individually, simultaneously or sequentially to examine repeatability and scaling. Mobile porosities, estimated from first arrival times in observation wells and from model fitting ranged from 0.02 to 0.14. The low mobile porosities measured rule out a purely advective-dispersive system and support a conceptual model of a highly preferential dual-porosity flow system with localised heterogeneity. A dual-porosity model was used to interpret the results. The model gave a good fit to the test data in 7 out of 11 tests (where R2 ≥ 0.98), and the parameters derived are compatible with previous experiments in MSW. Block diffusion times were estimated to range from 12 to 6,630 h, with a scaling relationship apparent between the size of the test (volume of tracer used and/or the duration) and the observed block diffusion time. This scaling relationship means affordable small-scale tests can inform larger-scale flushing operations.

Future challenges of coastal landfills exacerbated by sea level rise.

In England and Wales, there are at least 1700 coastal landfills in the coastal flood plain and at least 60 threatened by erosion, illustrating a global problem. These landfills are a major issue in shoreline management planning (SMP) which aims to manage the risks associated with flooding and coastal erosion. Where landfills exist, "hold the line" (requiring the building or upgrading of artificial defences to maintain the current shoreline) is often selected as the preferred SMP option, although government funding is not available at present. To investigate these issues in detail, three case-study landfills are used to examine the risks of future flooding and erosion together with potential mitigation options. These cases represent a contrasting range of coastal landfill settings. The study includes consideration of sea-level rise and climate change which exacerbates risks of erosion and flooding of landfills. It is fundamental to recognise that the release of solid waste in coastal zones is a problem with a geological timescale and these problems will not go away if ignored. Future erosion and release of solid waste is found to be more of a threat than flooding and leachate release from landfills. However, while leachate release can be assessed, there is presently a lack of methods to assess the risks from the release of solid waste. Hence, a lack of science constrains the design of remediation options.

Measuring methane emissions from a UK landfill using the tracer dispersion method and the influence of operational and environmental factors.

The methane emissions from a landfill in south-east, UK were successfully quantified during a six-day measurement campaign using the tracer dispersion method. The fair weather conditions made it necessary to perform measurements in the late afternoon and in the evening when the lower solar flux resulted in a more stable troposphere with a lower inversion layer. This caused a slower mixing of the gasses, but allowed plume measurements up to 6700 m downwind from the landfill. The average methane emission varied between 217 ±â€¯14 and 410 ±â€¯18 kg h-1 within the individual measurement days, but the measured emission rates were higher on the first three days (333 ±â€¯27, 371 ±â€¯42 and 410 ±â€¯18 kg h-1) compared to the last three days (217 ±â€¯14, 249 ±â€¯20 and 263 ±â€¯22 kg h-1). It was not possible to completely isolate the extent to which these variations were a consequence of measuring artefacts, such as wind/measurement direction and measurement distance, or from an actual change in the fugitive emission. Such emission change is known to occur with changes in the atmospheric pressure. The higher emissions measured during the first three days of the campaign were measured during a period with an overall decrease in atmospheric pressure (from approximately 1014 mbar on day 1 to 987 mbar on day 6). The lower emissions measured during the last three days of the campaign were carried out during a period with an initial pressure increase followed by a period of slowly reducing pressure. The average daily methane recovery flow varied between 633 and 679 kg h-1 at STP (1 atm, 0 °C). The methane emitted to the atmosphere accounted for approximately 31% of the total methane generated, assuming that the methane generated is the sum of the methane recovered and the methane emitted to the atmosphere, thus not including a potential methane oxidation in the landfill cover soil.

AERMOD as a Gaussian dispersion model for planning tracer gas dispersion tests for landfill methane emission quantification.

The measurement of methane emissions from landfills is important to the understanding of landfills' contribution to greenhouse gas emissions. The Tracer Dispersion Method (TDM) is becoming widely accepted as a technique, which allows landfill emissions to be quantified accurately provided that measurements are taken where the plumes of a released tracer-gas and landfill-gas are well-mixed. However, the distance at which full mixing of the gases occurs is generally unknown prior to any experimental campaign. To overcome this problem the present paper demonstrates that, for any specific TDM application, a simple Gaussian dispersion model (AERMOD) can be run beforehand to help determine the distance from the source at which full mixing conditions occur, and the likely associated measurement errors. An AERMOD model was created to simulate a series of TDM trials carried out at a UK landfill, and was benchmarked against the experimental data obtained. The model was used to investigate the impact of different factors (e.g. tracer cylinder placements, wind directions, atmospheric stability parameters) on TDM results to identify appropriate experimental set ups for different conditions. The contribution of incomplete vertical mixing of tracer and landfill gas on TDM measurement error was explored using the model. It was observed that full mixing conditions at ground level do not imply full mixing over the entire plume height. However, when full mixing conditions were satisfied at ground level, then the error introduced by variations in mixing higher up were always less than 10%.

Validation and error assessment of the mobile tracer gas dispersion method for measurement of fugitive emissions from area sources.

A controlled release test was carried out to assess the accuracy of the tracer gas dispersion method, which is used to measure whole-site landfill methane (CH4) emissions as well as fugitive emissions from other area sources. Two teams performed measurements using analytical instruments installed in two vehicles, to measure downwind concentrations of target (CH4) and tracer gases at distances of 1.2-3.5 km from the release locations. The controlled target gas release rates were either 5.3 or 10.9 kg CH4 h-1, and target and tracer gases were released at distances between 12 m and 140 m from each other. Five measurement campaigns were performed, where the plume was traversed between 2 and 31 times. The measured target gas emissions agreed well with the controlled releases, with rate differences no greater than 1.1 kg CH4 h-1 for Team A and 1.0 kg CH4 h-1 for Team B when quantifying a controlled release of 10.9 kg CH4 h-1. This corresponds to a maximum error of ±10%. A larger error of up to 18% was seen in the campaign with a lower target gas release rate (5.3 kg CH4 h-1). Using a cross plume integration method to calculate tracer gas to target gas ratios provided the most accurate results (lowest error), whereas larger errors (up to 49%) were observed when using other calculation methods. By establishment of an error budget and comparison with the measured error based on the release test, it could be concluded that following best practice when performing measurements, the overall error of a tracer gas dispersion measurement is very likely to be less than 20%.

Doublet tracer tests to determine the contaminant flushing properties of a municipal solid waste landfill.

This paper describes a programme of research investigating horizontal fluid flow and solute transport through saturated municipal solid waste (MSW) landfill. The purpose is to inform engineering strategies for future contaminant flushing. Solute transport between injection/abstraction well pairs (doublets) is investigated using three tracers over five separate tests at well separations between 5m and 20m. Two inorganic tracers (lithium and bromide) were used, plus the fluorescent dye tracer, rhodamine-WT. There was no evidence for persistent preferential horizons or pathways at the inter-well scale. The time for tracer movement to the abstraction wells varied with well spacing as predicted for a homogeneous isotropic continuum. The time for tracer movement to remote observation wells was also as expected. Mobile porosity was estimated as ~0.02 (~4% of total porosity). Good fits to the tracer breakthrough data were achieved using a dual-porosity model, with immobile regions characterised by block diffusion timescales in the range of about one to ten years. This implies that diffusional exchanges are likely to be very significant for engineering of whole-site contaminant flushing and possibly rate-limiting.

Multiple-tracer tests for contaminant transport process identification in saturated municipal solid waste.

Two column tests were performed in conditions emulating vertical flow beneath the leachate table in a biologically active landfill to determine dominant transport mechanisms occurring in landfills. An improved understanding of contaminant transport process in wastes is required for developing better predictions about potential length of the long term aftercare of landfills, currently measured in timescales of centuries. Three tracers (lithium, bromide and deuterium) were used. Lithium did not behave conservatively. Given that lithium has been used extensively for tracing in landfill wastes, the tracer itself and the findings of previous tests which assume that it has behaved conservatively may need revisiting. The smaller column test could not be fitted with continuum models, probably because the volume of waste was below a representative elemental volume. Modelling compared advection-dispersion (AD), dual porosity (DP) and hybrid AD-DP models. Of these models, the DP model was found to be the most suitable. Although there is good evidence to suggest that diffusion is an important transport mechanism, the breakthrough curves of the different tracers did not differ from each other as would be predicted based on the free-water diffusion coefficients. This suggested that solute diffusion in wastes requires further study.

Investigating the effect of compression on solute transport through degrading municipal solid waste.

The effect of applied compression on the nature of liquid flow and hence the movement of contaminants within municipal solid waste was examined by means of thirteen tracer tests conducted on five separate waste samples. The conservative nature of bromide, lithium and deuterium tracers was evaluated and linked to the presence of degradation in the sample. Lithium and deuterium tracers were non-conservative in the presence of degradation, whereas the bromide remained effectively conservative under all conditions. Solute diffusion times into and out of less mobile blocks of waste were compared for each test under the assumption of dominantly dual-porosity flow. Despite the fact that hydraulic conductivity changed strongly with applied stress, the block diffusion times were found to be much less sensitive to compression. A simple conceptual model, whereby flow is dominated by sub-parallel low permeability obstructions which define predominantly horizontally aligned less mobile zones, is able to explain this result. Compression tends to narrow the gap between the obstructions, but not significantly alter the horizontal length scale. Irrespective of knowledge of the true flow pattern, these results show that simple models of solute flushing from landfill which do not include depth dependent changes in solute transport parameters are justified.

A multi-component two-phase flow algorithm for use in landfill processes modelling.

This paper describes the finite difference algorithm that has been developed for the flow sub-model of the University of Southampton landfill degradation and transport model LDAT. The liquid and gas phase flow components are first decoupled from the solid phase of the full multi-phase, multi-component landfill process constitutive equations and are then rearranged into a format that can be applied as a calculation procedure within the framework of a three dimensional array of finite difference rectangular elements. The algorithm contains a source term which accommodates the non-flow landfill processes of degradation, gas solubility, and leachate chemical equilibrium, sub-models that have been described in White and Beaven (2013). The paper includes an illustration of the application of the flow sub-model in the context of the leachate recirculation tests carried out at the Beddington landfill project. This illustration demonstrates the ability of the sub-model to track movement in the gas phase as well as the liquid phase, and to simulate multi-directional flow patterns that are different in each of the phases.

A new economic instrument for financing accelerated landfill aftercare.

The key aspects of landfill operation that remain unresolved are the extended timescale and uncertain funding of the post-closure period. This paper reviews the topic and proposes an economic instrument to resolve the unsustainable nature of the current situation. Unsustainability arises from the sluggish degradation of organic material and also the slow flushing of potential pollutants that is exacerbated by low-permeability capping. A landfill tax or aftercare provision rebate is proposed as an economic instrument to encourage operators to actively advance the stabilization of landfilled waste. The rebate could be accommodated within existing regulatory and tax regimes and would be paid for: (i) every tonne of nitrogen (or other agreed leachate marker) whose removal is advanced via the accelerated production and extraction of leachate; (ii) every tonne of non-commercially viable carbon removed via landfill gas collection and treatment. The rebates would be set at a level that would make it financially attractive to operators and would encourage measures such as leachate recirculation, in situ aeration, and enhanced flushing. Illustrative calculations suggest that a maximum rebate of up to ∼€50/tonne MSW would provide an adequate incentive.

Quantifying the effect of settlement and gas on solute flow and transport through treated municipal solid waste.

The effect of degradation and settlement on transport properties of mechanically and biologically treated (MBT) waste was examined by applying three different tracers to two waste columns (~0.5 m diameter) in a series of closed-loop experiments. One column was allowed to biodegrade and the other was bio-suppressed. Permeability and drainable porosity were reduced by settlement, in line with previous results. A dual-porosity model performed well against the data and suggested that more preferential flow occurred early on in the un-degraded column. Diffusion timescales were found to be between 0.8 and 6 days. Volumetric water contents of the mobile region were found to be small in the bio-suppressed cell (~0.01) and even smaller values were found in the degrading waste, possibly due to displacement by gas. Once either settlement or gas production had disrupted this pattern into a more even flow, subsequent compression made little difference to the diffusion time-scale. This may indicate that transport was thereafter dominated by other aspects of the waste structure such as the distribution of low-permeability objects. The presence of gas in the degrading waste reduced the volumetric water content through displacement. The model indicated that the gas was primarily located in the more mobile porosity fraction. Primary compression of the degrading waste tended to squeeze this gas out of the waste in preference to water.

Developments to a landfill processes model following its application to two landfill modelling challenges.

The landfill model LDAT simulates the transport and bio-chemical behaviour of the solid, liquid and gas phases of waste contained in a landfill. LDAT was applied to the LMC1 and LMC2 landfill modelling challenges held in 2009 and 2011. These were blind modelling challenges with the model acting in a predictive mode based on limited early time sections of full datasets. The LMC1 challenge dataset was from a 0.34m deep 0.48m diameter laboratory test cell, and the LMC2 dataset was from a 55m×80m 8m deep landfill test cell which formed part of the Dutch sustainable landfill research programme at Landgraaf in the Netherlands. The paper describes developments in LDAT arising directly from the experience of responding to the two challenges, and discusses the model input and output data obtained from a calibration using the full datasets. The developments include the modularisation of the model into a set of linked sub-models, the strategy for converting conventional waste characteristics into model input parameters, the identification of flexible degradation pathways to control the CO2:CH4 ratio, and the application of a chemical equilibrium model that includes a stage in which the solid waste components dissolve into the leachate.

Clogging of landfill tyre and aggregate drainage layers by methanogenic leachate and implications for practice.

This paper reports the results of pilot scale tests carried out to investigate the clogging of shredded and baled tyres in comparison with aggregates when percolated by leachates representative of those generated by methanogenic stage landfills. Realistic lifetime loading rates of methanogenic leachate were applied, and clogging was not generally apparent in any of the drainage media studied. This is in apparent contrast to many other studies that have demonstrated the susceptibility of all forms of drainage media to biological and chemical clogging when percolated with high strength organic and calcium rich leachates. The reasons for this difference are identified, the implications for landfill drainage system design are discussed and some suggestions for operational practice are presented for discussion.

Leachate recirculation in a landfill: some insights obtained from the development of a simple 1-D model.

The re-introduction of leachate back into the waste can play an important part in landfill management. It can encourage biodegradation by raising the water content and transporting bacteria, nutrients and waste products. It also enables leachate to be stored within the body of the landfill, for example to help minimise temporal variations in the load on a leachate treatment plant. It is helpful for a landfill operator to be able to estimate the rate at which the landfill can accept leachate (the maximum infiltration or injection rate), the storage capacity of the landfill and the leachate retention time. This paper discusses some of the insights obtained from the development and application of a simple conceptual model of leachate recirculation that can be used to estimate key parameter values on the basis of the hydraulic properties of the waste. The model is described, partly validated against a more rigorous numerical analysis, and then used to interpret data obtained from field tests on a real site. The shortcomings of the model in its current form are discussed, and suggestions are made as to how these might be addressed in the context of developing the model as a design tool.

Modelling the compression behaviour of landfilled domestic waste.

This paper discusses the mechanisms involved in the compression of domestic waste, and how the resulting compression behaviour may be modelled. Reference is made to experimental data illustrating the effect of gas content and pore water pressure on bulk density and drainable porosity, and a theoretical model able to reproduce some but not all features of the data is presented.

Modelling flow to leachate wells in landfills.

Vertical wells are frequently used as a means of controlling leachate levels in landfills. They are often the only available dewatering option for both old landfills without any basal leachate collection layer and for newer sites where the installed drainage infrastructure has failed. When the well is pumped, a seepage face develops at the entry into the well so that the drawdown in the surrounding waste will not be as great as might be expected. The numerical groundwater flow model MODFLOW-SURFACT, which contains the functionality to model seepage surfaces, has been used to investigate the transient dewatering of a landfill. The study concludes that the position of the seepage face and information about the characteristics of the induced seepage flow field are important and should not be neglected when designing wells in landfills.