Hydrogeological techniques for the in-situ characterisation of saturated landfilled waste in relation to contaminant flushing

Abstract

Disposal of waste (landfilling) is considered to be the least desirable waste management option in terms of sustainability. However, it is clear that landfills are required to dispose of waste that cannot be reused or recycled, and for residues of waste incineration and waste treatment. Landfills have changed considerably throughout time and across the world. The UK has seen landfills that allowed natural processes of dilution and attenuation be replaced with landfills that hydraulically contain the waste mass. These landfills have covers to reduce the infiltration of water and therefore leachate production, leachate collection systems to collect any leachate created and basal liners to minimise the release of leachate. These measured are effectively creating a ‘dry tomb’ with the main aim being shortterm environmental protection. In the US, Canada and Australia, more sophisticated landfills have been envisaged, these ‘bioreactor landfills’ operate with optimum water content to promote biodegradation and production of methane, an exploitable resource. The concept of a sustainable landfills has received significant discussion. Such landfills must exploit the methane resource (as per the bioreactor landfill), protect the environment (as per the hydraulic containment landfill) and not pass the management burden to future generations. Flushing has been 3 considered as one way to meet the last requirement, whereby the circulation of treated water removes inorganic contaminants after the biodegradation phase reduces the organics. This concept of landfill, termed the high rate flushing bioreactor, is currently considered the only viable option to meet sustainable landfill criteria. Irrespective of current landfill practice, the UK has a backlog of 1,500 older closed landfill sites. Most of these sites do not currently pose an environmental risk, however many have the potential to in the future. A minority of these sites are currently polluting the environment (mainly controlled waters) at unacceptable levels. For most closed sites, in-situ remediation would be the only financially viable option to reduce the pollutant levels. When considering the inorganic fraction of the pollutant load, arguably the most effective in-situ remediation technique is flushing. For older landfill sites, or sites where the basal drainage layer has failed, the use of vertical injection wells may be the only viable option for the introduction and removal of liquid. For an effective flushing strategy, either as an on-going management technique (i.e. a high rate flushing bioreactor) or as a post-closure remediation measure, it is important to consider the hydraulic and contaminant transport properties of the waste. Considerable literature has been published regarding the hydraulic properties of saturated landfilled municipal solid waste, however much of this data is from small scale laboratory and variable size test cells and not field studies. Contaminant transport in waste is very much less understood with preliminary literature suggesting that the transport is dominated by a dual-porosity nature of the waste. This thesis presents two adapted hydrogeological field tracer tests, which take advantage of the single leachate wells present within most landfill cells, to determine the hydrogeological (hydraulic and contaminant) properties of saturated landfilled waste. Hydraulic data is presented from 32 dilution tests undertaken at four UK landfill sites. Hydraulic and contaminant transport data is also presented from 4 12 scales of single well injection-withdrawal tracer tests undertaken at three UK sites. Hydraulic observations show that hydraulic conductivity ranges over four orders of magnitude. The results also confirm the heterogeneous nature of the waste mass, and generally dispel the theory whereby hydraulic conductivity and therefore leachate flow decreases with burial depth within the sites. Contaminant transport observations support the dual-porosity theories and data has provided a good fit when applied to an advection-diffusion dual porosity model. The general conclusion, that waste is a heterogeneous medium with a dualporosity nature, has severe implications for flushing. Injected water will tend to follow preferential flow paths and/or zones and areas will be flushed better than others. The characteristics of the system are such that the contaminant removal time will be affected by the slow release (diffusion) of contaminants from negligible flow zones.

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ORCID for R.P. Beaven: orcid.org/0000-0002-1387-8299

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