Development of a conceptual framework of holistic risk assessment — Landfill as a particular type of contaminated land

Landfills can be regarded as a particular type of contaminated land that has a potential to directly and indirectly pollute all of the four main spheres of the environment which are the lithosphere, atmosphere, hydrosphere and eventually adversely impact the biosphere. Therefore, environmental risk assessment of a landfill has to be more integrated and holistic by virtue of its nature of being a multidimensional pollutant source. Despite this, although various risk assessment approaches have been adopted for landfill waste disposal sites, there are still wide-ranging knowledge gaps and limitations which need to be addressed. One important knowledge gap and limitation of current risk assessment approaches is the inability to fully identify, categorise and aggregate all individual risks from all combinations of hazards, pathways and targets/receptors (e.g. water, air, soil and biota) in connection to a certain landfill leachate and yet at any stage of the landfill cycle. So such an approach is required that could not only integrate all possible characteristics of varying scenarios but also contain the ability to establish an overall risk picture, irrespective of the lifecycle stage of the landfill (e.g. planning stage/pre-operation, in-operation or post-operation/closed). One such approach to address the wide-breadth of landfill impact risks is by developing a more holistic risk assessment methodology, whose conceptual framework is presented in this paper for landfill leachate in a whole-system format. This conceptual framework does not only draw together various constituting factors and sub-factors of risk assessment in a logical sequence and categorical order, but also indicates the "what, why, when and how" outputs of and inputs to these factors and sub-factors can be useful. The framework is designed to identify and quantify a range of risks associated with all stages of the landfill lifecycle, and yet in a more streamlined, logical, categorical and integrated format, offering a more standardised and unified whole-system approach. - Highlights: • For a whole-system risk management of landfill leachate, a holistic and integrated risk assessment is required, which could yield one overall (total) risk value via aggregating individual risks posed from all the respective combinations of all the leachate hazards via all the pathways to all the receptors. • Such a holistic and integrated methodology is presented for the first time for landfill leachate by assembling existing knowledge from varying disciplines and yet generating new building blocks (with innovative insights) to bridge a number of knowledge gaps. • For instance, an innovative notion of 'Least Bad' scenario of risk is developed as opposed to commonly used 'Most Likely' and 'Worst Case' scenarios of risk. Furthermore, the paper also introduces a new concept of how these three scenarios as a 'triple stream' approach can be employed as bench marks in risk assessment. • Hazard Index is generally attributed to non-carcinogenic hazards. This paper shows how hazard indices approach can also be applied to carcinogenic hazards. Similarly, unlike to the convention of considering risk quantification approach only for carcinogenic hazards, the paper additionally demonstrates how risk quantification approach can be considered even for non-carcinogenic hazards of landfill leachate. Furthermore, the paper explains why and how a risk assessment can be carried out beyond the stage of hazard indices, by explaining an innovative concept of dividing the receptor into two categories, namely: biota/living and non-biota/non-living. • The environmental legislation (examples described in the paper) is not only growing stringent but also integrated; therefore such a holistic risk analysis of landfill leachate is becoming an escalating need of the time. On the other hand, landfill leachate is a multi-dimensional pollutant source. Therefore, the paper's scope is multi-faceted bringing together appropriate aspects of the antroposphere/technosphere, biosphere, lithosphere, atmosphere and hydrosphere.