Flooding of municipal solid waste landfills
Flooding of municipal solid waste landfills is an environmental risk for the long term worldwide. Municipal solid waste (MSW) landfills pose a number of other environmental risks including leaching of pollutants into the groundwater and methane generation to the atmosphere.
The emissions occurring under average conditions in a landfill were subject to numerous research studies within the last decades and are therefore well documented. In contrast, landfill behaviour and associated emissions in extreme cases such as flooding are widely unknown. However, a review of existing reports about environmental damages caused by landfills during floods indicates, that the released substances might be of environmental concern. Hence, within this article the proportion of MSW landfills endangered by flooding is determined and their potential environmental significance is evaluated. Although the work presented in this article is largely based on Austrian conditions, the methodological aspects of the study might as well apply to other regions.
Figure: An old MSW deposit of approximately 25.000 m³ and an average age of 30 years was almost completely eroded during a major flood event in 2005 (top: aerial photograph in 2002, bottom: aerial photograph after the flood event in 2005)
Risk of flooding
The risk of flooding is determined for MSW landfill sites in Austria using areal data about flood risk zones (HORA) in Austria. The HORA-dataset provides information about potential flood inundation zones along rivers for discharges with a return period of T=30 years, T=100 years, and T=200 years. Information about technical flood protection is not included in the database, and hence, it provides an estimate on the potential risk of flooding for a specific site (for more details visit: http://www.hochwasserrisiko.at). The site coordinates of the landfill and information about designated flood risk zones are integrated in a geographic information system (GIS), which is used for a categorization of flood risk exposure:
- endangered landfills, are situated within a designated flood risk zone.
- probably endangered sites, are less than 150 m away from a designated flood risk zone.
- probably not endangered sites, do not have a designated flood risk zone within a distance of 150 m around the landfill coordinates.
The criteria above are a strong simplification of real landfill geometries (point coordinates are not necessarily in the centre and also landfills are normally not of a circular shape), but a visual validation of these criteria for all controlled landfill sites in Austria show that these assumptions are sufficiently accurate for a preliminary assessment of flood risk exposure. However, it should be noted that for a detailed evaluation of flood risk exposure a hydrodynamic model on a local scale is a prerequisite. Such models were used within three case studies, to assess the risk a flooded landfill is posing in view of the vulnerability of its local environment.
The results of the evaluation of flood risk exposure of Austrian MSW landfills indicate that one third of the landfills are located in flood prone areas (assigned either to the category “endangered” or “probably endangered” of flooding), with more than 10 % being located directly in a potential inundation zone corresponding to a recurrence interval flood of T=30 years.
Only a small portion of the endangered and probably endangered sites have flood protection facilities. Whereas around 60 % of active sites (only around 5 % of the investigated landfills) have some kind of flood protection facilities (i.e. dams), less then one third of the closed sites in flood prone areas reported technical flood protection.
Release of pollutants and environmental impact
The estimates on potential substance releases during a flood event are based on scenario evaluations. There is limited information available on flooded MSW landfills. The scenario of complete landfill erosion is defined as a worst case, and the fraction of soluble substances in the waste is estimated based on leaching experiments carried out with grounded waste samples excavated from a landfill (cf. Belevi and Baccini, 1989). It should be emphasized that the resulting emission loads or pollutant releases (total as well as potentially soluble) should be seen as potential rather than actual releases.
According to the scenario analysis, and compared to average landfill conditions, emissions during a flood event might increase by up to six orders of magnitude. For an average Austrian MSW landfill the subsequent total emission loads range from a few tonnes (e.g. Cd) to several thousands of tonnes (e.g. C org), with potentially dissolved substance loads being in the range of a few kilograms to a few hundred tonnes (for further details see Laner et al., 2009).
By estimating potential emission levels it is not possible to evaluate the environmental relevance of flooded MSW landfills. It is necessary to analyze the environmental impacts of the released pollutants depending on the dilution potential during the flood, the availability of these pollutants (i.e. solutes, particles), and the vulnerability of the affected environment (e.g. subjects of protection). As these factors are dependent on the characteristics of the individual landfill and the flood event, the potential pollution of a river system as a consequence of flooding a MSW landfill has to be discussed case by case. Hence, three sites, categorized as endangered during the preliminary assessment, were chosen for a detailed analysis of flood risk exposure on the one hand and the potential environmental impacts of substance releases during a flood event on the other hand. Out of the three sites, two were located in the inundation area of flood discharges below a 100 year recurrence interval. For one of these two sites the danger of erosion during flooding was estimated high (high water velocity, large water volume, non-engineered site and therefore reduced geotechnical stability), nevertheless the potential damages downstream of the site due to the increased substance concentrations in the river water are supposed to be on a similar or lower level than the pollution due to releases from waste water treatment plants or industrial areas along the river. The case studies indicate that flooded MSW landfills represent in general one amongst many potential pollutant sources adjacent to a river. However, it should be noted that many uncertainties exist with respect to flooded MSW landfills, especially the long-term behaviour of an initially saturated (during the flood) waste body might of special interest from the perspective of landfill aftercare.
- Belevi H, Baccini P. Long-term behaviour of municipal solid waste landfills. Waste Management and Research 1989;7:43–56.
- Laner D, Fellner J, Brunner PH. Flooding of municipal solid waste landfills — An environmental hazard? Science of the Total Environment 2009; 407(12):3674-80