Risk assessment for historic coastal landfills in the Waikato region

Report: TR 2021/15

Author: Jessica Green, Michelle Begbie and Stephen Hunt

Landfills are a widespread hazard. They can cause issues while they are operational, but also create ongoing potential hazards once they have closed. Historic landfills, those that operated prior to regulation and need for assessment of effects, pose the greatest uncertainty on hazard-related impacts (i.e., risk). Depending on the environment where these historic landfills were placed, they can cause significant human health and environmental impacts.

This report has designed a framework for relative risk assessment of historic coastal landfills in an accessible way. In this study, historic landfills are classified as coastal if they are within 200 m of the shoreline, either on the open coast or inside estuaries. Using national level datasets with localised scale where possible, parameters are defined and assigned severity scores ranging from one to five (least to highest risk, respectively). A likelihood index, consequence index, and overall risk score are generated by a summation and normalisation of parameter scores. A weighted risk score is also produced by weighting the likelihood index, given the risk can be minimised if a landfill breach can be avoided in the first place.

With the available data and records at time of publishing, no known sites in the Waikato region pose an extreme and imminent risk. However, the awareness that this risk-screening method is not failsafe needs to be kept in mind. Unknown site characteristics stemming from incomplete or inaccurate records as well as the potential for the existence of unknown sites adds uncertainty to the model. Additionally, there are some sites that do pose a relative risk above the midway score, which will require attention from the relevant policy makers and stakeholders.

This report presents a model to consistently analyse and evaluate the relative risk posed by historic coastal landfills. The model was validated by the historic Kaiaua landfill, which was actively eroding into the Firth of Thames, being ranked as the highest relative risk (noting this landfill has now been remediated). In the Waikato region, there are over 200 other sites that will need to be assessed in the near future. The recommendation is to use this risk assessment framework and modify the parameters to evaluate the impacts of the historic riverine landfills as a next step.