High Density Polyethylene (HDPE) geomembranes contain proprietary blends of antioxidants and stabilisers (known as the additive package) to improve the service life of the polymer. The service lifetime of HDPE liners is mainly dependent on the level and permanence of the additive package. The problem is manufacturers do not disclose the mix of additives to the end-user, and they regularly engage in resin substitution, masterbatch substitution and additive substitution based on price and local availability of the raw materials. This has led to a lot of uncertainty around the consistency and quality of HDPE geomembrane products available today. The way to overcome these issues is by reverse engineering the additive package through deformulation, and performing the newly developed rapid screening procedures outlined in this paper. Although conventional long-term immersion testing is now recognized as the preferred method to assess the durability of HDPE geomembranes in specific end-use media, the protracted amount of time required to develop meaningful ageing of the geomembrane is a disadvantage. Conventional accelerated ageing tests typically require a minimum of 3 to 6 months and sometimes years to complete, while the new rapid screening procedures can be completed within 2 to 4 weeks. The additive de-formulation combined with the knowledge of the acid and alkali tolerance of each additive (known as pKa values) allows one to predict the performance of the geomembrane for a given service environment, while the rapid screening procedure can ameliorate the risk level associated with using HDPE geomembranes and provide a greater degree of comfort to designers and facility owners, particularly when project timelines are tight.