Environmental contextThe majority of biosolids produced in Australia from wastewater treatment processes are applied to agricultural land for beneficial use. We have demonstrated, through improvements to the analytical method, that levels of PFAS in biosolids are significantly higher than historically understood. The land application of biosolids could result in sensitive environments being exposed to PFAS at levels higher than previously anticipated. AbstractThe current industry standard for per- and polyfluoroalkyl substances (PFAS) analysis is for the measurement of only 28 PFAS, even though there are greater than 4700 PFAS known to be in existence. The total oxidisable precursor (TOP) assay, originally published by Houtz and Sedlak, is widely used as an estimate of the total perfluoro alkyl acids (PFAA) content of a sample, particularly in wastewater and biosolid matrices. The total PFAA content is an important measure of potential environmental contamination, which assists in the inference of potential harm that may occur from both well characterised PFAS, such as perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA), as well as lesser known precursor compounds and their breakdown products. With the majority of Australian biosolids beneficially applied to land, it is important to understand the future PFAS capacity before they are land applied to maintain the preservation of our agricultural and environmental assets. Our investigation of the TOP method and its application to biosolids involves a comparison of the Houtz and Sedlak method with a modified version coupled with a hydrogen peroxide pretreatment. The underperformance of the previously published method is demonstrated by its inability to sufficiently digest PFAS within biosolids. Therefore, the Houtz and Sedlak method significantly underestimated the levels of PFAS compared with the modified method, which showed a 10-fold increase in the measured PFAS after digestion. Further improvement of this modified method may provide a greater degree of accuracy for the TOP assay. The significant underestimation of the total PFAS load and, therefore, potential environmental harm has significant implications for public and agricultural health and compliance with regulatory limits.