Evaluation of pH testing methods for sulfidic mine waste

Environmental assessment of solid mine waste is required throughout the life of a mine. This has driven the need for tools and practices to understand the current state of net acidity in mine wastes. Rinse and paste pH tests are commonly used in the initial screening of waste to provide a preliminary evaluation of its current net acidity. Such pH tests are particularly useful for assessing the chemistry of first flush waters draining sulfidic rocks and wastes. In this study, we compared nine different pH tests (rinse and paste pH tests as well as soil tests of the International Organization for Standardization ISO 10390:2005; American Society for Testing and Materials ASTM D4972-01 2007; Standards Australia AS4969.2-2008), using three different sulfidic rock samples and the acid–base accounting standard KZK-1. We observed significant variability in measured pH for the same samples using the different test methods. We show that different rinse and paste pH methods using different grain sizes and extraction solutions can result in different risk classification for ARD assessments. We recommend carrying out pH measurements using 0.01 M CaCl₂ solution, which results in more rapid, reproducible, and precise analyses than using deionised water.