Science, Technology and Engineering

Environmental Geoscience

Honours Projects

A comparison of methods for acid sulphate soil analysis

Silvana Santomartino

The management of acid sulphate soils requires analytical methods that provide accurate data on the quantity of reduced inorganic sulphur within a soil, as it is this fraction that generates acid upon oxidation. This study used sulphidic Coode Island Silt (CIS) samples, excavated from the Yarra Delta and deposited at the Werribee Rifle Range, to compare common analytical methods including TOS (Total Oxidisable Sulphur), chromium-reducible sulphur, and POCAS (Peroxide Oxidation-Combined Acidity and Sulphate) which consists of TSA (Total Sulphidic Acidity) and POS (Peroxide Oxidisable Sulphur).

Comparison of the soil sulphide content by chromium-reducible sulphur, POS and TOS methods indicate that the POS method, which uses ion chromatography to analyse for KCl extracted sulphate, produces highly inaccurate results due to the interferences of the sulphate peak by the chloride peak during analysis. TOS gives substantially higher values of sulphide due to its failure to account for organic sulphur, thereby overestimating the sulphide content of the soil by approximately 0.4%. Leco Carbon analyses verify that a substantial proportion of organic matter exists within the samples and there is a strong correlation between organic carbon content and estimated organic sulphur content (R²=0.8171). The TSA method for the analysis of the acid generating potential of the soil samples is an accurate measure of the potential acid hazard in a natural environment as it takes into account acid neutralisation by soil minerals including carbonates. A strong correlation was found between TSA and chromium-reducible sulphur methods.

The findings suggest that the chromium-reducible sulphur method is the most suitable technique for the analysis of soil sulphide content whilst the TSA method is a good indicator of acid conditions. According to the results obtained by these methods, the CIS sediments stockpiled at the Werribee Rifle Range are currently undergoing oxidation and generating substantial acidity and therefore require the implementation of a suitable management plane

This project was in collaboration with the Victorian EPA.