Reflectance spectral characterization of acid sulphate soil in South Yunderup, Western Australia

Abstract

Acid sulphate soils (ASS) are widely spread worldwide and are detrimental to the environment. South Yunderup is one of the coastal areas of Western Australia heavily affected by ASS. Conventional investigation is costly and time-consuming, and thus there is an urgent need to rapidly characterize and identify this type of soil. This paper aims to characterize these soils using reflectance spectra, which may be one of the most significant steps in effectively identifying them and mapping their extent by remote sensing. The ASS from the study area were divided into several groups and subtypes according to both pH measurements and mineral composition as identified by X-ray diffraction analysis. Each group and subtype was then characterized by its spectral reflectance features. We found that the spectral characteristics of ASS are governed by the spectral features of its compositional minerals. In particular, some secondary iron-bearing minerals produced by the formation of ASS, together with surrounding minerals such as carbonate, play vitally important roles in influencing the spectral characterization of ASS. These iron-bearing minerals, including iron oxides, hydroxides/oxyhydroxides (e.g. haematite, goethite, and ferrihydrite), and iron sulphates (e.g. jarosite and copiapite), have diagnostic spectral features and are therefore detectable in the reflectance range. Moreover, these secondary iron-bearing minerals could be indicators suggesting the pH conditions in which they formed. The results of this study include the overall mineral distribution of the study area, the spectral characterization of different groups and subtypes of ASS, and the linkages between spectral features and pH ranges.