Mapping magnesium sulfate salts from saline mine discharge with airborne hyperspectral data

Abstract

Managing saline water discharges from mining operations is a global environmental challenge. Measuring the location and extent of surface efflorescence can indicate solute movement before changes in electrical conductivity (EC) are detected in waterways. We hypothesised through the use of a case study that ground-based reflectance spectrometry and airborne hyperspectral (450–2500 nm) analysis of surface efflorescence could be a rapid method for monitoring large regions of the surrounding environment, including downstream of remote mines. X-ray diffraction and X-ray fluorescence were used to determine mineralogy and elemental composition of surface salts around a uranium mine. Salt samples were found to be mixtures of magnesium sulfate. The reflectance of field spectra varied depending on the hydration of the mineral, mainly hexahydrite and starkeyite. A constrained energy minimisation technique was used to match the field reflectance spectra to the airborne data. Airborne matches were confirmed at the field sampling sites and surrounds. Salts were also detected at lower matches at mine water irrigation areas where excess mine water had previously been applied. Hence, hyperspectral remote sensing is a potentially rapid and sensitive method for mapping magnesium sulfates over large areas in operating and rehabilitated mines. It was successfully demonstrated as a tool for monitoring and assessment of efflorescence as a result of saline processes.