Role of automated mineral analysis in the characterisation of mining-related contaminated land

The characterisation of soils or sediments contaminated as a result of historical metal mining activity is commonly carried out either by analysing the bulk geochemistry, or through methods which are aimed at assessing the biologically available component of the contaminant, such as physiologically based extraction techniques (P-BET tests). However, an understanding of the mineralogy or phase composition of particulate contaminants is critical in understanding both the long term geochemical stability of the contaminated sediments/soils, and also the development of potential remediation strategies, where required. Providing statistically robust mineralogical datasets based on traditional techniques is commonly difficult. However, modern advanced automated SEM-EDS mineral analysis systems have considerable potential in the characterisation of contaminated soils/sediments. Six sediment samples collected from a single shallow core, recovered from the Hayle Estuary, Cornwall, UK have been analysed using QEMSCAN® automated SEM-EDS analysis. This estuary was significantly contaminated as a result of the release of mine waste tailings, principally from tin, copper, arsenic and zinc mining operations particularly between the 1850s and the 1890s. The samples analysed contain between ∼2 and 6% sulphide and other ore minerals. In addition, there are significant depth-related changes in the overall bulk mineralogy of the samples reflecting the change in sediment supply to the estuary. For some elements (e.g. Sn) there is a reasonable correspondence between the measured bulk geochemistry of the sediments, with calculated elemental concentrations based on the measured mineralogical data. In this case study, Sn is probably occurring in cassiterite and possibly Sn slags and is relatively geochemically immobile. Other elements, such as arsenic and zinc show greater variance between the calculated elemental concentrations and the measured bulk chemistry, although the down-core trends are consistent. This can be interpreted as reflecting the increased geochemical mobility of these elements, resulting in them being under-reported. The data from automated mineralogy are however, extremely relevant in the assessment of metal contaminated land.