Selenium in volcanic environments: A review
Résumé
Selenium is a chemical element with major environmental implications, due to the extremely narrow range between essential and toxic concentrations. Volcanoes emit significant amounts of Se, which has an enormous impact on the Se cycle and potentially human and animal health. This paper presents a broad overview of the current state of knowledge of major aspects of volcanic derived Se, focussing on the processes in soils and aquifers. Volcanic soils, formed when volcanic products weather rapidly, display peculiar characteristics. Selenium contents can be relatively high (1-2 orders of magnitude higher) compared to the worldwide average (0.4 mg/kg), but often with low mobility. The soils can be impacted by local deposition of volcanic emissions, which are enriched in Se due to volatilization in the magma chamber. It seems that the chemical fate of Se in volcanic soils is controlled by sorption processes on poorly crystalline phases, which presence depends on the parental materials and weathering. These sorption processes also determine leaching to groundwaters. Some volcanic aquifers have Se concentrations above the WHO guideline of 10 mu/L. This might also be related to geothermal activity. In spite of the well documented interest on Se during the last decades a significant number of uncertainties exist on the behavior of this fascinating element. A preliminary model of the Se cycle around volcanoes can be constructed, but the well-known analytical problems associated with Se determinations have produced a lack of reliable information on some processes involved in the chemical interaction between soils and water. This review, therefore, also aims to identify the processes where new efforts are needed to produce good quality data to be compared and integrated in global models.