Volcanic regions, being very fertile, are often highly populated. Population pressure brings enhanced water demand both for drinking and agricultural purposes. Volcanic activity, however, can have a strong impact on water quality. Its effects, roughly proportional to the activity status, can be subdivided into direct and indirect impacts. Direct effects, due to the up-welling of volatile potentially harmful elements (PHE) that dissolve directly into groundwaters, are generally of minor importance. Indirect effects, due to the release of PHE from the rocks are generally more dangerous. Volcanoes often host geothermal systems, whose high temperature and acid compounds (CO2, SO2, H2S, HCl, HF) enhance the weathering of the aquifer's rocks. Examples of PHE found in volcanic aquifers are As, F, Hg, Rn Sb, Se, all of which can be toxic to human health. These elements sometimes exceed Maximum Admissible Concentrations (MAC) for drinking waters by orders of magnitude. For example, many groundwater samples collected from south Italian volcanic areas display arsenic concentrations above 1000 µg/l (MAC 10 µg/l).
Water quality problems are not limited to volcanoes hosting active geothermal systems. The cold aquifers near Mt. Etna, which are the dominant water resource for over a million people, suffer from water quality problems due to the large amounts of magma-derived CO2 which dissolves in groundwater, thereby enhancing rock weathering.
Protection of water resources is a generally important issue, but it represents a more crucial hazard in areas subjected to elevated natural backgrounds of PHE, such as active volcanoes, where even limited anthropogenic pollution could make groundwater unsuitable for drinking purposes.
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