CAN VOLCANIC ASH POISON WATER SUPPLIES?

Of all eruptive hazards, volcanic ash can affect the most people because of the large areas that can be covered by ashfall. Although ashfalls rarely endanger human life directly, they are a major cause of evacuation of residents as they can cause roof collapse as well as posing respiratory health hazards. Volcanic ash can also cause large-scale disruption of vital infrastructure such as electricity and water supplies, transport routes and communications. During and after an eruption, it is very common for there to be a high level of public concern and anxiety about contamination of water supplies by volcanic ash. The purpose of this work is to consider whether these concerns are well-founded.

Esquel is a city of 35,000 people in Chubut province, Argentina. It is located 110 km east of Volcan Chaitén, Chile. After more than 9000 years of inactivity, Volcan Chaitén began erupting on 2 May 2008, with emissions of ash to an altitude of 20 km. The prevailing westerly winds deposited much of this material over Argentina. Over the month of May, the city of Esquel received a total of about 5 cm ash, both from primary eruption plumes and wind remobilisation. Esquel’s water supply has both surface and subterranean sources, and public authorities were immediately concerned about the water supply as residents reported a strong metallic taste in the drinking water. The water supply authority immediately started a water sampling and monitoring programme aimed at determining public health risks from the ashfall contamination. On the basis of this monitoring, public officials were able to reassure residents that the water was unlikely to be toxic due to the presence of the volcanic ash.

We suggest that except for special cases where volcanic systems are high in fluorine, volcanic ash is in general unlikely to cause toxic hazards in water supplies. The main issues for water supply managers are likely to be managing water demand to conserve drinking water supplies, and ensuring that disinfection of water supplies allows for the effects of suspended ash. The case study of Esquel described here highlights the value of a timely, well-designed monitoring and sampling programme in the event of an ashfall, of which the results were clearly communicated to the public and were effective in allaying concerns.