Contamination of water supplies by volcanic ashfall: A literature review and simple impact modelling

Volcanic ash is the most widely-distributed product of explosive volcanic eruptions, and can disrupt vital infrastructure on a large scale. Previous studies of effects of ashfall on natural waters and water supplies have focused mainly on the consequences of increased levels of turbidity (ash suspended in water), acidity and fluoride, with very little attention paid to other contaminants associated with volcanic ash. The aims of this paper are twofold: firstly, to review previous studies of the effects of volcanic ashfall on water supplies and identify information gaps; and secondly, to propose a simple model for predicting effects of ashfall on water supplies using available information on ash composition.We reviewed reported impacts of historic eruptions on water supplies, drawing on case studies from New Zealand, Vanuatu, Argentina, the USA, Costa Rica, Montserrat, Iceland and Guadeloupe. Elevated concentrations of fluoride, iron, sulphate and chloride, as well as turbidity and acidity, have been reported in water supplies. From a public health perspective, the two main issues appear to be: (1) outbreaks of infectious disease caused by the inhibition of disinfection by high levels of suspended ash, and (2) elevated fluoride concentrations.We devised a simple model using volcanic ash leachate composition data to predict effects on receiving waters. Applying this model to the effects of Ruapehu ash, from the 1995/1996 eruptions, suggests that the primary effects of concern are likely to be an increase in acidity (decrease in pH), and increases in concentrations of the metals aluminium, iron and manganese. These metals are not normally considered to pose health risks, and are regulated only by secondary, non-enforceable guidelines. However, exceedences of guideline values for Al, Mn, Fe and pH will cause water to become undrinkable due to a bitter metallic taste and dark colour, and may also cause corrosion, staining and scale deposition problems in water tanks and pipes. Therefore, the main issues following volcanic ashfall of similar composition to Ruapehu ash are likely to be shortages of potable water and damage to distribution systems, rather than risks to public health.