Volcanic eruptions are one of the most spectacular natural phenomena and, at the same time, the most polluting and damaging to the environment. Their effect on the area near the volcano is immediate and devastating, due to the lava flows, gases, solid materials and mud avalanches (lahars) that can be produced by the melting of snow deposited on the slopes of the volcano or by heavy rains mixed with the ash emitted during these eruptive processes (Rafferti 2021).  

However, the effects of a volcanic eruption are not only felt in the closest areas, but, depending on its explosiveness, both the gases released and the finest ash dust can be transported through the Earth's atmosphere and affect, although less intensely, ecosystems and populations located at great distances from the source of the volcanic explosion.

Volcanic activity can be an important source of water contamination because, when flows from the volcano reach surface or groundwater bodies, dissolved gases and entrained solid particles can affect water quality, limiting the supply of drinking water for humans and the availability of water for animals.

One of the main concerns of the inhabitants of areas affected by this type of phenomena is the supply of drinking water. Studies conducted in areas affected by volcanic eruptions have shown that the water quality parameters most affected are turbidity, pH and increased concentrations of elements that may be toxic, essential for life, and others that may be both toxic and essential, depending on the concentration reached(Flaathen 2006). Therefore, whether surface waters are fertilized or polluted will depend on the type of ash and the chemical characteristics of the water.

Turbidity is a measure of the loss of transparency of water due to the presence of suspended particles. Ashes produced during volcanic eruptions can increase the turbidity of water if they remain in suspension, although they will slowly settle to the bottom of rivers, lakes or reservoirs. This increase in turbidity can cause some problems in water treatment and potabilization plants, since filtration processes and the effectiveness of disinfection treatments can be hindered.

In 1980 Mount St. Helens (Washington State, USA) erupted for about nine hours. The Environmental Protection Agency described the effects of the eruption on drinking water and water treatment facilities, as well as an estimate of the damage suffered by these facilities as a result of the eruption(EPA, Volcanic activity). According to the document, acid pH values and high turbidity were detected in surface water, but the drinking water did not show anomalous values of elements that could be present in the ash.

The changes in pH and the increase in the concentration of certain elements are due to the fact that the outer part of the ash particles contains acidic compounds and water-soluble salts. In general, when ash reaches surface water bodies such as lakes or reservoirs, which contain high amounts of water, the effect on the change in water composition is negligible (the pH will generally not drop below 6.5), but this effect will depend on the amount of ash deposited, which in turn will be related to the type of eruption and the time that the process lasts. In the case of rivers and streams, the natural stirring process induced by water velocity will allow dilution to be rapid, so the pH change and high metal concentration will not last long, except in the case of long-lasting eruptive events.

Up to 55 soluble compounds have been detected in ash-contaminated water, of which sodium, calcium, magnesium, chloride, sulfate, and fluoride are those that may appear in the highest concentrations(Stewart 2006). In the case of eruptive events limited to a short period of time, the concentrations of these elements were reduced to values below those established in the quality standards, but in the case of intermittent or semi-continuous eruptive events, these elevated values can become chronic, which implies the need for exhaustive control of the quality of these waters.

Another aspect to consider is what happens when the lava reaches the coast and comes into contact with seawater. The gaseous cloud observed in these cases will be composed mainly of water vapor and hydrochloric acid (HCl, a toxic and corrosive gas) that comes from the chloride anion present in seawater, as was proven in the eruption of the Kilauea volcano (Hawaii, USA) in 2004(Edmonds 2006). On the other hand, the possibility of toxic elements present in lava being transferred to seawater is low, since it has been determined that the fraction of lava that comes into contact with water is very small, so this route is not considered a source of contamination.

But however polluting these natural phenomena may appear to be, it should not be forgotten that anthropogenic activity is still much more harmful to the environment, as evidenced by the fact that global volcanic activity accounts for only 2% of the CO2 emitted annually by human activity.

This article was originally published in The Conversation. Read the original.