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 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 immediate vicinity. Depending on its explosiveness, the gases released and the finest ash dust can be transported through the Earth's atmosphere and affect, albeit less intensely, ecosystems and populations located at great distances from the source of the volcanic explosion.

source Volcanic activity can be an important source of water pollution, as 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 supply of water for animals.

One of the main concerns of the inhabitants of areas affected by this subject phenomenon is the supply of drinking water. programs of study conducted in areas affected by volcanic eruptions showed that the most affected water quality parameters are turbidity, pH and increased concentration of elements that can be toxic, essential for life and others that can be both toxic and essential, depending on the concentration reached(Flaathen 2006). Therefore, whether surface waters are fertilised or polluted will depend on the subject 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. Ash produced during volcanic eruptions can increase the turbidity of water if it remains in suspension, although it will slowly settle to the bottom of rivers, lakes or reservoirs. This increase in turbidity can cause some problems in water treatment and purification plants, as filtration processes and the effectiveness of disinfection treatments can be hampered.

In 1980, Mount St. Helens (Washington State, USA) erupted for about nine hours. The Environmental Protection Agency described the effects of the eruption on water treatment and drinking water 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 abnormal 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 fall below 6.5), but this effect will depend on the amount of ash deposited, which in turn will be related to the subject eruption and the time of the eruption process. In the case of rivers and streams, the natural stirring process induced by the velocity of the water 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, sulphate and fluoride can occur 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 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 a thorough control of the quality of these waters.

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

But as polluting as these natural phenomena may seem, it should not be forgotten that anthropogenic activity is still far more damaging 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.