Volcanic Ash, Water Systems, the Biosphere and Agriculture

Water

Volcanic ash can have adverse effects on the aquatic environments and water systems. There are several major river and lake systems around the North Island that may be affected by ash from Mt Taranaki. These include but are not limited to the Whanganui River, Lake Taupo and the Waikato River, the Mokau River, the Patea River, the Rangitikei River and the Whangaehu River. The Whangaehu River already has heightened acidity levels due to the interaction of the river water with volcanic materials from the river's source on Mt Ruapehu. The acidity of the Whangaehu so profound that the road sign on the bridge at Tangiwai states "Water Acidic."

Fresh ash is very acidic due to the coating of strong mineral salts, mainly
sulfuric acid (H₂SO₄), hydrochloric acid (HCl) and hydrofluoric acid (HF), on the exterior of the particles. While suspended in the eruption column, the ash particles can also pick up traces of heavy metal elements such as mercury, arsenic and lead, which are toxic to people and livestock. Therefore the chemical traces associated with ash fall have the capability to contaminate any water bodies with which the ash comes into contact (Stewart et al. 2009). This means that supplies of drinking water for both people and animals can easily be rendered unpalatable.

On the 2nd of May, 2008, a Chilean volcano, Volcán Chaitén (Vol-khan Chai-ten), erupted and ash was blown east into Argentina. A city by the name of Esquel (ess-kell), located 110 km from the volcano, received a major ash fall of about 5 cm on the 5th of May. The city's water supplies, both above ground and subterranean, were contaminated. Residents reported that the drinking water developed a strong, metallic flavour. Leachate from the ash fall was found to be moderately acidic with a pH of 4.76 when tested as a 1 to 25 ratio of volcanic ash to distilled water. However the water canal system in the city was tested to be approximately neutral. This was presumed to be due to quality control chemicals in the canal having enough buffering capacity to neutralise the acid (Stewart et al. 2009).

Generally, it is found that soluble components from volcanic ash will make water unpalatable before they make the water present any serious health hazards. There is an exception to this though. There are some volcanic systems throughout the world which have large amounts of fluorine dissolved in their magma. In New Zealand, the Ruapehu eruption is thought to have caused the deaths of approximately two thousand sheep and lambs due to fluoride contamination of livestock water supplies by the volcanic ash (Wilson et al. 2010).

The volcanic ash also has the capability of causing blockages in equipment and water conduits. After an eruption of Volcán Hudson in Chile in 1991, pumping equipment that was necessary for providing water to livestock were blocked and abraded. Metallic fittings of irrigations equipment also suffered abrasion and corrosion due to the ash particles (Wilson et al. 2010).

Therefore volcanic ash could have a profound effect on water sources around the region. Both surface water systems and subterranean water systems could be rendered unpalatable to people and livestock due to leaching of soluble components. High concentrations of certain chemicals could prove toxic to people and livestock. Also, if Taranaki is like Ruapehu in that it erupts magma with considerable concentrations of Fluorine, this could prove to be a very toxic hazard.

The Biosphere/Agriculture/Livestock

An eruption of Mt Taranaki would have very profound effects to the biosphere. An eruption would not only affect the national park around the mountain, but also pasture and forest further afield. The images below show how ash has been estimated to accumulate in zones around the volcano:

(Neall & Alloway, 1991)



(Neall & Alloway, 1991)

The Map 4 shows the Taranaki region, where Zone A is expected to experience ash fall of over 25 centimetres deep. Zone B is expected to experience ash depths of between 10 and 25 cm, and Zone C is expected to have ash fall between 1 and 10 cm. Map 2 shows how Zone D covers a large portion of the North Island. Here, ash fall between 1 millimetre and 1 cm is expected (Neall & Alloway, 1991).

The effects that ash has on plant life varies. The area affected by the ash fall will be large, and the effects on vegetation could last from years to decades. The effect that ash and other tephra will have on vegetation will depend on the depth to which the tephra settles. Plants are buried by the ash as a direct effect, and close to the volcanic vent, where the ash fall will be at its thickest, the settling ash may be so thick that it cannot grow through the ash to the surface (Dale, et al., 2005).

The ash fall will have a profound effect on the agricultural pastures surrounding the mountain. According the diagrams above, much of the Taranaki region can expect at least a centimetre of ash fall. However, depths of greater than ten centimetres are expected in many areas around the mountain. Therefore it is highly likely that many farms will be entirely covered by ash, with all palatable grass covered up. This will have a profound effect on the dairy industry. Firstly, it is predicted to bury the pastures with over ten centimetres of ash over a considerable area in the region. This will severely affect livestock food supplies, and may make the importing of stock feed necessary to minimise livestock casualties. This would be extremely difficult in some areas closer to the mountain, where 35 cm of ash would make roads impassable (Neall & Alloway, 1991). Remedial work with bulldozers and other excavation equipment would be required in order to allow the delivery of hay, silage and other livestock foodstuffs.

Also, even in areas where the pasture is not fully buried by ash, any ash that may be present can still present dangerous health issues to livestock. After the eruption of Volcán Hudson in Chile in 1991, livestock were found to suffer gastrointestinal problems due to consuming feed that was tainted with ash. Furthermore, sheep were found to be exhausted by the heavy weight of ash that collected in their wool. Also, in extreme cases, blindness tended to ensue due to ash-induced damage to the cornea of the eyes of livestock (Wilson et al., 2011). Although agriculture in the Taranaki region is predominantly dairying, there are many sheep farms in the ranges of inland Taranaki, east of the volcanic plain. These farms would be right in the path of the most likely vector of an ash cloud, so flocks of sheep in these areas may end up experiencing similar health issues as those in Chile in 1991. Losses due to such health issues could severely impact the livelihoods of many sheep farmers throughout the region. Cattle on dairy farms located further from the volcano may also experience similar health issues due to ingesting ash.