Volcanic Ash

Volcanic ash can have a detrimental impact on the environment which can be difficult to predict due to the large variety of environmental conditions that exist within the ash fall zone. Natural waterways can be impacted in the same way as urban water supply networks. Ash will increase water turbidity which can reduce the amount of light reaching lower depths, which can inhibit growth of submerged aquatic plants and consequently affect species which are dependent on them such as fish and shellfish. High turbidity can also affect the ability of fish gills to absorb dissolved oxygen. Acidification will also occur, which will reduce the pH of the water and impact the fauna and flora living in the environment. Fluoride contamination will occur if the ash contains high concentrations of fluoride. Ash accumulation will also affect pasture, plants and trees which are part of the horticulture and agriculture industries. Thin ash falls (<20 mm) may put livestock off eating, and can inhibit transpiration and photosynthesis and alter growth. There may be an increase in pasture production due to a mulching effect and slight fertilizing effect, such as occurred following the 1980 Mount St. Helens and 1995/96 Mt Ruapehu eruptions. Heavier falls will completely bury pastures and soil leading to death of pasture and sterilization of the soil due to oxygen deprivation. Plant survival is dependent on ash thickness, ash chemistry, compaction of ash, amount of rainfall, duration of burial and the length of plant stalks at the time of ash fall.The acidic nature of ash will lead to elevated soil sulfur levels and lowered soil pH, which can reduce the availability of essential minerals and alter the soil’s characteristics so that crops and plants will not survive. Ash will also impact upon arable crops, such as fruit, vegetables and grain. Ash can burn plant and crop tissue reducing quality, contaminate crops during harvest and damage plants from ash loading. Young forests (trees <2 years old) are most at risk from ash falls and are likely to be destroyed by ash deposits >100 mm. Ash fall is unlikely to kill mature trees, but ash loading may break large branches during heavy ash falls (>500 mm). Defoliation of trees may also occur, especially if there is a coarse ash component within the ash fall.

Land rehabilitation after ash fall may be possible depending on the ash deposit thickness. Rehabilitation treatment may include: direct seeding of deposit; mixing of deposit with buried soil; scraping of ash deposit from land surface; and application of new topsoil over the ash deposit.