Volcanic ash leachate compositions and assessment of health and agricultural hazards from 2012 hydrothermal eruptions, Tongariro, New Zealand

Abstract

After almost 80years of quiescence, the upper Te Maari vent on Mt. Tongariro erupted suddenly at 2352h (NZ time) on 6 August 2012. The short-lived hydrothermal eruption distributed a fine ash of minor volume (~5×105m3) over 200km from source. The threat of further eruptions prompted an investigation of the possible health and agricultural impacts of any future eruptions from this volcano, particularly since the most recent large-scale ash falls in New Zealand in 1995–1996 had generated significant agricultural problems, including livestock deaths. Deposited ash was sampled between 5 and 200km from the volcano as soon as possible after the eruption. Two sub-lobes of ash were identified from different vent areas and displayed subtly different leaching properties. The first was an initial small lobe directed NNE, likely formed from drifting low-level clouds associated with the initial lateral explosive blast and surges. The main fall lobe, directed eastward, was sourced from a short-lived vertical plume that rose up to c. 8km. Ash from the initial fall lobe had higher concentrations of F and Al, in single-step leaches as well as in the totals of three, sequential extractions. Further, the initial lobe showed a higher proportion of soluble F and Al extracted in the first leach, compared to totals. A linear relationship between concentrations of Al and F in single leaches from the 6 August eruption was highly significant (Pearson correlation coefficient r=0.987 for 1:20 leaching ratios and r=0.971 for 1:100), suggesting the presence of soluble alumino-fluoride complexes (AlFx+3−x). An even more significant 1:1 ratio is displayed for the largest concentration leached ions of Ca and SO4, which correspond to the presence of crystalline gypsum throughout the newly excavated hydrothermal system. Although no fresh magma was erupted in this event, a shallow intrusion prior to the hydrothermal explosion apparently provided significant contents of volcanic gas that was dissolved within the hydrothermal fluids and adhering to ejected particles. This and the ubiquitous presence of gypsum dominated the soluble components of these ash deposits leading to a complex leaching profile.The leaching study carried out here showed that agricultural and human health hazard assessment (particularly of F and S) is not straightforward, particularly because F solubility may be complex and not well characterised by simple leaching studies. In the case of S, which is agriculturally important, saturation effects are apparent using normal leaching protocols and also imply a need for modification of standard methods.