Geospatial and statistical analysis of volcanic ash leachate data from Mt. St. Helens

Abstract:

Upon contact with water, freshly-fallen volcanic ash releases a pulse of readily soluble material, derived from dissolution of S-, Cl- and F-bearing salts formed on ash surfaces during transport through the volcanic eruption plume. Analysis of leachate solutions can provide insight into the spatial and temporal variations in surface salt loadings, and hence the processes by which they were emplaced, and the hazards which they may induce upon mobilisation within receiving environments. However, excluding a small number of publications from the 1970’s and 1980’s, leachate studies are often limited by the use of small datasets with an uncertain capacity to adequately represent their parent ash deposit. Here we illustrate the significance of such limitations through the compilation and interrogation of a database of 96 published leachate compositions from 6 studies which investigated the May 18^th, 1980 eruption of Mt. St. Helens. Utilising statistical analysis techniques, we removed outliers and biases between studies by linear transformation in order to produce a useable ash leachate dataset. The corrected data were mapped by kriging method to derive the spatial distribution of soluble S and Cl concentrations downwind of the volcano. Our treatment highlighted spatial trends in leachate data which may reflect various volcanic and atmospheric processes. In order to be able to disentangle these processes, we emphasise the need to obtain a homogeneous spatial distribution when sampling ash for leaching purposes, and to conduct those analyses according to a standardized protocol.