Alteration of Tephra Conductivity Resulting From Secondary Pyroclast Disaggregation

Abstract:

In addition to influencing the electrical conductivity of volcanic ash, leachates bound to ash grain surfaces may pose significant hazards to water quality through the contribution of sulfates, fluoride, metals, and acidic compounds to local water supplies by either direct ash fallout or incorporation into precipitation runoff. In regions of active volcanism, remobilization of pyroclastic units may be a regular occurrence due to landslides or lahars, but the resultant effects of secondary disaggregation of pyroclasts have not been examined. Laboratory analyses of tephras from several eruptive centers have revealed variations in the pH, conductivity, and total dissolved solid (TDS) concentration of water-soluble compounds as a result of pyroclast disaggregation. Analyses were conducted using the standardized protocols for ash leachate analysis. TDS, conductivity, and pH were then measured and the tephra samples were allowed to air dry before being retested using identical methods. When pyroclast disaggregation was not performed, results show a progressive decrease in water conductivity, acidity, and TDS concentration following each stage of sonication. However, when an additional step of clast disaggregation (through crushing and grinding of samples) preceded the sonication step, water samples showed increases in measured properties. Mass spectroscopic analyses of water samples are in progress and results will be presented. These results indicate that secondary comminution of pyroclastic deposits will release water-soluble components from the pyroclast interior that may result in a renewed series of environmental hazards many years after the initial eruptive event. Break up of pyroclasts during transport in landslides or lahars (or even during cleanup efforts) will not only alter the size distribution of the deposit, but will also release metals, sulfates, and fluoride from the tephra interiors, altering the chemical and electrical properties of the tephra. This poses important implications for evaluating the hazards to water supplies and electrical infrastructure resulting from volcanic activity, and the need to consider how the physical modification of volcanic deposits due to surface processes may spark a renewed set of hazards in volcanically active regions.