Environmental mobility of potentially toxic trace elements of Andean volcanic ashes

Abstract

This research investigate the geochemical environmental impact of ancient ash deposits erupted to hundreds of thousands to several million years ago in the southern Puna and neighbouring areas (Argentina), Uruguay and five historical eruptions occurred over a 20-year period in the Andes. In the latter case, pristine ash samples of Cophue, 2000, Lonquimay, 1988, Llaima, 2008, Chaiten, 2008, and Hudson, 1999 eruptions were studied. The main purpose of this research is to investigate the geochemical environmental hazard posed by different types of volcanic ashes in general, and from the Andes, in particular. The specifics aims of this research are:(1) to develop an analytical methodology for the physico-chemical characterisation of volcanic ash;(3) to propose a new ash leaching methodology that can be useful both for recent and ancient volcanic ashes (Single Batch Leaching Test, SBLT);(4) to recognize the geochemical impact occurring after the deposition of volcanic ash on the environment through leaching experiments (Batch leaching tests, Column leaching tests, and Sequential Extraction Scheme). The main conclusions extracted from this research can be summarised as follows:(1) The physico-chemical information of ashes (mainly the morphological features by SEM, grain-size distribution by laser diffraction and Potential toxic trace elements identification and quantification of the bulk ashes) highlights the main environmental concerns related to the studied ashes and it lays the foundation of the basic knowledge required to understand the ash leaching. (2) The main body of this research has been to propose a proved methodology for a single batch leaching test (SBLT) with deionised water in order to obtain information on short term leaching behaviour of volcanic ash identifying, qualitatively and quantitatively, the potentially hazardous elements being rapidly released from both recently erupted and ancient volcanic ashes. The proposed procedure is completed within few hours and its design is simple, concise and reliable.(3) The application of the SBLT (with deionised water) method developed in this research have allowed to define the potential geochemical impact of the volcanic ashes investigated on the environment. This study shows that major elements with high mobility are Ca, Mg, Na, Si, K, and Al. Puna and Uruguay ashes could release in the environment the same trace elements (i.e., P, Ti, Fe, Mn, Zn), however, in different concentrations. These are macro- and micronutrients evidencing the fertilizing potential of the ashes. Chaiten ashes showed high concentrations for As, Pb, Fe, and Zn, whilst the other samples (i.e., COP, LON, LLA, and HUD) could release principally P, Ti, Mn, Ni, Zn, and Pb. Most of these are potentially toxic trace elements (PTTE) even at very low concentration. They are included in the drinking water guidelines due to their potential toxicity and must be especially monitored in the environmental assessment of these ashfall deposits. (4) Dynamic leaching tests conducted on raw ashes and grain-size fractions allowed to define the element implied during glass and mineral dissolution. From the data nowadays available on the temporal leaching of Andean rhyolitic ashes (from CVZ to SVZ), the association between ash dissolution and some trace elements (As, Mo, V, B, and U) through laboratory experiments was demonstrated. This study represents a great advance on the knowledge of the geochemical environmental hazard posed by volcanic ashes, in general, and from the Andes, in particular. In terms of methodological approach, the developed method of SBLT is a very important contribution because there is a need to harmonize the batch tests applied to volcanic ash. In addition, column leaching test and sequential extraction scheme complement the information obtained by SBLT and facilitate decision making on management of geochemical hazard associated with the volcanic ash fall.