Natural Occurrence of Arsenic in Groundwater from Lesvos Island, Greece

Abstract

A geochemical analysis and modelling was carried out to investigate the As occurrence and release in groundwater from two different geological environments on Lesvos Island: (i) the volcanic area of Mandamados (ignimbrite of rhyolithic to rhyodacitic composition) and (ii) the metamorphic area of Tarti (schists and marbles) that comprises the geologic basement under ignimbrite. Seven sampling campaigns were conducted between October 2010 and October 2011, including 65 groundwater samples from 11 wells and springs. Chemical analyses showed As concentrations exceeding the 10-μg/L national drinking water limit in 46 % of the samples from Mandamados. Groundwater composition in Mandamados evolved from Ca-HCO3 type, to mixed type and finally to Na-Cl type along the groundwater flow direction, indicating the contribution of ion exchange in groundwater chemical composition, while Ca-HCO3 type waters were observed in the Tarti area. Arsenic speciation analysis showed that As(V) was the main species in all samples, indicating that As was released under oxidizing conditions. Statistical analysis suggested silicate weathering as the prime mechanism of As release in groundwater in both cases, while, in the Tarti area, carbonate dissolution may represent a secondary mechanism which could be related to the observed relatively low As concentrations in the region. In both areas, pH-related desorption of As, primarily from Fe mineral phases, was found to be the most important factor controlling the mobilisation of As, while the contribution of the redox control to As release in groundwater was generally found to be less significant.