Groundwater arsenic and the role of secondary Fe/Mn hydroxides in a volcaniclastic aquifer, Guanacaste, Costa Rica

Abstract

Elevated arsenic (As) in the regional water supply from a volcaniclastic aquifer in Costa Rica is an example of the increasing problem of As-contaminated waters in aquifers throughout Latin America. In this study, potential sources of As were assessed by (1) measuring the solid-phase chemical composition of volcaniclastic aquifer sediments and intercalated paleosols, (2) analyzing As speciation and availability by sub-millimeter scale elemental mapping and targeted chemical extractions, and (3) mapping spatial trends of As in shallow drinking water wells, surficial hot springs, and deep wells producing geothermal waters. Iron-manganese hydroxides (Fe/Mn hydroxides) in this aquifer system contain 210–530 ppm As that sorbs to this solid phase when released by chemical weathering of As-bearing volcanic glass. Arsenic is primarily transferred from solid to aqueous state when groundwater conditions shift from oxidizing to reducing, likely driven by presence of organic matter or periods of water logging (e.g. rainy season or irrigation). The occurrence of isolated hot springs raises the possibility that arsenic in groundwater may result from mixing of low-As meteoric waters with localized high-As thermal waters (hot springs). However, experimental evidence for release of As upon reductive dissolution of Fe/Mn hydroxides in solid-phase aquifer sediments plus the homogeneous and widespread 10–60 ppb As content in drinking water wells indicate that Fe/Mn hydroxide breakdown is the primary cause of the widespread As contamination in the regional aquifer. Locally, natural upwelling of thermal waters may affect wells in close proximity to surficial hot springs, but minimal evidence of this was observed in this study. Given the high incidence of elevated As in groundwater in volcaniclastic aquifers in Latin America and similar environments globally, these results are applicable to understanding As speciation and source, in particular drawing attention to the significance of secondary Fe/Mn hydroxides in arsenic cycling in volcaniclastic aquifers.