IRON HYDROXIDES AS AN ARSENIC SOURCE IN THE VOLCANICLASTIC AQUIFER OF GUANACASTE COSTA RICA

Elevated geogenic arsenic (As) in drinking water and irrigation water produced from a volcaniclastic aquifer in Costa Rica is an example of an increasingly evident problem in much of Latin America, especially where aquifers are situated in volcanic sediments. In this study, potential sources of As were assessed by (1) measuring the solid-phase chemical composition of volcaniclastic aquifer sediments, (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. Fe/Mn hydroxides in this aquifer system contain 70 - 150 ppm As that is incorporated into this solid phase after chemical weathering of As-bearing volcanic glass; subsequently, As can be released to solution in groundwater when redox conditions shift from oxidizing to reducing, e.g. by introduction of organic matter to the aquifer system or water-logging. The presence of hot springs with highly variable As concentrations raises the possibility that arsenic in groundwater may result from mixing with localized high-As waters. However, experimental evidence for release of As upon reductive dissolution of Fe/Mn hydroxides combined with the relatively homogeneous and widespread 10-60 ppb As content in drinking water wells indicates Fe/Mn hydroxide breakdown as the dominant source of As throughout the regional aquifer. Upwelling of deep geothermal waters contribute to heterogeneous As concentrations in hot springs (<1 to 1188 ppb As), a source that has the potential to influence nearby wells, whereas dissolution of Fe/Mn hydroxides is the apparent source of As disseminated throughout the majority of the aquifer. 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 role of volcanic glass and Fe/Mn hydroxides.