GEOGENIC ARSENIC IN SEDIMENTARY AQUIFERS OF SOUTHWESTERN SEBACO VALLEY, NICARAGUA

In Nicaragua, arsenic contamination of drinking water resources due to geogenic sources can be identified in the southwestern part of Sébaco Valley. The arsenic sources are weathering products of Tertiary volcanic rocks of the Coyol group that form the alluvial aquifer of Sébaco Valley. This valley is located at the external eastern margin of the Nicaraguan depression (or Central American Graben). The region is characterized by intensive tectonic stress and the presence of different active and non-active fault systems, intensive fracturing and hydrothermal alteration along faults. The volcanic rocks outcrop at the margins and at several locations within the valley overlying the alluvial aquifer. This aquifer is used by several communities for drinking water supply. In 21 of 57 water samples collected from wells of this alluvial aquifer, arsenic concentrations ranged from 10 to 122 µg/l, exceeding the national and the WHO limit of 10 µg/l for arsenic in drinking water. Several hot spots with high groundwater arsenic were identified, e.g. El Zapote Village with 122 µg/l. This is the same locality where in 1996 a high concentration of 1320 µg As/l was found in a well drilled into the highly fractured hardrock aquifer, and which supplied drinking water to the population since 1994. As a consequence, the water from this well has produced severe health impacts on the population. In comparison, the localities of Las Mangas and Tatazcame, were identified as areas with safe groundwater, most suitable for future drinking water supply projects. The high arsenic concentration in El Zapote water is correlated with elevated arsenic concentrations in rocks and soils (14.98 and 57.19 μg/kg, respectively). Arsenic sources are the volcanic hard rocks, the alluvial sediments composing the alluvial aquifer and the unsaturated zone, and the hydrothermally altered volcanic rocks. Chemical mineral weathering and changing redox conditions increase mobility of arsenic and its transfer from solid to dissolved form. Similar Tertiary volcanic rocks cover wide parts of Nicaragua and other Central American countries. It can be expected that many other not yet identified sites with groundwater arsenic problems exist in the region.