Impact of volcanic emissions on rainwater chemistry: The case of Mt. Nyiragongo in the Virunga volcanic region (DRC)

Chemical composition of rainwater is strictly related to atmosphere scavenging. Active degassing volcanoes release acid gases and solid particulate in the surrounding environment. Nyiragongo Volcano (DRC) is characterized by a high degassing activity from an active lava lake hosted within the crater. Chemistry of rainwater in the area is clearly dependent on the influence of the volcanic plume, especially at the rim of the Nyiragongo summit crater. Rainwater collected from this zone has pH values as low as 2, high salinity (EC 28-1800 mu S/cm), and high contents of F- and Cl- (up to 193 and 270 ppm, respectively), NH4+ (up to 146 ppm) and SO42- ions (up to 340 ppm) relative to worldwide rainwater. The chemical composition of rainwater after interaction with the volcanic plume tends to shift towards the condensable fraction of fumarolic fluids discharged from the summit crater. Rainwater acidified by the volcanic plume also removes metals from particulate suspended in the atmosphere, thus undergoing metal enrichment. Displacement of the Nyiragongo volcanic plume by predominantly westward-directed wind causes natural contamination of rainwater collected for drinking purposes in villages located on the western flank of the volcano. Rainwater falling in urban centers located S-SE of the Nyiragongo Volcano is not usually affected by the rain-plume interactions which strictly depends on wind directions. However, areas of possible contamination by the volcanic plume might create a further emergency and critical situations on top of an already existing severe humanitarian crisis. Rainwater is the principal drinking water supply in the Nyiragongo area, thus the geochemical monitoring of rainwater quality is of great importance to mitigate the hazard of natural contamination of this fundamental resource for the local communities. (C) 2012 Elsevier B.V. All rights reserved.