Rain-plume interactions at Nyiragongo and Nyamulagira volcanoes and associated rainwater hazards, East Africa

Abstract

A rain-gauge network consisting of 13 stations was installed on the volcanic fields around Nyiragongo and Nyamulagira volcanoes, as well as in the surrounding villages and Goma city (DR Congo) from December 2013 to October 2015. The rain gauges were then sampled on a monthly basis in order to evaluate the influence of volcanic emissions on rain chemistry and rainwater quality. This is the first temporally distributed dataset of rain chemistry from this densely populated region, where the two Africa's most active volcanoes, 14 km apart, continuously eject gases and ash to the atmosphere and where rainwater represents an important water resource. The results revealed that volcanic emissions are the primary source of the dissolved loads. Wind-blown dust dissolution is in fact occasionally the dominant source of major cations at cities and villages that are remote with respect to the volcano summits. A few sites located in the forested Virunga National Park are neither significantly impacted by volcanic emissions nor wind-blown dust. The combined contribution of volcanic gases and the dissolution of volcanic ash and soil dust determined the pH of the rainwater. Thus, areas with limited volcanic impact showed higher pH values (up to 7.6), while those that experience major volcanic impact showed lower pH values (as low as 3.1), as a consequence of the continuous input of acidic volcanic gases. The chemical composition of rain varies according to location and was driven by changes in wind direction. Nearly all the sites showed prominent temporal variations in dissolved loads, which were essentially related to the variations in precipitation amount due to seasonal changes. The rain dissolved loads, the bulk atmospheric deposition fluxes (e.g., 0.5–24.6 t km−2 yr−1 for fluoride and 22.6–176.6 t km−2 yr−1 for total dissolved substances), and the spatial variations converged to show that localities to the west and southwest of Mt Nyiragongo are the most strongly impacted by volcanic emissions. These areas to the west and southwest, which experience higher rates of F deposition, also coincide with the locations of villages and small cities where endemic dental fluorosis occurs. Such a relationship is explained by the use of F-rich water in all domestic activities, including as drinking water, in the region.