ALBERT

Description: 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.

Description: Published

Description: 76-89

Description: 6A. Geochimica per l'ambiente

Description: JCR Journal

Description: Single rainwater samples were collected in the city of Goma (~1,1 million inhabitants), eastern Democratic Republic of the Congo, from January to June 2013 to draw a baseline for rainwater chemical composition and quality as influenced by the permanent plume of Nyiragongo volcano. This was a better period for a baseline as the neighboring Nyamulagira volcano, only 15 km apart, had no important degassing from its central crater, and hence the recorded volcanic influence on the rainwater chemistry was solely from Nyiragongo's lava lake which has been active since May 2002. The baseline for the rainwater chemistry and quality is important for this highly populated region where rainwater is the unique potable water source for the inhabitants of many villages surrounding the volcanoes, and for some of the inhabitants of the city of Goma. Our results show that samples collected at the crater rim of Nyiragongo were more acidic with pH ranging from 3.70 to 3.82, while the majority of rainwater samples collected in downtown Goma city and to the northeastern zone of the volcano had pH close to 5.7; which represents the value for rainwater from unpolluted continental areas (Berner and Berner, 2012). However, the pH was as low as 3.93 to the west of Nyiragongo volcano because the volcanic plume is directed westward by the dominant local wind direction. The western part of the city of Goma as well as the small town of Sake and many villages (e.g. Rusayo, Mubambiro, Kingi, …) are located in this zone, and experience endemic fluorosis caused by high fluoride in the available water. The mean F- in this zone was 0.38 mg/L, while the southern and northeastern zones had mean F- concentrations on 0.44 and 0.01 mg/L respectively; even though concentrations higher than the WHO guidelines were found in few samples from the western zone (1.69 mg/L) and from the southern zone (3.44 mg/L). Compared to data from Cuoco et al. (2012) obtained during the Nyamulagira 2010 eruption, and from Balagizi et al.2017 and Liotta et al., 2017 obtained during the intense degassing of both Nyiragongo and Nyamulagira lava lakes; we have noted similarity in the spatial variation of the pH, but samples from the present study showed notable lower concentrations of major elements. This is the case for fluoride which is strictly of volcanic origin. For the other major elements, anthropogenic sources, mainly the traffic and wind-blown dust; or other non-volcanic natural sources influenced their concentrations. Thus, the anions (Cl- and SO42-) and cations (Na+, K+, Mg2+, and Ca2+) from the present study are either lower compared to that previously reported in the literature for the Virunga, or are both comparable for the zones impacted by anthropogenic activities.

Description: Published

Description: 130859

Description: 6A. Geochimica per l'ambiente e geologia medica

Description: JCR Journal

Description: The δ18O and δ2H of rivers and springs were investigated in order to characterize the groundwater recharge sources around Nyiragongo and Nyamulagira volcanoes, in the Democratic Republic of the Congo and Rwanda. Water samples were collected monthly between November 2013 and October 2014 from 5 major rivers, 3 major cold springs, 3 tepid springs and 1 hot spring. The temperatures of each spring were nearly constant over the sampling period attesting for their groundwater character, while the temperatures of the rivers were much more variable. The rivers monthly δ2H and δ18O range from 􀀀 6.8‰ to 1.9‰ and 􀀀 3.1‰ to 1.6‰, respectively, while springs showed depleted values that span from 􀀀 10.2 to 􀀀 1.1‰ for δ2H and 􀀀 3.6 to 􀀀 1.9‰ for δ18O. Catchment morphology (formed of depression, upper footslope and medium to high gradient-mountains) and the local tectonic discontinuity (fissures and faults) regulate the surface runoff and subsurface flow, control the precipitation infiltration zones and hence the aquifers recharge areas. Chemical and isotopic (δ18O and δ2H) compositions of springs and rivers reveal the presence of shallow and deep aquifers, with some waters having intermediate isotope composition. Three different recharge zones characterized by different altitudes were identified: the first is found at low altitude ranging from ~1800 m to ~2150 m, the second and intermediate recharge zone in the altitude range from ~2180 m to ~2500 m at the upper footslope area, while the third and highest recharge area is located in the altitudes range from ~2620 to ~3220 m. The two upper recharge areas are the most fractured and fissured zones allowing rapid infiltration of depleted precipitations which recharge deep aquifers found in the tepid and hot springs. Based on their chemical and isotopic composition, waters from the shallow and deep aquifers have been considered representative of mixing end members. During their ascent to the surface, water from the deep aquifer mixes with that of shallow aquifer yielding the tepid springs of intermediate chemical and isotopic composition, while the other keep their original fingerprint corresponding to the isotopically depleted hot spring.

Description: Published

Description: 120778

Description: 6A. Geochimica per l'ambiente e geologia medica

Description: JCR Journal

Description: We report the first δ18O and δ2H data of Virunga rainfall in the Eastern Democratic Republic of the Congo, situated on the limit between Central and Eastern Africa. The dataset is from 13 rain gauges deployed at Mount Nyiragongo and its surroundings sampled monthly between December 2013 and October 2015. The δ18O and δ2H vary from -6.44 to 6.16‰, and -32.53 to 58.89‰ respectively, and allowed us to define a LMWL of δ2H = 7.60δ18O + 16.18. Three main wind directions, i.e. NE, E and SE, were identified in the upper atmosphere corresponding to three major moisture source regions. On the contrary, lower atmospheric winds are weaker in nature and originate mainly from the S and SW, creating a topographically-driven, more local moisture regime. The latter is due to the accumulation in the floor of the rift of water vapor from Lake Kivu forming a layer of isotopically enriched vapor that mediates the isotope enrichment of the falling raindrops. A strong seasonality is observed in both δ18O and δ2H data, and is primarily driven by combined seasonal and spatial variation in the moisture sources. The δ18O and δ2H seasonality is thus correlated to weather patterns, as the latter control the wet to dry season shifting, and vice versa. The key characteristic of seasonality is the variation of monthly precipitation amounts, since the mean monthly air temperature is nearly constant on an annual scale. Two regionally relevant hydrological processes contribute to the isotopic signature: namely moisture uptake from the isotopically enriched surface waters of East African lakes and from the depleted soil-water and plants. Consequently, the proportion of water vapor from each of these reservoirs in the atmosphere drives the enrichment or depletion of δ2H and δ18O in the precipitation. Thus, during wet periods the vapor from soil-plants evapotranspiration dominates yielding isotopically depleted precipitation, contrary to dry periods when vapor from lakes surface evaporation dominates, yielding isotopically enriched precipitation. At the global scale, our dataset reduces gaps in this region that has been poorly studied for δ18O and δ2H in precipitation. At the regional scale, the improved understanding of the ways land cover, moisture source seasonal and spatial dynamics, and atmospheric patterns impact precipitation spatial and temporal variabilities in Central-East African will contribute to the ongoing research on mitigating the impacts of ongoing climate change in Sub-Saharan Africa. The reduction of gaps and uncertainties in δ2H and δ18O of precipitation, and the understanding of their interrelation with weather patterns are essential for a better past, present and future environmental and climatic modelling at both local and regional scales.

Description: Published

Description: 1058-1078

Description: 6A. Geochimica per l'ambiente e geologia medica

Description: JCR Journal