Journal of African Earth Sciences xxx (2010) xxx–xxx

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Journal of African Earth Sciences

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Dry gas vents (‘‘mazuku”) in Goma region (North-Kivu, Democratic Republicof Congo): Formation and risk assessment

Smets Benoît a,*, Tedesco Dario b,c, Kervyn François a,d, Kies Antoine e, Vaselli Orlando f,g,Yalire Mathieu Mapendano h

a Royal Museum for Central Africa, Geology Department, Leuvensesteenweg 13, B-3080 Tervuren, Belgiumb Second University of Naples, Department of Environmental Sciences, Via Vivaldi 43, 81100 Caserta, Italyc National Research Council, Institute of Environmental Geology and Geo-Engineering, Rome, Italyd Centre d’Informations Géographiques, Goma, Congoe University of Luxembourg, Physics and Material Research Unit, Avenue de la Faïencerie 162A, L-1511 Luxembourg, Luxembourgf University of Florence, Department of Earth Sciences, Via La Pira 4, 50121 Florence, Italyg CNR – Institute of Geosciences and Earth Resources, Via La Pira 4, 50121 Florence, Italyh Goma Volcanological Observatory, Goma, Congo

a r t i c l e i n f o

Article history:Received 10 March 2009Received in revised form 4 March 2010Accepted 23 April 2010Available online xxxx

Keywords:MazukuGomaKivuDry gas ventHazard

1464-343X/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.jafrearsci.2010.04.008

* Corresponding author. Tel.: +32 (0)2 769 54 48; fE-mail address: benoit.smets@africamuseum.be (S

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a b s t r a c t

The word ‘‘mazuku” in Swahili means ‘‘evil wind”. It corresponds to lowland (depressions) where carbondioxide is released and, being heavier than air, accumulates at high – often lethal – concentrations(10 vol.% of CO2 in atmosphere can be considered as the deadly threshold, even for a short time exposure).Mazuku are abundant in Goma and surrounding areas and particularly in the area south of the large vol-canic edifices of Nyiragongo and Nyamulagira volcanoes located in the most eastern part of DR Congo, Wbranch of the East African Rift System (EARS). Our extensive field surveys have indicated that mazuku areconcentrated within to and around the densely populated city of Goma close to the N shores of Lake Kivu,mainly near fault or fissure networks. At a more local scale, depressions allowing CO2-rich gas accumu-lation are created by lava flow superposition, lava tunnels or cavity collapses, or directly associated withopen fractures. People are killed by mazuku every year. Given political and social unrest coupled with thecurrent important demographic and urban growths around Goma, the risks associated to mazuku areincreasing accordingly. Mazuku are currently the most important natural risk in terms of human lossfor the area and there is an urgent need for further research, more systematic mapping and monitoringof mazuku and for appropriate risk management to be implemented. This paper summarizes the currentscientific knowledge on mazuku as well as new advances and a preliminary risk assessment performedrecently in the frame of the GORISK project.

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1. Introduction

Goma city is built on the N shore of Lake Kivu (Fig. 1). This areais located in the W branch of the East African Rift System (EARS),under the threat of several natural hazards mainly associated withtwo highly active volcanoes: Nyiragongo and Nyamulagira. Thelatter volcano is the most active one in Africa and one of the mostactive worldwide. Presently, �1 million people live at the foothillsof these volcanoes. Taking into account the continuous political/social unrest triggered by local and national armed conflicts, therecent exponential demographic and urban growths, the Gomaregion can be regarded as one of the most dangerous placeson Earth to live in. The impact of future eruptive events is then

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ax: +32 (0)2 769 54 32.. Benoît).

al. Dry gas vents (‘‘mazuku”) in.1016/j.jafrearsci.2010.04.008

considered as one of the main concerns for the area, able to causea huge number of casualties and extremely important infrastruc-tural damages. In the last eruption of Nyiragongo, in January2002, several lava flows were emitted by a NS-oriented fracturesystem opened in the S Nyiragongo flank. Forty-five to 147 personswere killed, �10% of Goma city destroyed and 80% of the regionaleconomy paralyzed (Baxter and Ancia, 2002; Tedesco et al.,2002a,b, 2007b; van Overbeke et al., in press).

In May 2002, volcanic activity resumed in the central crater (e.g.Tedesco et al., 2007b), with a lava lake that continues to release apermanent gas plume, with high amounts of acid gases, SO2, HFand HCl (Carn, 2002/2003; Sawyer et al., 2008; Vaselli et al., inpress). This plume is responsible for continuous and important acidrains in surrounding areas with a pH �1–4, mainly west of Nyirag-ongo due to local dominant winds. Impacts include damage to veg-etation, crops and human infrastructures, as well as air, soil and

Goma region (North-Kivu, Democratic Republic of Congo): Formation and

Fig. 1. General map of the Goma region with the main volcanic fractures and the main escarpments in the rift. Nyam = Nyamulagira; Nyir = Nyiragongo; Mik = Mikeno;Kar = Karisimbi; Vis = Visoke; Sab = Sabinyo; Gah = Gahinga; Muh = Muhavura. Volcanic fractures from Thonnard et al. (1965) and field data of the Royal Museum for CentralAfrica; sinuous and straight escarpments based on Pouclet (1977), Ebinger (1989), Wauthier et al. (2008).

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water pollution with related health problems (e.g. Baxter andAncia, 2002; Vaselli et al., in press). Recent field campaigns (e.g.Vaselli et al., 2002/2003; Smets, 2007; Tedesco et al., 2010) re-vealed another well-known hazard among the indigenous popula-tion: the mazuku. These are CO2-rich dry gases that emit fromvents commonly located in morphologically depressed areas. Nodetailed studies about these manifestations are available andresearchers still debate about their origin. In the framework ofthe GORISK project (Smets, 2007; van Overbeke et al., in press),mazuku from this portion of the western of the EARS were system-atically investigated. All known mazuku along the Lake Kivu shore-line were mapped using GPS, the geomorphology of the mazukuarea as well as the morphology of each mazuku were describedin detail and three campaigns of systematic gas measurementswere performed. Those fieldworks allowed us to distinguish somepreferential areas where gas escapes from the ground and accumu-lates by gravity. This work is a new contribution to the mazukuphenomenon in the Goma region, in the light of our recent find-ings. A preliminary risk assessment is also proposed, as a basisfor first actions of mitigation.

2. Formation of mazuku

2.1. A common phenomenon?

From Kiswahili ‘‘Evil winds that travel and kill during the night”(Tuttle et al., 1990), mazuku (invariable in plural) are dry and cold-

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ambient gas vents that correspond to depressions from where gas,mainly consisting of CO2, is discharged. CO2 is denser than air andcan accumulate at high to extreme concentrations into porousground or in enclosed spaces such as depressions, lava caves/tun-nels or house’s cellars. Usually, CO2 rapidly dissipates and mixeswith the air when wind, solar irradiation and high pressures andtemperatures are present (e.g. Sorey et al., 1996; Tassi et al.,2009a). Nevertheless, CO2 concentrations of P1 vol.% become toxicfor humans and animals. A CO2 increase in the atmosphere also in-duces an oxygen deficiency. The combination of high CO2 and lowto extremely low O2 is a perfect deadly cocktail for any living being.The effects of short and long time exposures to CO2, and those re-lated to O2 deficiency are summarized in Tables 1 and 2.

Natural CO2, degassing from geological sources, is known fromactive and dormant volcanoes, submarine volcanism, major faultzones, geothermal systems and thermal decomposition of organicmatter are important sources of non-anthropogenic CO2 (see Ker-rick (2001) for an overall and general review). Carbon dioxide en-hanced levels can also be generated in karstic systems (e.g. Ek,1979; Crowther, 1983; Renault, 1985), especially those developedunder a high density of vegetation cover. All literature about natu-ral CO2 degassing is an important resource for the comprehensionof certain aspects of mazuku like gas vent specific features or phys-iological effects of high CO2 concentrations on humans andvegetation.

Thus, the Goma region is not the only place where thishazardous phenomenon occurs. A very similar example is the

Goma region (North-Kivu, Democratic Republic of Congo): Formation and

Table 2Effects of O2 deficiency to human. Only few minutes of high oxygen deficiency areenough to trigger death. Sources: Gellhorn (1936), INRS (1978).

12–16 vol.% of O2 Increased respiratory rate, coordination of movementsperturbed

10–14 vol.% of O2 Abnormal tiredness, unequal respiratory, headhache,sweating

6–10 vol.% of O2 Nausea, vomiting, impaired consciousness,uncoordinated movements, amnesia

<6 vol.% of O2 Unconsciousness, heart and respiratory arrest, death

Table 1Effects of short and long term exposure to carbon dioxide on human. Sources: Sechzeret al. (1960), Williams-Jones and Rymer (2000), Bonnard et al. (2005), Langford(2005), Toxnet (2005).

Short term exposure(minutes to tens ofminutes)

2%: increase in respiratory amplitude4%: increase in respiratory frequency5%: headhache, dizziness, increase in cardiacfrequency and arterial pressure, peripheralvasodilatation10%: visual blurred, arterial hypertension,hypersudation, impaired consciousness>10%: convulsion, coma, death

Long term gas exposure(8 h/day, 40 h/week)

2%: increased Pco2 in blood and decrease inblood pH3%: acidosis in blood, increased respiratoryrate4%: headache, tiredness, tachycardia, extrasystole>4%: tolerance threshold exceeded;unconsciousness

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peri-Tyrrhenian belt in central Italy, which is characterized bynumerous CO2 gas vents (e.g. Minissale et al., 1997; Chiodiniet al., 1999; Minissale, 2004), associated to deep reservoir(s) ormantle degassing, set along local or regional faults. These manifes-tations are often deadly traps for wild animals and human beings(e.g. Carapezza et al., 2003; Tassi et al., 2009a).

As already emphasized, dry gas vents in the African volcanicareas or in volcano-tectonic contexts have never been studied,especially for their lethal consequences, even though they are lo-cally known to indigenous people in every volcanically or tectoni-cally active area from across Central Africa (from Cameroon all theway to Uganda, Tanzania, Kenya and Ethiopia); in SW Tanzania, aparticular CO2 gas vent at the N-end of the Rungwe Volcanic Prov-ince (RVP) is known to have killed about 100 people in the 20thcentury, whilst another one associated with the Kiejo volcano atthe S-end of the RVP is mined for its CO2 for a multimillion fizzydrink industry generating important income for the local and na-tional economy (GGJ. Ernst, pers. comm., 2009; see also Fontijnet al. (this issue), this JAES AVCOR Special Issue).

2.2. Specific local-scale features of mazuku

Mazuku in the Kivu volcanic region are mainly located at lowelevation (about 1500 m a.s.l.) just before reaching the vicinity ofLake Kivu, in the lava fields of both Nyiragongo and Nyamulagiraand sometimes directly discharge close to or within the manyperipheral volcanic edifices (e.g. Rumoka and Bulengo craters) thatcharacterize the area (Capaccioni et al., 2002/2003). The depressionwhere gas accumulates can strongly vary in shape (from round/elliptical to fissure-like) and surface (generally from 5 to4500 m2). Furthermore, high vegetation density in this subequato-rial area makes field and geophysical surveys challenging at best(e.g. geoelectric surveys could help map subsurface lithologies,

Please cite this article in press as: Benoît, S., et al. Dry gas vents (‘‘mazuku”) inrisk assessment. J. Afr. Earth Sci. (2010), doi:10.1016/j.jafrearsci.2010.04.008

locate aquifers and gas/fluid escape structures in the future). None-theless, some typical features allow to identify mazuku.

If vegetation makes the interpretation of geomorphology diffi-cult, it also helps detect mazuku. Some plant species indeed preferhigh CO2 concentration and natural CO2 springs often lead to dis-tinct vegetation zoning, such as that at Bossoleto (Toscany, Italy;van Gardingen et al., 1997).

In the Goma region, mazuku are detected by the presence ofpapyrus, light green grass and sometimes kaki moss through whichgas seems to be released. However, where gas concentrations aretoo high (generally up to 40–50 vol.% for this case study), vegeta-tion does not develop or is killed off, leaving only bare and weath-ered rocks.

Weathered lava surfaces where gas accumulates are also a typ-ical feature. Silicate weathering is a natural sink for atmosphericCO2 (e.g. Volk, 1987; Navarre-Sitchler and Thyne, 2007). If thegas effect on rocks is sometimes considered as insignificant (Golu-bev et al., 2005), CO2 concentration in mazuku, usually much higherthan the deadly 10 vol.% threshold, is consistent with an increasedrock weathering by this process. Lavas in mazuku have a black orvery dark grey colour instead of the common grey–brown colourin the area. The alteration zone affects a deeper part of lava, reach-ing sometimes a thickness of tens of centimetres.

A third way to detect high CO2 concentration is the warm feel-ing near the ground surface. Mazuku being cold gas emanations,only a high CO2 gas concentration can account for this slight buteasy to feel increase in temperature. This feeling could correspondto a sort of extremely localized greenhouse effect and/or to the re-duced respiration of the skin due to high-CO2/low-O2 concentra-tions, which is felt as heat.

Finally, an accurate mazuku detector is the almost systematicpresence of dead animals. Given that the high gas concentrationsgenerally affect the first tens of centimetres or the first meter ofair above the ground, animals that are preferentially killed aremainly small-to-medium-size, e.g. insects, birds, goats or dogs. Insome mazuku where CO2-rich gas accumulation is thicker, animalslike cows or monkeys were also found dead.

2.3. Hypothesis of formation

During its ascent, gas tends to follow the easiest path towardsthe surface, through faults, fractures, more permeable rocks (e.g.at least partly open channelized lava flows typically have brecciat-ed rubble at their base, top and sides, these parts being more per-meable than the core of a massive lava flow) and/or morepermeable contacts in a stack of rock layers. In the Goma region,frequent lava flow and rapid dense vegetation coverage rates makedifficult a correlation between mazuku and the underlying geolog-ical structures. In order to allow distinguishing preferential areaswhere gas escapes from the ground and where it accumulates, fieldsurveys were carried out in 2007 and 2008. A handheld GPS Gar-min eTrex and two differential GPS Leica Systems 1200 were usedto locate mazuku and map the closely associated topographic con-text. A gas analyzer GA2000 with a metallic pipe ended by an inletfilter was also used for spot gas measurements at ground level and/or up to the first centimetres into the ground.

Mazuku were found to correspond to a convergence betweentwo phenomena: (1) the link between a CO2 source and a networkof fractures allowing the gas to reach the surface and (2) a depres-sion where the dense CO2-rich gas can gravitationally accumulate.Systematic measurements showed high CO2 concentrations(P10 vol.%) at the edge of embankments that seem to correspondto lava flow borders. Thus, if a new lava flow covers a gas vent, CO2

will use fractures in cooled lava or the brecciated and more perme-able interface between successive lava flow deposits to reach thesurface. If a morphological depression is present, gas accumulates

Goma region (North-Kivu, Democratic Republic of Congo): Formation and

Fig. 2. Map of the mazuku of Bulengo Seminaire. The black patches are the areas without vegetation due to very high CO2 concentrations (commonly 50–70 vol.%). This mapshows that the gas escapes mainly at foot of lava flows. Lava flows and mazuku were mapped on the basis of an IKONOS image acquired in the framework of the GORISKproject. Fieldworks were performed and confirmed the features mapped using the very high resolution satellite image.

Fig. 3. 3D map of the mazuku ‘‘Nyabyunyu 1”. High CO2 concentrations are located at foot of an embankment that corresponds to the border of a lava flow. Gas measurementswere performed in the first 10 cm of the ground.

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Please cite this article in press as: Benoît, S., et al. Dry gas vents (‘‘mazuku”) in Goma region (North-Kivu, Democratic Republic of Congo): Formation andrisk assessment. J. Afr. Earth Sci. (2010), doi:10.1016/j.jafrearsci.2010.04.008

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by gravity and a mazuku is formed. The mazuku called ‘‘Katwa” and‘‘Bulengo seminaire” typically correspond to this description. Kat-wa is located at the foot of a lava flow emitted from the Nyamula-gira volcano that overlies a lava flow from Nyiragongo. The mazukuof Bulengo Seminaire are set at the intersection of three differentlava flows, creating a wide depression with the largest mazuku evermet (�100 m long; �4700 m2) during our fieldwork (Fig. 2). In thearea of Sake, gas accumulates at the foot of lava flows in poroussuperficial deposits, which prevents mixing of the CO2-rich volca-nic gas with atmospheric air. Carbon dioxide-rich gas is heremainly released in the atmosphere through micro-landslides lo-

Fig. 4. Simplified model of the suggested mechanisms for gas release in homogenous and(B and D), based on our interpretation of mazuku. As shown in Fig. 3B, gas reach surface bdepression or cavity. The grey curves in the frame A and B model the variation of CO2 a

Fig. 5. Simplified models that show the diversity of mazuku. (A) Sketch of mazuku formedflow and on fissures are mazuku. (B) Mazuku formed by the collapse of a cavity roof. (C) Slayer (tephra).

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cated in the lava flow front. These porous deposits correspond totephra layers associated with the activity of Nyamulagira. In orderto identify gas release between successive lava flow deposits inmazuku, very high resolution topographic maps were generatedfor selected sites by using differential GPS positioning and the re-sults are consistent with our hypothesis (Fig. 3).

Sometime, mazuku depressions are filled by rock debris and areoften linked to cavities or partially-drained lava flow tunnels.These kinds of depressions are interpreted as collapse cavitiesallowing gas accumulation when gas is sourced into them. Openfractures at ground surface are also typical depressions from where

porous context (A and C) versus in active volcanic context such as the Goma regiony travelling in fractures or between lava flows and concentrate in any morphologicalt the ground surface along the profiles.

near lava flows and along the shore of the Lake Kivu. Dark patches at foot of the lavachematic vertical profile of the ground in Sake. Gas accumulates in the porous lapilli

Goma region (North-Kivu, Democratic Republic of Congo): Formation and

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CO2-rich gas can accumulate. These fissures can correspond tocooling joints in lava flows, to transverse fractures developing inlava flows at local topographic slope breaks (when slope suddenlyincreases extensional open fractures are formed; those fracturesare comparable to seracs on mountain glaciers) to volcanic fissures,or to local or regional faults reaching the surface.

All these different geomorphologic structures and the mazuku’sspatial distribution are summarized in simplified models in Figs. 4and 5.

3. Origin of gas and transport from source to surface

As previously discussed, CO2 degassing can occur within differ-ent tectonic and geologic settings (Kerrick, 2001). According toVaselli et al. (2002/2003) and Tedesco et al. (2007a), gas isotopicsignatures (C and He ratios) in mazuku indicate a deep magmaticorigin. Samples were collected at least at 25 different vents, span-ning the whole N side of the Lake Kivu shorelands, over a distanceof almost 25 km, from Goma to Sake towns. The results of theiranalyses are consistent with the previous studies (see Tedescoet al., 2010).

Based on helium isotope ratios, all gas emissions are unambig-uously related to a deep-mantle source, with a R/Ra (Helium ratio)of about 8 (Tedesco et al., 2010). Further constraints are obtainedfrom carbon isotope ratios. Two distinct clusters of data can be rec-ognized: (a) those related to central and eastern side of the riftwith mildly lighter carbon than that of the fluids from Nyiragongo(�5.3‰ to �6.8‰ V-PBD at eastern and central mazuku versus�3.5‰ to �4.0‰ V-PBD at the Nyiragongo crater) and (b) from�10.48‰ to �11.65‰ V-PBD at western Sake’s mazuku. The lattercarbon data can be interpreted in two ways: (1) severe fraction-ation of carbon isotopes (inconsistent with above deep mantle sig-nature from He isotopic ratio data) and/or (2) late-stage addition ofbiogenic CO2 (Tedesco et al., 2010). A third possible option could bea unique carbon source, related to this area also corresponding tothe Kabuno basin, Sake mazuku and possibly Nyamulagira volcano.This is matter of debate and further studies are needed to give a fi-nal answer.

Daily variations in CO2 concentrations have been also measuredduring fieldworks and using a Rn-CO2 monitoring station installed

Fig. 6. Variations of carbon dioxide between the 3rd and the 12th September 2009, in thCO2 concentrations in the ground (�20 cm depth) and in the air (�2 m above the groundof the solar irradiation. As shown in the graph, solar irradiation has a great influence on ato three times higher in the air. This influence is also noticeable in the variations of grounwind, rain or atmospheric pressure influence the gas concentrations inside the mazuku

Please cite this article in press as: Benoît, S., et al. Dry gas vents (‘‘mazuku”) inrisk assessment. J. Afr. Earth Sci. (2010), doi:10.1016/j.jafrearsci.2010.04.008

inside the mazuku called ‘‘Le Chalet”. These variations seem tomainly result from the effects of solar irradiation variability(Fig. 6), atmospheric pressure, wind velocity and rain. No link be-tween gas concentration and volcano-tectonic activity was foundyet. This is mainly due to the combination of the very few contin-uous measurements of CO2 (or other gases) yet carried out in theregion and the absence of volcano-tectonic event (e.g. seismic cri-sis, eruption, variation in volcanic activity). A critical aspect to beunderstood is how volcanic and seismic activity in the VVP is re-lated to CO2-mazuku degassing, and how the CO2-rich gas fluxingat mazuku may respond to continuous rift-wide volcano-tectonicevents. Nevertheless, mazuku degassing shows differences withvolcanic degassing along the 1997–2002 eruptive fracture networklocated south of Nyiragongo. Volcanic degassing often containswater vapour, sometimes sulphur dioxide (e.g. on Nyiragongosouth flank) and tends to slightly decrease since the NyiragongoJanuary 2002 eruption, which is not observed in mazuku.

The possible link between mazuku location and fracture or faultnetwork can be highlighted by considering the mazuku spatial dis-tribution map, as derived using handheld GPS positioning of sites(Fig. 7). This map shows a preferential location of mazuku alongthe N border of Lake Kivu and especially near major fracture net-works, which allows to hypothesize a clear link between the maz-uku and the main fractures (local and regional, and more generallya fault network). For example, both spatial distribution and spatialdensity maps for mazuku in the Bulengo area (Fig. 8) indicate alikely link between a NE–SW fracture network located at South-West of Nyiragongo and the presence of the lethal gas vents.

Some interesting insights can be obtained by what is known tocontrol the spatial distribution of deep-sea black smokers. Likeventing or magmatic vents in segmented continental rifts, theyare related to (1) deep and/or shallower magma sources, (2) to riftand rift relay faulting (e.g. transform faults), and (3) to rate ofextension/spreading. The discoveries concerning the control ondeep-sea venting areas (say from moderately extending rift –2 cm/y or less) show that venting should be concentrated or morefrequent by clustering toward the extremity rather than the mid-section of individual rift segments (e.g. Murton et al., 1994). Thishas been explained by the much higher fracture density associatedwith both rift faulting and transform faulting at rift segment ends

e mazuku ‘‘Le Chalet”. The black and dark grey lines correspond respectively to the). The light grey line corresponds to the battery voltage and gives an indirect insighttmospheric CO2 as after its inset, carbon dioxide concentrations directly increase upd-CO2 concentrations. As observed in the field, other meteorological parameters likeand thus should at least partly explain the other observed variations.

Goma region (North-Kivu, Democratic Republic of Congo): Formation and

Fig. 7. Location of mazuku along the northern border of Lake Kivu and schematic structural context. The mazuku in the Goma area seem to have a strong link with Lake Kivu.The mazuku of Bulengo are also located near Lake Kivu, but their high density in the area suggests a link with the tectonic/volcanic structures. The mazuku of Sake, Mubambiroand Rumoka seem to be also linked to volcanic and/or tectonic structures. The volcanic fractures are derived from Thonnard et al. (1965) and field data of the Royal Museumfor Central Africa. Sinuous and straight escarpments are based on Pouclet (1977), Ebinger (1989) and Wauthier et al. (2008). The shaded relief in the background is derivedfrom the SRTM DEM.

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and leading to easier pathways for both magma and fluid/gas vent-ing. If we transfer this understanding to the Kivu rift segment, themajor volcanoes and their associated mazuku are indeed only con-centrated toward the northern end of the Lake Kivu basin, whereintersecting faults are providing pathways for both magmas andgas to concentrate there. Similar arguments strikingly account forthe spatial distribution of vents and major volcanoes at the RVPin SW Tanzania that are also away from the main centre rift seg-ments (see Fontijn et al., this issue). Of course the strong controlexerted by intersecting fracture/faults networks is expected at allscales – at the most local scale, small fracture networks influencethe final outbreak locations where mazuku are found. The aboveis consistent with all observations made on the Goma – N Kivumazuku.

Another key point is that mazuku are located near Lake Kivu(generally <3 km far from Lake Kivu shoreline) where volcaniccones have a phreatic or phreatomagmatic origin, which suggestsa possible link between mazuku and a hydrothermal and/orgroundwater system(s). This is evidenced in the Sake area wheremazuku are associated with the presence of mineral-rich watersources. According to Capaccioni et al. (2002/2003) and Vaselliet al. (2002/2003), hydrothermalized products, discovered at thebase of the eruptive sequence of the phreatomagmatic cone called‘‘Lac Vert”, support the presence of a past and probably still activehydrothermal system. However, nowadays there are no clear evi-dences of the existence of any active (or fossil) superficial and/ordeep hydrothermal system in the area between Lake Kivu and Nyi-ragongo. We suspect that the extremely local active tectonics and

Please cite this article in press as: Benoît, S., et al. Dry gas vents (‘‘mazuku”) inrisk assessment. J. Afr. Earth Sci. (2010), doi:10.1016/j.jafrearsci.2010.04.008

huge fracturation (with subsequent high permeability) does not al-low to find such evidences. Most likely, the Lake Vert hydrothermalfeatures may be due to a very local hydrothermal event formedimmediately after the formation of the Lake Vert cone structure,when hot fluids and possibly superficial and/or rain waters werestill available.

4. Preliminary risk assessment

4.1. Definition of the risk

The usual definition of a specific risk (R) refers to the expecteddegree of loss associated with a particular hazard (Bell, 2003). Itcan be expressed by the equation R = H � V � E (Thouret, 1994;Newhall, 2000; Bell, 2003; Nott, 2006), which is used to assessthe risk. The term H is the hazard or the probability of occurrenceof a certain event within a specified time period (which is in gen-erally not always constrained), for a given area; ‘‘V” corresponds tothe vulnerability, which expresses the consequences of a givenevent by taking into account the response capacity of humans incase of a crisis; eventually, ‘‘E” refers to elements at risk, and tothe ‘‘economic” value assigned to them. The value given to the lat-ter parameter is usually given by decisional (local political) author-ities and is purely a matter of societal, economic or political choice.Thus, a scientific risk assessment can only enumerate the differentelements at risk without assigning them a ‘‘real” value. Here, ele-ments at risk from mazuku are human and animal lives.

Goma region (North-Kivu, Democratic Republic of Congo): Formation and

Fig. 8. Mazuku of the Bulengo area. Alignments of mazuku highlight a possible relation between their location and hidden fractures or faults. (A) Map with mazuku location inBulengo area. The light grey dots show surveyed mazuku and the dark grey dots show the potential mazuku identified on very high spatial resolution imagery with theirvegetal zonation, but not confirmed on the field yet. The location of the mazuku Katwa agrees with the hypothesis of mazuku formation at foot of lava flow. (B) The mazukudensity map (realized with the ESRI ArcMap� software, for a search radius of 100 m) emphasizes a possible alignment of mazuku, which is parallel with the neighbouringfractures and lineaments. (C) Map with a larger view on lineaments around the Bulengo area. The volcanic fractures and lineaments come from Thonnard et al. (1965) anddata of the Royal Museum for Central Africa. Sinuous and straight escarpments are based on Pouclet (1977), Ebinger (1989) and Wauthier et al. (2008).

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As a mazuku is not a probable event but an existing and perma-nent phenomenon, the risk equation is not really appropriatedhere. The term H is a constant parameter and the only way to as-sess it is to map systematically all areas where gas concentrationsare toxic or lethal. Given H is constant and E is identified, V is theonly remaining parameter that can change the importance of therisk.

4.2. Risk assessment

The aim of this preliminary risk assessment is to describe forthe first time the mazuku as a real hazard and their current impactin the Goma region. A quantitative risk assessment, which is notproposed here (see next chapter), will have to pass through theproduction of hazard maps with hazard zonation, and by regularsystematic vulnerability surveys showing the consequences ofmazuku for a given period of time.

Gas measurements from 2 detailed field surveying campaignsshow that more than 98% of surveyed mazuku exceed the deadlyCO2 level in the first 10 cm above the ground surface (Fig. 9). Also,a key finding from our research data is that mazuku, over time-scales of our measurements, are permanent both in space and intime, except when high winds allow more thorough mixing be-tween CO2 and air, sometimes reducing the CO2 concentration be-low the deadly level. Other meteorological parameters like rain,atmospheric pressure and solar irradiation can also affect gas con-centrations inside a mazuku, as previously discussed. All thesevariations in gas concentration can ‘‘temporarily dilute or dissi-

Please cite this article in press as: Benoît, S., et al. Dry gas vents (‘‘mazuku”) inrisk assessment. J. Afr. Earth Sci. (2010), doi:10.1016/j.jafrearsci.2010.04.008

pate the risk”. Mazuku are consequently much more dangerousfor humans and animals as the CO2 level at the ground surfacecan quickly change and affect any living form travelling insidethe depression.

If the local population is aware about the mazuku hazard, polit-ical and social unrests are frequently triggering important move-ments of non-native populations. More than 400,000 persons inGoma are individuals and families from areas where security isminimal and, during the late 2008 riot, about 100,000 refugeeswere settled in camps West of town. Hence, mazuku risk-awarepeople no longer make up the majority of the Goma region popu-lation. On the other hand, the rapidly increasing demographicgrowth and uncontrolled urban developments in the region arealso greatly increasing vulnerability and risk. Two vulnerabilitysurveys carried out in the Bulengo area, close to 2 recently builtrefugee camps, by a Congolese NGO revealed the impressing newsthat at least 37 people were killed by mazuku in 2007 (Buhendwa,2007). This is especially alarming as it is based on a survey that isrelated to a relatively short time period and the only one of theseveral areas where mazuku are concentrated. One can only con-clude that mazuku are currently the most acute natural risk interms of human loss in the region and that there is a urgent needfor mitigation/remediation actions coupled with further more sys-tematic mapping, research, continuous monitoring of those lethalmazuku, as well as for further urgent risk-awareness-raising fieldcampaigns, e.g. through community information sessions and byposting clear weather-resistant danger signposts around each rec-ognized mazuku.

Goma region (North-Kivu, Democratic Republic of Congo): Formation and

Fig. 9. Histogram showing CO2 concentration at subsurface in surveyed mazuku. From 10% to higher concentration, CO2 exposure can be considered as lethal. As shown onthis histogram, mazuku are most of the case lethal.

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4.3. Risk mitigation

Two kinds of mitigation are possible: prevention and remedia-tion. Information and preventive campaigns were already carriedout and signposts installed at main known mazuku sites. However,several additional efforts are to be performed in this direction bytaking into account for example the very recently settled popula-tions and newly urbanized areas. In some places, houses are al-ready built on mazuku, e.g. some old houses along the shore ofLake Kivu, in Goma. Being built in concrete, the hazard is mainlylocalized in areas surrounding the houses, e.g. in parts of the gar-dens. More recently, the whole area of the mazuku Himbi, the wellknown mazuku area in Goma, just in front of the Governor house,has been the site of new built concrete houses. The CO2 flux cannotbe stopped but can be only slightly deviated and the deadly dangeris still there. On the other hand, in the area of Sake, less sophisti-cated shelter-style houses mainly built with wood, rock debrisand sheet metal allow a faster CO2 accumulation. Some basic mod-ifications are possible and could be implemented to decrease theexisting deadly risk (Fig. 10). In such a perspective both preventiveand curative low-cost solutions have already been suggested bySmets (2007).

Fig. 10. Example of a basic modification for wood houses built on mazuku. As CO2 is heaopenings along the walls would drastically reduce the risk of lethal concentration insid

Please cite this article in press as: Benoît, S., et al. Dry gas vents (‘‘mazuku”) inrisk assessment. J. Afr. Earth Sci. (2010), doi:10.1016/j.jafrearsci.2010.04.008

5. Towards a quantitative risk assessment

In order to better understand the phenomenon and improvehazard assessment, a second step is absolutely required. Hazardmaps must be created, the possible link between mazuku and theVirunga volcano-tectonic activity must be studied and the link be-tween gas concentration inside mazuku and the variations of mete-orological parameters must be quantified. As political and socialunrest often restrict field access, scientific investigations take timeto be performed and this contrasts with the urgent need of theseactions. However, several possible measures have been developedand are still in progress. The first results with our future expecta-tions are presented hereafter.

5.1. Mapping mazuku by remote sensing (RS)

RS could be used to extend and more systematically map maz-uku over the whole region associated with Nyiragongo and Nyamu-lagira. Pickles and Cover (2004) and Bateson et al. (2008) showedrecently the usefulness of very high spatial resolution airborneRS techniques. However, planning flight campaigns in suchunstable regions like Goma is dangerous, expensive and difficult

vier than air, a raised, if possible non-porous, and bulging ground with ventilatione the house.

Goma region (North-Kivu, Democratic Republic of Congo): Formation and

Fig. 11. Detection of mazuku in the Bulengo area using an IKONOS image. This image allowed us to detect new mazuku which were confirmed by fieldworks. However, thistechnique has some limitations: Small mazuku are difficult to detect and black patches can correspond to crops or private properties without vegetation. (A) Thepanchromatic image shows several patches that correspond to mazuku in the field. (B) The NDVI image increases the contrast between mazuku and vegetation, allowingsometimes to distinguish mazuku and other surfaces without vegetation. The grey scale in the frame B ranges from �1 (black) for unhealthy/no vegetation to +1 (white) tohealthy vegetation. The IKONOS image has been acquired in the framework of the GORISK project.

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to organize. In the meanwhile, space-borne RS could be used to ac-quire very high spatial resolution images. Currently, existing satel-lites with adequate sensors are scarce and the frequent cloud coverin the region makes data acquisition difficult. In the framework ofthe GORISK project, an IKONOS image (respectively 1 m and 4 mspatial resolutions for the panchromatic image and the four spec-tral bands) has been acquired and already allowed the detectionof the main mazuku in the Bulengo area (see Fig. 11). This imageis currently used to detect and map mazuku (both known and po-tential sites) based on their associated vegetation zoning and ad-vanced lava weathering.

5.2. Ground-based mapping of areas affected by abnormal gasconcentration

Bateson et al. (2008) showed that RS can help detect and mon-itor CO2 emissions, provided this is combined with field ground-truthing of RS data. Small mazuku are difficult to spot with thistechnique and can also be hazardous if specific physical/ambientparameters allow CO2 to accumulate. In order to accurately mapmazuku and validate RS mapping, ground-based mapping has be-gan and must be carried out in the near future. Four features ofmazuku are used to locate and map them in the field: gas concen-tration, depression morphology, vegetation zoning and rockweathering. Further field and geophysical (e.g. geoelectric) surveyswould also allow more crucial constraining and mapping of rele-vant geological and geomorphological structures, e.g. lava tunnels,collapsed tumuli, fractures and faults which are typically not visi-ble on air- and space-borne acquisitions, or at least not along theirfull extension.

5.3. Delimiting gas isotopic zones and studying gas dynamics inmazuku

The ultimate origin of the gases discharged by mazuku is a deepmagma-related mantle source (Tedesco et al., 2007a, 2010; Tassiet al., in press). Presently, a relation between gas dynamics, volca-nic and/or tectonic activity is not assessed. It is both crucial toquantify risk from mazuku more accurately and systematicallyand, if an influence of volcanism and tectonics on the mazuku gasconcentrations is evidenced, to evaluate the potential benefit for

Please cite this article in press as: Benoît, S., et al. Dry gas vents (‘‘mazuku”) inrisk assessment. J. Afr. Earth Sci. (2010), doi:10.1016/j.jafrearsci.2010.04.008

near-real-time monitoring and early warning for diking, tectonicfaulting or eruptive events, which can also all be associated withdamage and loss of life at the surface. First results from real-timeRn-CO2 monitoring inside the mazuku ‘‘Le Chalet” (in a privateproperty of Goma) show only a link with meteorological parame-ters, as previously mentioned. In order to quantify this influence,a weather station was recently installed. We plan in the future todevelop a network of such a monitoring station all along the LakeKivu shoreline between Goma and Sake, where security allows it.This network is required to determine if a link exists betweenthe gas dynamics inside mazuku and the Virunga volcano-tectonicactivity.

6. Conclusions

In this paper, the very first systematic study of mazuku (dry andcold-ambient CO2-rich gas vents) in an African active volcanismand tectonic context has been carried out. We focused in the Gomaregion although mazuku are known to be common to indigenouspeople across central Africa of the EARS. Typically, they are consid-ered as lethal sites due to the high to definitely deadly accumula-tion of CO2. Mazuku were found to exist where geomorphologicaldepressions allow gravitational gas accumulation and range in sizefrom 1 to 100 m across and in shape from round to highly irregular.These depressions can be created at foot of lava flow, by the col-lapse of a cavity roof or by joint fractures themselves. Faults, fis-sures, permeability variations and contact between lava layersare the preferential ways that allow the gas reaching the surface.The present study offers a first insight of the mazuku in the Gomaregion, both as a geological phenomenon and as a clear hazard. Ithas enabled to start to systematically map in the N Kivu regiontheir spatial distribution and density, to document that the ulti-mate source of the CO2-rich gas is a deep magmatic (mantle)one, that the studied mazuku are permanent in space and time,and that they are usually concentrated within 3 km far from theLake Kivu shoreline, where fault and fracture networks seem tobe combined with an hydrothermal and/or most possibly with asuperficial groundwater system(s).

No link between gas venting at mazuku and the Virunga volca-nic activity has been evidenced due to the lack of continuous CO2

data to be coupled with the seismic and volcanic signals. More

Goma region (North-Kivu, Democratic Republic of Congo): Formation and

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studies are needed to better constrain them. According to ourstudy, a deadly risk due to mazuku exists and an urgent need foran appropriate risk management is therefore strongly required.Mazuku are thus highly significant both scientifically and also asa direct local threat to life. In the Goma region, recent relocationof populations has introduced a large mazuku risk for unawarepopulation into the mazuku danger zone. There is a truly urgentneed to provide a precise mapping of their location and geologi-cal/topographic context, including a combination of RS/field map-ping and low cost geophysical surveys, to study and monitoringof all such lethal sites and to continue implementing effectivewarning signposts or other community-driven participative ac-tions to let people know where each of these numerous gas emis-sions sites are located.

Acknowledgements

Fieldworks were performed in the framework of a MasterThesis in Natural Hazards Management at the University of Liège(Belgium) and GORISK, a ‘‘product and service” project supportedby the Belgian Federal Science Policy (STEREO II programme, pro-ject SR/00/113) and the Luxembourg National Research Fund(FNR/STEREOII/06/01). The Rn-CO2 monitoring station is providedby the University of Luxembourg. We thank all the GVO team insharing their field experience and collecting data. We also thankG.G.J. Ernst and B. Capaccioni who have greatly improved the man-uscript with their reviews and encouragements.

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