Development and sensitivity analysis of a model for assessing stratification and safety of Lake Nyos during artificial degassing

To prevent the recurrence of a disastrous eruption of carbon dioxide (CO2) from Lake Nyos, a degassing plan has been set up for the lake. Since there are concerns that the degassing of the lake may reduce the stability of the density stratification, there is an urgent need for a simulation tool to predict the evolution of the lake stratification in different scenarios. This paper describes the development of a numerical model to predict the CO2 and dissolved solids concentrations, and the temperature structure as well as the stability of the water column of Lake Nyos. The model is tested with profiles of CO2 concentrations and temperature taken in the years 1986 to 1996. It reproduces well the general mixing patterns observed in the lake. However, the intensity of the mixing tends to be overestimated in the epilimnion and underestimated in the monimolimnion. The overestimation of the mixing depth in the epilimnion is caused either by the parameterization of the k-epsilon model, or by the uncertainty in the calculation of the surface heat fluxes. The simulated mixing depth is highly sensitive to the surface heat fluxes, and errors in the mixing depth propagate from one year to the following. A precise simulation of the mixolimnion deepening therefore requires high accuracy in the meteorological forcing and the parameterization of the heat fluxes. Neither the meteorological data nor the formulae for the calculation of the heat fluxes are available with the necessary precision. Consequently, it will be indispensable to consider different forcing scenarios in the safety analysis in order to obtain robust boundary conditions for safe degassing. The input of temperature and CO2 to the lake bottom can be adequately simulated for the years 1986 to 1996 with a constant sublacustrine source of 18 l s−1 with a CO2 concentration of 0.395 mol l−1 and a temperature of 26 °C. The results of this study indicate that the model needs to be calibrated with more detailed field data before using it for its final purpose: the prediction of the stability and the safety of Lake Nyos during the degassing proces

Major ions, δ18O, δ13C and 87Sr/86Sr compositions of water and precipitates from springs along the Cameroon Volcanic Line (Cameroon, West Africa): Implications for provenance and volcanic hazards

A combined study of major ions, δ18O, δD, 13C, 87Sr/86Sr isotopes, X-ray diffraction, scanning electron microscopy, and electron probe microanalyses on springs and spring mineral precipitates along the Cameroon Volcanic Line (CVL) was undertaken to understand water chemistry, and infer the type and origin of the precipitates. The waters are of evaporated Na + KCl and non-evaporated Ca + MgHCO3 types, with the more mineralized (electrical conductivity-EC of 13130 μS/cm) Lobe spring inferred to result from interaction of circulating 49 °C waters with magmatic volatiles of the active Mt. Cameroon. Water mineralization in the other springs follows the order: Sabga A > Sabga B > Bambui B > Bambui A > Nyos Cave. But for the Nyos Cave spring, all other springs contain fluoride (up to 0.5–35.6 mg/l above WHO potable water upper limit). The Sabga spring contains arsenic (up to 1.3 mg/l above the WHO limits). The springs show low fractionation temperatures in the range of 19–43 ᵒC. The Lobe and Sabga A springs are precipitating dolomite (CaMg(CO3)2), while the Nyos Cave, Bambui A, Bambui B and Sabga B springs precipitate trona ((Na3H(CO3)2.H2O). Our data suggest a marine provenance for the carbonates, and point to a volcanic input for the Lobe, Nyos, Sabga A, and Bambui A springs. The latter springs are therefore proposed as proxies for monitoring volcanic activity for hazard mitigation along the CVL

Increased risk of fluorosis and methemoglobinemia diseases from climate change: evidence from groundwater quality in Mayo Tsanaga River Basin, Cameroon

Current assessments of the impacts of climate variability and change on water resources commonly exclude groundwater. Thus, the identification of actual and potential health threatening elements in the groundwater, and linking up to climate variation and change at hydrologic catchment scale is an important ingredient for identifying feasible local-scaled adaptation strategies. Against these backdrops the focus of this paper was to assess the implications of climate change on groundwater-derived methemoglobinemia, and fluorosis which have been identified in Mayo Tsanaga River Basin (MTRB), North Cameroon. The basic approach of the study involved collection and analyses of previously published reports and articles that are related to the impact of climate change on water resources in Cameroon. Moreover in addition to groundwater samples that were collected from hand dug wells and boreholes in the dry season, streams, rivers, springs, and dams were sampled in the rainy season. In-situ measurements, and determination of electrical conductivity, pH, water temperature, atmospheric temperature, and alkalinity, respectively, were done. Laboratory analysis of potassium, sodium, calcium, magnesium, chloride, sulfate, nitrate, and fluoride was done by ion chromatography. The succinct results showed that atmospheric averaged annual temperature has increased from 28ºC to 29ºC over the past 40 years. Projected temperature for the year 2030 is 30ºC. Twenty seven percent of the sampled drinking water sources were contaminated by fluoride, which is causing fluorosis. The variation in nitrate suggests that during the dry season water in rivers, springs, dams, boreholes, and shallow wells contained nitrate below the WHO upper limit of 45 mg/l, while in the rainy season some shallow wells were polluted by nitrate. In contrast to the relationship of fluoride with groundwater age and depth, nitrate concentrations increased with decreasing age and depth of the groundwater. Based on the premises that a complex nexus exists between climate change, groundwater quality and health in the study area, adaptation and mitigation strategies were identified, and summarized with the accronym “ADAPT” for: Avoid untreated groundwater from deeper aquifers, Drink water from rain, rivers, and springs, Adopt local drinking water norms, Prohibit shallow well water in the rainy season and Treat young groundwater for nitrate and old groundwater for fluoride before drinking.Keywords: Climate change, Groundwater quality, Fluorosis, Methemoglobinemia, Adaptation strategies, Mayo Tsanaga River BasinJOURNAL OF THE CAMEROON ACADEMY OF SCIENCES Vol. 11 No. 1 (2013

A comparative review of petrogenetic processes beneath the Cameroon Volcanic Line: Geochemical constraints

The origin and petrogenesis of the Cameroon Volcanic Line (CVL), composed of volcanoes that form on both the ocean floor and the continental crust, are difficult to understand because of the diversity, heterogeneity, and nature of available data. Major and trace elements, and Sr-Nd-Pb isotope data of volcanic rocks of the CVL spanning four decades have been compiled to reinterpret their origin and petrogenesis. Volcanic rocks range from nephelinite, basanite and alkali basalts to phonolite, trachyte and rhyolite with the presence of a compositional gap between SiO2 58–64 wt.%. Similarities in geochemical characteristics, modeled results for two component mixing, and the existence of mantle xenoliths in most mafic rocks argue against significant crustal contamination. Major and trace element evidences indicate that the melting of mantle rocks to generate the CVL magma occurred dominantly in the garnet lherzolite stability field. Melting models suggest small degree (<3%) partial melting of mantle bearing (6–10%) garnet for Mt. Etinde, the Ngaoundere Plateau and the Biu Plateau, and <5% of garnet for the oceanic sector of the CVL, Mt. Cameroon, Mt. Bambouto, Mt. Manengouba and the Oku Volcanic Group. The Sr-Nd-Pb isotope systematics suggest that mixing in various proportions of Depleted MORB Mantle (DMM) with enriched mantle 1 and 2 (EM1 and EM2) could account for the complex isotopic characteristics of the CVL lavas. Low Mg number (Mg# = 100 × MgO/(MgO + FeO)) and Ni, Cr and Co contents of the CVL mafic lavas reveal their crystallization from fractionated melts. The absence of systematic variation in Nb/Ta and Zr/Hf ratios, and Sr-Nd isotope compositions between the mafic and felsic lavas indicates progressive evolution of magmas by fractional crystallization. Trace element ratios and their plots corroborate mantle heterogeneity and reveal distinct geochemical signatures for individual the CVL volcanoes

Major elements, trace elements and Sr-Nd-Pb isotopes form lavas of lakes Nyos, Wum, Elum and Oku sampled in the Oku Volcanic Group of the Cameroon Volcanic Line

Lake Nyos is located at the summit of a stratovolcano in the Oku Volcanic Group (OVG) along the Cameroon Volcanic Line. The sudden release of magmatic CO2 trapped at the bottom of Lake Nyos in August 1986 caused historical casualties of 1750 people and over 3000 cattle. New geochemical data of volcanic rocks from the Nyos volcano and the first available data for volcanic rocks from other maar-bearing volcanoes (Lakes Elum, Wum and Oku) in the OVG are presented and compared. Lavas from the Nyos, Elum and Wum volcanoes show similarities in major and trace elements and Sr?Nd?Pb isotopes, suggestive of a similar mantle source. However, this source is slightly different from that of the Oku volcano. The samples from Lake Oku have lower alkali, higher TiO2 and more depletion and enrichment in most incompatible trace elements than those from the Nyos, Elum and Wum volcanoes. These differences and those observed in the Sr?Nd?Pb results are consistent with a heterogeneous source for lavas in the OVG. Trace element compositions suggested the presence of garnet in the source (< 6% garnet) and modelled melting results indicate < 2% partial melting of the source material. Isotope data plot within the focal zone, extending towards enriched mantle 1 (EM1; e.g. Lakes Oku and Nyos samples). This indicates the involvement of at least three mantle components: depleted mid-ocean ridge basalt mantle, high-µ and EM1 components in the magmatism of the lavas studied. The contributions of these components in different proportions, originating from asthenospheric and subcontinental lithospheric mantle sources, can account for the observed variations in geochemical characteristics. The geochemical characteristics of the studied lavas indicate that the magma source need not necessarily have an abnormal CO2 concentration to pose a potential threat. Degassing of an ordinary magma chamber and the migration of gas to the bottom of the lakes through cracks and faults can lead to the accumulation of CO2 in lake bottoms. This is controlled by tectonic parameters (fractures and faults) that enhance degassing from the magma chamber to the lake bottom and physical parameters of the lake (e.g. size, depth, temperature and solubility) that control CO2 stability

Disaster prevention, disaster preparedness and local community resilience within the context of disaster risk management in Cameroon

Cameroon was one of the 57 countries that participated in the Global Network of Civil Society Organizations for Disaster Reduction (GNDR) Views from the Frontline (VFL) 2013 project on everyday disasters, community resilience and disaster preparedness. Working with 6 other civil society organizations, Geotechnology, Environmental Assessment and Disaster Risk Reduction administered 400 questionnaires to frontliners in 7 administrative regions of the country on 14 disaster indicators that assessed the underlying causes of disasters and the level of preparedness and resilience of the communities. Scores from the 89% of informants who responded show that Cameroon occupied the 43rd position globally, was 15th out of the 23 African countries, and was 7th out of the 9 West African countries surveyed. Cameroon average scores for all 14 indicators were lower (poorer) than the West African average, suggesting that a lot more effort is needed in managing disaster risks in the country, i.e., reducing vulnerabilities and increasing preparedness and resilience. At the national level, the Center and Adamaoua Regions recorded the lowest scores of the survey. Above-average scores recorded for some indicators in the Southwest, Northwest and Far North Regions are interpreted to be due to disaster prevention activities like monitoring via early warning systems, resilience building and outreach exercises carried out for disasters like landslides, floods, gas explosions from lakes, and volcanic eruptions, in these areas. Cameroon presently has many laws relating to disaster risk management matters, but an analysis of how the laws are applied shows that the expected results have not been attained, mainly because of over-centralization and a reactive, rather than a proactive approach to disaster risk management. Given her current disaster risk profile, Cameroon has to increase research, better manage, and make disaster risks a central tenet in her development project decision-making, if the goal earmarked in her development vision to become a newly industrialized country by 2035 has to be realized. We propose the creation of an autonomous statutory National Disaster Risk Management Agency which will have a local community-driven bottom-top approach to disaster risk management, and disseminate appropriately tailored disaster risk information to promote a proactive community-based resilience and disaster prevention framework. This will fulfill the post-2015 Sendai framework priority of action No. 2 (strengthening disaster risk governance to manage disaster risk) and appropriately prepare Cameroon to face the challenges of the Sustainable Development Goals (SDGs)