An automatic observation buoy (AOB) was installed at Lake Nyos in March 2014. The temperature and electrical conductivity (EC) of lake water were continuously observed from March 10, 2014 to April 15, 2016. The thermocline and chemocline located deeper than -90 m subsided 15 m in the observation period due to the bottom water removal by the CO2 degassing pipes. The velocity of subsidence was estimated quantitatively by correlating the time variation of EC obtained by AOB and the depth profile of EC. The velocity of subsidence was not constant, namely, it was fast in dry season and slow in rainy season. The time variation in the velocity of subsidence can be attributed to the seasonal variation of water flux in the degassing pipes. Considering the water balance in the deep part of lake, the flux of ground water supplied to lake was estimated to be 6.5*10(6) (m(3)/year), which is comparable to the recharge rate of ground water within the catchment area surrounding Lake Nyos. Abrupt drops in temperature and conductivity were observed at -120 m in January 2016, when the temperature at -4 m reached 21.8 degrees C as the lowest value. The abrupt drops are due to the sinking of surface cold water.

A limnic eruption in 1986 at Lake Nyos in Cameroon was caused by sudden release of magmatic CO2 dissolved in the lake water. For frequent measurements of CO2 in the lake water, we developed a simple method of CO2 monitoring using sound speed (SS) measurement. We measured the depth profile of sound speed at Lake Nyos in 2012, 2013, 2014, and 2015. Furthermore, in the 2014 and 2015 survey, we took movies of the underwater and the bottom of the lake using an underwater camera housed in a pressure-resistant container. The vertical change of transparency of water was checked by the visibility of reflectors set in front of the camera. A pressure sensor monitored the depth simultaneously. In both years, in addition to well-known red surface water, we discovered a cloudy layer near the upper chemocline around 100 m depth. The depth of chemocline is a good indicator of the change of total amount of CO2 in Lake Nyos. Because the transparency can be detected by a simple optical sensor, we can now apply our low-cost monitoring tool to find the depth of chemocline for Lake Nyos.

Acoustic equipment called the underwater data logger and the Multibeam Echosounder (MBES) are used in this study to provide a better resolution of the morphology of the Lake Monoun's basin and improve the assessment of adequate volume of its water and gas contents. The use of the high resolution multibeam sonar allowed to capture all the features of the lake borders and produce a highly accurate hypsographic curve and hence, provide a revised estimate of the gas contains in the lake. The presence of depressions, located on the floor of the main basin containing CO2 enriched fluid constitute probable sources of the gas feeding Lake Monoun.

Coelacanth remains from the Lower Cretaceous (pre-Aptian) freshwater deposits of the Babouri Figuil Basin, Northern Cameroon are described as a new taxon Mawsonia soba sp. nov. based on two characters: four openings in the dentary for the branch of the trigeminal nerves and an extremely rugose and disorganized ornamentation of the bones. M. soba sp. nov. known by some three-dimensionally partially preserved specimens represents the first actinistian described from Cameroon. This discovery attests the presence of Mawsonia in the African counterpart of the continental pre-rift and rift environments where only Brazilian members were known so far. The fact that the Cameroonian coelacanth belongs to a new species suggest a certain endemism at least in this African basin. (C) 2017 Elsevier Ltd. All rights reserved.

Abstract Vertical profiles of sound speed (SS) in lake water were measured at lakes Nyos and Monoun (Cameroon) in 2012 and 2014. A significant linear correlation with Pearson's r of 0.987 to 0.995 between total CO2 concentration ([CO2]=[CO2(aq)]+[HCO3−]) and SS excess (Δv) in water was found. Based on this correlation, we propose the SS method as a new simple tool to monitor CO2 concentration in lakes Nyos and Monoun. We applied this method to multipoint measurements of CO2 in lakes Nyos and Monoun, and found horizontal differences between CO2 distributions in the two lakes. The results indicated that Lake Nyos water was stably stratified, and the total CO2 decreased from 2012 to 2014. At Lake Monoun, which consists of three basins (the western basin, the central basin, and the main basin), waters in the western and central basins exhibited a higher dissolved CO2 content than those in the main basin at the same depth. An increase in CO2 was also detected in the main basin from 2012 to 2014.

The amount of dissolved CO2 (CO2aq) in Lake Nyos, Cameroon, has been measured annually since January 2011. First, HCO3- concentration was estimated by a chemical analysis with the assumption of equilibria between CO2aq, HCO3- and CO32-. The estimated [HCO3-] was correlated with the observed electrical conductivity of lake water. The profile of [CO2aq] was generated from the electrical conductivity, pH and temperature profiles. The [CO2aq] profile was integrated over the water column to obtain the total amount of CO2aq. The amounts were 8.4, 6.8, 5.5, 4.4 and 3.9 Gmol (=10(9) mol) in 2011, 2012, 2013, 2014 and 2015, respectively. The fast reduction between 2011 and 2013 was attributed to the installation of two additional degassing pipes in April 2011. However, the rate of reduction decreased between 2014 and 2015 due to the reduced [CO2aq] at the inlet of the degassing pipes.

Anew method to measure theCO(2) concentration in lakes Nyos and Monoun using sound speed (SS) and electrical conductivity (EC) of the lake water was developed. Limnic eruptions at lakes Monoun and Nyos in Cameroon, characterized by a risk related to possible sudden release of magmatic CO2 gas dissolved in the lake water, occurred in 1984 and 1986 respectively, killing about 1800 residents. CO2 monitoring of the lake waters was initiated to prevent further disasters. Classical methods of CO2 measurement, which require chemical analysis of water samples, are not suitable for frequent and multi-point monitoring. Inspired by the method of measuring salt concentration using SS, we obtained data for dissolved CO2 (CO2(aq)) assuming the following formula Delta v = k1[CO2(aq)] + k(2) [HCO3-], where Delta v is additional SS due to dissolved ions and k(1) and k(2) are coefficients determined through experiments. The coefficient k(1) was determined with temperature T (degrees C) dependence as 0.0323-0.000523T [l m s(-1) mmol(-1)]. k(2) was estimated to be 0.132 at Lake Nyos and 0.0930 at Lake Monoun. Once k(1) and k(2) values are obtained, [CO2(aq)] and [HCO3-] are calculated using the SS, T and EC.

Abstract A large amount of CO2 is stored in the deep waters of Lake Nyos, a volcanic crater lake in Cameroon. The lake is meromictic and thus anoxic in the deeper areas, where dissolved iron exists as Fe2+. Since 2001, a controlled degassing of the lake has been underway. The degassing brings deep water containing Fe2+ to the lake surface as a fountain. This resulted in the formation of Fe(OH)3 precipitates and turned the lake surface red-brown. This coloration was accelerated after the addition of two further degassing pipes in 2011. The Fe(OH)3 precipitates sink to deeper parts of the lake, re-dissolve and are reduced back to Fe2+, which is thought to be precipitating as siderite. The rates of dissolution and precipitation of siderite in Lake Nyos were examined. Fragments of siderite crystals were covered with gel (Epikote) and placed in lake water for 54 h at several depths characterized by chemical features. The change in the thickness of the crystal surface before and after the reaction was analysed by an interferometer in the laboratory. The siderite dissolves at a rate of −0.09 µm a−1 for samples placed at a depth of 50 m, whereas precipitation took place in deeper waters. The precipitation rate of siderite increased by 0.29 µm a−1 with an increase in depth from 100 to 200 m, and decreased to 0.25 and 0.09 µm a−1 at depths of 208 and 210 m, respectively. However, the calculated saturation indices of siderite in the lake waters increased with depth. The objective of this study is to examine the observed rates of dissolution and precipitation of siderite and to compare them with the saturation index of siderite calculated from the chemistry of the lake water. We also discuss the change in Fe species from the oxidation of Fe2+ to Fe3+ at the surface and reduction to Fe2+ and consequent precipitation as siderite in deeper parts of the lake by degassing of CO2 from bottom water in Lake Nyos.

Abstract We assessed the potential for limnic eruptions at lakes Nyos and Monoun, Cameroon on the basis of numerical modelling and CO2 profiles obtained by regular monitoring of the lakes. The change through time of the profiles suggests one particular scenario for producing an eruption: a supply of CO2-undersaturated fluid from the lake bottom that induces upwards growth of the CO2-rich bottom layer, leading eventually to CO2 saturation at mid-depths of the lake. By using a numerical model for the ascent of a plume of CO2 bubbles, we found that under realistic conditions (e.g. a profile of CO2 as deduced from the regular monitoring data), a bubble plume generated from the middle depths of the lake can reach the lake surface with a high flux of CO2, which corresponds to a limnic eruption. In addition, we developed a numerical model to investigate how changes in the CO2 concentration at the lake bottom affect the dynamics of a two-phase flow in the controlled degassing pipe, using the recently observed CO2 profiles. This model enables us to estimate the CO2 concentrations at the lake bottom from the heights of fountains that are observable at the lake surface.

With the use of conventional hydrogeochemical techniques, multivariate statistical analysis, and stable isotope approaches, this paper investigates for the first time surface water and groundwater from the surrounding areas of Lake Monoun (LM), West Cameroon. The results reveal that waters are generally slightly acidic to neutral. The relative abundance of major dissolved species are Ca2+ > Mg2+ > Na+ > K+ for cations and HCO3 − ≫ NO3 − > Cl− > SO4 2− for anions. The main water type is Ca-Mg-HCO3. Observed salinity is related to water-rock interaction, ion exchange process, and anthropogenic activities. Nitrate and chloride have been identified as the most common pollutants. These pollutants are attributed to the chlorination of wells and leaching from pit latrines and refuse dumps. The stable isotopic compositions in the investigated water sources suggest evidence of evaporation before recharge. Four major groups of waters were identified by salinity and NO3 concentrations using the Q-mode hierarchical cluster analysis (HCA). Consistent with the isotopic results, group 1 represents fresh unpolluted water occurring near the recharge zone in the general flow regime; groups 2 and 3 are mixed water whose composition is controlled by both weathering of rock-forming minerals and anthropogenic activities; group 4 represents water under high vulnerability of anthropogenic pollution. Moreover, the isotopic results and the HCA showed that the CO2-rich bottom water of LM belongs to an isolated hydrological system within the Foumbot plain. Except for some springs, groundwater water in the area is inappropriate for drinking and domestic purposes but good to excellent for irrigation.

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.

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.

For the first time, comprehensive study of hydrogeochemistry of water seeps, role of chemical weathering on dam failure, estimation of minimum width of dam to resist failure and simulation of changes in dissolved ions and secondary mineral was conducted on the Lake Nyos dam. The salient results and conclusions were; the dam spring water represented a mixture of 60-70% rainwater and 30-40% Lake water (from 0 to -40 m). The chemistry of the observed waters was Ca-HCO3 for rainwater, Ca-Mg-HCO3 in boreholes, and Mg-Ca-HCO3- for spring water. The relative rate at which ions dissolved in water was HCO3- > Mg2+> Ca2+ > Na+> SiO2 > K > NO3- > SO42- > Cl-. Weathering of rocks resulted in the formation of clay minerals such as kaolinite and smectite. Relative mobility of elements compared to Alumina (Al2O3) indicated that in monzonites there was a loss of CaO, Na2O, K2O, P2O5 and gain of SiO2, Fe2O3, TiO2, MnO and MgO, while in basalts there was a loss of SiO2, Fe2O3, Ca2O, NaO, MgO and gain of TiO2, K2O and P2O5. Values of chemical alteration index that ranged from 49 to 82 suggest a weak to intermediate categories of chemical weathering that occurred at a rate of 5.7 mm/year. Paired to that rate, which suggests that the dam is not vulnerable to failure at the previously thought time scale, some other processes (physical weathering, secondary mineral formation and lake overflow) can cause instant failure. Hydrostatic pressure of 1.6 GN generated by Lake water can be supported only when the width of the dam is greater than 19 m. PHREEQC-based simulation for 10 years indicates decoupling of Ca and Mg, and Na and Mg. Multidisciplinary monitoring of the dam is advocated. (C) 2014 Elsevier Ltd. All rights reserved.

Lake Nyos (2 x 1.2 km), a maar volcano in northwestern Cameroon, exploded a large amount of CO2 that killed 1746 people 1986. We performed field survey in and around Lake Nyos to establish its eruptive history and formation processes. Eruptive deposits of Nyos maar lie on Pre-Cambrian granitic basement and can be seen at the north to eastern lakeshore. These include tuff breccia (Unit A-1), scoria fall (A-2), lava flow (A-3) and base surge (A-4), in ascending order. We see no evidence of time breaks between these units as can be typified by paleosol and reworked deposits. A-1 is rich in lithic fragments, such as basaltic bombs with chilled margins (juvenile) and granitic (crustal) and peridotitic (mantle) xenoliths. Its limited areal distribution in the eastern lakeside indicates a nearby vent location (Vent 1). A-2 is clast-supported and mainly composed of well sorted basaltic scoria. The thickness increases from east to north lakeshore. A-3 is deposited 20 m above lake level with covering basement rock and A-2 in the north lakeshore. Its depositional level decreases to the lake level in other parts surrounding the lake. These evidences suggest that the vent that erupted A-2 and -3 was located at the north part of the lake (Vent 2). Distribution of A-3 extends northeastward for more than 10 km along valleys from Lake Nyos. A-4 (most voluminous) is base surge deposits characterized by cross-laminated and fines-poor facies. It consists of dominant basalt, isolated crystals and accidental lithics. The thickness is more than 30 m at the lakeside and it makes a depositional surface 1 km around Vent 2. The distal facies shows well-sorting and parallel laminations containing accretionary lapilli, which can be interpreted as air fall deposits. A scoria cone is situated 1.5 km northeast of Lake Nyos. Ejecta from the cone (scoria fall) overlies A-4 around the lake, suggesting that activity of the cone started immediately after the Nyos maar-forming event. The cone is not a single cone but a complex structure. The main cone structure is divided into west and east, and the southern slope is collapsed. Several hummocks, which might be debris avalanche deposits from the sector collapse are found at the south to southwestern foot of the main cone. A remarkable crater (NE Vent), cutting part of the main cone is located near the northeast side. Products from the scoria cone consist of scoria fall (B-1), volcanic bombs (B-2) and lava flow (B-3). B-1 is characterized by highly vesiculated basaltic scoria whose thickness and diameter increase towards the main cone. B-2, made of basaltic bombs, was emplaced on B-1 and scattered within about 500 m around the cone. There are two types of bombs: xenolith-poor and -rich. Xenolith-poor bombs are a few meters in size and occur near the main cone and on the hummocks. Xenolith-rich bombs are abundant near the NE vent. This kind of distribution suggests that xenolith-poor bombs were produced from the main cone and xenolith-rich bombs from the NE Vent. B-3 is a small, blocky lava flow less than 500 m long on the southwest side of the main cone. Existence of accretionary lapilli and chilled margins of juvenile materials in Unit A suggest that phreatomagmatic eruption occurred during the Nyos maar-forming eruption. Although conditions of conduit changed from wet to dry during A-1 to A-3 with shifting of the vent locations (Vent 1 to 2), catastrophic base surge caused by interaction of water and voluminous magma formed Lake Nyos consequently. After that, the locus of eruption moved to the scoria cone. B-1 formed the main part of the scoria cone, and then the cone collapsed to generate debris avalanche deposits on the southern foot. Finally, eruptive activity terminated with small effusion of lava flow from the collapsed cone and formation of NE crater ejecting xenolith-rich bombs. The volcanic activity is characterized by various products and styles of eruptions with changes in vent locations northeastwards.

Rainwater characteristics can reveal emissions from various anthropogenic and natural sources into the atmosphere. The physico-chemical characteristics of 44 monthly rainfall events (collected between January and December 2012) from 4 weather stations (Bamenda, Ndop plain, Ndawara and Kumbo) in the Bamenda Highlands (BH) were investigated. The purpose was to determine the sources of chemical species, their seasonal inputs and suitability of the rainwater for drinking. The mean pH of 5 indicated the slightly acidic nature of the rainwater. Average total dissolved solids (TDS) were low (6.7 mg/L), characteristic of unpolluted atmospheric moisture/air. Major ion concentrations (mg/L) were low and in the order K+ > Ca2+ > Mg2+ > Na+ for cations and NO3−≫HCO3−>SO42−>Cl−>PO43−>F− for anions. The average rainwater in the area was mixed Ca-Mg-SO4-Cl water type. The Cl–/Na+ ratio (1.04) was comparable to that of seawater (1.16), an indication that Na+ and Cl– originated mainly from marine (Atlantic Ocean) aerosols. High enrichments of Ca2+, Mg2+ and SO2–4 to Na+ ratios relative to seawater ratios (constituting 44% of the total ions) demonstrated their terrigenous origin, mainly from Saharan and Sahelian arid dusts. The K+/Na+ ratio (2.24), which was similar to tropical vegetation ash (2.38), and NO3– was essentially from biomass burning. Light (< 100 mm) pre-monsoon and post-monsoon convective rains were enriched in major ions than the heavy (> 100 mm) monsoon rains, indicating a high contribution of major ions during the low convective showers. Despite the acidic nature, the TDS and major ion concentrations classified the rainwater as potable based on the WHO guidelines.

In 1986, carbon dioxide gas exploded from Lake Nyos and killed about 1,800 people. After that disaster, various administrative and research activities have been conducted to mitigate subsequent disasters. However, none of those endeavors have characterized the groundwater chemistry to identify hydrogeochemical processes that control the water chemistry, and the quality of the water for domestic and agricultural uses that support the lives of un-official resettlers around Lake Nyos. Conventional hydrochemical techniques coupled with statistical and graphical analysis were therefore employed to establish the baseline hydrochemical conditions, assess processes controlling solutes distribution in shallow groundwater in the Lake Nyos catchment and explore its usability. Groundwater samples were analyzed for their physical and chemical properties. The wide ranges of electrical conductivity and total dissolved solid values reveal the heterogeneous distribution of groundwater within the watershed. The relative abundance of major dissolved species was Ca > Mg > Na > K for cations and HCO3 >>> Cl > SO4 > NO3 for anions. Piper diagram classified almost all water samples into mixed CaMg–HCO3 water type. Major ion geochemistry reveals that, in addition to silicates weathering (water–rock interaction), ion exchange processes regulate the groundwater chemistry. Principal component analysis supports the occurrence of water rock interaction. Hierarchical cluster analysis showed that the chemistry of groundwater in the study area is controlled by three main factors, and suggests no hydraulic connectivity between deep lake water and groundwater in the catchment. The quality assessment of the groundwater showed that groundwater parameters are within the acceptable limit of the World Health Organization and Nigeria guidelines for drinking and domestic uses, and water found to be good for irrigation.

his study presents the first and detail field investigations of exposed deposits at proximal sections of the Barombi Mbo Maar (BMM), NE Mt Cameroon, with the aim of documenting its past activity, providing insight on the stratigraphic distribution, depositional process, and evolution of the eruptive sequences during its formation. Field evidence reveals that the BMM deposit is about 126m thick, of which about 20m is buried lowermost under the lake level and covered by vegetation. Based on variation in pyroclastic facies within the deposit, it can be divided into three main stratigraphic units: U-1, U-2 and U-3. Interpretation of these features indicates that U-1 consists of alternating lapilli-ash-lapilli beds series, in which fallout derived individual lapilli-rich beds are demarcated by surges deposits made up of thin, fine-grained and consolidated ash-beds that are well-defined, well-sorted and laterally continuous in outcrop scale. U-2, a pyroclastic fall-derived unit, shows crudely lenticular stratified scoriaceous layers, in which many fluidal and spindle bombs-rich lapilli-beds are separated by very thin, coarse-vesiculatedash-beds, overlain by a mantle xenolith- and accidental lithic-rich explosive breccia, and massive lapilli tuff and lapillistone. U3 displays a series of surges and pyroclastic fall layers. Emplacement processes were largely controlled by fallout deposition and turbulent diluted pyroclastic density currents under "dry" and "wet" conditions. The eruptive activity evolved in a series of initial phreatic eruptions, which gradually became phreatomagmatic, followed by a phreato-Strombolian and a violent phreatomagmatic fragmentation. A relatively long-time break, demonstrated by a paleosol between U-2 and U-3, would have permitted the feeding of the root zone or the prominent crater by the water that sustained the next eruptive episode, dominated by subsequent phreatomagmatic eruptions. These preliminary results require complementary studies, such as geochemistry, for a better understanding of the changes in the eruptive styles, and to develop more constraints on the maar's polygenetic origin.

This study was conducted on the hydrochemistry of shallow groundwater and surface water in the Ndop plain, North West Cameroon. The objectives were to determine the physico-chemical characteristics of water, controls on water chemistry and suitability for drinking and irrigation. Forty-six shallow groundwater and 26 surface water samples were investigated. Field measurements of physical parameters were preceded by chemical analyses of the samples for major ions and F-. About 69% of the water sources had pH values below 6.5, classifying the water as barely acidic. Electrical conductivity (<282 µS/cm) and total dissolved solids (<183 mg/l) were low suggesting low-mineralised and freshwater. The relative abundance of major ions (meq/l) was Na2+>Ca2+>Mg2+>K+ for cations and HCO3->>NO3-≥Cl->SO42- for anions. Major ion concentrations were low, and within the WHO guidelines for drinking water, but F-concentrations were much lower (<0.39 mg/l) and below guideline value. Main water types and proportions were Na-HCO3 (53 %), Ca-HCO3 (35 %) and a mixed Na-Ca-HCO3 (11%). Predominant processes influencing water chemistry were incongruent dissolution of silicate minerals and cation-exchange of Na+ in rocks for Ca2+ in water. The low major ion concentrations indicated low water-rock interactions and short residence time. The analysed water was suitable for irrigation. Key words: Water chemistry, hydrochemical controls, drinking-irrigation quality, Ndop plain, Cameroon.

A portable gas-analyser was used to perform on-site gas analysis at lakes Nyos and Monoun. Results were used to evaluate the effect of the dissolved CH4 on the gas saturation level. Although CH4 represents marginal proportion in term of the total gas concentration (similar to 2.2 mol % in Lake Monoun and similar to 0.6 mol % in Lake Nyos) it contributes similar to 37% and similar to 19% to the total dissolved gas pressure and similar to 23% and similar to 13% to saturation level in lakes Monoun and Nyos respectively. The risk due to CH4 increase is evaluated and monitoring and surveillance measures proposed. Because methane is less more soluble than carbon dioxide, its increase may accelerate saturation of the lake waters with dissolved gas making gas eruptions more frequent. It is therefore recommended to regularly monitor the lakes in order to assess CH4 evolution for hazard mitigation purposes. If future monitoring surveys confirm current CH4 increase, a device to extract deep rich-gas water must be experimented and put in place without delay.