Hydrogeochemical features of Lake Ngozi (SW Tanzania)

Mouëllic, Manuëlla Delalande-Le; Gherardi, Fabrizio; Williamson, David; Kajula, Stephen; Kraml, Michael; Noret, Aurélie; Abdallah, Issah; Mwandapile, Ezekiel; Massault, M.; Majule, Amos; Bouchet, L.

doi:10.1016/j.jafrearsci.2014.11.004

Cited by 10 publications

(11 citation statements)

References 53 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…The Shiwaga gas emanations, as many other RVP sites, are mostly composed of CO 2 (more than 95% up to 99%; Ebinger et al 1993;Delalande et al 2008Delalande et al , 2011Kraml et al 2010;de Moor et al 2013). High 3 He/ 4 He ratios measured in Lake Ngozi water (R/Ra = 8.3; Delalande et al 2008;Delalande-Le Mouëllic et al 2015), RVP hydrothermal waters and gases (Barry et al 2013), or volcanic RVP rocks (Hilton et al 2011) evidence a gas mantle source and highlight plume-like ratios under the RVP. During the last decade, the RVP had been explored, investigated, and recognized for its promising geothermal potential (Kraml et al 2010(Kraml et al , 2014Kalerkamp et al 2010;Kabaka et al 2016).…”

Section: The Shiwaga Gas Fieldmentioning

confidence: 97%

“…It culminates around 2500 m a.s.l., and shows a 3 km in diameter caldera, occupied by Lake Ngozi (around 2060 m a.s.l. ; Delalande-Le Mouëllic et al 2015). The Ngozi volcano had undergone at least two major eruptions in the Holocene, responsible for both the caldera formation and important ash deposition at regional scale (Fontijn et al 2010(Fontijn et al , 2012.…”

Section: The Shiwaga Gas Fieldmentioning

confidence: 99%

See 1 more Smart Citation

Evidence of a water δ18O negative shift driven by intensive deep CO2 upflow at Shiwaga gas field (Rungwe, Tanzania)

et al. 2018

Self Cite

View full text Add to dashboard Cite

Located on the flank of Ngozi volcanoes (Tanzania), the Shiwaga gas field is a spot of intense CO 2(g) emanations. Physicochemical measurements on different types of waters (rivers, puddles, and springs) as water and gas sampling were discontinuously performed over 10 years for equilibrated partial CO 2 pressure calculations and stable isotopic analyses. The most striking result shows that meteoric H 2 O and deep originated CO 2(g) exchanges are responsible for a negative 18 O-shift of the studied waters in relation with waters electrical conductivity, pH, and pCO 2 eq changes. In spring waters, a maximum shift of − 11.2‰ in δ 18 O was observed and pCO 2 eq values up to 1196 mbar were computed. Although this trend has already been reported around the world, such extended shift is rarely measured and requires an important amount of CO 2(g) , with a CO 2(g) / H 2 O ratio up more than 0.5 mol/mol. This approach is useful to better understand the hydro-geochemical processes involved in such environments. Moreover, this study evidences that an inventory as a monitoring of these gas fields are needed for the management of natural hazards and local resources.

show abstract

Section: The Shiwaga Gas Fieldmentioning

confidence: 97%

Section: The Shiwaga Gas Fieldmentioning

confidence: 99%

Evidence of a water δ18O negative shift driven by intensive deep CO2 upflow at Shiwaga gas field (Rungwe, Tanzania)

et al. 2018

Self Cite

View full text Add to dashboard Cite

show abstract

“…The high salinity and gas content of the lake is explained by assumed fault-controlled inflows of gas-rich, mantle-derived fluids 10 ; however, the existence of such structures could not be proved so far. Geochemical analyses 10 of Lake Ngozis water column do neither indicate a chemical nor thermal stratification as described for Lake Nyos and Monoun 11 . High Na-Cl and low Mg concentrations suggest an inflow of a mature geothermal fluid into the lake, combined with CO 2 input from regional deep degassing sources and H 2 S, indicated by high pCO 2 and medium SO 4 content 10 , respectively.…”

Section: Introductionmentioning

confidence: 97%

“…Lake Ngozi (maximum depth 83 m) is located within the Ngozi caldera and its maximum extent is 2.6 km (W-E) by 1.6 km (N-S). The annual average of the surface water temperature (undefined depth) is 21 °C with temperature variations from 20.0 °C in August to 21.5 °C in March/October 10 . The high salinity and gas content of the lake is explained by assumed fault-controlled inflows of gas-rich, mantle-derived fluids 10 ; however, the existence of such structures could not be proved so far.…”

Section: Introductionmentioning

confidence: 99%

“…The annual average of the surface water temperature (undefined depth) is 21 °C with temperature variations from 20.0 °C in August to 21.5 °C in March/October 10 . The high salinity and gas content of the lake is explained by assumed fault-controlled inflows of gas-rich, mantle-derived fluids 10 ; however, the existence of such structures could not be proved so far. Geochemical analyses 10 of Lake Ngozis water column do neither indicate a chemical nor thermal stratification as described for Lake Nyos and Monoun 11 .…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Detecting gas-rich hydrothermal vents in Ngozi Crater Lake using integrated exploration tools

Jolie

2019

Sci Rep

View full text Add to dashboard Cite

Gas-rich hydrothermal vents in crater lakes might pose an acute danger to people living nearby due to the risk of limnic eruptions as a result of gas accumulation in the water column. This phenomenon has been reported from several incidents, e.g., the catastrophic Lake Nyos limnic eruption. CO 2 accumulation has been determined from a variety of lakes worldwide, which does not always evolve in the same way as in Lake Nyos and consequently requires a site-specific hazard assessment. This paper discusses the current state of Lake Ngozi in Tanzania and presents an efficient approach how major gas-rich hydrothermal feed zones can be identified based on a multi-disciplinary concept. The concept combines bathymetry, thermal mapping of the lake floor and gas emission studies on the water surface. The approach is fully transferable to other volcanic lakes, and results will help to identify high-risk areas and develop suitable monitoring and risk mitigation measures. Due to the absence of a chemical and thermal stratification of Lake Ngozi the risk of limnic eruptions is rather unlikely at present, but an adapted monitoring concept is strongly advised as sudden CO 2 input into the lake could occur as a result of changes in the magmatic system.

show abstract

Water budget of tropical volcanic lakes in center‐north Cameroon: Reconciling the stable isotope and chloride mass balance

Abba

Hamelin

Michelot

et al. 2023

Hydrological Processes

View full text Add to dashboard Cite

The Cameroon volcanic line (CVL) hosts numerous volcanic lakes whose internal processes and hydrological functioning remain poorly documented. A detailed understanding of these hydro-systems is however essential, both for the consideration of these lakes as sentinels of the regional hydro-climatic changes and for the calibration of palaeoenvironmental proxies. Here, we present a hydrological and geochemical investigation of five of these lakes (Mbalang, Tabere, Tizon, Gegouba and Baledjam) around Ngaoundere on the Adamawa Plateau, based on repeated sampling of water profiles and monthly monitoring of rain and lake water samples over two seasonal cycles. We show that each of these throughflow lakes bears a distinct geochemical and isotopic signature, despite quite similar morphometric characteristics and a common climatic regime, due to varying contribution of the watersheds to the watermass balance and different partitioning between evaporation (E) and outflow (Inflow minus E). We use these differences as a benchmark for a sensitivity analysis of the classical budget equations of conservative tracers. The results demonstrate that to reconcile chloride and stable isotope data with the standard single-box steady state model would require unusual values of the physical parameters of Craig and Gordon's equation such as the n.θ term that would have to be significantly lower than its usual value (n.θ = 0.5). We also show that the data can be simulated more easily by including the inflow from the watershed while assuming that transpiration exceeds evaporation for this compartment. Using this conceptualization of the throughflow lakes, we were able to constrain the different fluxes. Transpiration from the watershed and evaporation from the lake are on the same order of magnitude, or slightly in favour of † Died October 22, 2022. This work is dedicated to the memory of our colleague and friend, Jean-Luc Michelot, who passed away too early on October 22, 2022.

show abstract

Hydrogeochemical features of Lake Ngozi (SW Tanzania)

Cited by 10 publications

References 53 publications

Evidence of a water δ18O negative shift driven by intensive deep CO2 upflow at Shiwaga gas field (Rungwe, Tanzania)

Evidence of a water δ18O negative shift driven by intensive deep CO2 upflow at Shiwaga gas field (Rungwe, Tanzania)

Detecting gas-rich hydrothermal vents in Ngozi Crater Lake using integrated exploration tools

Water budget of tropical volcanic lakes in center‐north Cameroon: Reconciling the stable isotope and chloride mass balance

Contact Info

Product

Resources

About