[1] Many lakes show vertical stratification of their water masses, at least for some extended time periods. Density differences in water bodies facilitate an evolution of chemical differences with many consequences for living organisms in lakes. Temperature and dissolved substances contribute to density differences in water. The atmosphere imposes a temperature signal on the lake surface. As a result, thermal stratification can be established during the warm season if a lake is sufficiently deep. On the contrary, during the cold period, surface cooling forces vertical circulation of water masses and removal of gradients of water properties. However, gradients of dissolved substances may be sustained for periods much longer than one annual cycle. Such lakes do not experience full overturns. Gradients may be a consequence of external inflows or groundwater seepage. In addition, photosynthesis at the lake surface and subsequent decomposition of organic material in the deeper layers of a lake can sustain a gradient of dissolved substances. Three more geochemical cycles, namely, calcite precipitation, iron cycle, and manganese cycle, are known for sustaining meromixis. A limited number of lakes do not experience a complete overturn because of pressure dependence of temperature of maximum density. Such lakes must be sufficiently deep and lie in the appropriate climate zone. Although these lakes are permanently stratified, deep waters are well ventilated, and chemical differences are small. Turbulent mixing and convective deep water renewal must be very effective. As a consequence, these lakes usually are not termed meromictic. Permanent stratification may also be created by episodic partial recharging of the deep water layer. This mechanism resembles the cycling of the ocean: horizontal gradients result from gradients at the surface, such as differential cooling or enhanced evaporation in adjacent shallow side bays. Dense water parcels can be formed which intrude the deep water layer. In the final section, stratification relevant physical properties, such as sound speed, hydrostatic pressure, electrical conductivity, and density, are discussed. The assumptions behind salinity, electrical conductance, potential density, and potential temperature are introduced. Finally, empirical and theoretical approaches for quantitative evaluation from easy to measure properties conclude this contribution.

https://www.eoas.ubc.ca/~mjelline/453website/eosc453/E_prints/newfer09/boehrer_stratlakeRG08.pdf

Stratification of lakes

Instances of gas leakage from naturally occurring CO2 reservoirs and natural gas storage sites serve as analogues for the potential release of CO2 from geologic storage sites. This paper summarizes and compares the features, events, and processes that can be identified from these analogues, which include both naturally occurring releases and those associated with industrial processes. The following conclusions are drawn: (1) carbon dioxide can accumulate beneath, and be released from, primary and secondary shallower reservoirs with capping units located at a wide range of depths; (2) many natural releases of CO2 are correlated with a specific event that triggered the release; (3) unsealed fault and fracture zones may act as conduits for CO2 flow from depth to the surface; (4) improperly constructed or abandoned wells can rapidly release large quantities of CO2; (5) the types of CO2 release at the surface vary widely between and within different leakage sites; (6) the hazard to human health was small in most cases, possibly because of implementation of post-leakage public education and monitoring pro- grams; (7) while changes in groundwater chemistry were related to CO2 leakage, waters often remained potable. Lessons learned for risk assessment associated with geologic carbon sequestration are discussed.

/pdf/natural-and-industrial-analogues-for-leakage-of-co2-from-5g0jg377zm.pdf

Natural and industrial analogues for leakage of CO2 from storage reservoirs: identification of features, events, and processes and lessons learned

[1] The deep waters of the East African Rift Lake Kivu contain large amounts of dissolved carbon dioxide and methane The release of a fraction of these gases, which could be triggered by a magma eruption within the lake, would have catastrophic consequences for the two million people living on its shore Up to now the safety assessment of the lake was based on the assumption that the gas concentrations in the deep waters are in a steady state with a residence time of 400 years Turbulent transport was regarded as the main pathway of vertical exchange Recent measurements and the analysis of the vertical transport processes in the lake radically change this evaluation The vertical turbulent exchange is negligible, as documented by a spectacular set of several hundred double-diffusive layers Gases are mainly transported out of the deep zones by a slow upwelling with a residence time of 800–1000 years Our results indicate that the methane production within the sediment has recently increased, leading to a gas accumulation in the deep waters and consequently decreasing the heat input needed to trigger a devastating gas release With the estimated current CH4 production, the gas concentrations could approach saturation within this century

/pdf/weak-mixing-in-lake-kivu-new-insights-indicate-increasing-2ktzepr4m9.pdf

Weak mixing in Lake Kivu: New insights indicate increasing risk of uncontrolled gas eruption

Since the catastrophic releases of CO2 in the 1980s, Lakes Nyos and Monoun in Cameroon experienced CO2 recharge at alarming rates of up to 80 mol/m2 per yr. Total gas pressures reached 8.3 and 15.6 bar in Monoun (2003) and Nyos (2001), respectively, resulting in gas saturation levels up to 97%. These natural hazards are distinguished by the potential for mitigation to prevent future disasters. Controlled degassing was initiated at Nyos (2001) and Monoun (2003) amid speculation it could inadvertently destabilize the lakes and trigger another gas burst. Our measurements indicate that water column structure has not been compromised by the degassing and local stability is increasing in the zones of degassing. Furthermore, gas content has been reduced in the lakes ≈12-14%. However, as gas is removed, the pressure at pipe inlets is reduced, and the removal rate will decrease over time. Based on 12 years of limnological measurements we developed a model of future removal rates and gas inventory, which predicts that in Monoun the current pipe will remove ≈30% of the gas remaining before the natural gas recharge balances the removal rate. In Nyos the single pipe will remove ≈25% of the gas remaining by 2015; this slow removal extends the present risk to local populations. More pipes and continued vigilance are required to reduce the risk of repeat disasters. Our model indicates that 75-99% of the gas remaining would be removed by 2010 with two pipes in Monoun and five pipes in Nyos, substantially reducing the risks.

/pdf/degassing-lakes-nyos-and-monoun-defusing-certain-disaster-2s95nhqhl6.pdf

Degassing Lakes Nyos and Monoun: Defusing certain disaster

Geologic carbon sequestration is the capture of anthropogenic carbon dioxide (CO2) and its storage in deep geologic formations. The processes of CO2 seepage into surface water after migration through water-saturated sediments are reviewed. Natural CO2 and CH4 fluxes are pervasive in surface-water environments and are good analogues to potential leakage and seepage of CO2. Buoyancy-driven bubble rise in surface water reaches a maximum velocity of approximately 30 cm s−1. CO2 rise in saturated porous media tends to occur as channel flow rather than bubble flow. A comparison of ebullition versus dispersive gas transport for CO2 and CH4 shows that bubble flow will dominate over dispersion in surface water. Gaseous CO2 solubility in variable-salinity waters decreases as pressure decreases leading to greater likelihood of ebullition and bubble flow in surface water as CO2 migrates upward.

On leakage and seepage of CO 2 from geologic storage sites into surface water

better understood, thanks to the almost continuous scrutiny of the two lakes by scientists. Lakes Nyos and Monoun occupy the crater of a supposedly extinct volcano, in a region known by geologists for its numerous soda (not thermal) water springs, a common feature of old volcanic areas.The region belongs to the so-called volcanic chain of Cameroon, which culminates and ends 300 km further southwest at the still-active Mount Cameroon (4000 m asl.). Accumulated evidence strengthens the early hypothesis that the Monoun gas burst originated in the release of huge amounts of

https://mhalb.pagesperso-orange.fr/nyos/2004EO300001_papier%20EOS%20Nyos.pdf

Jean-Claude Tochon-Danguy

Papers

Degassing the “Killer Lakes” Nyos and Monoun, Cameroon