An additional challenge of Lake Kivu in Central Africa - upward movement of the chemoclines

Abstract

In Lake Kivu, gases accumulate, exposing the riparian population to the perils of a limnic eruption. This primary challenge has been known for years. Measurements have now established that the chemoclines in Lake Kivu are moving upward at the following total vertical flow rates (in km3 y-1): ~0.05 below 390 m, ~0.12 below 313 m, ~0.13 below 257 m, ~0.25 below 191 m, and ~1.39 below ~140 m depth. The upward movement is caused by inflows to the deeper parts of the lake from water sources with different salinities and thus of different densities. As a result, they stratify at their different isopycnal levels (especially at 350 m and at 220 to 245 m depth) and mix into the waters at those levels, thus pushing up all water above, chemoclines and mixed zones alike. Water from rivers in the catchment area remains in the mixolimnion and when cold, some may sink down to the uppermost mixolimnion. Depending on the wind direction, it is suggested that alternating outflows from Kabuno Bay with variable density ends up either in the mixolimnion or at 245 m depth. In Lake Kivu, only the uppermost chemocline remains at the same level; in this case the upward movement results in transport of mass across the chemocline. In the uppermost mixed zone of the monimolimnion, from 130 to 180 m depth, the natural dilution might suffice to outbalance methane accumulation and thus to prevent any future limnic eruption from this zone. In the next mixed zone below (at 200 to 250 m depth), the upward displacement of dissolved gases to higher levels increases the risk of future limnic eruption. In fact, over time this risk is changed to certainty, even if gas is removed from the deeper (commercially attractive) layers. The limited dilution in this zone probably does not suffice to outbalance methane accumulation. Furthermore, if this upward movement is not prevented, probably within a decade or two a point of no return is likely to be passed, leaving us with no reliable means of responding to this challenge. It is therefore necessary to stop the upward movement by disposing ~0.25 km3 y-1 of degassed water into the mixolimnion. It is what gas extraction plants do to the lake in the next decades that determines whether realistically a future limnic eruption from this layer may be prevented.

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