Abstract
Lake Kivu is a 485 m deep, Central-East African rift lake with huge amounts of carbon dioxide and methane dissolved in its stably stratified deep waters. In view of future large-scale methane extraction, one-dimensional numerical modelling is an important and computationally inexpensive tool to analyze the evolution of stratification and the content of gases in Lake Kivu. For this purpose, we coupled the physical lake model Simstrat to the biogeochemical library AED2. Compared to an earlier modelling approach, this coupled approach offers several key improvements, most importantly the dynamic evaluation of mixing processes over the whole water column, including a parameterization for double-diffusive transport, and the density-dependent stratification of groundwater inflows. The coupled model successfully reproduces today's near steady-state of Lake Kivu, and we demonstrate that a complete mixing event ∼2000 years ago is compatible with today's physical and biogeochemical state.
Highlights
Lake Kivu is a large (2386 km2) and deep (485 m) tropical rift lake, situated on the boundary between Rwanda and the Democratic Republic of the Congo (DRC), directly south of the Virunga volcano chain
We chose to neglect this warming rate in our simulations for two reasons: i) the observed warming trend seems variable in time and is comparably slow, and ii) we focus on the present state and the past evolution of Lake Kivu in this work, not on its future evolution
We developed and calibrated a one-dimensional model for Lake Kivu, exploiting the fact that the lake is well mixed horizontally below ~60 m
Summary
Lake Kivu is a large (2386 km2) and deep (485 m) tropical rift lake, situated on the boundary between Rwanda and the Democratic Republic of the Congo (DRC), directly south of the Virunga volcano chain It is fed by numerous small streams, and discharges, via the Ruzizi River, into Lake Tanganyika (Fig. 1a). In addition to surface streams, ~45% of the inflow into Lake Kivu is provided by subaqueous groundwater sources (Schmid and Wüest, 2012), some of which were identified in the northern part of the lake using temperature and salinity profiles (Ross et al, 2015a and Fig. 1a) Part of this intruding groundwater is hydro thermal, meaning that it is warm, salty and rich in carbon dioxide (CO2). The model of Schmid et al (2005) suggested that the discharge of the cooler sources is one magnitude larger than the hy drothermal discharge, and that they explain the strong thermo- and chemoclines at around 190 and 250 m (see Fig. 1b for an overview)
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