Characterisation of the Subaquatic Groundwater Discharge That Maintains the Permanent Stratification within Lake Kivu; East Africa

Warm and cold subaquatic groundwater discharge into Lake Kivu forms the large-scale density gradients presently observed in the lake. This structure is pertinent to maintaining the stratification that locks the high volume of gases in the deepwater. Our research presents the first characterisation of these inflows. Temperature and conductivity profiling was conducted from January 2010 to March 2013 to map the locations of groundwater discharge. Water samples were obtained within the lake at the locations of the greatest temperature anomalies observed from the background lake-profile. The isotopic and chemical signatures of the groundwater were applied to assess how these inflows contribute to the overall stratification. It is inferred that Lake Kivu’s deepwater has not been completely recharged by the groundwater inflows since its turnover that is speculated to have occurred within the last ~1000 yrs. Given a recent salinity increase in the lake constrained to within months of seismic activity measured beneath the basin, it is plausible that increased hydrothermal-groundwater inflows into the deep basin are correlated with episodic geologic events. These results invalidate the simple two-component end-member mixing regime that has been postulated up to now, and indicate the importance of monitoring this potentially explosive lake.

冷热水下地下水向基伍湖（Lake Kivu）的排泄，形成了当前在该湖中观测到的大规模密度梯度。该结构与维持深水层封存大量气体的水体分层状态密切相关。本研究首次对这类入流进行了特征刻画。2010年1月至2013年3月期间，我们开展了温度与电导率剖面测量，以绘制地下水排泄点位的分布范围。在湖中依据背景湖体剖面观测到的温度异常最显著的点位采集了湖水样本。通过分析地下水的同位素与化学特征标识，评估了这类入流对整体水体分层的贡献。据推断，基伍湖深水层自其末次湖水翻转事件（推测发生于约1000年内）以来，尚未完全经由地下水入流获得补给。鉴于近期该湖的盐度升高事件与湖盆下方观测到的地震活动时间仅相隔数月，据此可以合理推断，流入深盆的热液地下水流量增加，与幕式地质事件存在显著关联。上述研究结果推翻了此前一直被假定的简单双端元混合模式，同时凸显了对这座具有潜在爆发风险的湖泊开展长期监测的重要性。