Extensive groundwater development resulting from increased water demand from rapid population growth in many African cities poses a significant risk to groundwater resources and associated environmental flows. The Nairobi volcano-sedimentary regional aquifer system (NAS) hosting >6M people including 4.7M people in Nairobi city is a typical example. Here, we combine analysis of multi decadal in-situ water-level data with numerical groundwater modelling to provide an assessment of the past and likely future evolution of its groundwater resources. Since the mid-1970s groundwater abstraction has increased 10-fold at a rate similar to urban population growth; groundwater levels have declined at a median rate of 36 m/decade underneath Nairobi since 1950; whilst built-up areas have increased by 70% since 2000. Despite the absence of significant trends in climatic data since the 1970s, more recently, drought conditions have resulted in increased applications for borehole licences. Based on a new conceptual understanding of the NAS (including insights from geophysics and stable isotopes), numerical simulations provide further quantitative estimates of the accelerating negative impact of abstraction and capture the historical groundwater levels quite well. Analysis suggests a 4 m average decline over the entire aquifer area and up to 46 m below Nairobi, net groundwater storage loss of 1.5 billion m3 29 and 9% river baseflow reduction since 1950. Given current practices and trajectories these figures are predicted to increase 6-fold by 2120. Modelled future management scenarios suggest that future groundwater abstraction required to meet Nairobi projected water demand is unsustainable and that the regional, anthropogenically-driven depletion trend can be partially mitigated through conjunctive water use. The presented approach can inform groundwater assessment for other major African cities undergoing similar rapid groundwater development.
- Long-term groundwater records
- Aquifer depletion
- Hydrogeological conceptual model and numerical modelling
- Nairobi aquifer system
Oiro, S., Comte, J-C., Soulsby, C., Macdonald, A., & Mwakamba, C. (Accepted/In press). Depletion of groundwater resources under rapid urbanisation in Africa: recent and future trends in the Nairobi Aquifer System, Kenya. Hydrogeology Journal. https://doi.org/10.1007/s10040-020-02236-5