부산대학교 도서관

Groundwater is crucial to Africa's environment and socio-economic development and to date remains Africa's main source of domestic water supply both in rural and urban areas. The overall aim of the research was to provide new understanding on the historical and future response of East African aquifer systems of high societal importance to climatic change and human development, using two juxtaposed but representative case studies: the Nairobi aquifer system (NAS) and the Kenyan South Coast aquifer (SC). Population growth rate in Kenya over the last 60 years has been twice the rate of the world and until recently, about 50% higher than the average African continent population growth rate. Currently, Kenya's population growth rate is 2.52% while that of Nairobi City is at 4.5%, Nairobi Aquifer System Area at 3.4%, and that of Kwale County (where the SC aquifer is located) at 3.1%. These very high rates have come with high water demand thereby putting pressure on water resources and particularly groundwater, which has been specifically targeted in Kenya and wider Africa for coping with increased demand. Groundwater abstraction from the Nairobi aquifer system (NAS) currently sustains the steady flow of supply and bridges the gap of piped water supply shortage for over 6 million people while the south coast aquifer (SC) supplies over 1 M people in both Kwale and Mombasa counties. Fresh groundwater resources in coastal East Africa, including Kenya, are under threat of salinization caused by changes in recharge patterns and increasing abstraction. The two aquifer systems chosen for research are categorised as strategic by Water Resources Authority-Kenya (WRA) due to their socioeconomic importance vs. resource availability. Their response to climate change and anthropogenic processes are not yet understood and analysed. Groundwater-surface water interaction investigations within the country/region are still lagging and mostly unknown. Groundwater modelling is not yet embraced in Kenya and East Africa while it is an integral part of groundwater management globally. East African coastal aquifers reported saltwater intrusion, but underlying processes and drivers have gone unanswered for a long time and scrutiny of the extent and encroachment rate is yet to be provided. Hence, taking Kenyan SC aquifer for study acted as a pilot study to providing answers for similar aquifers in the region. Additionally, the impact of declining groundwater levels due to over-abstraction in the NAS affects the socio-economic growth of Nairobi and its surrounding. The NAS site was considered for study to make available intuitions for similar cities in the region experiencing increased private borehole developments. An integrated multi-technique hydrogeological methodology was employed using existing historical groundwater data analysis, water stable isotope investigations, geophysical investigations and numerical groundwater flow modelling. Refined and adapted methodological approaches were uniquely carried out in specific sites. The saltwater intrusion investigation was carried only on the SC aquifer due to its coastal location while groundwater numerical modelling was piloted first to NAS area due to data availability as well as the urgency of better hydrogeological understanding because of its high developmental rate, global importance, and the ongoing management crisis. For the SC area, this work provides the foundation for future modelling by the Water Resource Authority of Kenya to inform management and the design of monitoring networks. For both study regions, the analysis of long-term climate data demonstrated statistically insignificant trends. The low statistical significance obtained for both rainfall and temperature trends over the observation period suggests that no clear conclusion can be made with regards to long-term climate impact on groundwater recharge. More generally, large uncertainties surround the East African climatic projections (Powell and Chadwick, 2018), which makes it currently challenging to forecast its effect on groundwater resources with high probability as observed in the analyses of data in this research. For Kenya specifically, longer-term data and more meteorological stations are required for precise predictions/projections. The analysis of NAS groundwater time series showed a declining water table at an average rate of 0.5 - 20 m/decade (from 2006 – 2014) and an exponential increase of groundwater abstraction by a factor of 7 within a span of 40 years from 20,000 m3/day in 1970 to 140,000 m3/day in 2017. Over the same span of time (40 years), SC groundwater electrical conductivity values at the Tiwi well field increased by about three times and groundwater levels declined by 1 to 3 m over the last decade. In the NAS, EC values increased towards groundwater flow direction, an indication of residence time and geological formation influence through mineral dissolution. Spatial analysis of groundwater stable isotopes uncovered predominant spatiotemporal controls on groundwater recharge in the Nairobi Aquifer System (NAS) and South Coast aquifer (SC). 368 samples were analysed for stable isotopes and basic physico-chemical parameters. The NAS groundwater isotopes are controlled by precipitation orographic effects and enriched recharge from impounded water bodies (dams, ponds, water pans) and wetlands. SC isotopes are correlated with water-table depth influencing evapotranspiration. Global Network of Isotopes in Precipitation-GNIP data revealed groundwater recharge during months of heavy rains in NAS, whilst SC experiences spatiotemporally diffuse recharge. Inferred groundwater 'isoscapes' showed; in NAS, (1) direct, rapid recharge favoured by faults, well-drained soils and ample rainfall in uplands, (2) delayed recharge from impounded-lakes and wetlands in midlands, (3) focussed, event-based recharge in floodplains; in SC, diffuse recharge is complicated by significant water-table evapotranspiration processes. The usefulness of the approach is supported by other findings in south Africa where water stable isotopes was applied in delineating groundwater discharge areas sustaining river flows (Levy and Xu, 2012) and in delineating groundwater recharge zones in greater Maputo region in Mozambique (Nogueira et al., 2019). Since water stable isotope compositional analyses provides information for delineating groundwater recharge origins and discharge zones, the approach can be adapted by water sector institutions for understanding surface water/groundwater interaction for managing both baseflows and protecting recharge zones in any country. In the SC specifically, groundwater trends, when put in perspective with the long-term climate (rainfall, temperature) and abstraction records, appear to be primarily driven by increased borehole abstraction (+400 m3/day per year in average) rather than climate change. Groundwater quality mapping in SC showed that proximity to the ocean, presence of abstraction well-fields and regional geology control groundwater salinity patterns at regional scale. Locally, geophysical data showed that, saltwater intrusion spatial patterns are controlled by local aquifer lithology, groundwater abstraction and freshwater recharge in floodplains. Comparison with previous (1984) resistivity data showed that the saltwater front has advanced toward the well-field by up to 2 km and rose by up to 80 m over the last 30 years, which corresponds to a maximal velocity of about 60 m/y horizontally and 2 m/y vertically. The saltwater extent is almost similar to recent observations by Van Camp et al., (2014) in the neighbouring Dar es Salaam coastal aquifer which reported a saltwater intrusion extent of between 2 and 3 Km inland. In the NAS specifically, high resolution land-use change mapping showed that built-up area surfaces have increased by 70% from the year 2000 to 2017 from 14.5% to 24.2%, and that forest cover has decreased by 23% from 20.3% in 2000 to 15.7% in 2017. Geophysics (ERT) revealed subsurface structural diversity controlling groundwater movement and connectivity between surface water and groundwater. Numerical modelling confirmed a resounding negative impact of abstraction on groundwater levels (17 m decline) and estimated a storage loss of 4.6% (1.9 x 1010 m3 loss) from 1950 to 2018. The observed declining groundwater levels in Nairobi area is similar in scale to what was recently reported by Foster et al. (2018) for other exploited, semi-confined aquifers of Africa's urban areas. Management steps are therefore recommended for sustainable groundwater use in the region. Modelling scenarios suggested that continued unsustainable abstraction of groundwater in the NAS is likely to convert permanent springs and rivers to ephemeral waterbodies. Scenarios suggested however that a hybrid of regulated groundwater abstraction and conjunctive water supply approach has potential for more sustainable groundwater development and management of the NAS. This hybrid approach could reduce pressure on Tana Catchment resources where piped water is being and still expected to be outsourced from in future while at the same time controlling the pressure on Nairobi aquifer system. Human driven depletion of the NAS can be further mitigated through enactment of groundwater conservation laws, practicing sustainable use, and strategic management such as managed/artificial recharge. Sustainable groundwater exploitation, sourced alternative water supply, and managed aquifer recharge are required to mitigate the effects of seawater intrusion along the East African coastal strip.