Assessment of a groundwater system under global change scenarios: the case of Kwale (Kenya)

One of the main drivers of the current global change is the climate change characterized by increased number and increased length of drought periods. Another relevant driver that will imply greater pressure on natural processes is the expected population growth, increasing the demand and competition for water for domestic, industrial, agricultural, and municipal uses. Global change effects on water resources are profound and need to be explored deeply, especially, in the developing countries. The main goal focus on develop a combination and integration of different types of hydrogeological tools, climatic episodes, and social variables, in order to better understand the effects of global change on Sub-Saharan Africa. To do it, the coastal aquifer system of Kwale (Kenya) is taken as a reference, where local communities share groundwater resources with new water-reliant activities as mining, agriculture and tourism. The final goal is to understand the risks and impacts in this context to improve water resources management in benefit of the poorest. The aquifer system has been characterized integrating kilometric geophysical transects, hydrochemistry, environmental isotopes and groundwater level data. The main quality groundwater problems, contamination by faecal bacteria (E. coli), have been characterized. Several qualitative and quantitative variables as geology, hydrology, geochemistry, sanitary risk factors, well types, and maintenance have been statistical analysed to study its correlation with E. coli concentration. The other main quality problem in the area is the saline intrusion, so geochemical models have been developed using PHREEQC software. The groundwater sustainability of the system has been determined under the new abstraction regime of the water-reliant users. This sustainability has been evaluated during La Nina drought 2016/17. A numerical groundwater flow model integrates all the information available and it is used as a tool to study how the global change may affect the groundwater system. In addition, a new index has been defined and tested to define the risk for a given household to have no access to drinking water (in terms of either quantity or quality). The aquifer is a multi-layered system formed by a shallow and deep aquifer. The statistical analyses indicate that low Eh, short water column and areas with fast infiltration are factors related with the presence of faecal bacteria. The geochemical mix models point out that sea water intrusion will tend to increase calcite dissolution. The main effect of La Nina has been a reduction of the recharge of 69 % compared to a year with average annual rainfall as 2013 and a groundwater level decline in 86 % of the measured shallow wells. The total anticipated volume abstracted will increase by around 85 % compared with current abstraction. Despite groundwater level decline observed during prolonged dry periods and abstraction increment, a dry period followed by a humid period leads to the relatively swift recovery of the groundwater system in less than 3 years. The numerical groundwater flow model has been to use the outputs of future scenarios together with household data to test the new developed index. This index helps to understand better the effect of the global change on households under a “transient state” instead of a “steady state”, as most of the current indexes do. The present dissertation contributes to the hydrogeological knowledge in a context that represents much of East Africa. Different tools have been presented in order to study these systems by limited lack of data. It shows the importance of integrating the hydrogeological data of stakeholders, and the alternative sources of information to advance the knowledge in areas with lack of data. Integrating hydrogeological and social household data let a bigger understanding of how the groundwater system changes can affect the groundwater availability to the water-reliant users.