Drivers, patterns and velocity of saltwater intrusion in a stressed aquifer of the East African coast: Joint analysis of groundwater and geophysical data in southern Kenya
Introduction
About half of the global population resides within coastal areas (IPCC and UNEP, 1997, Jahanshahi and Zare, 2016, Sonkamble et al., 2014), most of them, particularly in the developing world being subject to high demographic increase resulting in higher groundwater demand. In these areas, groundwater is a vital freshwater resource for human needs (Ketabchi et al., 2016) and the broader environment through acting as exchange zones separating marine and terrestrial hydro-biogeochemical cycles (Colombani et al., 2015, Post and Werner, 2017). They are highly sensitive to changes triggered by both natural and anthropogenic forcing such as climate, land use, pollution and over-pumping (Comte et al., 2014, Hsieh et al., 2015, Iyalomhe et al., 2015, Klassen and Allen, 2017, Lathashri and Mahesha, 2015). The use of coastal groundwater for drinking, agriculture or industry is however globally compromised by the salinization problem (Ahmed et al., 2017, Argamasilla et al., 2017, Himi et al., 2017). Progressing saltwater intrusion into freshwater aquifers due to over-abstraction have prompted global alert and concern (Ahmed, 2017, Priyanka and Mahesha, 2015, Sonkamble et al., 2014). Salinization due to over-abstraction leads, in the most severe cases, to water supply wells being abandoned (Argamasilla et al., 2017, Himi et al., 2017, Ketabchi et al., 2016, Klassen and Allen, 2017, Post and Werner, 2017). Aquifer recharge reduction caused by climate change is another important factor of salinization (Green et al., 2011; Singh et al., 2014).
In Africa, over 50% of population live in coastal zones (Altchenko and Villholth, 2013, Arthurton, 1998, Steyl and Dennis, 2010) with freshwater demand as a basic human need being at centre stage (Arthurton, 1998, Steyl and Dennis, 2010, Taylor et al., 2012). Human developments strongly affect the coastal strip hydrosystems with 38% of the African coast categorized by UNEP in 1998 as under severe threat from over-development (Steyl and Dennis, 2010). In Sub-Saharan coastal areas particularly, higher resilience of groundwater than surface water systems to the impacts of climate change and pollution is promoting rapid, unprecedented development of groundwater resources, which in turn increases their dependability by population (Steyl and Dennis, 2010). Groundwater data scarcity and lack of high level interstates cooperation has been pointed out as major obstacle in managing Africa's coastal aquifers of which many are transboundary (Steyl and Dennis, 2010, Wangati and Said, 1997). Yet improved management of groundwater resource requires acquisition of suitable groundwater inventory information including assessment of groundwater limiting factors, and how to disseminate the information for the benefit of coastal communities (Arthurton, 1998).
In East Africa more specifically, coastal aquifers are the main source of freshwater supply for all the economic sectors and domestic use and groundwater exploitation has increased with time (Comte et al., 2016). In Kenya, supply of quality freshwater was flagged long ago as a major challenge facing the coastal communities with problem intensity dependent on seasons (Arthurton, 1998). Up to 1972, most water boreholes within south coast of Kenya, which supported over 0.5 Million people including the major city of Mombasa, were drilled in the coastal fringe areas underlain by Pleistocene coral reef. Saline contamination is greatly pronounced in areas of coral limestone (Tole, 1997) due to high permeability and low hydraulic gradients resulting from large primary porosity and secondary karstification. After 1972, less saline sandy facies aquifers of back-reef/lagoon origin (Magarini and Kilindini sands), located further inland, were explored and drilled (Buckley, 1981). The first borehole in 1973 was installed near the Tiwi village and was then followed by other eight exploratory wells with two being able to be used for production yielding good quality and quantity water (Buckley, 1981). The so-called Tiwi Aquifer is among the highest yielding sedimentary-rock aquifers of Kenya (Wangati and Said, 1997). The Kenyan south coast aquifer system (Tiwi) is also amongst the most threatened aquifers in East Africa by seawater intrusion fashioned by groundwater over-exploitation (Tole, 1997, Wangati and Said, 1997).
The research aimed at assessing the current extent, past evolution and drivers of coastal aquifer salinization in the East African coast using climatic records, current and historical groundwater monitoring data including basic in-situ measured water quality parameters and geophysical (electrical resistivity tomography – ERT) investigations in the Kenyan South Coast aquifer. The specific objectives of the groundwater mapping and geophysical investigations were: (1) understanding the regional impact of the observed changing climate and groundwater abstraction on freshwater availability and saltwater intrusion; and (2) delineating sub-regional and local spatial patterns and evolution of seawater intrusion into the main exploited aquifer systems.
Section snippets
Study area
The study area is located in Kwale County, on the Kenyan South Coast, East Africa. It lies within one of the five major catchment areas of Kenya, the Athi River catchment. The altitude ranges from 0 m near the ocean to 229 m inland, within latitude 4° 1.5′ to 4° 40.6′ S, and longitude 39° 5.4′ to 39° 43.2′ E. Slightly less than a million people live in the Kwale County, however, groundwater from the area supports over 1M people in two counties of Kwale and Mombasa. The population density of the
Climate data analysis
The historical climate data available for eleven weather stations across the coastal region were purchased from the Kenya Meteorological Department (Head Office, Nairobi). The nearest station from the study site (Moi International Airport, Mombasa; average annual rainfall of 1200 mm) had precipitation data available from 1970 to 2016 (46 years), and temperature data from 1972 to 2016 (44 years). Data were compiled, re-organized, and analysed using linear regression to highlight climate trends
Long-term climatic trends
The analysis of annual precipitation over the last 45 years revealed significant interannual variability with periods of heavy annual rainfall occurring at an average frequency of 10 years, which correspond to the major El Nino (high rainfall) events. The El Nino effect is particularly clear in 1982 and more spectacularly in 1997. Over the whole period, rainfall displays an almost insignificant decreasing trend, at an average rate of 0.8 mm per annum (Fig. 4b). Annual temperatures are less
Discussion
Coastal aquifers are vital for the socio-economic development of towns along the coastal areas (Kenyan coast not exempted) as water needs are met by groundwater exploitation. Understanding factors affecting the sustainability of coastal fresh groundwater resources is crucial. The integrative approach considered under this study in mapping out the extent of seawater intrusion and investigating its driving forces is vital. Widespread problem of seawater intrusion is increasing in coastal aquifers
Conclusion
Population is growing along the coastal areas of Kenya, which is also accompanied by increasing water demand. The water need in the area as well as along the whole, ca. 4000 km long East African coast from Somalia to Mozambique is mostly met by exploitation of the coastal groundwater resources. This has pushed both the government and communities to sink more wells ranging from open wells to hand-pumped wells and electric power-driven boreholes. Climatic data recorded over the last few decades
Acknowledgements
We acknowledge the Royal Geographical Society (with IBG) Environment and sustainability Research Grant, United Kingdom, Grant number ESRG3/16 for financial support for all field activities. Acknowledgments extend to the Water Resources Authority, Kenya, Grant number WRA and the University of Aberdeen, United Kingdom, Grant number UoA for supporting S. Oiro's PhD scholarship. We thank Security Officers and Government of Kenya Administrators who facilitated field access by informing the public of
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