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

Samson Oiro (Corresponding Author), Jean-Christophe Comte

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Abstract

Fresh groundwater resources in coastal East Africa are crucial for the region's socio-economic development but are under threat of salinization caused by changes in recharge patterns and increasing abstraction. With the aim of establishing the drivers behind saltwater intrusion and its current spatial extent, we studied the Kenyan South Coast aquifer, a representative, strategic aquifer under increased pressure. Investigations included electrical resistivity tomography (ERT) surveys and in-situ groundwater measurements (water table and basic quality) together with the analysis of available long-term climatic and borehole monitoring data. Over the last 40 years, groundwater electrical conductivity values at the well field increased by about three times and groundwater levels declined by 1–3 m over the last decade. When put in perspective with the long-term climate (rainfall, temperature) and abstraction records, these trends in groundwater appear to be primarily driven by increased borehole abstraction (+400 m3/day per year in average), whereas observed increasing temperature (+0.02 °C per year) and decreasing rainfall (−0.8 mm per year) could potentially act as a secondary control through reduced recharge. However 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. Groundwater quality mapping 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. Implementation of groundwater management strategies such as 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.
Original languageEnglish
Pages (from-to)334-347
Number of pages14
JournalJournal of African Earth Sciences
Volume149
Issue number1
Early online date27 Aug 2018
DOIs
Publication statusPublished - 1 Jan 2019

Fingerprint

saline intrusion
aquifer
groundwater
coast
recharge
rainfall
electrical resistivity
borehole
groundwater control
groundwater abstraction
regional geology
analysis
temperature
climate effect
salinization
groundwater resource
tomography
electrical conductivity
water table
floodplain

Keywords

  • coastal aquifer system
  • seawater intrusion
  • electrical resistivity tomography
  • recharge
  • groundwater time series

Cite this

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title = "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",
abstract = "Fresh groundwater resources in coastal East Africa are crucial for the region's socio-economic development but are under threat of salinization caused by changes in recharge patterns and increasing abstraction. With the aim of establishing the drivers behind saltwater intrusion and its current spatial extent, we studied the Kenyan South Coast aquifer, a representative, strategic aquifer under increased pressure. Investigations included electrical resistivity tomography (ERT) surveys and in-situ groundwater measurements (water table and basic quality) together with the analysis of available long-term climatic and borehole monitoring data. Over the last 40 years, groundwater electrical conductivity values at the well field increased by about three times and groundwater levels declined by 1–3 m over the last decade. When put in perspective with the long-term climate (rainfall, temperature) and abstraction records, these trends in groundwater appear to be primarily driven by increased borehole abstraction (+400 m3/day per year in average), whereas observed increasing temperature (+0.02 °C per year) and decreasing rainfall (−0.8 mm per year) could potentially act as a secondary control through reduced recharge. However 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. Groundwater quality mapping 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. Implementation of groundwater management strategies such as 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.",
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note = "We acknowledge the Royal Geographical Society (with IBG) Environment and Sustainability Research Grant for financial support for all field activities. Acknowledgments extend to the Kenyan Water Resources Authority (WRA) and the University of Aberdeen 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 our presence in the area as well as the assistance by local casuals during the geophysical surveys.",
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N1 - We acknowledge the Royal Geographical Society (with IBG) Environment and Sustainability Research Grant for financial support for all field activities. Acknowledgments extend to the Kenyan Water Resources Authority (WRA) and the University of Aberdeen 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 our presence in the area as well as the assistance by local casuals during the geophysical surveys.

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N2 - Fresh groundwater resources in coastal East Africa are crucial for the region's socio-economic development but are under threat of salinization caused by changes in recharge patterns and increasing abstraction. With the aim of establishing the drivers behind saltwater intrusion and its current spatial extent, we studied the Kenyan South Coast aquifer, a representative, strategic aquifer under increased pressure. Investigations included electrical resistivity tomography (ERT) surveys and in-situ groundwater measurements (water table and basic quality) together with the analysis of available long-term climatic and borehole monitoring data. Over the last 40 years, groundwater electrical conductivity values at the well field increased by about three times and groundwater levels declined by 1–3 m over the last decade. When put in perspective with the long-term climate (rainfall, temperature) and abstraction records, these trends in groundwater appear to be primarily driven by increased borehole abstraction (+400 m3/day per year in average), whereas observed increasing temperature (+0.02 °C per year) and decreasing rainfall (−0.8 mm per year) could potentially act as a secondary control through reduced recharge. However 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. Groundwater quality mapping 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. Implementation of groundwater management strategies such as 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.

AB - Fresh groundwater resources in coastal East Africa are crucial for the region's socio-economic development but are under threat of salinization caused by changes in recharge patterns and increasing abstraction. With the aim of establishing the drivers behind saltwater intrusion and its current spatial extent, we studied the Kenyan South Coast aquifer, a representative, strategic aquifer under increased pressure. Investigations included electrical resistivity tomography (ERT) surveys and in-situ groundwater measurements (water table and basic quality) together with the analysis of available long-term climatic and borehole monitoring data. Over the last 40 years, groundwater electrical conductivity values at the well field increased by about three times and groundwater levels declined by 1–3 m over the last decade. When put in perspective with the long-term climate (rainfall, temperature) and abstraction records, these trends in groundwater appear to be primarily driven by increased borehole abstraction (+400 m3/day per year in average), whereas observed increasing temperature (+0.02 °C per year) and decreasing rainfall (−0.8 mm per year) could potentially act as a secondary control through reduced recharge. However 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. Groundwater quality mapping 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. Implementation of groundwater management strategies such as 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.

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