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 socioeconomic 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 longterm 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 to 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 m 3 /day per year in average), whereas observed increasing temperature (+0.02 ℃ 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 longterm climate impact on groundwater. Groundwater quality mapping showed that proximity to the ocean, ACCEPTED MANUSCRIPT 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.

Toward Contamination in Coastal Regions for Groundwater Quality, Fluoride, Nitrate and Human Health Risk Assessments Within Multi-aquifer Al-Hassa, Saudi Arabia

“…A recent study on coastal groundwater in Bhola District, Bangladesh, reported an average EC of 588.15 µS/cm [46], indicating a notable difference between the two reports. However, the EC values observed in this study align with those found in coastal aquifers of Southern Kenya [47], Western Cape Province, South Africa [48], and South [49]. Typically, fresh groundwater unaffected by saltwater intrusion exhibits mild salinity and, thus, low EC values.…”

Water security in sub‐Saharan Africa: Understanding the status of sustainable development goal 6

“…Untreated sewerage discharged or leakage of organic and microbial pollutants to water bodies also contribute to the deterioration of water quality in many countries resulting in several public health threats and disease outbreaks in SSA (Gumbo et al, 2016; Houéménou et al, 2020). Seawater intrusion particularly in coastal areas is another source of shallow groundwater contamination, rendering it undrinkable, and not suitable for irrigation (Oiro & Comte, 2019; Takem et al, 2015). Emerging organic contaminants have equally been detected in wastewater, surface water and groundwater wells across many countries (Gwenzi & Chaukura, 2018; Sorensen et al, 2015).…”

Assessing groundwater salinity across Africa

“…In Northeastern Tunisia, as the result of overexploitation of the coastal Ras el Jabel aquifer the range of groundwater salinity has increased from 1.5-4 g/L in 1969 to 5-10 g/L in the 1990s (Khater, 2019). Well fields in coastal southern Kenya show a gradual increase from less than 400 S/Cm in the 1970s to 1100 S/cm in 2017 (Oiro and Comte, 2019). In Djibouti a similar increase in the salinity of coastal aquifers has been observed (Houssein and Jalludin, 1996).…”