Abstract
Urban groundwater in Sub-Saharan Africa provides vital freshwater to rapidly growing cities. In the Thiaroye aquifer of Dakar (Senegal), groundwater within Quaternary unconsolidated sands provided nearly half of the city’s water supply into the 1980s. Rising nitrate concentrations traced to faecal contamination sharply curtailed groundwater withdrawals, which now contribute just 5% to Dakar’s water supply. To understand the attenuation capacity of this urban aquifer under a monsoonal semi-arid climate, stable-isotope ratios of O and H and radioactive tritium (3H), compiled over several studies, are used together with piezometric data to trace the origin of groundwater recharge and groundwater flowpaths. Shallow groundwaters derive predominantly from modern rainfall (tritium >2 TU in 85% of sampled wells). δ18O and δ2H values in groundwater vary by >4 and 20‰, respectively, reflecting substantial variability in evaporative enrichment prior to recharge. These signatures in groundwater regress to a value on the local meteoric water line that is depleted in heavy isotopes relative to the weighted-mean average composition of local rainfall, a bias that suggests recharge derives preferentially from isotopically depleted rainfall observed during the latter part of the monsoon (September). The distribution of tritium in groundwater is consistent with groundwater flowpaths to seasonal lakes and wetlands, defined by piezometric records. Piezometric data further confirm the diffuse nature and seasonality of rain-fed recharge. The conceptual understanding of groundwater recharge and flow provides a context to evaluate attenuation of anthropogenic recharge that is effectively diffuse and constant from the vast network of sanitation facilities that drain to this aquifer.
Highlights
Over the last half century, rates of urban population growth in Sub-Saharan Africa have been the highest among the world’s regions (UNDESA 2017)
During the 2008 monsoon, 14 monthly values of the stable isotope ratios of O and H in rainfall derive from sampling twice at eight locations in the Thiaroye area of Dakar (Table 1). These data regress (R2 = 0.99) along a local meteoric waterline (LMWL), δ2H = 7.4 · δ18O + 5.6, that reflects the impacts of evaporative enrichment relative to a previously computed LMWL by Travi et al (1987), δ2H = 7.9 · δ18O + 10 (R2 = 0.97), based on a set of seven monthly samples collected from rainfall stations across
A combination of environmental isotopic (18O, 2H, and 3H) tracers shows that shallow groundwater of the unconfined Thiaroye aquifer in the rapidly growing conurbation of Dakar (Senegal) derives primarily from modern rainfall following the bomb pulse of atmospheric tritium that began in 1963
Summary
Over the last half century, rates of urban population growth in Sub-Saharan Africa have been the highest among the world’s regions (UNDESA 2017). The urban population of Sub-Saharan Africa was estimated to be 294 million; by 2030, this is projected to grow to 621 million. Such rapid urbanisation presents serious challenges to the provision of universal access to safe water and sanitation by 2030 outlined in United Nations’ Sustainable Development Goal (UN SDG) 6. A recent overview of urban water-supply sources in 10 African cities (Foster et al 2018) reveals that urban groundwater represents a substantial, strategic freshwater resource to meet rising demand under accelerating rates of urbanisation and reduced river-intake due to pollution and climate change. As recognised by Adelana et al (2008), there is a critical need to manage groundwater storage as a strategic reserve, used conjunctively with surface-water sources, to improve security of urban water supplies.
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
