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Tracing natural groundwater recharge to the Thiaroye aquifer of Dakar, Senegal

Faye, SC; Diongue, ML; Pouye, A; Gaye, CB; Travi, Y; Wohnlich, S; Faye, S; (2019) Tracing natural groundwater recharge to the Thiaroye aquifer of Dakar, Senegal. Hydrogeology Journal 10.1007/s10040-018-01923-8. (In press). Green open access

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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.

Type: Article
Title: Tracing natural groundwater recharge to the Thiaroye aquifer of Dakar, Senegal
Open access status: An open access version is available from UCL Discovery
DOI: 10.1007/s10040-018-01923-8
Publisher version: https://doi.org/10.1007/s10040-018-01923-8
Language: English
Additional information: © The Author(s) 2019. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http:// creativecommons.org/licenses/by/4.0/).
Keywords: Urban groundwater, Groundwater recharge, Environmental isotopes, Semi-arid regions, Sub-Saharan Africa
UCL classification: UCL
UCL > Provost and Vice Provost Offices > UCL SLASH
UCL > Provost and Vice Provost Offices > UCL SLASH > Faculty of S&HS
UCL > Provost and Vice Provost Offices > UCL SLASH > Faculty of S&HS > Dept of Geography
URI: https://discovery.ucl.ac.uk/id/eprint/10069622
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