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

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-isotoperatiosofOandHandradioactivetritium(3H),compiledoverseveralstudies,areusedtogetherwithpiezometricdata to trace the origin of groundwater recharge and groundwater flowpaths. Shallow groundwaters derive predominantly from modern rainfall tritium>2TUin85%ofsampledwells).?18Oand?2Hvaluesingroundwatervaryby>4and20‰,respectively,reflecting substantial variability in evaporative enrichment prior to recharge. These signatures in groundwater regress to a value on the local meteoricwaterlinethatisdepletedinheavyisotopesrelativetotheweighted-meanaveragecompositionoflocalrainfall,abiasthat 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.

Auteur(s) : Seynabou C. Faye, M. L. Diongue, Abdoulaye Pouye, Cheikh B. Gaye, Yves Travi , Stefan Wohnlich, Serigne Faye & Richard G. Taylor
Année de publication : 2018
Revue : Hydrogeolgy Journal
N° de volume : Published online: 12 Fébruary 2019
Type : Article
Mise en ligne par : CISSE Seynabou