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

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 (3 H), 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). δ 18 O and δ 2 H 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.

Groundwater recharge from heavy rainfall in the southwestern Lake Chad Basin: evidence from isotopic observations

“…S7). Similar deductions of a bias in groundwater recharge to months of heavy rainfall in tropical drylands have been reached from stable isotope tracers in Senegal (Faye et al 2019) (Fig. 2) and southern Africa (Jasechko 2019) (Fig.…”

Global Isotope Hydrogeology―Review

“…Groundwater isotope compositions have been applied to evaluate processes and identify recharge sources in many tropical aquifers (Adomako et al, , ; Ayenew et al, ; De Vries et al, ; Fantong et al, , ; Faye et al, ; Gibrilla et al, ; Joshi et al, ; Kamtchueng et al, ; Mukherjee et al, ; Njitchoua et al, , ; Olaka et al, ; Sánchez‐Murillo et al, ; Shamsuddin et al, ; Taylor & Howard, ; Yeh & Lee, ). Evidence is mounting in support of the following hypothesis: that high‐intensity rainfall contributes disproportionately to recharge—that is, recharge ratios of intensive rainfall exceed recharge ratios of less intensive rainfall.…”

Tryptophan-like and humic-like fluorophores are extracellular in groundwater: implications as real-time faecal indicators

“…The four study areas have contrasting hydrogeological settings. The unconfined shallow Thiaroye aquifer of Dakar comprises Quaternary fine-and medium-grained sands with a shallow water table less than 2 m below ground level (bgl) [32][33][34] . The heterogeneous volcano-sedimentary Kisumu aquifer system is a suite of Archaean age metasediments, Tertiary volcanics, Quaternary sediments, colluvium and lateritic soils [35][36][37] .…”