Abstract
ABSTRACT We examine groundwater recharge processes and their relationship to rainfall intensity in the semi-arid, southwestern Lake Chad Basin of Nigeria using a newly compiled database of stable isotope data (δ2H, δ18O) from groundwater and rainfall. δ18O signatures in groundwater proximate to surface waters are enriched in 18O relative to regional rainfall and trace focused groundwater recharge from evaporated waters via ephemeral river discharge and Lake Chad; groundwater remote from river channels is comparatively depleted and associated with diffuse recharge, often via sand dunes. Stable isotope ratios of O and H (δ2H, δ18O) in groundwater samples regress to a value along the local meteoric waterline that is depleted relative to weighted mean composition of rainfall, consistent with rainfall exceeding the 60th percentile of monthly precipitation intensity. The observed bias in groundwater recharge to heavy monthly rainfall suggests that the intensification of tropical rainfall under global warming favours groundwater recharge in this basin.
Highlights
In tropical drylands where river flow is often episodic or seasonal, groundwater is commonly the only perennial source of freshwater sustaining vital ecosystems (Leblanc et al 2006) and freshwater withdrawals for agricultural, domestic and industrial uses (Taylor et al 2013a)
The δ18O values in groundwater, which are enriched in the heavy isotope relative to the weighted mean composition of rainfall in the SW Chad Basin (≥ −3.2‰; N = 44) with a deuterium excess of ≤ +3‰, are more commonly observed proximate to perennial and ephemeral surface drainage including Lake Chad. δ18O values between these two categories (−3.3 to −4.2‰; N = 26) show no clear bias in their geographical position within the SW Lake Chad Basin (LCB). δ18O values from wells in which groundwater was sampled below a depth of 30 m bgl are consistently depleted (≤-4.3‰) in 18O (Fig. 5)
Of note is that sampled groundwaters from the Quaternary Aquifer (QA) of Chad (IAEA 2017, fig. 32) and Niger (Leduc et al 2000) (Fig. 2) in the LCB regress to a rainfall composition on the LMWL that is depleted in heavy isotopes relative to the weighted mean composi tion of monthly rainfall at N’Djamena
Summary
In tropical drylands where river flow is often episodic or seasonal, groundwater is commonly the only perennial source of freshwater sustaining vital ecosystems (Leblanc et al 2006) and freshwater withdrawals for agricultural, domestic and industrial uses (Taylor et al 2013a). In the semi-arid region of the Sahel in West Africa, for example, groundwater with drawals are rising and projected to increase substantially over the few decades as nations expand irrigated agriculture and access to safe water in pursuit of the United Nations (UN)’s Sustainable Development Goals 2 and 6 (e.g. World Bank 2017, Commission Climat pour la Région du Sahel (CCRS) 2018). Wada et al 2012, Hanasaki et al 2018) is commonly restricted to the direct infiltration of precipitation (i.e. diffuse recharge), and neglects focused groundwater recharge that occurs via seepage from surface drainage that includes lake and rivers as well as ephemeral ponds and stream discharges The latter has, been shown to contribute substantially to groundwater replenishment in drylands (e.g. Scanlon et al 2006, Dahan et al 2008, Favreau et al 2009, Villeneuve et al 2015, Cuthbert et al 2019, Acworth et al 2021). Evidence from the few long-term piezometric observa tions that have been made in tropical semi-arid Africa suggests that groundwater recharge, whether focused or dif fuse, is strongly influenced by climate variability (e.g. Taylor et al 2013b, Cuthbert et al 2019, Kolusu et al 2019) and landuse change (e.g. Favreau et al 2009, Le Coz et al 2013, Ibrahim et al 2014)
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