Tracing natural groundwater recharge to the Thiaroye aquifer of Dakar, Senegal
Tracing natural groundwater recharge to the Thiaroye aquifer of Dakar, Senegal
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DOI：
10.1007/s10040-018-01923-8
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发表时间：
2019-05-01
影响因子：
2.8
通讯作者：
Taylor, Richard G.
中科院分区：
文献类型：
--
作者：
Faye, Seynabou C.;Diongue, M. L.;Taylor, Richard G.
撒哈拉以南非洲的城市地下水为快速发展的城市提供了重要的淡水。在达喀尔（塞内加尔）的Thiaroye含水层，第四纪松散砂层中的地下水为1980年代的城市供水提供了近一半。由于粪便污染导致硝酸盐浓度上升，地下水的抽取量急剧减少，目前仅占达喀尔供水量的5%。为了了解这个城市含水层的衰减能力，在季风半干旱气候下，稳定同位素比的O和H和放射性氚（H-3），编译了几项研究，一起使用测压数据来追踪地下水补给和地下水流路径的起源。浅层地下水主要来自现代降雨（85%的取样威尔斯井中氚含量>2 TU）。地下水中O-18和H-2值的变化分别大于4和20，反映了补给前蒸发富集的巨大变化。这些签名在地下水回归到当地的大气降水线上的值，相对于当地降雨的加权平均组成，表明补给优先来自同位素耗尽的降雨在季风后期（9月）的一个偏见，是在重同位素耗尽的线。地下水中氚的分布是一致的地下水流向季节性湖泊和湿地，定义的测压记录。测压数据进一步证实了雨水补给的扩散性质和季节性。对地下水补给和流动的概念性理解提供了一个背景，以评估从排入这一含水层的庞大卫生设施网络中有效扩散和持续的人为补给的衰减。
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 (H-3), 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). O-18 and H-2 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.