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Shallow groundwater resources, especially in hard rock environment, constitute an important part of urban water supply in developing countries, appropriate to the low level of economic development. However, increasing urban population and dependence on shallow groundwater systems make it imperative to evaluate the availability and the contamination of these resources, and define new strategies of water exploitation taking into accounts these findings and constrains. This study has been carried out on the shallow groundwaters of Yaounde, central Africa. Based on head slug-in tests, chemical and isotope analyses, we demonstrate the importance of geomorphological and geological settings that constrain hydrogeology, urban occupation and therefore, water exploitation and contamination. Slug test results show spatial variability of well recovery rates with higher values recorded in the valleys compare to the hills, presenting saturated hydraulic conductivity of 10-6-10-8 m/s. Groundwater evolves from recharge zone as Ca-HCO3 in the hillside lateritic system to discharge zone in the slope/valley colluvium/alluvium system as NaK-NO3. The groundwater composition dominated by silicates/water interaction in the hillside lateritic system, and anthropogenic processes in the slopes and valleys. δ15N and δ18O of nitrates indicates that nitrate pollution of groundwater is mainly from sewage and human waste. Shallow groundwater in the hillside/new urban district and to a lesser extent slopes should therefore be protected and prioritised for usability and sustainability of the resources while ensuring the abstraction of the deeper part of the shallow aquifer in the valley/central districts due to the presence of denitrification. The proposed conceptual scheme for Yaounde can then be used as a guide in the development, exploitation and management of local wells in hard rocks system of Africa.

The Bamenda Highlands contains numerous dilute and soda springs of unknown hydrological provenance. These dilute springs are a vital source of drinking water to the inhabitants. As a contribution to water management, this study investigated the spring water recharge, residence time and sub-surface circulation using δ18O, δD, 3H, CFCs, SF6 and TDS. A plot of both water types along the local meteoric water line indicated the meteoric origin and rapid recharge following precipitation. Dilute springs with a δ18O altitude effect of 0.27/100 m suggested recharge at different elevations. The CFC-12, CFC-11 and CFC-113 dating (complemented by 3H) showed reasonable young apparent ages of dilute springs ranging from 21 to > 52 (mean/median of 25) years and old soda springs (> 52 years). SF6 concentrations in dilute springs revealed exceptionally young ages relative to CFCs modelled ages suggesting a terrigenous enrichment of the former. Thus, groundwater dating with SF6 is unreliable in the area. Besides the old apparent ages of soda springs, their highly depleted δ18O and high TDS (mean of 1396 mg/l) indicated palaeo-recharge and high water–rock interaction, respectively. In contrast, the low TDS of dilute springs (

Hydrogen (D) and oxygen (18O) stable isotopes in precipitation are useful tools in groundwater recharge and climatological investigations. This study investigated the isotopes in rainfall during the 2013 and 2014 hydrological years in the Douala and Yaounde urban cities. The objectives were to generate local meteoric water lines (LMWLs), define the spatial–temporal variations of the isotopes in rainwater and their relationship to the regional precipitation cycle, and determine the factors controlling the isotopic variation. The LWMLs in Douala and Yaounde were δD = 7.92δ18O + 12.99 and δD = 8.35δ18O + 15.29, respectively. The slopes indicate isotopic equilibrium conditions during rain formation and negligible evaporation effect during rainfall. Precipitation showed similar wide ranges in δ18O values from −5.26 to −0.75 ‰ in Douala and −5.8 to +1.81 ‰ in Yaounde suggesting a common moisture source from the Atlantic Ocean. Enriched weighted mean δ18O (wδ18O) values during the low pre- and post-monsoon showers coincided with low convective activity across the entire region. Enriched isotopic signatures also marked the West African monsoon transition phase during each hydrological year. Abrupt wδ18O depletion after the transition coincided with the monsoon onset in the region. Peak periods of monsoonal rainfall, associated with high convective activities, were characterised by the most depleted wδ18O values. Controls on isotopic variations are the amount effect and moisture recycling. The stable isotope data provide a tool for groundwater recharge studies while the isotopic correlation with regional rainfall cycle demonstrate their use as markers of moisture circulation and detecting climatic changes in precipitation.

WOS:000438892600002

WOS:000432663000002; International audience

最高標高が海抜4090m のカメルーン山(口絵 写真 A)は,カメルーン西部に位置し(図 1), キリマンジャロ(5895m)に次ぐアフリカ第二の 高峰である。また,山体南西地域は世界で最も降 水量の多い地域の一つとしても有名である。大規 模な山体における豊富な降水,また,成層火山特 有の地形が相まって,山麓には山体を取り囲むよ うに大規模な湧水群が発達し,地域住民の重要な 水源として利用されている(口絵写真 B)。加え て,山体南西の裾野はギニア湾に大きく張り出す 地形的特徴を有し,大規模な海岸湧水や海底湧水 が観察できる。このように稀有な自然環境にあ り,かつ,学術的に興味深い特徴を有しているに もかかわらず,カメルーン山周辺の湧水群につい ての記述は国内においてほとんど存在しない。 筆者は現地が乾季に相当する2010年 1 月25日か ら 2月 7日にかけてと,雨季に相当する同年10月 15日から30日にかけての 2度にわたり,当時熊本 大学大学院博士後期課程学生であったカメルーン 人国費留学生 Ako Andrew Ako 氏(2008年10月か ら2011年09月末まで熊本大学水文学研究室に在 籍)の研究調査に同行するため現地入りした。 調査では水質,同位体トレーサー(δO,δD, δS),年代トレーサー(CFCs,トリチウム)分 析用試料を採水し,また,これらをラボに持ち 帰った後,データ解析により水質特徴(Ako et al., 2012),流動機構(Ako et al., 2013),水質進化 機構(Ako et al., 2014)を明らかにしてきた。一 方,国際誌に掲載した学術論文では調査地域の自 然の素晴らしさを含め,現場状況やアクセス方法 が十分に紹介できていなかったため,これを紹介 できる機会があれば是非原稿を執筆しようと前向 きに考えていたところであった。 名水を訪ねては,日本国内の湧水や地下水を対 象とし,歴史や名水たる所以と絡めてその水質ま でを紹介する場合が一般的であったが,最近では 諸外国の名水についても紹介されるようになって きた。カメルーン山周辺の湧水群は水源として未 訪 問 記

High water demand for domestic use in Douala with over 3 million inhabitants is met mainly by shallow groundwater. Field measurements and water sampling in January 2015 were carried out to examine the major controls on the groundwater composition and spatial view of ions in the water, timing of recharge and link between the recharge process and quality of the water. Fifty-two water samples were analysed for major ions and stable hydrogen and oxygen isotopes. Low pH values (3.61–6.92) in the groundwater indicated an acidic aquifer; thus, prone to acidification. The dominant water type was Na–Cl. Nitrate, which exceeded the WHO guide value of 50 mg/l in 22% of the groundwater, poses a health problem. Mass ratios of Cl−/Br− in the water ranged from 54 to 3249 and scattered mostly along the mixing lines between dilute waters, septic-tank effluent and domestic sewage. A majority of the samples, especially the high NO3 − shallow wells, clustered around the septic-tank effluent-end-member indicating high contamination by seepage from pit latrines; hence, vulnerable to pollution. Stable isotopes in the groundwater indicated its meteoric origin and rapid infiltration after rainfall. The δ18O values showed narrow ranges and overlaps in rivers, springs, open wells and boreholes. These observations depict hydraulic connectivity, good water mixing and a homogeneous aquifer system mainly receiving local direct uniform areal recharge from rainfall. The rapid and diffused recharge favours the leaching of effluent from the pit toilets into the aquifer; hence, the high NO3 − and Cl− in shallow wells. Silicate weathering, ion exchange and leaching of waste from pit toilets are the dominant controls on the groundwater chemistry. Drilling of deep boreholes is highly recommended for good-quality water supply. However, due the hydraulic connection to the shallow aquifer, geochemical modelling of future effects of such an exploitation of the deeper aquifer should support groundwater management and be ahead of the field actions.