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标题：Hydrogeochemical and multi-tracer investigations of arsenic-affected aquifers in semi-arid West Africa
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作者：Anja Bretzler ; Lucien Stolze ; Julien Nikiema 等
期刊名称：Geoscience Frontiers
印刷版ISSN：1674-9871
出版年度：2019
卷号：10
期号：5
页码：1685-1699
DOI：10.1016/j.gsf.2018.06.004
语种：English
出版社：Elsevier
摘要：AbstractThe semi-arid Sahel regions of West Africa rely heavily on groundwater from shallow to moderately deep (<100 m b.g.l.) crystalline bedrock aquifers for drinking water production. Groundwater quality may be affected by high geogenic arsenic (As) concentrations (>10 μg/L) stemming from the oxidation of sulphide minerals (pyrite, arsenopyrite) in mineralised zones. These aquifers are still little investigated, especially concerning groundwater residence times and the influence of the annual monsoon season on groundwater chemistry. To gain insights on the temporal aspects of As contamination, we have used isotope tracers (noble gases,3H, stable water isotopes (2H,18O)) and performed hydrochemical analyses on groundwater abstracted from tube wells and dug wells in a small study area in southwestern Burkina Faso. Results revealed a great variability in groundwater properties (e.g. redox conditions, As concentrations, water level, residence time) over spatial scales of only a few hundred metres, characteristic of the highly heterogeneous fractured underground. Elevated As levels are found in oxic groundwater of circum-neutral pH and show little relation with any of the measured parameters. Arsenic concentrations are relatively stable over the course of the year, with little effect seen by the monsoon. Groundwater residence time does not seem to have an influence on As concentrations, as elevated As can be found both in groundwater with short (<50 a) and long (>103 a) residence times as indicated by3He/4He ratios spanning three orders of magnitude. These results support the hypothesis that the proximity to mineralised zones is the most crucial factor controlling As concentrations in the observed redox/pH conditions. The existence of very old water portions with residence times >103years already at depths of <50 m b.g.l. is a new finding for the shallow fractured bedrock aquifers of Burkina Faso, suggesting that overexploitation of these relatively low-yielding aquifers may be an issue in the future.Graphical abstractDisplay OmittedHighlights•Fractured bedrock aquifers provide drinking water for millions in West Africa.•Highly variable groundwater chemistry and residence time over small spatial scales.•Groundwater As concentrations (As(V)) controlled by proximity to sulphide minerals.•Low3He/4He points to groundwater with ages >103years already at <50 m depth.•Geogenic As contamination occurs both in waters with short and long residence times.
关键词：KeywordsenArsenicGroundwater chemistryWest AfricaFractured aquifersResidence timeNoble gases