Groundwater flow system analysis in the regolith of Dodowa on the Accra Plains, Ghana
Groundwater flow system analysis in the regolith of Dodowa on the Accra Plains, Ghana
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DOI：
10.1016/j.ejrh.2020.100663
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发表时间：
2020-04-01
影响因子：
4.7
通讯作者：
Minkah, Obed
中科院分区：
文献类型：
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作者：
Foppen, Jan Willem;Lutterodt, George;Minkah, Obed
研究区域：阿克拉平原。研究重点：我们进行了实地地质测绘，井库存演习，使用ERT，在8个位置（15-60米深）钻孔，钻孔岩心样品，我们进行了稀酸浸出实验，安装压力计，并配备了压力传感器，在高频地下水水文分析潮汐信号，进行抽水试验，最后，采取了49个地下水样品。新见解：我们的研究结果表明了一个多样化的地下水系统。一方面，在多哥构造单元（大津）的腐岩浅层发现了地下水，该腐岩处于未风化状态，由千枚岩、片岩和石英岩组成。潮汐分析显示，该系统较浅，且主要是无限制的。此外，大津腐泥土的透射率（均在<6 e-5 m2/s的量级）随深度而降低，这表明缺乏层状破碎层或存在超出钻探深度的此类层。另一方面，在达荷美构造单元（DSU）的片麻岩裂缝中发现了地下水。该系统可能更深，但DSU透射率显着低于那些大津腐泥土。水化学方面，地下水的主要特点是废水的渗透，证明了离子浓度升高，包括硝酸盐。由于基岩风化较薄，该地区地下水系统的开发是局部的，仅限于Dodowa地区。
Study Region: Accra Plains. Study Focus: We conducted a field geology mapping, a well inventory exercise, used ERT, drilled boreholes at 8 locations (15-60 m depth), took drill core samples which we subjected to dilute acid leaching experiments, installed piezometers and equipped them with pressure transducers, analyzed tidal signals in high frequency groundwater hydrographs, carried out pumping tests, and, finally, took 49 groundwater samples. New Insights: Our results indicated a diverse groundwater system. On the one hand, groundwater was found at shallow depths in the saprolite of the Togo Structural Unit (TSU), which, in unweathered state, is composed of phyllites, schists, and quartzites. This system was shallow and predominantly unconfined, as revealed by tidal analysis. In addition, transmissivities of TSU saprolite, all in the order of < 6e-5 m2/s, reduced with depth, which indicated either the lack of a stratiform fractured layer or the presence of such layer beyond drilled depths. On the other hand, groundwater was found in fractures of the gneisses of the Dahomeyan Structural Unit (DSU). This system was potentially deeper, but DSU transmissivities were significantly lower than those of TSU saprolite. Hydrochemically, groundwater was mainly characterized by infiltration of wastewater, evidenced by elevated ion concentrations, including nitrate. Due to the thinly weathered basement, groundwater system development in the area is local and restricted to the Dodowa area.