Investigation hydrogéologique au nord du Togo

Symposium AIEA

1. Methods Conclusion International Symposium on Isotope Hydrology: Sustainable Water Resources in a Changing World Vienna, Austria, 3-7 July 2023 ISOTOPE INVESTIGATION ON GROUNDWATER RECHARGE AND DYNAMICS IN AQUIFERS OF SAVANNAH REGION IN THE NORTHERN TOGO A. BLEZA 1 , G. BOGUIDO 1 , R. BARRY 2 , M. GNAZOU 1 , R. TRABELSI 2 , K. ZOUARI 2 , L. ARAGUAS 3 1 Laboratory of Applied Hydrology and Environment (LHAE), Faculty of Science; University of Lomé, Togo 2 Laboratory of Radio-Analysis and Environment, National School of Engineering of Sfax, ENIS, Tunisia 3 Isotope Hydrology Section, International Atomic Energy Agency, Vienna, Austria . Introduction Results The Savannah region (Fig. 1) is shared between two major geological units: a crystalline basement in the north and a hardened sedimentary basin in the south . In this area, groundwater constitutes the main perennial water resource for domestic and agricultural purposes. However, the increase in water demand associated with population growth, economic development, and the effects of climate variability can lead to a decline in the resource's availability. This study aims at determining the origin of groundwater resources and assessing their sustainability for effective and efficient management of water resources . Fig.1 : Location of the study area Four campaigns were organized in the framework of the RAF7019 project, with approximately 95 boreholes sampled (Fig.2) Major elements were analyzed using the AFNOR methods at the LHAE (University of Lomé), while 18O/16O and 2H/1H  → the Laser spectrometry at LRAE (Sfax), Tritium → electrolytic enrichment and liquid scintillation spectrometry . at LRAE (Sfax), 14 C → RMS method ; 13 C → mass spectrometer at Groningen. Fig. 2: Location of sampling points Fig. 5: Piezometric map of the study area Fig.3 : Geological map showing the cross section The geology (Fig. 3) of the Savannah region consists of two main geologic domains: the Birrimian basement in the north and the Volta basin in the south. The Birimian basement is part of the West African shield and consists of plutonic and metamorphic rocks with ages ranging from the Archean to the Paleozoic inferior. The Volta Basin is comprised of the Bombouaka and Oti supergroups and is made up of sandstones, quartzite-sandstones, conglomerate, and silt. A weathered horizon and fractured layer are accounted for when it comes to the hydrogeological context ( Fig. 4). Fig 4 : Hydrogeological section of the study area 2. Isotopic investigation Fig 7 : Spatial distribution of tritium Fig. 6 : Fréquence des teneurs en tritium Fig 8 : Spatial Distribution of carbon The chemical results vary according to the granitic basement and the sedimentary basin of the Volta. The TDS values range from 96 mg/l to 528 mg/l in the granitic basement and 41 mg/l to 1425 mg/l in the sedimentary basin of the Volta, with average values of 236 mg/l and 493 mg/l, respectively. The groundwater in the basement area is less mineral-bearing compared to the water in the basin of Volta. The piezometric map shows a watershed at the edge of the boundary between the Birrimian basement and the Volta Basin. However, the groundwater flow is on both sides of this line and is essentially converging to the Oti River , which is the main drainage axis of the waters, and this is in accordance with the topography of the area (Fig.5). The values of tritium in surface waters range from 3.0 to 3.6 UT, for an average of 3.3 UT. However, groundwater values vary between the detection limit and 3.8 UT. The overall average value is 1.5 ± 0.9 UT for waters in the crystalline basement zone and 0.5 ± 0.6 UT for waters in the Volta Basin . Figure 6 shows the different frequencies of tritium contents according to the two geological sets. It shows that for tritium contents below 1 UT, 76% of these waters are located in the Volta Basin and 24% in the basement zone (Fig. 7). This indicates that the oldest waters are located in the Volta Basin. Tritium contents show that there are two major recharge periods: the pre-nuclear period with 3H values < 1 UT and the post-nuclear period with 3H values > 1 UT . Dating using 14C shows that groundwater has an age from the current to 16800 years BP with the ancient waters residing in the Volta Basin (Fig. 8).   1. Hydrogeology and chemical investigation Groundwater in the Savannah region is low- to medium-mineralized, with the highest salinities found in the Volta Basin. The tracing of water using the radioactive isotopes of the water molecule and the TIDC helped identify the ages and main recharge episodes of the aquifer system studied. Tritium activities have highlighted two major recharge periods. A post-nuclear recharge that has fed the aquifer system in the north, corresponding to the crystalline basement zone, by the rains of recent decades, and a pre-nuclear recharge in the south, corresponding to the Volta basin, materialized by low tritium contents. Carbon-14 groundwater dating confirms the two major recharge periods: the current recharge in the Birrimian basement zone and an ancient recharge that dates from the wet periods of the Holocene and the late Pleistocene. ID: 404