Values of δD and δ¹⁸O and concentrations of ³H and ¹⁴C of deep groundwater in Djibouti

Abstract

Djibouti is located at the northeast end of the African Continent. Rivers are not formed normally in this country due to the extremely low amount of rainfall (c.a. 150 mm annually) and rain falls irregularly in a short time. Consequently, deep groundwater in the volcanic aquifer has been thought to be the only available resource of fresh water in most of the region of the country. As it is difficult for the habitants to grow crops due to the extremely arid condition, nomads occupy over 30% of the national population. The nomads are strongly dependent on fresh water from groundwater and some advanced farmers have started to use groundwater for growing cash crops in rural area of this country. Although some research have been conducted to find out the depth, quality and age of groundwater, the data is not still enough to construct a scheme of agricultural development using groundwater in larger area of Djibouti. Then in this study, the purposes are firstly to estimate the original recharged area of deep groundwater of this country, using stable isotope analysis (δD, δ¹⁸O), and secondary to calculate the age of groundwater, using radioactive isotope analysis (³H, ¹⁴C). We have collected groundwater samples from 16 wells in 4 area (Djibouti City, Arta, Ali-Sabieh, and Dikihil) in 2019. This report shows the results obtained so far. The results on EC indicated higher than 0.8 mScm-1 for all the samples. According to the clustering analysis based on the ionic constructions, the samples collected from 2 areas (Djibouti City and Arta area) were classified into the Na-Cl type and the other two (Ari-Sabieh area and Dikihil area) into the Na-HCO₃ type. The results on δD and δ¹⁸O showed a possibility that there are multiple recharge areas of the groundwater. The concentration of 3H on the 6 samples was very low (under 0.3 T. U.), indicating that the age of the groundwater samples was older than 70 years.