The use of isotopes to assess the long term impact of the GERD on the groundwater level in agricultural areas in Sudan

Eng. Mohamed Fadlelseed, Dr. Nadir M Hassanein

(Version I, proposal under preparation April 2023)

?1.1 INTRODUCTION

?On April 2, 2011, Ethiopia began construction of the Grand Ethiopian Renaissance Dam (GERD) on the Blue Nile 475 km northwest of Addis Ababa and 15 km from the Sudan border. The Grand Ethiopian Renaissance Dam will be the largest dam in Ethiopia: its length is 1800 meters, its height is 155 meters, and its total volume is 74 cubic kilometers. The dam is located in a Precambrian bedrock-dominated area containing granite and metamorphic rocks. The Renaissance Dam was known as the border dam in the study of the American Bureau of Reclamation (1958-1964). The name was changed to "X-Project" in March 2011, and within two weeks it was renamed to the "Millennium Dam", and then in April 2011 the Cabinet renamed it for the fourth time to the Grand Ethiopian Renaissance Dam. There are some advantages of the Renaissance Dam for Sudan including managing the Blue Nile sediments, controlling floods, providing water flow throughout the year, and increasing the efficiency of electricity production from Sudanese dams (1).

Unlike surface water, groundwater is not visible to the eye, and therefore it is difficult to understand and predict in terms of volume and quality. However, obtaining quick information on new water resources such as groundwater is essential to ensure security of supply (2).

Nuclear technologies are used to make the invisible visible. Environmental isotope tracers provide fast and reliable answers to the age, quantity, and source of groundwater pollution. Isotope hydrology has been applied in several IAEA technical cooperation projects to relieve pressure on populations and help them identify alternative, sustainable sources of water. In the Sahel region of Africa, a series of droughts associated with an increase in water demand due to population growth has led to problems with the safety and availability of clean water. Although scarce on the surface, this semi-arid region has rich water bodies underground. The IAEA works closely with water authorities to help them understand unseen groundwater resources. This information is essential to support long-term access to clean water (2).

Groundwater is of great importance in our daily life, and its importance is due to its multiple uses, whether in agriculture, industry, or other uses. An increase in the groundwater level (GWL) in any area is of great benefit due to its importance and versatility, but in the city of Aswan, the matter is different, as the increase in the GWL causes severe damage to buildings and leads to poor quality of agricultural lands and destruction of infrastructure due to lack of proper management (3).

1.2 Materials and Methods

?To develop an integrated approach to assess the impact of the GERD on the groundwater level in agricultural projects in Sudan, two methods based on assessing the impact of the GERD on the groundwater level in the Gezira Agricultural Project in Sudan are proposed as follows:

?1.2.1 Stable and Unstable (Radioactive) Isotopes

?In 2017, a study was conducted with the aim of evaluating the impact of the GERD on groundwater in the Gezira project in Sudan, which is considered the most important economic and agricultural project in Sudan since 1925, where 31 groundwater samples were collected from pumping wells, and only two out of 31 samples were from the Gezira aquifer and the rest Samples from the Nubian Sandstone aquifer in the area (1), as shown in Figure 1 and Figure 2.

Figure 1: Piezometric water levels of the Nubian Sandstone aquifer and the Gezira aquifer

Figure 2: The location map of water samples in the Gezira area

Stable isotopes and some other chemical and physical factors were also analyzed in that study for 11 samples of groundwater samples from pumping wells and compared with the values of stable isotopes in samples from previous studies that were taken from groundwater in the same area and from the surface water of the Blue and White Nile rivers and rainwater as shown in Figure 3 and Figure 4. The study concluded from the stable isotope analysis that the closer the well is to the Blue Nile river, the higher the stable isotope value of oxygen and hydrogen and vice versa, which indicates the feeding of groundwater in the Gezira aquifer from the Blue Nile river as a main source of recharge and that samples No. 7, 8 and 20 contain lower values for? stable isotopes to hydrogen and oxygen due firstly to its distance from the Blue Nile River and secondly to its high chloride values, which indicates that it is more affected by the evaporation process. The study concluded that since the GERD will maintain a high level of the Blue Nile water throughout the year and since it is the main source of groundwater recharge in the Gezira project, this will increase the groundwater level in the Gezira project (1).

Figure 3: Data of δ2H and δ18O for rainfall and surface Water (Blue and White Niles)

Figure 4: Isotopic composition of the Nubian Sandstone Aquifer samples

Other studies were also conducted using the radioactive (unstable) isotope of hydrogen known as tritium, where the researchers were able to estimate the ages of recent groundwater (nearly 50 years ago) in a number of wells, and they were able to know the presence of modern recharge due to the high tritium values in those wells (4) and (5).

In this research, I analyzed the data of 11 wells for the study in 2013 and found that the closer the well is to the Blue Nile River, the higher the groundwater level, as well as the positive relationship between the stable isotopes of oxygen and hydrogen, as shown in Figure 5. Therefore, I suggest sampling Groundwater from the same 11 wells in the 2013 study and comparison of groundwater levels in 2013 and the present time in those wells. I also suggest doing new analyzes of the stable isotopes of hydrogen and oxygen in those wells and comparing them to the study in 2013 and finding out if the values of the stable isotopes increased for distant samples from the Blue Nile, which means an increase in recharge. I also propose an analysis of the "tritium" radioisotope of the same 11 wells to find out the presence of new feeding in the wells far from the Blue Nile, in addition to chloride to see the effect of the evaporation process. I also suggest taking samples of stable isotopes from the Blue Nile River itself to see if the stable isotope values of hydrogen and oxygen for wells far from the Blue Nile will be closer to the samples taken from the Blue Nile River or not, which indicates direct recharge from the Blue Nile River (1) as it is Shown in Figure 6 and Figure 7 for samples near the Blue Nile in a 2013 study.

Figure 5: The relationship between the value of oxygen and hydrogen isotopes, the distance of each sample from the Blue Nile River, and the depth to the water level in the well

Figure 6: The spatial distribution of piezometric levels in the Gezira and Nubian Sandstone Aquifers

Figure 7: Plot of δ18O vs δD of average rains in Khartoum, Blue and While Niles, and groundwater in Gezira, Sudan (GMWL is from Edmunds et al. [62])

1.2.2 Groundwater Model

To make a groundwater model in the Gezira Agricultural Project in Sudan, and to assess the impact of the GERD on the groundwater level in this agricultural areas, we must first know that the impact of the GERD on the groundwater level in the Gezira, which is mainly represented in recharge from the Blue Nile River, It will not be visible until after the Roseries Dam, because the geology of the area before the Roseries Dam is a basement rock and groundwater does not seep into it significantly (Figure 8). A study was conducted using the Modflow model to assess the impact of the Aswan Dam on the groundwater level on agricultural areas in Aswan, where the researchers first calibrated the model to give a level very close to the groundwater level in a number of selected wells in their study area, then posed two scenarios to find out the impact of the Aswan Dam on groundwater levels and assess its impact in the long term. The first scenario was to increase the recharge of the aquifer in Aswan, and the other scenario is to increase the water level in the Aswan Dam lake, which will provide an opportunity to know the impact of the dam on groundwater levels in the long term (3). Therefore, I suggest making a groundwater model and calibrating the model with groundwater level data for the same 11 wells in the 2013 study, and then imposing scenarios for increasing the rate of groundwater recharge in the Gezira project area, by the same rate of increasing the groundwater level in 11 wells from 2013 until 2022, for example, or Any of the past three years to assess the impact of the GERD on the groundwater level during the coming years in the Gezira project.

Figure 8: Geological map based on Schandelmeier & Reynolds (1997), GRAS

?1.3 Excepted outputs

?Good management of groundwater in the long term and a good understanding of the impact of the GERD on the groundwater level in agricultural projects in Sudan.

1.4; Requirements

?1.5 Timeline

Results can be generated on an annual basis to assess long term impact

?1.6 REFRENCES

?1. ????? Negm M, Abdel-Fattah S. Grand Ethiopian Renaissance Dam Versus Aswan High Dam: A View from Egypt [Internet]. Vol. 79, The Handbook of Environmental Chemistry. 2019. 1–579 p. Available from: https://www.springer.com/series/698

2. ????? Nuclear Techniques to Assess and Manage Groundwater in Critical Situations | IAEA [Internet]. [cited 2022 Dec 8]. Available from: https://www.iaea.org/newscenter/news/world-water-day-2022-making-invisible-visible-using-nuclear-techniques-to-assess-and-manage-groundwater-in-critical-situations

3. ????? Zelenakova M, Abd-elaty I. Investigation of Groundwater Logging for Possible Changes in Recharge Boundaries and Conditions in the City of Aswan , Egypt. 2022;

4. ????? R. Trabelsi ? KZ??. Use of stable and radio active isotopes in the determination of the recharge rate in Djeffara aquifer system southern Tunisia. 2012;(December). Available from: https://inis.iaea.org/collection/NCLCollectionStore/_Public/44/066/44066250.pdf

5. ????? Telloli C, Rizzo A, Salvi S, Pozzobon A, Marrocchino E, Vaccaro C. Characterization of groundwater recharge through tritium measurements. Adv Geosci. 2022;57(2015):21–36.