Groundwater Vulnerability mapping of Mekelle city and surroundings, Tigray region, Ethiopia

Mekelle, the regional capital of Tigray, faces difficulties managing and monitoring the quality of its water resources. The identified geological formations in the area are fractured limestone, fractured limestone shale-marl intercalation, and fractured dolerites. The hydrogeological setting of the Mekelle area is strongly controlled by geology and associated tectonic and non-tectonic geological structures. In comparison to other locations of Tigray, Mekelle and its environs have a higher concentration of various governmental and nongovernmental industries. Agricultural and mining activities, landfill sites, fuel and gas stations, health centers, car wash stations, garment and textile factories, and mega industries, are among the identified possible sources of water resource contamination in the area. Most of the water wells are found either within or around the agricultural lands and industrial zones. Chemicals and wastewater discharged from those industries, as affluent, are the sources of anthropogenic water resources pollution. The impacts and extent of pollution sources are not thoroughly investigated in the area. In this context, water resource vulnerability mapping was performed using the DRASTIC method and hydrogeological settings evaluation to identify the most vulnerable areas to anthropogenic pollution. The potential sources of groundwater pollution in the area were inventoried and mapped. The final DRASTIC vulnerability map with indexes ranging from 54 to 140 is classified into four classes: low (54 –75.5), moderate (75.5 – 97), high (97 – 118.5), and very high (118.5 – 140) vulnerability indexes. A modified DRASTIC vulnerability method was developed, that includes lineament density and land use thematic layers. The modified DRASTIC vulnerability map with indexes ranging from 91 to 192 is also similarly classified into four classes: low (91 – 116), moderate (116 -141), high (141 – 166), and very high (166 – 192). Single parameter sensitivity analysis and map removal sensitivity analysis were performed. The results from single parameter analysis show the impact of the vadose zone, depth to the water table and aquifer media have higher effective weightings (mean effective weightings = 23.7%, 22.9%, and 16.9%, respectively). In addition, the map removal sensitivity analysis confirms high variation of the vulnerability index is expected from the removal of impact of the vadose zone (1.5 %) and depth to the water table (1.4 %), respectively. Nitrate, used to validate both models, indicates a positive, moderate correlation (with Pearson’s correlation coefficient, r, of 0.681 and 0.702) for the DRASTIC and modified DRASTIC indexes, respectively. A comparison of both maps shows that the significant pollution sites are located in areas with high to very high vulnerability. The results of this research will directly aid water resources protection, pollution studies, and monitoring tasks of the Mekelle area