Characterization of Atmospheric Bulk Deposition: Implications on the Quality of Rainwater Harvesting Systems in the Semi-Arid City of Mekelle, Northern Ethiopia

It is estimated that about 1.2 billion people, mostly in arid and semi-arid regions of the Middle East and Africa, live in regions where water is physically scarce. Recently, rainwater harvesting and more effective water storage are suggested as important means for investment to cope against water scarcity and as an adaption to climate change. Cognizant of this fact, contribution of atmospheric deposition contaminant to rainwater harvesting systems, in the semi-arid city of Mekelle was investigated at four locations (industrial, commercial, traffic and residential areas). A total of 24 bulk samples were collected during the rainy season from May to September 2014. Physicochemical water quality analysis was performed following standard protocols. The results revealed that alkaline pH (6.95–10.29) was recorded in all the locations, with a mean pH of 8.11. The major ions analysis indicated the dominance of bicarbonates and carbonate types and to some extent sulfate and chloride type mixtures justifying the source of alkaline pH. Mean concentrations of ions follow the order of SO 4 2− > Ca 2+ > Mg 2+ > CI − > NO 3 − > Na + > NH 4 + > K. Correlation between SO 4 2− and NO 3 − ( r = 0.436), and Mg 2+ and K + ( r = 0.707) suggested their anthropogenic and crustal origin, respectively, while the correlation between NH 4 + with Ca 2+ and Mg 2+ ascribed their role in alkalization of the rainwater. Moreover, principal component analysis showed spatiotemporal variation of atmospheric bulk components in the study area. Thus, this primary investigation demonstrated that atmospheric bulk deposition might pose public health risk due to poor rainwater quality in Mekelle city. Therefore, development of rainwater harvesting systems for semi-arid urban centers should be supplemented with low-cost water treatment technologies.