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International Institute for Geo-Information Science and Earth Observation (ITC) 2014

Hydrogeochemical and water quality investigation on irrigation and drinking water supplies in the Mekelle region, northern Ethiopia.

Abreha, A.G. (Asmelash Gebreyohanns)

Titre : Hydrogeochemical and water quality investigation on irrigation and drinking water supplies in the Mekelle region, northern Ethiopia.

Auteur : Abreha, A.G. (Asmelash Gebreyohanns)

Etablissement de soutenance : University of Twente International Institute for Geo-Information Science and Earth Observation (ITC)

Grade : Master of Science in Geo-Information Science and Earth Observation 2014

Résumé This study focused on investigating water use problems, related to water quality and hydrogeochemical processes ( evolutions, origins and mixing ) of water resources in Mekelle region, Northern Ethiopia (Tigray). Hydrogeochemical (PHREEQC) and Aquachem modeling approaches combined with stable water isotopes and geospatial data analysis were employed for this purpose. Both primary and secondary geochemical data were used to understand the general hydrochemistry, correlation of major ions with TDS and types of water in the region. A statistical approach of hierarchical clustering analysis was also applied to classify t he water resources in to distinct groups and sub- groups, which are used to calculate hydrogeochemical equilibrium speci ation , saturation indices of solid phases and inverse modeling among the sub- groups. Moreover, stable isotopes were used in the hydrogeochemical analysis. A water quality assessment was also carried out against WHO and Ethiopian standards for domestic use, irrigation and engineering works. The findings of the study show that the water resources in the study area are evolved from Ca- HCO 3 water types to Ca- SO 4 water types through Ca- HCO 3- SO 4 and Ca- SO 4- HCO 3 water types. But small numbers of observations with distinct sodium, chloride and nitrate signal were also identified possibly indicating contamination by urban and agriculture . The result of the study further revealed that outgassing and dissolving of carbon dioxide, loss and dilution of water, dissolution and precipitation of carbonate and evaporite of halite and sylvite, and dissolution of sulphate minerals and anthropogenic effects are the main hydrogeochemical processes observed during hydrogeochemical evolution of the water resources. It is also observed that the isotopes signature of deuterium and oxygen- 18 from water of boreholes, hand- dug wells , springs and rivers show depletion in deuterium and oxygen- 18 when compared to VSMOW due to effects of evaporation processes. Moreover, lower D- excess was observed when compared to local rainfall of KOBO station data and this is mainly due to secondary fr actionation effect and isotopic exchanges in the aquifer . The origin of the water resources in the study area is found to be primarily precipitation and there is mixing of ground and river waters . By large, the study revealed that a significant number of water resources observations don’t meet the WHO water quality standards for domestic uses including 38.5 % for TDS, 82.5 % for Total Hardness and 19 % for Nitrate. Moreover, 83% of the water resources data have corrosive character though 100 % not aggress ive. Generally, the water resources in the region are characterized with high salinity and low alkalinity controlled by geology, land- use, water- rock interaction, evolution and anthropogenic effects.

Mots clés : Water Quality, Hydrogeochemical processes , evolution, inverse modeling, isotopes fractionation, salinity, equilibrium speciation and saturation Indices

Version intégrale (ITC)

Page publiée le 5 avril 2018