Hydrogeological System Analysis In Ziwayshala Lakes Area Using Hydrochemistry And Isotope Techniques Central Ethiopia

In this work, hydrochemical and isotopic study has been carried out in order tornconceptualize the groundwater circulation and to evaluate the subsurface hydraulicrnconnection between the lakes in the Ziway-Shala lakes basin, located 190km south ofrnAddis Ababa in the central sector of the Main Ethiopian Rift and covers a large portionrnof the lakes district in Ethiopia. Lacustrine sediments and volcano-clasitic succession (inrnthe floor), volcanics (on the highlands and escarpment) are the main geological units inrnthe area. The area is highly affected by rift structures and recent tectonic features.rnThe hydraulic characteristics of the aquifers in the area show high spatial variability,rnwhich is the result of the complex nature of the lacustrine sediments and the degree ofrnfracturing of the volcanic rocks. The most permeable unit is the basalts and localrnignimbrites with hydraulic conductivity grater than 40m/day, where as the least arernthose rift volcanoes and volcanic ridges having less than 1m/day. The others volcanicrnrocks lie in between depending on the degree of weathering, fracturing or faulting.rnThe hydrochemical and isotopic analysis of δ2H, δ18O & 3H result signifies thatrngroundwater of the floor, escarpment and highland is found in different flow regime.rn75% of the total groundwater samples collected in the area has concentration range ofrnsodium (the dominant cation) and range of bicarbonate (the dominant anion) betweenrn60-300 mg/l and between 200-600 mg/l respectively. These high ionic variabilities showrnthe presence of different groundwater flow systems.rnGroundwater in the recharge area evolves from a dilute Na -Ca– HCO3 type waterrn(statistically determined Subgroup-1: average TDS is ~332 mg/l ) to a fresh Na–HCO3rntype water (Subgroup-2: average TDS is ~475 mg/l) to a more concentrated Na–HCO3rntype water (Subgroup-3: average TDS is ~1184 mg/l) to a brackish Na–HCO3-Cl typernwater (Subgroup-4: average TDS is ~6538 mg/l) along the topographic flowpathrnOverall, the waters from the area can be classified as recharge area waters (Subgroup-1),rntransition zone waters (Subgroup-2) and discharge area waters (Subgroup-3 andrnSubgroup-4).Hydrochemical and Isotope concentrations of groundwater and surface water samplesrnbetween the lakes help to study the subsurface interconnection between the lakes.18Ornand 2H signatures in relation with the hydrochemistry of waters from the highlands,rnescarpments and rift valley helps to conceptualize the groundwater flow system in thernarea.

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