Hydrogeological System Analysis in Ziway–Shala Lakes Area Using Hydrochemistry and Isotope Techniques, Central Ethiopia
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Date
2006-06
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Addis Ababa Universty
Abstract
In this work, hydrochemical and isotopic study has been carried out in order to
conceptualize the groundwater circulation and to evaluate the subsurface hydraulic
connection between the lakes in the Ziway-Shala lakes basin, located 190km south of
Addis Ababa in the central sector of the Main Ethiopian Rift and covers a large portion
of the lakes district in Ethiopia. Lacustrine sediments and volcano-clasitic succession (in
the floor), volcanics (on the highlands and escarpment) are the main geological units in
the area. The area is highly affected by rift structures and recent tectonic features.
The hydraulic characteristics of the aquifers in the area show high spatial variability,
which is the result of the complex nature of the lacustrine sediments and the degree of
fracturing of the volcanic rocks. The most permeable unit is the basalts and local
ignimbrites with hydraulic conductivity grater than 40m/day, where as the least are
those rift volcanoes and volcanic ridges having less than 1m/day. The others volcanic
rocks lie in between depending on the degree of weathering, fracturing or faulting.
The hydrochemical and isotopic analysis of δ2H, δ18O & 3H result signifies that
groundwater of the floor, escarpment and highland is found in different flow regime.
75% of the total groundwater samples collected in the area has concentration range of
sodium (the dominant cation) and range of bicarbonate (the dominant anion) between
60-300 mg/l and between 200-600 mg/l respectively. These high ionic variabilities show
the presence of different groundwater flow systems.
Groundwater in the recharge area evolves from a dilute Na -Ca– HCO3 type water
(statistically determined Subgroup-1: average TDS is ~332 mg/l ) to a fresh Na–HCO3
type water (Subgroup-2: average TDS is ~475 mg/l) to a more concentrated Na–HCO3
type water (Subgroup-3: average TDS is ~1184 mg/l) to a brackish Na–HCO3-Cl type
water (Subgroup-4: average TDS is ~6538 mg/l) along the topographic flowpath
Overall, the waters from the area can be classified as recharge area waters (Subgroup-1),
transition zone waters (Subgroup-2) and discharge area waters (Subgroup-3 and
Subgroup-4).Hydrochemical and Isotope concentrations of groundwater and surface water samples
between the lakes help to study the subsurface interconnection between the lakes.18O
and 2H signatures in relation with the hydrochemistry of waters from the highlands,
escarpments and rift valley helps to conceptualize the groundwater flow system in the
area.
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Keywords
Hydrochemical and Isotopic