Hydrogeological Investigation of the Upper and Middle Borkena River Catchment, Northern Ethiopia, Wollo
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Date
2001-06
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Addis Ababa University
Abstract
The geology, hydrology, hydrogeology and hydrochemistry of the Borkena catchment which is
located on the western edge of the Afar rift was studied based on conventional geologic and
hyd rogeological mapping, river di scharge measurements, water sample anal ysis and assessment
and interpretation of hydrometeorological data. The main objective of this work is to
understand the different lithologic unit and to characterize and group them into
hydrostratigraphic unit and map them, to analyze the water balance and to study the water
quality.
Four hydrostratigraphic units were recognized and mapped. These are: coll uvial and river
channel deposits, alluvial deposits, scoraceous and basaltic lava flow and fractured massive
basalt, welded tuff and rhyolite. Generally the volcanic rocks lie with in the recharge zone
wllile the Quaternary sediments lie in the discharge zone. Accordingly the hydrostratigraphic
unit located on the Quaternary sediments are characterized by many springs and hand dug well
where as low or rare in the volcanic regions.
From storage, utilization and abstraction point of view, the colluvial and river channel deposits
are the most promising units than other units as a result most of the springs, hand dug wells
and boreholes are located in this hydrostratigraphic units.
Generally seven rock units and three set of fau lts are identified and mapped at the scale of
I :50,000. Among the rock units, scoraceous and basaltic lava flow and alluvial deposits cover
more than 70 percent of the catchment.Usually the exposure of the volcanic unit is limited to the topographically elevated and steep
slope regions of the catchment where as the Quaternary sediments out crop with in the floor of
the graben and gentle slope areas. The oldest rock unit (Rhyolite) corresponds to the AJaji
Formation (28-32Ma) while basalts are correlated to the Tarmaber Formation (26-28Ma).
Structurally the N-S trending faults are more common than the E-W ones.
The analysis of precipitation shows that the area experience 1028 mm of mean annual rainfall
with 84 percents obtained during the rainy seasons. Potential evapotranspiration was calculated
by modified Penman method and is 1339 mm per annum while actual evapotranspiration was
computed using Thornthwaite and Mather Soil-Water Balance Model, (1955) and is found to
be 860 mm per annum.
Computation of water balance was done in two steps: one for each subcatchment and the other
for the entire catchment. Accordingly the entire catchment receives 50.6 mcm (million cubic
meter) of recharge per annum while the Dessie and Kombolcha Subcatchments receive 18 .2
mcm and 32.4 mcm of recharge per annum respectively.
Hydrochemical analysis of water samples indicate that the river water is enriched with respect
to both cations and anions than the groundwater bodies. The plotting of ionic concentration of
water samples was done through Aquachem software. From the plot it is found that the river
water is a sodium-calcium bicarbonate type water where as the groundwater (springs, hand
dug well and borehole) is calcium-sodium bicarbonate type water.From hydrochemical analysis, measurement of TDS, EC and other variables, the water bodies
are characterized by low total dissolved solids (TDS) and low electri cal conduct ivity (EC)
values as a result they are classified as fresh water and their utilization for various purposes
scarcely bring danger.
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Hydrogeological Investigation