,CENTRAL ETHIOPIA

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,CENTRAL ETHIOPIA
HYDROGEOLOGICAL SYSTEM ANALYSIS
IN ZIWAY SHALA LAKES AREA USING
HYDROCHEMISTRY AND ISOTOPE TECHNIQUES
Shemelis Fikre Addis Ababa University,Department
of Earth Sciences POBOX 1176, Addis Ababa,
Ethiopia
Are these lakes connected?
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Topography of Ziway-Shala lakes area
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Objectives

• Conceptualize the groundwater flow system in the
  area
• Identify the subsurface hydraulic connections of
  the lakes in the area
• Investigate the correspondence between spatial
  locations and statistical groups.
• Assess surface water- groundwater interactions in
  the lake watershed system.
• Conceptualize the role of geological structures
  on the groundwater movement

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Methodology

• Sampling and laboratory analysis
• Hydrochemical techniques
• Physico chemical analysis
• Statistical cluster analysis (HCA) and
  distributions of the clusters
• Isotope techniques
• 18O, 2H and 3H are analysed using plots for
  samples from the different water bodies
• Both hydrochemical and isotope techniques
  together
• Scatter plots for the values of 18O versus EC,
  18O versus chloride, tritium versus EC and
  tritium versus chloride are prepared and
  interpreted
• Spatial variation of isotopes in relation with
  hydrochemistry using maps

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Structural map of Ziway- Shala lakes area
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Hydrogeology

• 1 Ignimbrite, tuff, local rhyolite
• 2 Ignimbrite, tuff, local basalt
• 3 Ignimbrite covered with lacustrine soils,
  recent regression
• 4 Ignimbrite covered with lacustrine deposit
• 5 Rift volcanoes and volcanic ridges
• 6 Basalt, local ignimbrite
• 7 Lake
• 8 Volcano-tectonic structures
• 9 Drainage
• 10Groundwater level contour
• 11Cold spring
• 12Hot spring
• 13Groundwater flow direction, Circles represent
  the Hydraulic conductivity.

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The locations of sampling sites
Hydrogeological System Analysis
Using Hydrochemistry Data

• Physico chemical analysis
• Major ion variability

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Major ion chemistry ..
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Water types and physiography
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• Electrical conductivity
• There are clear zonations in EC of natural waters
  following the rift ward directions

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Statistical analysis
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Statistical analysis

• Spatial distribution and chemical differences of
  the HCA derived subgroups for groundwaters

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Hydrochemical evolutions and groundwater flow
Groundwater-lake water interactions

• Starting from highland and escarpment waters and
  ending with rift floor waters 1?2?3?4. Total
  dissolved solids (TDS) concentration increases
  with increasing subgroup number

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Hydrogeological System Analysis
Using Isotope Data
Deuterium (2H) and oxygen(18O ) isotope
sampling sites for isotope analysis-
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Deuterium (2H) and oxygen(18O ) isotopes

• The LMWL is plotted above the GMWL this is due
  to the isotopic concentrations of precipitation
  in the study area has more deuterium excess (that
  is D excess2.35) than the global averaged
  precipitations.
• The majority of groundwaters, river waters and
  rain waters are plotted near the LMWL .This
  indicate the importance of present day
  precipitation for groundwater recharge.
• The lake Waters are plotted far to the right and
  shifted right down of the LMWL .This shows that
  the lakes are more enriched with 18O and 2H
  resulted from substantial evaporative loss of the
  lake waters as compared to the present day
  precipitation .
• Groundwaters (waters from hot spring, cold
  springs, cold wells, and geothermal wells) are
  scattered at different positions on the plot and
  have differences in 18O and 2H concentrations.

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The relations between 18O with Cl
Both hydrochemistry and isotopes together

• The relations between 18O with EC

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The relations between 18O with EC

• The cold spring waters have negative 18O
  constituents and low EC similar to the rivers(
  except Bulbula and Horakelo) indicates that they
  are rechrged by shallow circulating groundwaters
  which have undergone little rock- water
  interactions.Groundwater flows from lake Ziway to
  lake Langano
• The source of some hot springs is surface waters
  and shallow groundwaters.

The relations between 18O with Cl ...

• The lake water labeled c12 (lake Ziway) has
  similar chloride concentration with c9 (lake
  Langano) but the 18O enrichment in c9 is higher
  than c12.This is due to the high evaporation
  water loss from lake Langano. From this it is
  evidence to conclude that there is southward
  migration of lake Ziway waters towards lake
  Langano
• The geothermal water labeled f19 has similar 18O
  enrichment but higher Cl concentration than lake
  Ziway waters (labeled c12).Lake waters labeled
  c13 has also similar 18O enrichment and Cl
  concentrations with the geothermal water labeled
  f20. This shows that there is dilution of the
  geothermal water by the lake Ziway water. From
  this it is evidence to conclude that there is
  mixing of the lake waters with the geothermal
  waters.

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Tritium versus EC
Tritium versus Chloride

• The borehole waters near the lakes have similar
  EC and tritium values to the lake waters
  indicating the interaction of the lake waters
  with the surrounding groundwaters.
• Boreholes near the geothermal system has higher
  EC and lower tritium values may indicate mixing

• The lake Ziway water shows a decrease in tritium
  content and chloride concentration with time
  indicates the decrease in the amount of surface
  inflow in to the lake

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Spatial variation of isotopes in relation with
hydrochemistry
symbol label begins with a are cold spring,
with d are rain and e are river samples
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CONCLUSIONS

• The high spatial ionic variations follows
  systematic trend. This reflects the different
  groundwater flow systems and the existence of
  hydrochemical evolution of waters along the flow
  path
• On the highlands and escarpments there is shallow
  circulation of groundwaters from direct recharge
  of precipitation and these waters have undergone
  no marked rock-water interactions.
• The low EC TDS and isotope depleted waters in
  highly faulted rift waters which have similarity
  in EC, TDS and depletion with the highland and
  escarpment waters indicates the southward
  migration of highland and escarpment waters
  through faults and finally to lake Langano. The
  tectonic structures play a great role on the
  groundwater flow and chemical evolution
• There is deeper groundwater circulation of old
  age on the highly faulted areas

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CONCLUSIONS

• The lake waters, the majority of the groundwaters
  and surface waters have similar tritium contents.
  This shows that these waters have similar
  recharge source.
• Tritium contents of water from deep wells and hot
  springs are different from lake waters indicates
  they have different sources
• Groundwaters north of lake Langano have similar
  18O content with the lake Ziway waters may show
  that there is subsurface hydraulic connection
  between lake Ziway and lake Langano
• The chemical composition of borehole waters
  between lake Abiyata and Langano is similar for
  the nearby lake waters. This shows that there is
  flow of waters from lake Langano to lake Abiyata
  along the NE-SW trending fault

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Thank You