Geophysical and geochemical characterisation of groundwater ...

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Geophysical and geochemical characterisation of groundwater ...

Geophysical and geochemical characterisation of

groundwater resources in Western Zambia

Mkhuzo

Chongo

Kawawa

Banda

Peter Bauer-Gottwein, DTU Environment

Flemming Larsen, Anders Vest Christiansen, GEUS/AU

Imasiku Nyambe, UNZA IWRM Centre


Mozambique

Zambia’s world

ranking

DRC

Tanzania

Botswana

Zim

• GDP per capita: 124 th of 196 countries

• GDP growth rate: 37/196

• Copper production: top 10

• Income inequality: 20/196

• Area: 39/196

• Population: 69/196

• Population growth rate: 11/196

• Water availability: 68/196


Groundwater in Zambia

• Groundwater is the backbone of rural water

supply in large parts of Zambia

• Widespread occurrence of saline groundwater

• How much fresh groundwater is available?

• What is the best way to manage the resource?

• A case study: Sesheke District


Spatial Variation: Groundwater EC (S/cm)


Airborne and Ground TEM Resistivity


West – East Profile Resistivity Section

West

North – South Profile Resistivity Section

East

South

North


Why saline groundwater in Sesheke?

Moore et al., 2007

Late Pleistocene (126 – 11.7 Ka (thousands of years)

Displacement along the major north-east-trending Linyanti and Chobe Faults

temporarily severs the link between the Upper and Middle Zambezi, and

diverts the flow of the Kafue and Zambezi into Lake Palaeo-Makgadikgadi,

which filled to the 945 m shoreline level.


Extent of the Palaeo-Lake at

various water levels


Area (km 2 )

Volume (km 3 )

Current level-area and levelvolume

relationships

70000

Level - Area

1400

Level - Volume

60000

y = 1,59x 3 - 4391,4x 2 + 4E+06x - 1E+09

1200

y = 0,7315x 2 - 1326,3x + 601171

50000

1000

40000

800

30000

600

20000

400

10000

200

0

910 920 930 940 950

Level (m)

0

910 920 930 940 950

Level (m)

Lake Palaeo-Makgadikgadi


Water Balance of Lake Palaeo-

Makgadikgadi

River Inflow Precipitation Evaporation River Outflow

Q I

AP AE

Q O

dV

dt

= Q + A × P -

I

A × E - Q ± Q

O

GW

Q GW

Current Precipitation: 500 mm/yr

Current Evaporation: 1500 mm/yr

River

Upper Zambezi 1200

Kafue 250

Kwando 50

Luangwa 500

Okavango 450

Current

Discharge

(m 3 /s) - Q s

2450

Zambezi + Kwando +

Okavango

Zambezi + Kwando +

Okavango + Kafue

Zambezi + Kwando +

Okavango + Kafue

+Luangwa

Equilibrium Surface

area (km 2 )

52000 938

60000 942

76000 950

Equilibrium water

level (mamsl)


Palaeo-lake shorelines

Thomas and Shaw,1991

• There has been a lake for the majority of the past 50 000 years

• The lake seems to have had an overflow at level 945 mamsl

• The lake was most likely endorheic for the past 10 000 years

and became highly saline

• Hypothesis: Saline lake sediments from Lake Palaeo-

Makgadikgadi are the cause of present saline water occurrence

in Machile


Working on the Lake Hypothesis...

• Phreeqc modeling of lake

water quality evolution

• Complications:

• Various mineral

phases and reactions

• The lake closed and

opened intermittently

depending on climate

and capture

Field data collection

Geophysical borehole logs

• Groundwater and

sediment sampling and

dating

• Sediment characterization

based on fossiles


Back to groundwater resources:

Working hypotheses

• We have a generally saline environment because of the

lake

• We have local freshwater pockets due to either surface

water – groundwater interaction or recharge from precip

• Using coupled hydrogeophysical inversion, the

geophysical data can tell us something about hydrology

• Phreatic Evapotranspiration is a key process

• Probably, we are dealing with a very vulnerable resource

which must be carefully managed. We need reliable

variable-density groundwater models for the relevant

areas.


Conceptual Model of surface water – groundwater

interaction: Fresh water from e.g. Zambezi infiltrates

into saline environment, driven by ET

Transpiration

Transpiration

Evaporation

Evaporation

Freshwater = 0.1kg/m 3

River

Channel

Saline Aquifer TDS = 35kg/m 3 (1 Ohm-m)


Conceptual Model of fresh groundwater

recharge from local precipitation

GW Recharge

Fresh Aquifer TDS = 0.5kg/m 3 (50 Ohm-m)

River

Channel

Saline Aquifer TDS = 35kg/m 3 (1 Ohm-m)


Conclusions

• Safe and sustainable water supply is a key

development issue in Zambia and elsewhere

• Groundwater is a highly valuable resource

• Groundwater management is complex and sciencebased

decision support is urgently needed

• This projects builds essential capacity for the

Zambian water sector and addresses some exciting

research questions:

– Origin and extent of saline groundwater in

Western Zambia

- Coupled hydrogeophysical modeling of GW

quality evolution

- Optimal management strategies for limited

freshwater resources in saline environments

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