Hydrogeology of Malawi

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Africa Groundwater Atlas >> Hydrogeology by country >> Hydrogeology of Malawi

Authors

Dr Kirsty Upton, Brighid Ó Dochartaigh, British Geological Survey, UK


Geographical Setting

Map of Malawi (For more information on the datasets used in the map see the geography resources section)

General

Malawi includes four main physiographic zones: highlands or uplands; plateau areas; the rift valley escarpment; and the rift valley floor/plains.

The uplands cover a relatively small area but reach 2000 m to 3000 m in elevation. The plateau areas cover much of the country, and are gently undulating surfaces from 750 m to 1300 m in altitude. They are cut into by wide valleys, or 'dambos', with grassy, swampy bases, which largely drain towards the rift valley floor. The rift valley escarpment consists of parallel faults with the land stepping down to the rift valley floor. The rift valley floor and plains extend along the shores of Lake Malawi and the upper Shire Valley; they have low relief and range from 100 m to 600 m in altitude.

Estimated Population in 2013* 16,400,000
Rural Population (% of total) (2013)* 84%
Total Surface Area* 94,280 sq km
Agricultural Land (% of total area) (2012)* 60.8%
Capital City Lilongwe
Region Eastern Africa
Border Countries Tanzania, Mozambique, Zambia
Annual Freshwater Withdrawal (2013)* 1,357 Million cubic metres
Annual Freshwater Withdrawal for Agriculture (2013)* 85.9%
Annual Freshwater Withdrawal for Domestic Use (2013)* 10.6%
Annual Freshwater Withdrawal for Industry (2013)* 3.5%
Rural Population with Access to Improved Water Source (2012)* 83.2%
Urban Population with Access to Improved Water Source (2012)* 94.6%

* Source: World Bank


Climate

Malawi has two main seasons: a wet season from November to April and a dry season from May to October. Rainfall is influenced by topography, with the highest rainfall in upland areas and on slopes explosed to the prevailing northeasterly wet season winds. Annual rainfall exceeds 800 mm over most of the country. Lake Malawi and the upper Shire Valley receive relatively low rainfall. There are large interannual variations in rainfall.

Rainfall time-series and graphs of monthly average rainfall and temperature for each individual climate zone can be found on the Malawi Climate Page.


Average monthly precipitation for Malawi showing minimum and maximum (light blue), 25th and 75th percentile (blue), and median (dark blue) rainfall Average monthly temperature for Malawi showing minimum and maximum (orange), 25th and 75th percentile (red), and median (black) temperature Quarterly precipitation over the period 1950-2012 Monthly precipitation (blue) over the period 2000-2012 compared with the long term monthly average (red)

For further detail on the climate datasets used see the climate resources section.

Surface water

Lake Malawi is the largest water body in Malawi, and is the dominant control on the surface water drainage network in the country. The main rivers flowing into the lake are the Linthipe, Bua, Dwanga and Rukuru. These flow from wide 'dambos' that drain the plateau areas, which have steep valley sides tht shallow as they reach the rift valley.

Lake Malawi is drained entirely by the Shire River, which is a tributary of the Zambezi River. The upper Shire Valley has a wide alluvial plain, changing to a narrower valley with gorges and rapids in the lower part.

Lake Chilwa forms an internal drainage basin, mainly draining the northern uplands. Rivers flowing into the Chilwa basin tend to be ephemeral in their lower courses, losing water to permeable valley alluvial deposits.

Surface Water Map of Malawi (For more information on the datasets used in the map see the surface water resources section)

Soil

Soil compositions tend to be closely related to the underlying geology. Sandy soils occur on many granitic areas of the basement complex, and at the edges of alluvial plains. Latosols, including ferrosols, are common. Many soils are lateritic, typically 1 to 3 m thick (Smith-Carington and Chilton 1983), and sometimes 5 to 20 m thick (BGS 2004). Other soils include lithosols on steep slopes of the uplands and rift valley escarpment; hydromorphic (water logged) soils, including vertisols, in dambos; and calcimorphic soils on alluvial plains (Smith-Carington and Chilton 1983).

Soil Map of Malawi (For map key and more information on the datasets used in the map see the soil resources section)

Land cover

Land Cover Map of Malawi (For map key and more information on the datasets used in the map see the land cover resources section)


Geology

This section provides a summary of the geology of Malawi. More detail can be found in the references listed at the bottom of this page. Many of these references can be accessed through the Africa Groundwater Literature Archive.

The geology map on this page shows a simplified version of the geology at a national scale (see the Geology resources page for more details).

Malawi Geology2.png

Summary

Most of the country is underlain by Precambrian to Lower Palaeozoic crystalline basement rocks. These are unconformably overlain in places, particularly in the north and south, by more recent sedimentary rocks or volcanic rocks. There is extensive Quaternary alluvium in valley bottoms and the rift floor plain.

The main structural feature is the rift valley, which was formed by a subsidence fault during the Upper Mesozoic and Cenozoic, and is still active. Lake Malawi and the Shire Valley lie on the floor of the rift valley (UN 1989).


Geological Environments
Key Formations Period Lithology Structure
Unconsolidated
Alluvial and lacustrine deposits Quaternary Clays, silts, sands and occasionally gravels, deposited in the floor of the rift valley, and along the lakes in the major valley floors (UN 1989). The lithology of the deposits is highly variable. They occur particularly in several basins along the rift valley shore, including at Karonga and Salima-Nkhotajota lakeshores and the in upper and lower Shire Valley (Chavula 2012). They vary in thickness along Lake Malawi, tending to increase closer to the lake shore to a maximum of 60 m. In the centre of the Shire Valley, alluvium is 40 to 80 m thick, and reaches 150 m thick in the lower Shire Valley (UN 1989). The alluvial deposits are faulted, caused by ongoing subsidence of the rift valley.
Cretaceous to Quaternary sedimentary rocks
Sungwa, Chiwondo, Chitembe, Dinosauric and Lupata Cretaceous to Quaternary These sedimentary rocks are found in small, narrow, elongated basins in the north of the country, running parallel to the shores of Lake Malawi. They include loosely consolidated sandstones and unconsolidated sands, sandy marls, clays and conglomerates, but are dominanted by aeolian desert sandstones, with abundant evaporite deposits in a limestone matrix (UN 1989).
Igneous
Intrusive plutonic rocks Jurassic These occur in the Chilwa region in the south, and comprise granitic and syenitic plutonic rocks.
Stormberg volcanic rocks Jurassic A series of basaltic lava flows, interbedded with layers of sandstone and tuff (UN 1989).
Karoo
Karoo sedimentary series Permian-Triassic Karoo sedimentary rocks crop out in the north of Malawi, in north-south trending basins, and also to the southwest of the Shire Valley. They lie unconformably on crystalline basement. The base of the sedimentary sequence comprises conglomerates, sandstones, argillaceous rocks and coal seams; at the top of the sequence are arkosic sandstones, marls and more argillaceous rocks. The rocks are largely well cemented by calcite. They can be over 3500 m thick (UN 1989).
Basement Complex
Crystalline Basement Precambrian to Lower Palaezoic Dominantly gneiss and granulite; also some metamorphic schists, quartzites and marbles (UN 1989). The Mafinga and Mchinjin groups in the north and east comprise mainly unmetamorphosed sandstones and conglomgerates. Across the whole basement complex are outcrops of intrusive igneous rocks, such as the Nyika and Dzanalyama granites, syenites, and pegmatite and dolerite dykes (UN 1989).

Hydrogeology

This section provides a summary of the hydrogeology of the main aquifers in Malawi. More information is available in the references listed at the bottom of this page. Many of these references can be accessed through the Africa Groundwater Literature Archive.


The hydrogeology map on this page shows a simplified version of the type and productivity of the main aquifers at a national scale (see the Hydrogeology Map resource page for more details).

Malawi Hydrogeology2.png

Summary

The two most important aquifer systems in Malawi are (Chavula 2012):

- unconsolidated alluvium in the major valleys and lake shore plains, which forms a high yielding aquifer in many areas; and
- crystalline basement rocks, which cover much of the country, and form a low yielding aquifer.

There are only small occurences of other rock types, and although these may be important locally, little is known of their hydrogeological properties.


Unconsolidated

Named Aquifers General Description Water quantity issues Water quality issues Recharge
Alluvial and lacustrine deposits Alluvial and lacustrine deposits can form highly productive aquifers where they are better sorted and dominated by coarse grained sediment - sand and, sometimes, gravel - and where they are thick enough.

The aquifer tends to be unconfined. Groundwater can be in hydraulic continuity with adjacent rivers or lakes. Water table depth is typically between 5 m and 10 m below the ground surface (Chavula 2012).

Studies show that the average borehole yield from alluvial aquifers in Malawi is 15 l/s (litres/second); hydraulic conductivity ranges from 1 to 10 m/d; transmissivity ranges from 50 to 300 m²/day; and storage coefficient ranges from 1 x 10-2 to 5 x 10-2, and average borehole depth is 60 m (Chavula 2012).

Groundwater in alluvial aquifers is typically more mineralised than in basement aquifers, in some cases with salinity levels that are too high for potable water, such as in the lower Shire Valley and the eastern part of the Bwanje Valley (Chavula 2012). Chloride concentrations of up to 4000 mg/l and sodium concentrations up to 3600 mg/l have been recorded (Bath 1980). Salinity appears to be linked to the dissolution of evaporate minerals and/or evaporative concentration (BGS 2004). High fluoride concentrations of 2 to 20 mg/l have been found in some parts of the alluvial aquifers (Bath 1980). High iron concentrations of up to 80 mg/l and more are common (BGS 2004). Recharge occurs from direct rainfall infiltration and, where the aquifers are in hydraulic contact with rivers or lake water, by losses from surface water. Studies estimate recharge to the eastern side of the lower Shire Valley alluvial aquifer at more than 200 mm/year (Chavula 2012).

Sedimentary - Intergranular Flow

Named Aquifers General Description Water quantity issues Water quality issues Recharge
Cretaceous to Quaternary sedimentary rocks These rocks are likely to have relatively high porosity and intergranular permeability. They may form moderate to highly productive local aquifers.

Igneous Volcanic

Named Aquifers General Description Water quantity issues Water quality issues Recharge
Stormberg volcanic rocks The most porous and permeable parts are in the weathered zones of contact between lava flows. These may form moderate productivity local aquifers.

Igneous Intrusive

Named Aquifers General Description Water quantity issues Water quality issues Recharge
Plutonic intrusions of granite and syenites in the Chilwa province Likely to have very low groundwater potential.

Sedimentary - Fracture Flow

Named Aquifers General Description Water quantity issues Water quality issues Recharge
Karoo sedimentary sequence The rocks of the Karoo are generally well-cemented with low porosity and intergranular permeability. Groundwater storage and flow occurs largely in fractures in the rocks. Groundwater levels are typically 20 m to 30 m below ground surface (UN 1989). These rocks may form a low to moderate productivity local aquifer.

Basement

Named Aquifers General Description Water quantity issues Water quality issues Recharge
The basement complex rocks have virtually no primary (intergranular) permeability, but can form an aquifer where they have been sufficiently weathered in-situ, to form a layer of unconsolidated saprolitic weathered material. This weathered zone is best developed in the plateau areas, where it is often 15 m to 30 m thick, and locally even thicker (Chavula 2012). The best development of weathered saprolite tends to be associated with fractures, often along fracture zones.

The aquifer tends to be unconfined. The depth to water table is typically between 15 m and 25 m below the ground surface (Chavula 2012).

Studies have shown the average borehole yield is 1 to 2 l/s; hydraulic conductivity ranges from 0.5 to 1.5 m/d; transmissivity ranges from 5 to 35 m²/day; and storage coefficient ranges from 1 x 10-2 to 5 x 10-2. Boreholes tend to be 45 to 50 m deep (Chavula 2012).

Groundwater is typically of good quality, with slightly acidic to neutral pH (~6.4 to 7.0). Total dissolved solids (TDS) are generally less than 1000 mg/l, and typically around 350 mg/l (Chavula 2012), but occasionally elevated salinity is seen, sometimes linked to high sulphate concentrations, possibly linked to the dissolution of evaporate minerals (BGS 2004). Fluoride concentrations are typically relatively low, less than 1 mg/l (BGS 2004).

Groundwater Status

Groundwater quantity

Groundwater quality

The quality of groundwater in Malawi is highly dependent on aquifer lithology, and is highly variable spatially. Nationally, groundwater is generally suitable for drinking water (Chavula 2012). Groundwater in alluvial aquifers is genearlly more mineralised than that in basement aquifers, and a number of boreholes in alluvial aquifers have been abandoned due to high salinity (Chavula 2012). Chemical parameters that are elevated in different areas include fluoride, sulphate, iron, chloride and nitrate. Fluoride concentrations of more than 1.5 mg/l are common in groundwater in alluvial aquifers in the Salima/Nkhotakota and Karonga lakeshore areas (Chavula 2012).

Generally, groundwater from boreholes is of better microbiological quality than groundwater from dug wells, which tend to be more vulnerable to contamination (Chavula 2012).

An overview of in groundwater quality is given in Bath 1980, BGS 2004, and Water Department/UNDP (1986).

Groundwater use and management

Groundwater use

Groundwater is widely used across Malawi for both domestic and agricultural purposes. There were about 30,000 water boreholes and 8,000 protected hand dug wells in 2012 (Chavula 2012). Drought and poor water quality have meant more people are turning from hand dug wells to drilled boreholes. Estimates show that 65% of the population depends on groundwater for domestic supply: in rural areas, this rises to 82%, while in urban areas is it closer to 20%, although a number of towns or districts within towns get most of their water supply from groundwater (Chavula 2012).

Groundwater is increasingly used for irrigation by smallholder farmers, often from hand dug wells, and increasingly using treadle pumps, particularly in dambo areas. There is also a large scale irrigation scheme in the Salima district, where groundwater from an alluvial aquifer is extensively used for horticultural crops and agro processing. Some farmers use groundwater for livestock watering (Chavula 2012).

Boreholes fitted with hand pumps are standard for rural groundwater supplies. The use of motorised pumps is widespread, particularly by farmers; and treadle pumps are becoming more widely used.


Groundwater management

The Ministry of Irrigation and Water Development has responsibility for management of groundwater resources. Within the Ministry, the Departments of Water Resources (Groundwater Division) and Water Supply are responsible for groundwater development policy. The Groundwater Division is responsible for groundwater management, including management of groundwater data and issuing abstraction rights, while the responsibility for operating and maintaining boreholes lies with the Water Supply department (Chavula 2012).

The practical development of groundwater resources is dominantly carried out by NGOs, the donor community and the private sector, in collaboration with the Ministry of Irrigation and Water Development (Chavula 2012).

Under the Water Resources Act of 1969, all individuals have the right to use public water for domestic purposes, without the need for water rights, but that all water abstractions must be licensed, with annual permits required for abstractions greater than 1,000 l/day. The exception is for general household municipal use (Chavula 2012). Both the Water Resources Act (1969) and the Irrigation Act (2001) provide for the formation of water user associations (WUAs) or irrigation management authorities, to promote community or farmer participation in the development of water resources (Chavula 2012).


Groundwater monitoring

Transboundary aquifers

For further information about transboundary aquifers, please see the Transboundary aquifers resources page


References

The following references provide more information on the geology and hydrogeology of Malawi. These, and others, can be accessed through the Africa Groundwater Literature Archive.


Geology: key references

Smith-Carington AK and Chilton PJ. 1983. Groundwater resources of Malawi. Department of Lands, Valuation and Water; Republic of Malawi, Lilongwe.

Hydrogeology: key references

Bath AH. 1980. Hydrochemistry in groundwater development: report on an advisory visit to Malawi. British Geological Survey Report WD/OS/80/20.

BGS. 2004. Groundwater Quality: Malawi. British Geological Survey/WaterAid. http://www.wateraid.org/~/media/Publications/groundwater-quality-information-malawi.pdf

Chavula GMS. 2012. Malawi, in Groundwater Availability and Use in Sub-Saharan Africa: a review of fifteen countries. Pavelic P et al. (Eds). International Water Management Institute, Sri Lanka.

Smith-Carington AK and Chilton PJ. 1983. Groundwater resources of Malawi. Department of Lands, Valuation and Water; Republic of Malawi, Lilongwe.

UN. 1989. Malawi: Ground water in Eastern, Central and Southern Africa. Natural Resources/Water Series No. 19, ST/TCD/6. United Nations Department of Technical Cooperation for Development.

Water Department/UNDP. 1986. National Water Resources Master Plan, Malawi Government.

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