Draft:Groundwater in Mozambique
Groundwater is the main domestic water source for the rural population in Mozambique, but to date is used only sparsely for irrigation, mostly on small scales for subsistence farming. Large scale irrigation schemes all rely on surface water. A lack of hydrogeological information is a factor restricting groundwater development.
The United Nations Food and Agriculture Organisation (FAO) estimates that Mozambique has 17,000 million m3/year of renewable groundwater resources .
Mozambique can be divided into three major hydrogeological provinces (DNA 1987). These are:
- Sedimentary rock basins;
- Volcanic (and other igneous) terrains; and
- Basement Complex.
A fourth province is unconsolidated aquifers, which are sometimes in hydraulic continuity with underlying bedrock aquifers.
The sedimentary basins comprise mainly Cretaceous-Tertiary rocks, with small outcrops of older Mesozoic-Palaeozoic rocks mainly of Karoo-type. The most significant sedimentary basin aquifer is the Mozambique Sedimentary Basin to the south of the Save River, in southern Mozambique. Other, smaller sedimentary basins are the Mozambique Sedimentary basin to the north of the Save River; the Northern/Rovuma Sedimentary Basin, with which has a narrow linear outcrop in the northeast of the country; the Middle Zambeze Sedimentary Basin, in the centre-west of the country; and the Maniamba Sedimentary Basin, which has a very small outcrop in the northwest of the country. The sedimentary rocks form variably local/discontinuous to regional/continuous aquifers, usually with a mixture of intergranular (porous) and fracture/fissure permeability.
Large areas are overlain by unconsolidated sediments, particularly in valleys, dune fields and the coastal plain. These form variably local/discontinuous to regional/continuous aquifers. The unconsolidated aquifers are described separately in the table below, but where they overlie bedrock aquifers, they can be in hydraulic continuity with the underlying aquifer. This is particularly the case where they overlie consolidated sedimentary rock aquifers, but can also occur over basement and igneous aquifers.
Reliable information on natural aquifer recharge in Mozambique is very scarce. Only a few studies have been conducted, in a few small areas. Recharge to sandy soils around Maputo was studied by IWACO in 1985 and was found to be somewhere between 140 and 185 mm per year, which is around 20% of the total precipitation. In other areas with sandy soils and higher precipitation, recharge was found to be higher, for example, 210 and 350 mm per year, or around 30% of annual rainfall, near Pemba. In drier areas with less permeable soils, such as the semi-arid Chitima region near Tete, recharge values of less than 10 mm per year were estimated (IWACO 1985, DNA 1987).
There are many potential problems linked to the use of groundwater in Mozambique. Little information exists on the present status of groundwater quantity, and there is no quantitative information on groundwater use and recharge. Before groundwater can be used on a large scale for irrigation or other uses, extensive research is needed (DNA 1987; British Geological Survey 2002).
Groundwater has a significant role to play in the supply of drinking water, but is currently only used sparsely for irrigation due to the following factors:
- Mozambique still has enough areas that can be developed for irrigation using surface water resources.
- There is limited to no information on the potential of aquifers and yields of individual boreholes.
- Large areas in southern Mozambique are deemed unsuitable for groundwater abstraction due to salinity issues. The exact extent of these areas is undetermined.
It seems that the lack of information (which means there is an unknown risk of drilling low yielding boreholes and/or incurring high drilling costs) is a larger constraining factor than the actual potential of the groundwater. As well as this, there is a general consensus that surface water is more cost-effective for irrigation than groundwater. The potential for, in particular, large surface water irrigation schemes is still present in Mozambique, and thus the interest for groundwater is low. Furthermore, the following can be concluded:
- There is little knowledge of groundwater use around non-perennial rivers
- Groundwater for irrigation is mainly used for subsistence farming, except for certain labour intensive areas around Maputo.
- Legislation of groundwater abstraction is in its infancy in Mozambique. Capacity is very limited presently. Technical assistance and capacity training in this area is essential.
- Groundwater is probably underutilised at present, as potential users do not have knowledge of or access to the groundwater potential in their area
Little information is available on the quality of groundwater in the aquifers of Mozambique. The available information suggests that the groundwater is for the larger part fresh and of acceptable quality, though often of limited quantity, especially in the aquifers of the Basement Complex and the Volcanic Terrains.
Significant salinity problems are experienced in some parts of the Tertiary aquifers in the south, as a result of natural seawater intrusion, forming areas with brackish groundwater. In large parts of Gaza, Maputo and Inhambane Province, groundwater in the main aquifer (from 20 – 80 m depth) has Electrical Conductivity values well above the WHO drinking water standards of 2000 µS/cm. These values also make the groundwater unsuitable for many crops and for livestock watering. In some particular instances (e.g. at Chokwe and Chibugo), extra deep boreholes (100-150 m) have been drilled to reach fresh water in aquifers. Such boreholes are, however, too expensive to consider for irrigation (particularly as proper sealing of the upper saline aquifer needs to be achieved).
There is risk of pollution in the vicinity of industrial and urban developments, including from sewage effluent, from centres of petroleum and chemical manufacture, and from ports, as well as from agricultural activity. Pollution incidence is likely to be greatest in the coastal lowlands (DNA 1987, British Geological Survey 2002).
Localized groundwater pollution is known to be occurring in the Chokwe (Gaza Province) area, where extensive irrigation has taken place (using surface water) since the 1930s. This has raised the local groundwater table and has led to increased levels of salinity. An estimated 2,000 hectares (of the 30,000 irrigated) has already been lost for normal crop production (Interview with Eng. Rui Brito UEM, Faculty of Agronomy and Forestry).
Pollution by heavy metals is rarely measured or recorded. A recent GAS meeting indicated that after a relative extensive groundwater quality study led by UNICEF in the centre of Mozambique, only in some areas (around Gorongosa mountain) was mercury (Hg) found at levels above WHO standards (Minutes of meeting GAS, October 2009).
- FAO, 2016. AQUASTAT Main Database. Food and agriculture Organisation of the United Nations. http://www.fao.org/nr/water/aquastat/data/query/index.html?lang=en
- IWACO. 1985. Study of groundwater to supply Maputo.
- Ferro and Bouman / DNA. 1987. Explanatory Notes to the Hydrogeological Map of Mozambique: 1:1,000,000.
- DNA. 1999. Water resources of Mozambique.
- British Geological Survey (BGS). 2002. Groundwater Quality: Mozambique.