Difference between revisions of "Hydrogeology of Sierra Leone"

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||There is a near-surface weathered (regolith) layer that is often dominated by clay. Below this are ancient conosolidated (meta)sedimentary rocks, with very little intergranular porosity. Groundwater storage and flow occurs within fractures in the rock, which are often along old bedding plains, although there is little information on potential borehole yields (Lapworth et al. 2015).
 
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||There is typically a layer of highly weathered rock overlying the unweathered bedrock, which has often transformed to a thick tropical soil. This is generally up to 20 m thick, although up to 37 m thick has been seen. The upper section of this weathered zone often has relatively little clay - the clay minerals have often been leached out, leaving metal oxides. These metal oxides are often in the form of indurated or gravelly layers, which can be highly permeable, and can allow rapid horizontal groundwater flow. Towards the bottom of the weathered zone, the weathered rock is often dominated by clays, and therefore has much lower permeability.  
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||There is typically a layer of highly weathered rock (regolith) overlying the unweathered bedrock, which has often transformed to a thick tropical soil. This is generally up to 20 m thick, although up to 37 m thick has been seen. The upper section of this weathered zone often has relatively little clay - the clay minerals have often been leached out, leaving metal oxides. These metal oxides are often in the form of indurated or gravelly layers, which can be highly permeable, and can allow rapid horizontal groundwater flow. Towards the bottom of the weathered zone, the weathered rock is often dominated by clays, and therefore has much lower permeability. Yields from shallow boreholes abstracting from this zone are typically in the range 0.3 to 1.5 l/s. This shallow aquifer tends to dry up rapidly when the rains stop and groundwater drains rapidly away through the permeable material. It is vulnerable to contamination, because of limited attenuation potential in the subsurface and rapid horizontal and vertical groundwater flow pathways for seasonal rainfall recharge (Lapworth et al. 2015).
Yields from shallow boreholes abstracting from this zone are typically in the range 0.3 to 1.5 l/s. This shallow aquifer tends to dry up rapidly when the rains stop and groundwater drains rapidly away through the permeable material (Lapworth et al. 2015).
 
  
At the base of the weathered zone, the underlying crystalline bedrock is often extensively fractured and not clay rich, and can store and transmit groundwater through fractures. There can also be deeper fracture zones associated with faults. The average thickness of the fractured aquifer zone is 35 m, but it can be as much as  60 m. Borehole yields are typically between 0.3 and 1.5 l/s. Groundwater flows can be rapid over distances of tens of metres. This deeper, fractured aquifer zone is typically a more sustainable groundwater source than the upper weathered zone (Lapworth et al. 2015).
+
At the base of the weathered zone, the underlying crystalline bedrock is often extensively fractured and not clay rich, and can store and transmit groundwater through fractures. There can also be deeper fracture zones associated with faults. The average thickness of the fractured aquifer zone is 35 m, but it can be as much as  60 m. Borehole yields are typically between 0.3 and 1.5 l/s. Groundwater flowpaths are usually longer than in the shallow weathered aquifer, and groundwater flow can be rapid over distances of tens of metres. This deeper, fractured aquifer zone is typically a more sustainable groundwater source than the upper weathered zone. It also has more potential for the natural attenuation of contaminants, because of the overlying clay zone and the longer flowpaths (Lapworth et al. 2015).
 
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Revision as of 17:29, 13 October 2015

Africa Groundwater Atlas >> Hydrogeology by country >> Hydrogeology of Sierra Leone


Authors

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

Mustapha Thomas, Hydrenv Consulting, Sierra Leone

Geographical Setting

A coastal strip approximately 50 km in width extends over about 15% of the country. Inland are plains and plateaus. The lower plains, covering 43% of the country, rise from 40 m elevation in the west to 200 m elevation in the east. Swampy depressions in the west are known as bolilands. In the northeast and southeast, the plateaus range from 300 m to 700 m altitude, covering 22% of the country. Hills and mountains in the east reach a maximum elevation of nearly 2,000 m at Mount Bintumani in the Loma Mountains, while the hills formed by the Freetown Complex reach 800 m height around Sierra Leone’s capital (Lapworth et al. 2015).

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

General

Estimated Population in 2013* 6,092,075
Rural Population (% of total) (2013)* 60.8%
Total Surface Area* 72,180 sq km
Agricultural Land (% of total area) (2012)* 56.8%
Capital City Freetown
Region Western Africa
Border Countries Guinea, Liberia
Annual Freshwater Withdrawal (2013)* 212.2 Million cubic metres
Annual Freshwater Withdrawal for Agriculture (2013)* 21.5%
Annual Freshwater Withdrawal for Domestic Use (2013)* 52.3%
Annual Freshwater Withdrawal for Industry (2013)* 26.2%
Rural Population with Access to Improved Water Source (2012)* 42.4%
Urban Population with Access to Improved Water Source (2012)* 87.1%

* Source: World Bank


Climate

Sierra Leone has a humid tropical climate. Average annual rainfall is approximately twice the average annual potential evapotranspiration. Rainfall is highly seasonal, with a peak in August and a dry season from December to March. Inter-annual variation in rainfall is generally small, but there are some extreme rainfall events.

Temperatures are relatively uniform throughout the year, ranging from 24 to 28 degrees C. Lowest temperatures are from July to September, in the middle of the rainy season, and highest temperatures are in February and March, near the in end of the dry season (Lapworth et al. 2015).



Average monthly precipitation for Sierra Leone showing minimum and maximum (light blue), 25th and 75th percentile (blue), and median (dark blue) rainfall Average monthly temperature for Sierra Leone 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

Five main rivers flow from northeast to southwest across Sierra Leone: the Little Scarcies, Rokel, Jong, Sewa and Moa rivers. Between them draining most of the land surface. In addition, there are six smaller drainage basins: the Great Scarcies, Lokko, Rokel Estuary, Western, Robbi/Thauka and Sherbro Water Resources Areas. River runoff is highly seasonal, reflecting the seasonal distribution of rainfall. In the Rokel river, discharge increases from May, peaks in September and decreases to near-zero by March (Lapworth et al. 2015).


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

Soil

There is extensively weathered tropical soil, including distinctive duricrust development. The lowland area in the west of Sierra Leone is dominated by strongly weathered ferrasols with low nutrient levels. The upland area in the east has a partial cover of pisoplinthic plinthosols - soils with accumulations of iron that harden irreversibly when exposed to air and sunlight. Toward the coast these become yellow in colour. Elsewhere there are lithic leptosols - shallow soils over hard rock with bedrock close to the surface. In many cases, iron rich tropical soils contain openings and macropores which permit rapid infiltration and flow of water (Lapworth et al. 2015).

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

Land cover

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

Geology

The geology map shows a simplified version of the geology at a national scale. More information is available in the report UN (1988) (see References section, below).

Summary

Most of Sierra Leone is underlain by Precambrian cratonic rocks of the Archaean Basement Complex (Lapworth et al. 2015). A major belt of late Precambrian (Upper Proterozoic) to Lower Palaeozoic age metasedimentary rocks, with some (meta)volcanic rocks, occurs in the western part of the country (Camus and Cukor 2012, Lapworth et al. 2015). These are often capped by a weathered zone of unconsolidated material, where the Precambrian basement rocks have been weathered in-situ to sand, gravel and clay. They are also often capped by a layer of laterite or ferricrete.

There are outcrops of intrusive igneous rocks across the country, which are generally of granitic composition(Camus and Cukor 2012, Lapworth et al. 2015).

Across the country, in river valleys there are often unconsolidated alluvial deposits laid down by rivers. Along the coastal belt are extensive outcrops of coastal, marine and estuarine unconsolidated deposits.


Sierra Leone Geology.png


Geological Environments
Key Formations Period Description
Unconsolidated sediments
Bullem Group Tertiary to Quaternary Poorly consolidated marine and estuarine sediments, largely sands, gravels and kaolinitic clays with some lignite (Lapworth et al. 2015).
Igneous intrusive
Freetown Peninsula Complex and other intrusions Mostly Mesozoic
Consolidated (meta)sedimentary rocks; some volcanic rocks
Saionya Scarp and Rokel River groups Upper Proterozoic to Lower Palaeozoic Shales, schists, metaconglomerates and quartzites, metacherts and banded iron formations (BIF), with volcanic bands (Camus and Cukor 2012, Lapworth et al. 2015).
Basement Complex
Marampa and Kasila groups Precambrian (Archaean) Crystalline granitic gneisses with supracrustal metamorphosed volcanic and sedimentary belts. The Marampa Group is dominated by metasedimentary and volcanic rocks; the Kasila Group is dominated by granulites, basement granites, gneisses and migmatites, volcanic greenstone, amphibolite and gneiss. The granitic basement has a well-developed fracture network (Lapworth et al. 2015).


Hydrogeology

The hydrogeology map below 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).

More information on the hydrogeology of Sierra Leone is available in the documents listed in the References section, below.


Sierra Leone Hydrogeology.png Hydrogeology Key.png


Summary

The main hydrogeological distinction in Sierra Leone is between the relatively low permeability and productivity aquifers formed by the crystalline Precambrian Basement Complex, consolidated metasedimentary rocks and igneous intrusions; and the higher permeability and storage of the Bullom Group unconsolidated sand aquifer in the coastal zone. The weathered basement rocks form the most widespread and important aquifer across most of Sierra Leone.


Unconsolidated sedimentary aquifers

Named Aquifers General Description
Alluvial (valley fill) deposits Sands, gravels and clays that overlying the basement rocks, usually up to 15 m thick. They can have high permeability. Groundwater storage and flow is entirely intergranular. There is little data on borehole yields, but it is likely that yields of between 0.3 and 5 litres/second (l/s) will be possible (Lapworth et al. 2015).
Bullom Group Unconsolidated sands and clays (inland alluvial & coastal), usually 10 to 20 m thick, can form a moderately productive aquifer with potential borehole yields up to 3 l/s. Groundwater flow is intergranular and storage capacity can be high. Fracture flow is less common (Lapworth et al. 2015).

Below this are interbedded sands and clays which are typically 30 to 80 m thick. Boreholes can often abstract up 6 l/s (Lapworth et al. 2015).

Consolidated (meta)sedimentary aquifers

Named Aquifers General Description
Saionya Scarp / Rokel River groups There is a near-surface weathered (regolith) layer that is often dominated by clay. Below this are ancient conosolidated (meta)sedimentary rocks, with very little intergranular porosity. Groundwater storage and flow occurs within fractures in the rock, which are often along old bedding plains, although there is little information on potential borehole yields (Lapworth et al. 2015).

Igneous aquifers

Named Aquifers General Description
Granites, gabbros, dolerites Fractured gabbros are thought not to typically develop a thick weathered zone. Groundwater is likely to flow through the igneous rocks largely in fractures, although thin weathered zones may also contribute. There is little information on borehole yields (Lapworth et al. 2015).

Basement Complex

Named Aquifers General Description
There is typically a layer of highly weathered rock (regolith) overlying the unweathered bedrock, which has often transformed to a thick tropical soil. This is generally up to 20 m thick, although up to 37 m thick has been seen. The upper section of this weathered zone often has relatively little clay - the clay minerals have often been leached out, leaving metal oxides. These metal oxides are often in the form of indurated or gravelly layers, which can be highly permeable, and can allow rapid horizontal groundwater flow. Towards the bottom of the weathered zone, the weathered rock is often dominated by clays, and therefore has much lower permeability. Yields from shallow boreholes abstracting from this zone are typically in the range 0.3 to 1.5 l/s. This shallow aquifer tends to dry up rapidly when the rains stop and groundwater drains rapidly away through the permeable material. It is vulnerable to contamination, because of limited attenuation potential in the subsurface and rapid horizontal and vertical groundwater flow pathways for seasonal rainfall recharge (Lapworth et al. 2015).

At the base of the weathered zone, the underlying crystalline bedrock is often extensively fractured and not clay rich, and can store and transmit groundwater through fractures. There can also be deeper fracture zones associated with faults. The average thickness of the fractured aquifer zone is 35 m, but it can be as much as 60 m. Borehole yields are typically between 0.3 and 1.5 l/s. Groundwater flowpaths are usually longer than in the shallow weathered aquifer, and groundwater flow can be rapid over distances of tens of metres. This deeper, fractured aquifer zone is typically a more sustainable groundwater source than the upper weathered zone. It also has more potential for the natural attenuation of contaminants, because of the overlying clay zone and the longer flowpaths (Lapworth et al. 2015).


Groundwater management

Information on groundwater sources (water points) is collected in certain districts, but there is no central national database. Borehole logs with geological information are not readily available.


Transboundary aquifers

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


References

The following reports provide more information on the geology and hydrogeology of Sierra Leone. Some, and others, can be accessed through the Africa Groundwater Literature Archive

Camus Y and Cukor D. 2012. NI 43-101 Technical Report on the Resource Update Nimini Gold Project, Kono Region, Sierra Leone. SGS Canada Inc., submitted to Polo Resources Ltd.

Flinch JF, Huedo JL, Verzi H, Gonzalez H, Gerster R, Mansaray AK, Painuly LP, Rodriguez-Blanco L, Herra A, Brisson I and Gerard J. 2009. The Sierra Leone-Liberia Emerging Deepwater Province. Adapted from oral presentation at AAPG Annual Convention, Denver, Colorado, June 7-10, 2009.

Lapworth DJ, Carter RC, Pedley, S and MacDonald AM. 2015. Threats to groundwater supplies from contamination in Sierra Leone, with special reference to Ebola care facilities. British Geological Survey Technical Report OR/15/009, Nottingham, UK, 87pp.

United Nations. 1988. Groundwater in North and West Africa: Sierra Leone. United Nations Department of Technical Cooperation for Development and Economic Commission for Africa. Department of Technical Cooperation for Development and Economic Commission for Africa, Natural Resources/Water Series No. 18.


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Africa Groundwater Atlas >> Hydrogeology by country >> Hydrogeology of Sierra Leone