Difference between revisions of "Hydrogeology of Gambia"

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==Geology==
 
==Geology==
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Revision as of 15:10, 9 June 2015

Authors

Landing Bojang, Department of Water Resources, The Gambia

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


Geographical and Political Setting

Gambia is the smallest country in Africa. It borders the Atlantic Ocean to the west, and is otherwise entirely surrounded by the country of Senegal. The majority the country comprises the floodplain of the Gambia River, which originates in Guinea before flowing through Senegal and through Gambia to the sea. The country is therefore generally very flat, ranging from 0 to <100 m above sea level.

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

General

Estimated Population in 2013* 1,849,285
Rural Population (% of total)* 42%
Total Surface Area* 10,120 sq km
Agricultural Land (% of total area)* 60%
Capital City Banjul
Region West Africa
Border Countries Senegal
Annual Freshwater Withdrawal (2013)* 91 Million cubic metres
Annual Freshwater Withdrawal for Agriculture* 43%
Annual Freshwater Withdrawal for Domestic Use* 37%
Annual Freshwater Withdrawal for Industry* 19%
Rural Population with Access to Improved Water Source* 84%
Urban Population with Access to Improved Water Source* 94%

* Source: World Bank


Climate

The climate of Gambia is largely classified as tropical savannah, apart from the central north region which transitions into hot, arid steppe. There is little spatial variation in average annual precipitation and temperature, other than a slight reduction in rainfall in the central north region.


Gambia has a very distinct wet season between June and October, and is relatively dry from November to April. The wet season is relatively hot compared to the cooler dry season.

Rainfall time-series and graphs of monthly average rainfall and temperature for the two climate zones in Gambia can be found on the Gambia Climate Page.


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

Gambia is dominated by the perennial Gambia River, which flows along the entire length of the country from the border with Senegal in the east to its discharge point to the Atlantic Ocean to the west.

The Department of Water Resources is responsible for river flow gauging, and currently monitors the Gambia River close to its discharge point to the Atlantic Ocean.


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


Soil

Soil Map of Gambia (For more information on the datasets used in the map see the soil resources section)
Soils across the floodplain of the Gambia River are dominantly Gleysols, which are highly important for agriculture.

Lixisols in the north of the country are associated with fine-grained weathered parent material, and natural savannah or open woodland vegetation.

More acidic Acrisols are found in the coastal region. This soil type is common in the wetter parts of Africa, and is generally deficient in nutrients.

Regosols, which cover a significant area inland, are reflective of the largely unconsolidated underlying geological deposits.


Land cover

General information about COUNTRY land cover.
Land Cover Map of Gambia (For more information on the datasets used in the map see the land cover resources section)

Geology

Gambia Geology.png

Geological Environments
Key Formations Period Lithology Structure
Unconsolidated
Unconsolidated Sedimentary Deposits Quaternary Unconsolidated deposits of Quaternary age crop out across the whole country. They consist of Holocene aeolian sands/silts and alluvial clays, underlain by Pleistocene sandy clays and Pliocene fine-medium grained sands which contain varying amounts of silt, clay and laterites. The Holocene deposits are typically up to 20 m thick, while the underlying Pleistocene and Pliocene deposits have thicknesses of around 20 and 30 m, respectively.
Mauritania/Senegal Basin Sedimentary deposits Cretaceous-Tertiary Loosely consolidated deposits, variously of fluvial, lacustrine and marine origin. These Cretaceous and Tertiary deposits are part of the Mauritania/Senegal sedimentary basin, which formed as a result of the breakup of Gondwana in the late Palaeozoic/early Mesozoic.

The Tertiary sediments are largely composed of shales and marly limestones. These can be subdivided into 4 main units:

(1) Miocene shales and marly fine sands with subordinate limestones;

(2) Oligocene marly limestones, which are of limited extent;

(3) Eocene shales and marly limestones with bands of flint near the base;

(4) Paleocene limestones interbedded with dark grey marls.

Below this are two sequences of Cretaceous loosely consolidated sediments. The younger sediments, of Maestrichtian age, are composed of fine to coarse grained sandstones, with subordinate grey-black shales, phosphatic nodules and lignite bands. The older Cretaceous sediments (of Campanian age) consist of grey clays and marls interbedded with fine calcareous sandstones, dolomite limestones, and lignite bands.

The Tertiary sediments have a total thickness of around 180 m. The Maestrichtian sediments are typically around 200 m thick while the underlying Campanian sediments are >500m thick.

Hydrogeology

There are two main aquifers in Gambia: the upper Quaternary unconsolidated sands comprise a shallow sand aquifer (SSA), which is an important aquifer throughout Gambia. The deeper Cretaceous sediments form a deep sandstone aquifer (DSA). More detail is in the Aquifer Properties section below.


Aquifer properties

File:Gambia Hydrogeology.png Hydrogeology Key.png


Unconsolidated

Named Aquifers General Description Water quantity issues Water quality issues Recharge
Shallow Sand Aquifer (SSA) The shallow sand aquifer (SSA) is composed predominantly of fine to coarse sand, and is found and exploited across the extent of Gambia. It can be subdivided into 2 units: the phreatic aquifer, which comprises the Holocene sediments, and the semi-confined aquifer, which comprises the underlying Pliocene sediments. The two aquifers are separated by a 15-30 m clay-silt layer which allows limited hydraulic connection between them.

Yields are generally in the range of 1-30 l/s and can be greater than 30 l/s in the most productive areas.

Hydraulic conductivity and transmissivity generally ranges from 5-30 m/d and 100-10000 m²/d, respectively.

Storage is generally between 10-4 and 10-2.

The SSA typically varies from 5-25 m thick and the water table may sit between 4 and 50 m below ground level. Boreholes are generally drilled to depths of 35-100 m.

Groundwater abstraction is significantly less than recharge and water levels fully recover during the wet season. There are no major groundwater quality issues. Isolated instances of elevated iron concentrations have been reported. When mean annual precipitation is above 900 mm, recharge is generally in the range of 250 – 300 mm. This is a result of direct infiltration.
Deep Sandstone Aquifer (DSA) The deep sandstone aquifer (DSA) comprises mainly unconsolidated sands and loosely consolidated sandstones, typically at depths of 250-450m. Groundwater in the DSA is confined, and is very old water of 'fossil' origin, between 4000 and 40000 years old. Exploitation of the DSA would require deep boreholes (up to 380 m), and potential yields have been estimated at 40 l/s. Storage in the DSA has been estimated at 650,000 M cubic metres, of which only 80,000 M cubic metres is thought to be potable. In the east of Gambia, groundwater in the DSA is potable, but in the west the old confined groundwater is typically highly mineralised, with total dissolved solids in the range 1000 to 2000 mg/l, and fluoride concentrations between 2 and 5 mg/l. If required, highly mineralised water in the western parts of the DSA could be abstracted and mixed with groundwater from the SSA at a ratio of 2:1 to expand the exploitable water resources of Gambia. There is no appreciable modern recharge to the DSA.

Groundwater Status

Groundwater abstraction from the main unconsolidated aquifer (SSA) is less than average annual recharge. Total groundwater availability could be significantly increased by exploiting the DSA.

Groundwater use and management

Groundwater use

The national water supply in Gambia is derived entirely from groundwater.

The following groundwater abstractions are currently known: 207 boreholes with hand pumps, 260 boreholes with solar pumping systems, 84 boreholes with electric pumping systems, and 1634 hand dug wells with hand pumps (Department of Water Resources).

Groundwater management

The Department of Water Resources is responsible for the development, utilisation and protection of groundwater in Gambia. They issue permits, which are required for both borehole drilling and groundwater abstraction.

The National Water and Electricity Company (NAWEC) is mandated to provide water supply in the Greater Banjul Area and surrounding provinces.

Groundwater monitoring

The Department of Water Resources established a network of 38 groundwater level and quality monitoring boreholes in 2014. These are distributed across Gambia and are equipped with automatic data loggers. The recorded groundwater level observations are collected every 3 months, and the data is stored in the GeOdin database in the Department of Water Resources.

The same network is used to monitor groundwater quality.