Gambia has a “Good” and “Bad” situation with regard to water.
The “Bad” of Gambia water situation
The Bad is that the year round supply of rainwater is very seasonal, with a very heavy wet season in the months – June, July, August, September.
In this period falls almost all of the rainfall, so there is a very long “dry”, for the rest of the year where natural rainfall is not available, so must be stored to ensure all round supplies.
Also, much of the population and industry is in the lower reaches of the river and the water from the river is brackish and not drinkable for up to 250km from the mouth depending on the wet season rainfall.
Agriculture further than 250km up the river can utilise the water without the salty, tidal influence, but for the rest of the population, the water has to be obtained from the groundwater reservoir.
The “Good” of Gambia water situation
Now, the “Good” is that Gambia has two dominant, large, freshwater supplies.
The Gambia river, which has a very large overall flow, and the groundwater reservoir, another large amount of freshwater.
The actual amount of renewable water running through Gambia each year is very large – over 8 cubic kilometres of water, or 8,000,000,000,000 litres per year!
And the use of it for irrigation, industry and domestic use amounts to less than 1% of this supply, so there is adequate water flowing through the Gambia for all uses.
So, for most of the population near the coast, the water supply is available from rainwater for 4 – 5 months of the year and for the remainder of the year, unless in above ground storage, it can be obtained from ground aquifers..
The available groundwater reservoir in Gambia
Gambia is located in one of Africa’s major sedimentary basins which is referred to as the Mauritania/Senegal Basin.
It contains a deep and extensite aquifer called the “Maastrichtian aquifer” , stretching from the northern part of Mauritania to the South of Guinea Bissau, covering nearly 200,000 km2 and containing considerable groundwater resources.
Of this overall aquifer, there are two main aquifers available below ground in Gambia, one above the other, the Shallow Sand Aquifer (SSA) and the Deep Sandstone Aquifer (DSA).
The Shallow Sand Aquifer (SSA)
The SSA is the most important for Gambians and this is where the majority of bores obtain their water from.
The SSA varies in depth, but generally in the range of 4 to 50 m below ground level, although in places it can be found at 100m depths.
It is further divided into two reservoirs, one above the other separated by a 15-30 m clay-silt layer which ensures minimal mixing of this water.The upper aquifer is in sand laid down in very recent geological times and the lower layer in much older sediments.
Extraction of water from both is significantly less than recharge and while there is some lowering of water table during extraction in the dry season, the water levels fully recover during the wet season.
In general there are no major water quality issues with the water from this aquifer, although there have been instances of elevated iron concentrations and in urban areas ground pollution and underground pollution from drop toilet holes and septic tank effluent can contaminate the water.
Yields are generally in the range of 1-30 l/s and can be greater than 30 l/s in the most productive areas.
It is the SSA that we are almost always referring to when discussing drilling for groundwater in the Gambia.
There is also the deeper layer called the Deep Sandstone Aquifer (DSA).
The Deep Sandstone Aquifer (DSA)
The DSA is found at depths of 250-450m and the water is confined and is very old water of between 4000 and 40000 years old.
To get to this aquifer would need deep bores up to 380m.
There is a lot of water at this depth – estimated at 650,000 M cubic metres, of which only less than 10~% is thought to be drinkable.
The western parts of the aquifer are highly mineralised, with the more drinkable water found in the east of Gambia.
Summary
Water resources in the Gambia for most purposes are easily obtained from below ground in the upper level of the Shallow Sand Aquifer where good quality water is available, although caution should be made for urban extraction where pollutants may be found.
The deeper section of the Shallow Sand Aquifer can be drilled down to ensure best water quality, although this will be more expensive.
If you are interested in our water services such as bore drilling, solar pumping, water storage or plumbing, Click through to our WATER SERVICES page for more information.