Freshwater springs in the lagoons of Zanzibar are a unique feature of this island. Although such springs exist on many – or even most – coasts worldwide, these Zanzibarian springs can be used as a characteristic example of this phenomenon. Freshwater can be drunk directly at the exit points of these springs within low-tide lagoons of saltwater.
Relevant geographical features of Zanzibar
Zanzibar Island (called ‘Unguja’ in Swahili) is a relatively flat country with mountainous elevations of up to 130 meters above sea level in the island’s northern part. In the southern areas of Chwaka Bay, which is located roughly in the middle of the east coast, the elevation is 5 meters below sea level.
Geological features of Zanzibar
Except for the mangrove areas at Chwaka Bay, the east coast consists of coralline limestone. This limestone covers other limestone strata (Miocene limestone) or a mixture of limestones and sandy clays. All these limestone layers exhibit high soil infiltration. Water can flow easily through these layers, creating caves and feeding cave wells. The water table is formed above the contact area of the coralline limestone’s underlying strata.
Hydrological features of Zanzibar
According to Munishi et al. (2023), very few rivers flow throughout the year on the island, while most are perennial. After a short travel time, they quickly infiltrate and disappear into the limestone underground.
Therefore, groundwater remains the primary source of water in Zanzibar. Most wells, however, have increased salinity values, which can be up to seven times above the recommended limits. Also, the bacteriological pollutants are very high due to pit toilets near wells or contaminated ropes when lowering buckets into the wells. Due to the large limestone layers, Zanzibar has many submarine aquifers.
Freshwater on the coastline
This website’s article already discussed how to find fresh water on sandy beaches. The same effects can be seen at Zanzibar beaches. Therefore, we will not discuss it further.
Freshwater springs in the lagoons
The freshwater springs are the lagoons’ most interesting and distinctive feature on this island’s southern and eastern shores. The specific geographical, geological, and hydrological features of the landscape formed these springs. They are exposed during low tides and can be used for freshwater supply.
All of these springs provide a strong flow of freshwater. They can be spotted in the lagoons by their circular shapes and the bubbling sand in the middle.
Freshwater is often diluted with saltwater from the surrounding surface areas. Due to saltwater contamination, such springs can not always be used for freshwater supply. However, it is important to note that the human body will accept the isotonic value of human blood as a maximum salinity. This is about 0.9% or 9 grams per liter of water. Saltwater of the Indian Ocean has a salinity of about 3,5%. Therefore, some mixture of salt- and freshwater is acceptable when for example, sucking freshwater from such spring with a flexible pipe.
More examples of freshwater springs in Zanzibar
Human tears have the same isotonic value as blood. Their taste can be compared with the taste of brackish water to determine if it safe to drink.
Besides the salinity of the freshwater from such springs, it should be tasted if it contains a high magnesium content. It will taste bitter in such cases, although the freshwater will not be seriously harmful. The freshwater I tasted from these intertidal springs in the lagoons of southern Zanzibar tasted sweet, not bitter, and would be suitable for drinking even for a long time.
Freshwater can be drunk directly next to the sediment bubbles. A further improvement would be to shield these springs with flat stones, large bivalve shells, or another plate from any saltwater inflow from above. However, these springs showed such a strong flow of freshwater that such shields were unnecessary.
Lessons learned about freshwater springs in the lagoons of Zanzibar:
- Eastern- and southern Zanzibar coastlines consist of limestone, which facilitates the creation of freshwater springs beyond the beaches.
- Freshwater seeps through the sand at aquifer lines, forming strong springs in the lagoons.
- These springs can be spotted by their circular shape with bubbling sand in the middle.
- Sweet-tasting water can be drunk directly next to the spring bubbles.
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