Saline water intrusion in southeast Tanzania

Abstract

Geophysical method of electrical resistivity was used in the detection of saline water intrusion in the aquifers within the coastal belt of Mtwara region in southeast Tanzania. A formation saturated with saline water has a low value of the electrical resistivity.

The data from the electrical resistivity surveys were interpreted by the classical method of curve-fitting. Formations with low electrical resistivity ($\text{< 10 Ω·}\text{m}$) were inferred at depths of within 50 m, in the sedimentary coastal zone. Such formations were suspected to be saturated with saline water. Electrical conductivity tests, and drilling logs where feasible, were used to confirm the existence of saline water. The saltwater-freshwater interface is expected to follow the Ghybe-Herzberg lens model.

Formations of low electrical resistivity are observed to continue inland up to 20–30 km, from the shoreline. The top surface of these formations is relatively horizontal and hence does not follow the Ghybe-Herzberg lens model. Presently there is very limited abstraction of groundwater from the coastal aquifers. A moderate amount of annual recharge from rainfall is indicated. Hence saline water intrusion from the sea under prevailing conditions is very unlikely.

However, the relative elevation of the top surface of formations saturated with saline water suggests that it is possible to induce saline water intrusion even with moderate pumping rate. Development of groundwater resources in the coastal zone of Mtwara region in Tanzania ought therefore to be pursued cautiously.