Mapping of the freshwater lens in a coastal aquifer on the Keta Barrier (Ghana) by transient electromagnetic soundings

We present a model of the freshwater lens and saltwater intrusion in a 1000 m wide and 2500 m long portion of the Keta Barrier, Ghana, based on 96 transient electromagnetic (TEM) measurements. Saltwater intrusions from the Gulf of Guinea to the south of the barrier and from the Keta Lagoon to the north threaten the freshwater resources. The freshwater resources are essential for supporting the relatively large population and intensive farming on the Keta Barrier. It is therefore of utmost importance to identify and map the saltwater intrusion in order to get an overview of the remaining freshwater resources. The sedimentary column below the barrier is characterised by sand and gravel deposits with layers of clay and silt. The electrical resistivity of the subsurface is most likely primarily controlled by the salinity of the pore water. The TEM method is well suited for mapping the depth to and the resistivity of good conductors, and, therefore, a strong tool for identifying the top of the low-resistivity sediments which are saturated with saline pore water. The surface marking the top of the salt-water-saturated sediments has a saucer-like shape. It is situated at 0-5 m depth close to the shorelines of the Gulf of Guinea and the Keta Lagoon, whereas it is situated at up to ∼ 40-45 m depth in the central parts of the barrier. The freshwater lens is thin (0-5 m) close to the coasts of the Gulf of Guinea and the lagoon, whereas it may be up to ∼ 20 m thick in the central parts of the Keta Barrier. We interpret the existence of a mixing zone with brackish water between the freshwater lens and the layers with saline pore water. This mixing zone varies in thickness from 0-5 m close to the coastlines to ∼ 10-20 m in the central part of the barrier.