Geophysical Exploration of Coastal Saline Water Intrusion: A Study of Ikoyi and Banana Island, Lagos State

Abstract

Groundwater is the primary source of potable water in Lagos State, Nigeria, particularly in coastal districts such as Ikoyi and Banana Island. In recent years, over-abstraction from coastal aquifers has accelerated seawater intrusion, threatening the availability of safe freshwater. This study investigates the extent of saline water encroachment and the distribution of freshwater in selected boreholes using integrated geophysical borehole logging. Nine wells were logged with gamma-ray and electrical resistivity tools, and the results were analyzed and correlated using Petrel® software to produce three lithostratigraphic panels along west–east and northeast–southwest transects. The gamma-ray logs distinguished alternating sand, sandy clay, clayey sand, and clay layers, revealing a multi-aquifer system separated by interbedded aquitards. Resistivity data classified aquifers into saline (<40 Ωm), brackish (40–100 Ωm), and fresh (>100 Ωm) water zones. Saline water was detected in seven of the nine wells, occurring at depths of 10–155 m. Brackish water interfaces extended between 10–210 m, while freshwater was found at shallow depths of 12–55 m and in deeper aquifers at 150–250 m. In the west–east profile, saline intrusion advanced from both flanks toward the central wells, though intrusion was minimal where population density and groundwater abstraction were lower. The deepest aquifers (beyond 150 m) consistently showed the highest freshwater potential. These findings demonstrate that borehole geophysical logging is an effective approach for mapping aquifer geometry and identifying safe groundwater zones in urban coastal environments. By highlighting the depth and distribution of saline intrusion, this study provides valuable insights for managing and protecting groundwater resources in Lagos vulnerable coastal districts..