Assessment of Subsidence Risk from Groundwater Abstraction in Lagos Metropolis

Intensive groundwater abstraction in Lagos Metropolis, South West Nigeria, in conjunction with the compressible nature of the Holocene alluvial and coastal plain sediments that underlie much of the city, has raised concerns about land subsidence that could amplify flood risk and damage to building foundations and buried infrastructure. This study assesses the subsidence risk associated with ongoing groundwater abstraction in selected districts of Lagos Metropolis using a combination of geodetic monitoring and geotechnical modelling. Persistent scatterer interferometric synthetic aperture radar analysis of Sentinel-1 satellite data spanning three years was conducted to measure surface displacement rates across the study area. A network of continuous GPS benchmarks was installed and monitored to provide independent ground truth for the satellite-derived deformation measurements. Borehole data from a geotechnical database were used to characterise the thickness and compressibility of unconsolidated aquifer and aquitard units in the study area. One-dimensional consolidation modelling was applied to estimate the component of observed subsidence attributable to aquifer system compaction under measured drawdown conditions. The spatial correlation between subsidence rates and borehole abstraction densities is analysed statistically. Results are communicated to the Lagos State Ministry of the Environment and the Lagos Water Corporation to inform abstraction licensing reforms. Keywords: land subsidence, groundwater abstraction, Lagos Metropolis, InSAR, consolidation.