TITLE:
Variable-Density Modelling of Groundwater Abstraction, Sea-Level Change and Managed Recharge in the Tudor Coastal Aquifer, Mombasa, Kenya
AUTHORS:
Charles Ngala Kithome, Mary Makokha, Shadrack Murimi
KEYWORDS:
SEAWAT, Variable-Density Flow, Seawater Intrusion, Groundwater Abstraction, Managed Recharge, Sea-Level Change, Ghyben-Herzberg, Tudor Aquifer, Mombasa
JOURNAL NAME:
Journal of Geoscience and Environment Protection,
Vol.14 No.9,
September
28,
2026
ABSTRACT: Coastal groundwater management requires models that can distinguish the individual and combined effects of groundwater abstraction, a changing saline coastal boundary, and interventions that preserve freshwater head. A companion hydrochemical and statistical assessment of the Tudor aquifer, Mombasa County, Kenya (Kithome, Makokha, & Murimi, 2026) established strong empirical associations between licensed groundwater abstraction, local relative sea-level change, hydraulic-head decline and chloride concentration, but, being observational, could not simulate the aquifer’s process-based response to future stress or to candidate management interventions. This study addresses the gap by developing a calibrated two-dimensional SEAWAT variable-density flow and chloride-transport model for the Tudor aquifer, using the Ghyben-Herzberg relation only as a first-order analytical screening check. The 8-km cross-section comprised 80 columns and 20 layers (1600 active cells), with a specified coastal head and seawater-concentration boundary, a no-flow Precambrian basement, spatially distributed recharge, and 15 represented pumping centres. Calibration used hydraulic head at eight monitoring boreholes and chloride observations from 30 groundwater records. Hydraulic-head calibration yielded RMSE = 0.38 m, MAE = 0.29 m and R2 = 0.89; chloride calibration yielded RMSE = 892 mg/L, MAE = 671 mg/L and R2 = 0.94. Under the 2025 reference condition (+0.23 m relative coastal head, 11,500 m3/day represented pumping and 150 mm/year mean recharge), coastal chloride was 12,847 mg/L, the 5000 mg/L isochlor occurred about 2.5 km inland of the coast and the 1000 mg/L isochlor about 5.5 km inland of the coast. Reducing represented pumping by 30% (to 8050 m3/day) moved the system in a freshwater-preserving direction, and managed recharge to 650 mm/year in designated zones produced the largest simulated improvement, reducing coastal chloride from 12,847 mg/L to approximately 7837 mg/L (about 39% lower). A higher coastal-head condition (+0.50 m, giving +0.73 m relative coastal head) and a combined-stress scenario (represented pumping raised to 14,000 m3/day, recharge reduced by 20% to 120 mm/year, and coastal head raised to +0.73 m) both moved the system toward greater intrusion, with the combined-stress scenario producing the largest deterioration and shifting the 5000 mg/L chloride front from 2.50 km to approximately 4.54 km inland of the coast (an advance of about 2.04 km). Sensitivity tests varying hydraulic conductivity and dispersivity by ±20% changed simulated chloride by approximately 15% - 25% but did not change the ranking of scenarios by direction of response. As an independent, qualitative consistency check—not a transient 2015-2025 simulation, since the model is calibrated to 2025 conditions and each scenario is run to steady state—the 2025 reference simulation’s implied head and chloride levels are broadly consistent in direction and order of magnitude with the 0.80 m hydraulic-head decline and 2565 mg/L coastal chloride increase reported between 2015 and 2025 in the companion study. The results show that preserving freshwater head through coordinated abstraction control and carefully monitored managed recharge offers a stronger management response than treating sea-level change or pumping in isolation. The model is intended for comparative scenario assessment rather than parcel-scale prediction or deterministic forecasting.