Integrated groundwater flow modeling and prediction of septic-derived phosphate plume in shallow aquifers of the Niger Delta

The contamination of shallow aquifers by septic-derived phosphate poses significant threat to water security in the Niger Delta, Nigeria. This study developed a field-validated numerical model to simulate the groundwater flow and phosphate transport in a shallow aquifer system, to forecast the evolution of the contaminant plume over 20 years. The study integrated field hydrogeochemical surveys with a 3D finite-difference model constructed in Visual MODFLOW Flex. The model was calibrated using observed head data (13.7 to 14.5 m) and field-measured phosphate concentrations. The MT3DMS engine was employed for transport simulation under a no-reaction scenario, incorporating hydraulic conductivity (ranging from 2.157 × 10−7 m/s for an aquitard to 1.470 × 10−3 m/s for the upper aquifer), specified dispersion coefficients (longitudinal dispersivity = 10), and boundary conditions. Field observations confirmed a phosphate plume, with mean concentrations decreasing from 0.060 mg/L to 0.009 mg/L. The calibrated flow model established a hydraulic gradient 16.8 to 19.8 m directing groundwater flow from the septic source towards borehole sinks. The 20-years forecast revealed a persistent plume in the upper aquifer, with the source concentration (72.19 mg/L). However, significant vertical attenuation occurred, limited by subsurface stratigraphy. These findings underscore the need for stringent septic system standards and targeted monitoring along identified flow paths. Key words: Septic tank (ST) contamination, phosphate transport, Niger Delta aquifer, MODFLOW-MT3DMS modeling, groundwater vulnerability, persistent contaminant plume.