Quantifying Fluid-Rock Interaction Rates in the Benin Formation Using Reactive Geochemical Transport Modelling: Implications for Managed Aquifer Recharge

Managed aquifer recharge programmes being evaluated for the Benin Formation in the Niger Delta region require accurate characterisation of the rates and pathways of geochemical reactions occurring when treated surface water of different composition is injected into the aquifer, yet the kinetic parameters governing mineral dissolution and precipitation in this sandy formation at in-situ conditions have not been determined. This study quantifies fluid-rock interaction rates in the Benin Formation aquifer under managed aquifer recharge conditions using a combination of laboratory flow-through column experiments and field-scale reactive transport modelling. Column experiments were conducted using intact aquifer cores under controlled temperature, pore water velocity, and injected water chemistry conditions representing realistic recharge scenarios. Effluent geochemistry was monitored continuously for major ions, trace elements, and dissolved silica to track reaction progress. Mineral dissolution and precipitation rates were estimated from effluent composition changes using mass balance calculations. Rate laws derived from the laboratory data were incorporated into a three-dimensional reactive transport model constructed using TOUGHREACT, calibrated against geochemical observations from a pilot recharge test well in Delta State. Sensitivity analysis identifies the relative importance of calcite dissolution, kaolinite dissolution, and silica polymorph precipitation in controlling injected water quality evolution. An original contribution is made through the derivation of temperature-corrected dissolution rate constants for aquifer minerals applicable to tropical setting managed aquifer recharge modelling. Keywords: reactive transport modelling, managed aquifer recharge, Benin Formation, fluid-rock interaction, TOUGHREACT.