Abstract: Arsenic contamination in groundwater poses a persistent challenge in many urbanizing regions of the Philippines, particularly in Cavite, where mixed volcanic and carbonate aquifers are under increasing anthropogenic and geochemical stress. This study applies a quantitative geochemical simulation to evaluate the effects of pH, redox potential (Eh), and carbonate chemistry on arsenic speciation and mobility in Cavite groundwater. Using thermodynamic equilibrium modeling (PHREEQC) combined with logistic and adsorption-based parameterization, arsenite [As(III)] and arsenate [As(V)] species were simulated under Eh values 100–500 mV, pH 6.0–9.0, and total inorganic carbon (TIC) 1–10 mM. Results indicate that oxidizing conditions (Eh ≥ 300 mV) promote conversion of As(III) to As(V), while reducing conditions favor As(III), the more toxic and mobile form. The speciation transition between H2AsO4− and HAsO42− occurs near pH 6.9, consistent with experimental pKa values. Increasing carbonate concentrations significantly reduce arsenic adsorption, raising dissolved As from ≈17 μg/L to ≈30 μg/L. These patterns align with local hydrochemical observations in Cavite and comparable tropical aquifer systems. The integrated modeling framework highlights the coupled control of redox, pH, and carbonate equilibria on arsenic behavior in groundwater. The study provides a mechanistic basis for assessing arsenic transport and supports sustainable groundwater management and remediation strategies for Cavite and similar hydrogeologic settings.