This paper introduces a graphene–gold-metasurface sensor designed for rapid and precise detection of heavy metals, specifically Cu$^{2+}$ and Mg$^{2+}$ ions, in water samples. The sensor's performance was comprehensively analyzed using COMSOL simulations, demonstrating optimized sensitivities of 1140 GHz/RIU for Cu$^{2+}$ and 1149 GHz/RIU for Mg$^{2+}$ detection. The sensor achieved minimum figure of merit (FOM) values of 2.781 RIU$^{-1}$ and 2.855 RIU$^{-1}$ for Cu$^{2+}$ and Mg$^{2+}$ detection, respectively, and high quality factors (Q factors) of 11.244 and 11.247 for these ions. The sensor also exhibits very low detection limits of 0.466 for Cu$^{2+}$ and 0.485 for Mg$^{2+}$, indicating its ability to detect trace amounts of these ions. These features position the sensor as a promising candidate for high-accuracy and sensitive detection of Cu$^{2+}$ and Mg$^{2+}$ ions, with potential applications in environmental monitoring and other fields. Additionally, the sensor can function as a 2-bit encoder.This paper introduces a graphene–gold-metasurface sensor designed for rapid and precise detection of heavy metals, specifically Cu$^{2+}$ and Mg$^{2+}$ ions, in water samples. The sensor's performance was comprehensively analyzed using COMSOL simulations, demonstrating optimized sensitivities of 1140 GHz/RIU for Cu$^{2+}$ and 1149 GHz/RIU for Mg$^{2+}$ detection. The sensor achieved minimum figure of merit (FOM) values of 2.781 RIU$^{-1}$ and 2.855 RIU$^{-1}$ for Cu$^{2+}$ and Mg$^{2+}$ detection, respectively, and high quality factors (Q factors) of 11.244 and 11.247 for these ions. The sensor also exhibits very low detection limits of 0.466 for Cu$^{2+}$ and 0.485 for Mg$^{2+}$, indicating its ability to detect trace amounts of these ions. These features position the sensor as a promising candidate for high-accuracy and sensitive detection of Cu$^{2+}$ and Mg$^{2+}$ ions, with potential applications in environmental monitoring and other fields. Additionally, the sensor can function as a 2-bit encoder.