In this study, we have investigated the electrochemical properties of arsenic (\( As^{3+}\)) ions in standard solutions of arsenic ions and in samples of well water from the artesian wells in phosphate buffer pH = 7. The goal of the study was the electrochemical detection of arsenic in the model systems, and well water samples using unmodified and modified glassy carbon electrodes. Differences between the surface of unmodified and modified glassy carbon electrodes were determined by electrochemical impedance spectroscopy. For the detection of arsenic, two voltammetric methods were used, cyclic and differential pulse voltammetry. For voltammetric measurements, a three-electrode system with a working glassy carbon electrode, a reference Ag / AgCl electrode and a platinum counter electrode were used. The measurements were performed in a phosphate buffer solution (pH = 7), with the addition of \( As^{3+}\) ion (c (\( As^{3+}\)) =1 · \( 10^{-3}\) mol \( dm^{-3}\)) and in samples of well water with a modified glassy carbon electrode. In all samples of well water taken during the September and October, the presence of arsenic was found and additionally confirmed by the increase of oxidation and reduction peaks with the addition of \( As^{3+}\) ions in tested samples. After modification of the glassy carbon electrode, arsenic could be qualitatively detected by cyclic and differential pulse voltammetry with the advantage on differential pulse voltammetry because of the more pronounced oxidation peak currents.