In this study, the risk of environmental pollution by arsenic (As) and heavy metals from the tailings of Co-Ni arsenide ores at the Khovu-Aksy mine site (Russia) has been systematically evaluated over 20 years. The assessment results indicated that As poses the greatest concern, followed by Cu, Zn, Co, and Ni, among others. This article presents the results of a study on the mobile forms of metals (Cu, Ni, and Co, among others) and metalloids (e.g., As and Sb) in stored waste that was produced by the hydrometallurgical processing of CuCoNi arsenide ore from the Khovu-Aksy deposit. A comparative assessment of the changes in element forms during the 20-year of waste storage period was conducted. It was determined that in the pore solutions, the metal concentration decreased by an average of 710 times because they precipitated into secondary solid phases or were adsorbed on sulfides. Moreover, the As concentrations in the pore solutions remained practically unchanged, and in some cases, they slightly increased, i.e., this element remained immobile. In contrast, greater changes occurred in the fractional composition of the element forms, which were determined based on the compositions in water extracts. The concentrations of the water-soluble forms of the metals decreased by 100-700 times in 20 years. The concentrations of the soluble As compounds decreased by 1.6-4.6 times. A study of the mineralogy of the tailings showed that the secondary rims of the Fe hydroxides and Ca carbonates with admixtures of Cu, Zn, Co, Ni, and Ag formed on the grains of the sulfide and arsenide grains. The secondary As compounds on the rims are represented by aqueous Fe arsenate. The obtained data indicate active processes of metal deposition, while As remained mobile. The internal structure and geometries of the groundwater horizons were determined by using electrical resistivity tomography, which made it possible to calculate the quantities of dissolved metals and metalloids in the tailings. Condensates were collected to determine the concentrations of the volatile forms of the elements, and the amounts migrating with the vaporgas flows were estimated. At present, the tailings contain ˜2.1 tons of dissolved As (in pore water), ˜ 85 tons of soluble As (in water-soluble forms), and ˜ 23 kg of volatile As that enters the air with vaporgas flows during summer.