Assessment of Water Pollution in the Arieș River Induced by Acid Mine Drainage from the Valea Șesei Tailings Pond, Roșia Poieni Copper Mine, Romania

Tailings ponds associated with the beneficiation of low-grade sulphide ores are a recognized source of acid mine drainage (AMD) and heavy metal contamination in receiving watercourses. This study assesses the physicochemical quality of process water stored in the Valea Șesei tailings pond, which receives flotation tailings from the Roșia Poieni porphyry copper deposit (Apuseni Mountains, Romania), and evaluates the impact of its discharge on the Arieș River. Ten water samples were analysed for pH, dissolved oxygen, turbidity, conductivity, total dissolved solids, salinity, hardness, and dissolved copper and iron (ICP-OES). Water within the tailings pond was strongly acidic (pH as low as 3.27), with elevated conductivity and turbidity consistent with active AMD generation; pH and conductivity were negatively correlated (r = −0.68), a signature characteristic of sulphide oxidation. At the discharge point, copper and iron concentrations exceeded Romanian STAS 4706/1988 limits by two orders of magnitude and remained 30- to 40-fold above the limit in the Arieș River after dilution. Recomputing exceedances against the current NTPA-001/2002 normative altered this picture for iron, while copper remained non-compliant under both standards. These results confirm a measurable impact of tailings pond discharge on downstream water quality and highlight the need for regular monitoring against current regulatory thresholds.