Detailed chemical and mineralogical analyses were conducted to characterize slag wastes, leachate, and associated tufa precipitates at an iron slag dump near Kraków in southern Poland. The slag contained elevated content of Cr (1,800-6,500 mg/kg), V (500-1,800 mg/kg), and Zn (up to 4,500 mg/kg). The negative environmental impact posed by the wastes can be potentially caused by the formation of leachate with extremely high pH (12.5-12.9), high sulphate (720 mg/l) and K concentrations (420-520 mg/l), and elevated concentrations of fluoride (1.7-2.1 mg/l) and Al (up to 2.7 mg/l). A dominant feature of the leachate was massive precipitation of calcareous tufa forming a cascade of tufa barrier ponds at the base of the slag dump. The tufa preferentially concentrated Pb, Sr, Ba, Rb, sulfur, and phosphorus, while Fe, Zn, Mg, Mn, Cr, and V were depleted relative to the content in the slag material.