Geochemistry and stable sulfur and oxygen isotope ratios of the Podwiśniówka pit pond water generated by acid mine drainage (Holy Cross Mountains, south-central Poland)

The paper presents the results of a geochemical and isotopic study of acidic pond water in the abandoned Podwiśniówka quarry (Poland). The scope of investigations also encompassed mineralogical and isotopic studies of pyrite and related supergene minerals. Compared to similar sites throughout the world, the pit pond water examined is characterized by a very low pH averaging 2.64 ± 0.33 and simultaneously very low concentrations of SO 4 2 - (geometric mean of 237 ± 57 mg L −1), Fe(II) (4.8 ± 3.4 mg L −1), Fe(III) (10.0 ± 6.2 mg L −1) and other trace elements. This acidity has been generated by complex processes of As-rich pyrite oxidation combined primarily with hydrolysis, precipitation, and transformation of Fe oxyhydroxysulfates and oxyhydroxides into goethite. The specific mineralogy of ore and gangue minerals, but especially the lack of acid–buffering constituents, has additionally contributed to the very low pH and element concentrations. Of the toxic elements, the high content of As (1111–1879 mg kg −1) in the western part of pit pond sediment may be of great concern, especially when using lime as a neutralizing agent of the acidic water. The δ 34S of soluble SO 4 varied from −19.8‰ to −11.1‰ and was different from that of efflorescent sulfates (−25.7‰ to −25.4‰) and host pyrite (−25.4 ± 2.5‰). The comparison of the δ 18O– SO 4 2 − (−2.0 ± 1.2‰) and δ 18O–H 2O (−6.2 ± 3.5‰) values indicated that the pyrite underwent bacterially catalyzed oxidation by two natural oxidants, primarily by Fe 3+ and to a lesser extent O 2.