Geochemical modeling-based rehabilitation proposal for abandoned sulfidic flotation mill tailings, Kirki, Thrace, NE Greece

A total number of eight (8) tailings dams are found at the Kirki flotation plant and are either built on impermeable Neogene formations or permeable alluvial deposits of the Kirkalon and Eirini streams. The tailings dams were originally constructed without any environmental concerns, nowadays their embankments are highly eroded, and toxic material is readily transported to the nearby streams, resulting in degradation of the quality of both stream waters and sediments. Stream water sampling revealed that concentrations of dissolved Pb, Mn, Cd and SO₄ increase rapidly in the vicinity of the Kirki flotation plant. Four case-specific PHREEQC scenarios were employed including “Direct Precipitation,” “Evaporation,” “Mixing with Rain water” and “Mixing with Stream water” in order to investigate the effect of interaction between water accumulated on the surface of the tailings dams and the precipitation or dissolution of heavy metal-bearing phases that may control heavy metal solubility. All tests performed concluded that such interaction plays a very important role in the environmental behavior of the toxic tailings material of the Kirki flotation plant. Based on the results of the PHREEQC tests, the proposed rehabilitation plan includes the construction of a sealing system in order to eliminate tailings dam wall erosion and infiltration effects. The suggested plan is limited to dams 2, 3, 4, 5, 6, 7 and 8, whereas dam 1 is empty and requires no remediation. Additionally, and only for dams 5, 7 and 8, built on top of permeable alluvial formations a few meters above the ground water head, chemical improvement of the tailings material is suggested to limit toxic pollutants dissolution. In particular, installation of lime piles is proposed in order to further improve the quality of possible small-scale, low-quality percolating solutions that might migrate to underground aquifers.