The fate of major constituents and chromium and other trace elements when acid waters from the derelict Libiola mine (Italy) are mixed with stream waters

Geochemical modeling was used to investigate the fate of major constituents and Cr and other trace elements when acid waters from the derelict Libiola mine are mixed with the Gromolo creek waters, and related processes. This was carried out in two steps: (1) Mixing between acid waters and stream waters was simulated using the EQ3/6 software package; the results showed that mixing brings about the precipitation of some secondary solid phases, mainly a ferrihydrite-rich solid mixture of hydroxides and basaluminite. (2) The sorption of cations (Cr3+, Cu2+, Zn2+, Mn2+, Ni2+, Ca2+, and Mg2+) and anions (CrO4(2-) and SO4(2-)) on ferrihydrite was modeled by means of the code MINTEQA2, treating the concentrations of relevant solutes and the moles of ferrihydrite (computed by means of EQ3/6) as input data. The results of this two-step geochemical modeling generally agree with analytical data, supporting the following conclusions: (a) dissolved Mg, Ca, Na, SO4, Cl, HCO3 and SiO2 are mainly controlled by acid water–stream water mixing; (b) Fe is mainly incorporated into ferrihydrite and subordinately in schwertmannite and jarosite; (c) Al is chiefly sequestered in basaluminite and alunite; (d) K is temporarily incorporated into jarosite and alunite and is released into the aqueous solution upon their dissolution; (e) the mobility of Cu, Zn, Ni, Mn, and Cr(VI) is mainly controlled by sorption on precipitating ferrihydrite; (f) Cr(III) is immobilised by both incorporation into ferrihydrite and sorption on its surface. The effects of acid mine waters on the Gromolo aqueous system are not ameliorated to an acceptable extent by natural attenuation through dilution and related processes.