Assessing the impact on groundwater chemistry of an environmental restoration performed using industrial solid waste from TiO2 production

A hydrogeochemical study was performed to assess the impact on groundwater chemistry of industrial solid waste (ISW) used for environmental restoration of a site degraded by past quarry activities in southern Tuscany (Italy). Water samples were collected from springs, piezometers and drains around and within this site where solid waste generated by industrial processes for the production of titanium dioxide (TiO2) has been disposed. Water samples were analysed for the physico-chemical parameters (temperature, pH, electrical conductivity, redox potential), major ions (Na, K, Ca, Mg, Cl, HCO3, SO4), trace elements (Fe, Mn, Ti, B) and isotopes (δ18O, δ2H, δ13C, 3H). The study area is characterised by a complex hydrogeological system consisting of groundwater belonging to four hydrochemical facies. The leachate of the stored ISW had high salinity and concentrations of major ions (mainly SO4, Cl, Na, Mg) and trace elements (Fe, Mn). The chemical features of leachate in the stored ISW are similar to those of water found in the ISW before disposal, primarily due by the reactions between the raw material (ilmenite slags) and reagents (e.g. sulphuric acid, seawater and limestone) during production of TiO2. The hydrochemical and hydrogeological findings suggested that: i) no change of chemistry due to leakage of the ISW leachate occurred in groundwater around the ISW disposal site; ii) dilution of the ISW leachate by meteoric waters was found in the landslide zone of the ISW disposal site; iii) mixing between the ISW leachate and groundwater [Na–HCO3(Cl) waters] probably took place under the ISW disposal site in the period immediately following a landslide event.