Reactive transport model of an extreme acidic perched aquifer within roasted pyrite waste in a fully urbanized area (Zaragoza, Spain)

The abandonment of industrial waste frequently leads to acid drainage affecting groundwater and severely impacting the environment or urban infrastructure. The severe impacts of the acid drainage are related, besides the extremely low pH reached in the affected waters, to the subsequent mobilization and spread of toxic heavy metals such as As, Fe, Cu, Cd, Ni, Pb, and Zn in highly oxidizing environments. The hydrogeology and hydrochemistry of an urban area in Al-Mozara (Zaragoza, Spain), built over an old industrial zone with pyrite roasting waste deposits that experienced acid drainage problems, has been investigated and modelled. Drilling and piezometer construction, and groundwater samples taken during the activities of the SAGE4CAN project, revealed the existence of a perched aquifer within old sulfide mill tailings, where the building basements interrupted the groundwater flow leading to a water stagnation zone that reached extreme acidity values (pH < 2), by drastically increasing the reaction times. A groundwater flow reactive transport model was developed with the software PHAST to reproduce flow and groundwater chemistry, to be used as a predictive tool for guiding and selecting the optimal remediation actions. The model reproduced the measured groundwater chemistry by simulating the kinetically controlled pyrite and portlandite dissolution. Further, the model predicts that an extreme acidity front (pH < 2), coincident with the Fe (III) pyrite oxidation mechanism taking dominance, is propagating by 30 m/year. The incomplete dissolution of residual pyrite (up to 18 % dissolved by the end of the simulations) predicted by the model indicates that the acid drainage in this aquifer is limited by the flow regime rather than by sulfide availability. The construction of additional water collectors between the recharge source and the stagnation zone has been proposed, together with periodical pumping out of acid water from the stagnation zone. The study findings are expected to serve as a useful background for the assessment of acid drainage and remediation techniques in urban areas by numerical modelling, since urbanization of old industrial land is rapidly increasing worldwide.