Simulation on water quality of reservoir at construction phase by pollutant release from oxidation of waste rocks rich S and Fe

Abstract

The presence of sulfur and iron in waste rocks produced during reservoir construction can generate acid rock drainage (ARD). Its environmental impact however, has often been neglected. Therefore, lab experiments were firstly conducted to simulate the release of main pollutants when the waste rocks having been submerged at the bottom of a reservoir in Guizhou Province, China. The impact of pollutants released on water quality during impounding was then predicted by a two-dimensional model MIKE21 coupled with ECO Lab module. Results showed that the argillaceous rock mainly produced acid and sulfate through oxidation. Based on release experiment, the model suggested that the average concentrations of TP, SO 4 2- , and Fe 3＋ peaked rapidly in the cross section about 100 m downstream of the waste rock yards when water level reached 855 m. At this point, the average Fe 3＋ concentration exceeded the standards of drinking water source and remained for 6 days, while the average TP concentration downstream exceeded the standards for 60 days. The water quality would meet the standards because of the dilution effect on pollutants when the water level was above 883 m. However, the release of phosphate from combined cycles of “P-Fe-S” should be paid more attention during impoundment. Based on the simulation results and existing actual cases, it showed that the anaerobic environment caused by impoundment can significantly reduce the negative impact from ARD. The study would provide a new insight for the stacking of engineering waste rocks and environmental risk assessment of the reservoir under construction. • Impact on water quality by waste rocks during reservoir construction was simulated. • SO 4 2- , TP and Fe 3+ mainly came from argillaceous rock through microbial oxidation. • P released from waste rocks through combined cycles of “P-Fe-S” should be concerned. • The pollutants migrated downstream to the front of the dam with water level rise. • Anaerobic environment due to impoundment reduced the impact of ARD from waste rocks.