Analysing Contaminant Mixing and Dilution in River Waters Influenced by Mine Water Discharges

Mine water discharges can cause negative effects on recipient rivers. The magnitude of the effects depends on how quickly and efficiently contaminants dilute in the river. Electrical conductivity (EC) is linked to the water quality and can be utilized as a general tool to compare and detect the contaminant sources and concentration changes derived from mine water discharge. Hydrological and flow velocity profiling devices were tested to assess the impact as well as mixing and dilution of mine effluent discharge in a river next to a gold mine in Finland. Additionally, flow rate and velocity profiles of the cross-sections were measured. Recently, the greatest ecological impacts had been detected in the vicinity of the dewatering discharge point, which has the highest sulphate concentrations in rivers. According to EC measurements of this study, these were the same locations, where the mine effluent did not dilute and mix efficiently due to lower flow velocities and lesser turbulence. Further, EC values displayed a significant positive correlation with sulphate, magnesium, potassium, sodium, and calcium, whereas a lower correlation was observed with the trace elements. The tested study method revealed how changes in the river morphology and flow velocity affect behaviour, mixing, and dilution processes. Mixing and dilution of contaminants depended on the discharge location and method as well as on the density differences between the mine water discharge and fresh river water. This study highlights the importance of detailed hydrological and flow rate measurements when designing the location of mine water discharge to recipient rivers.