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

Study methods and results

The magnitude of adverse effects depends on how quickly and efficiently contaminants dilute in the recipient waterbody. EC and flow velocity rates are usually monitored with fixed stations at mines, which do not take into account the channel bathymetry and its effect on mixing and dilution. This was resolved by combining electrical conductivity data from CTD sensors with water velocity profiling data from an acoustic Doppler current profiler (ADCP) device to assess the river and contaminant transport dynamics at the Kittilä mine site. The study method can help in planning discharge locations for mine water pipelines or ditches in order to enhance the mixing and dilution processes, resulting in a smaller impact on the river ecosystem. Combining the monitoring method with water chemistry analysis will also help in assessing whether the recently set sulphate concentration limit for the environmental permit of the mine is adequate to diminish the adverse impacts on the river ecosystem.

A field campaign was conducted in the Seurujoki River in August 2014. The CTD device was used to measure horizontal and bathymetric, i.e. vertical EC and temperature profiles from selected river cross-sections. The flow velocity profiles were measured using a RiverSurveyor M9 device in the same river cross-sections. The vertical and horizontal distribution of contaminants in the river was coupled with the water flow patterns in order to estimate the transverse mixing processes. Finally, the field data were combined with water quality analysis (water samples analysed in a laboratory) and processed using data analysis to produce information on the behaviour, mixing and dilution of contaminants in the river.

The electrical conductivity in Seurujoki River increased from natural levels of 50–100 μS/cm to a level of 210–280 μS/cm downstream from the mine. The high conductivity was attributable to the levels of sulphate, magnesium, potassium, sodium and calcium, whereas there was no significant correlation with trace elements such as arsenic. The maximum concentrations of sulphate were detected in the same locations downstream from dewatering water discharge sites where other studies have revealed the greatest ecological impacts (e.g. Pöyry 2011, 2012, 2013). The mine waters mixed and diluted slowly in the river, resulting in (contaminant?) levels still being elevated 15 km downstream. In addition, the tendency to dilute and mix in the river differed significantly between process and dewatering water. Despite the lower conductivity, i.e. salinity, of the dewatering water, its impact on the river was greater. This was due to the three-fold greater volume of dewatering water and the way it was discharged continually through several natural ditches along the river. In contrast, as the process water was discharged along one ditch and there was a stronger flow into the river, which enhanced the mixing and dilution processes. Furthermore, the dewatering water plume was only distributed on the river surface, and different advection velocities caused strong vertical concentration gradients and a large diffusive vertical flux. As the plume continued to move further downriver, lateral turbulent diffusion smoothened out the vertical concentration gradients and mixed the plume in the transverse direction.

Conclusions

This study highlights the importance of detailed hydrological and flow-rate measurements when planning the location of mine water discharge into recipient rivers. The tested study method revealed how changes in river morphology and flow velocity affect the discharge behaviour, mixing and dilution processes. The 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. If the discharge of mine waters into the Seurujoki River was channelled along clear ditches, and into points where the river flow is naturally vigorous, their mixing and dilution in the river would be intensified, which would reduce the environmental impacts. Even though the method is understandably not applicable in routine monitoring at mine sites, these types of studies can be used to assess the accuracy of existing water quality policies and permits. They could also be used in the environmental impact assessment phase when assessing the environmental impact of mine waters on recipient rivers and planning mine water discharge methods and locations for a new mine.

This study was part of the SUSMIN project funded by the ERA-MIN Joint Call.

References

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Palmer, K., Ronkanen, A.-K. & Bjørn, K. 2015. Efficient removal of arsenic, antimony, and nickel from mine wastewaters in northern treatment peatlands and potential risks in their long-term use. Ecological Engineering, 75, 350–364. Available at: https://doi.org/10.1016/j.ecoleng.2014.11.045
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Text: Kaisa Turunen

Kaisa Turunen is a research scientist in the Water Solutions Unit. She has worked at the Geological Survey of Finland (GTK) since May 2012 and specializes in mine water-related emissions. She is currently also conducting PhD studies at the University of Helsinki on the research topic of “Mine effluent impacted surface and groundwaters: implications for tracer and hydraulic applications at the mine-site scale”.