Riverine flow-through of mine pit lakes: Improving both mine pit lake and river water quality values?
Proceedings of the 10th International Conference on Acid Rock Drainage and IMWA Annual Conference
International Mine Water Association
School of Science / Centre for Ecosystem Management
Coal mine pit lakes may form at mine closure when voids formed through mining extractions have extended below groundwater. Internationally, acid and metalliferous drainage (AMD) is a common problem for coal pit lake water quality. Even if not acidic, pit lake water quality may become degraded gradually through dissolution of contaminants and evapoconcentration. Contaminated coal pit lake waters can present significant risk to both surrounding and regional communities and natural environments. Pit lake waters may discharge into surface and groundwater; or directly present risks to wildlife, stock and human end users. Riverine flow-through is increasingly being proposed to mitigate pit lake water contamination. This paper presents the motivation for, and key processes and considerations regarding a flow-through final lake hydrology. International case studies as precedent and lessons for future application are also described from a review of literature describing pit lakes that use or propose surface water inflows and discharge as key components of their closure and pit lake management designs. Chemical and biological processes such as dilution, absorption and flocculation and sedimentation reduce solute loads from river and lake. We conclude that riverine lake flow-through may often be a valid mine closure strategy for pit lakes with poor water quality. Although, we caution that maintenance of existing riverine system values must be maintained first and foremost, we further suggest that decant river water quality may, in some circumstances, be improved; notably in examples of meso-eutrophic river waters flowing through slightly acidic pit lakes. Flow-through closure proposals for coal pit lakes must be scientifically justifiable and follow a risk assessment approach. Due to the high-uncertainty, biotic and physico-chemical attributes of both upper and lower river and lake should be well monitored. Monitoring should directly feed into an adaptive management framework approved by key stakeholders.