Consequences and opportunities from river breach and decant of an acidic mine pit lake

Mining typically diverts natural water courses during operations which are then not reinstated at closure allowing an isolated pit lake to form in the open cut void. Pit lake water quality may then degrade over time.A heavy rainfall event led a diverted river to breach a large acidic, coal mine pit lake allowing assessment of river flow-through for both lake and downstream river environments.Lake and river water quality samples were interpreted and compared to end use value guidelines. Fresher, more alkaline and nutrient-richer river water interacted with saline and acidic pit lake water improving upper lake water quality and enabling beneficial end use opportunities.There was no significant risk of toxicity to downstream river livestock drinking water during the period. However, water quality at all sites sampled (including a reference site) exceeded pH and Zn ecosystem protection guidelines, and some recreational and aesthetic guidelines.River flow-through is being trialled as the most sustainable long-term closure option for this lake. Flow-through may also represent the best closure scenarios for mine pit lakes with similar socio-environmental contexts.