TORONTO (miningweekly.com) – Toronto- and New York-listed Vista Gold on Friday said it has completed the first phase of water treatment at the Mt Todd mine site in Northern Territory, Australia, and has received authorisation to release treated water from the mine site into the Edith river, subject to stringent water quality and environmental standards.

From when the company first began the Mt Todd gold project, Vista had to manage acidic water accumulated on site. As part of the water management plan to protect the nearby Edith river system, about 10.3 gigalitres of water affected by acid rock drainage is stored on site in the Batman pit.

Vista had undertaken laboratory, bench-scale and pilot-scale testing programmes to evaluate the effectiveness of the water treatment methods to be used at the Mt Todd gold project and from October 2012, had added more than 10 000 t of finely ground limestone to the water in the Batman pit.

The water quality had rapidly improved with the pH rising from 3.3 to 6.4, with significant decreases in the concentrations of aluminium, copper, chromium, iron and lead as a result of precipitation. Vista said it continued to add about 2 000 t of hydrated lime and believed water quality would continue to improve as the water approached the pH target of 7.

Vista had been authorised to release treated water only when flow rates were sufficient to ensure that Australian drinking water standards for health were achieved downstream of the mixing zone.

The company carries out daily water-quality testing during periods of discharge, and yearly macro-invertebrate and sediment testing programmes in the Edith river to ensure that there is no long-term impact to the environment.

Based on historic flows in the Edith river, Vista estimates that two wet seasons would be required to complete the discharge of water from the pit.

To date, Vista had invested A$9-million in the water treatment programme and the construction of the automated management system.

Vista is currently working on completing a feasibility study for the Mt Todd gold project.