Science, Technology and Engineering

Environmental Geoscience

Honours Projects

Geochemistry and Arsenic Contamination of Gold-mine Waste Water

Susanna Finger

At a central Victorian gold mine, water pumped from underground workings has been found to contain concentrations of iron (Fe) and arsenic (As) above recommended EPA recommended levels. In addition the water contains manganese (Mn) levels which fluctuate, but often exceed EPA recommended levels.

The water is treated by passing it through an aeration tower; oxidising ferrous iron to ferric iron, which precipitates as poorly crystalline ferric oxyhydroxide. The precipitate is trapped in three settling ponds, which also have Typhus well established. This vegetation aids the removal of manganese through biologically mediated processes. The water is then delivered to a siltation dam, which also collects run-off from the site, before it travels to a wetland. Arsenic is removed by adsorption onto the Fe-oxyhydroxides, and this process has worked for much of the mine's history. However, in recent years elevated levels of arsenic have been detected at the siltation dam outlet, necessitating addition treatment by ferrous sulphate dosing (which increases the Fe:As ratio and thus removal of arsenic).

The main aim of the study was to identify where arsenic remobilisation was occurring, the mechanism for it's release and suggest ways of overcoming the problem. In addition a basic water analysis was required to better understand the chemistry of the treatment system.

Analysis of the water by ICP-AES, AAS, HPLC and simple titration indicated the high concentrations of many major ions in solution originating from the underground workings, namely bicarbonate (~360ppm), calcium (~130ppm), magnesium (~215ppm), chloride (~740ppm), sodium (~300-330ppm) and sulphate (~260ppm). Other major ions displayed concentrations of ~8-14ppm (potassium) and silica (<25ppm). The patterns observed for these major ions strongly indicates the influence of both CO₂ degassing (at the Aeration Tower) and calcium carbonate precipitation at the Siltation Dam Inlet (which was confirmed as aragonite and calcite by XRF and XRD analysis) with little other variation evident. There is only one apparent period where water chemistry is affected by evaporative effects due to climatic variation, this being the concentration of chloride, sulphate, sodium and potassium at the Wetland during the hottest month of the sampling period (February).

In general arsenic concentrations decreased through the system, and this observation is consistent over time. However, there was an increase in arsenic concentration between sites 5 and 6 (i.e. the Siltation Dam), with the magnitude of this increase decreasing from February to May, so that it was absent in June and July.

From the results obtained the main area of arsenic remobilisation was identified as the Siltation Dam, and due to the seasonal nature of the observed increase in concentration, and based on previous work by Sproal (1998) the reason was concluded to be re-dissolution of iron-oxyhydroxides (and adsorbed arsenic) into the bottom reducing water of the centre of the Siltation Dam during a period of stratification caused by a surface air temperature increase during warmer months.

Arsenic released by this mechanism then concentrates in the bottom waters of the dam and gradually diffuses into the rest of the dam, allowing the observation of a concentration increase. Iron concentrations do not increase at this point due to the oxidising conditions of the upper layer of the water column causing its re-precipitation as Fe-oxyhydroxides. Arsenic is not re-adsorbed due to an insufficient concentration of iron.

A simple, inexpensive remedy to the problem of As remobilisation in the siltation dam would be the positioning of an air bubbler on the floor of the deepest part of the dam. This would then be used to oxygenate the bottom waters, preventing water stratification.