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3.3  Prediction of water quality downstream of a mine

Site-specific factors such as dilution, natural alkalinity, baseline or background concentrations of dissolved components, and historical mine drainages will impact on the water quality at a proposed mine. To predict water quality at a point of interest downstream of a mine (e.g. consent compliance points), information on the hydrogeology and water chemistry and how these parameters change with time at the proposed mine site is required. This information can then be integrated with predicted chemistry and volume of potential mine drainage to predict the water quality downstream of a mine.

Site hydrogeology and background water quality information is integrated with information on mine drainage to predict downstream water quality using reactive transport modelling (Figure 10). Reactive transport modelling must be used to predict downstream water chemistry because reactive components (both acid and neutral) are present in stream waters and mine drainage. This means the prediction of downstream water chemistry based on only dilution ratios of different mine drainage components is inadequate. Reactive components include:

• Alkalinity and pH;
• Dissolved oxygen;
• Dissolved Fe2+ and Fe3+, Al;
• Fine-grained (colloidal) particulate Fe3+ and Al minerals; and
• Trace elements.

Reactive transport modelling requires specialist knowledge and should be undertaken by appropriately qualified and experienced personnel.

Figure 10. Basic process for determining water quality downstream of a mine.

The likely mine drainage chemistry can be determined from subsequent chapters, while collection of relevant site hydrogeological and background water quality is outlined below.

3.3.1     Site hydrogeology

Site hydrogeological data for predicting downstream water quality are similar to those used for determining baseline hydrogeology (section 2.3.1), but include a projected value of mine drainage volume. The volume of mine drainage relates to the type of mining (opencast vs underground), mine scheduling, the area of disturbance, as well as hydrogeological models that include calculation of the amount of rainfall that contributes to surface flows vs groundwater. These models are best completed by a suitably qualified specialist and are often developed and refined during the life of a mine.

A conservative mine drainage flow at an opencast mine could be calculated by assuming that 100% of rainfall or stream flow for the area of disturbance becomes mine drainage.

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