9.1.6 Step 5: Predict stream water chemistry
Section 3.3 and Figure 10 provide an outline of the process for predicting water chemistry downstream of a mine drainage discharge. Information on mine drainage chemistry is integrated with information on site hydrogeology and background water quality using reactive transport modelling. Reactive transport modelling must be used because reactive components (both acid and neutral) are present in stream waters and mine drainage.
For the proposed mine under typical conditions, dewatering and surface runoff from the pit are considered to produce a discharge of 100 L/s into the receiving environment (the river has a flow of 2000 L/s and pristine water quality). Based on the predicted chemistry (Section 9.1.5) and likely flow from the pit, mine drainage is likely to impact downstream water quality. However, reactive transport modelling undertaken by a qualified and experienced professional would provide more detailed information on the likely impact on water quality. In this case, reactive transport modelling indicates that downstream water chemistry will be pH 3.4, Fe 9 mg/L, and Al 40 mg/L.
A checklist for the steps involved in predicting downstream water chemistry is provided in section 3.4.
9.1.7 Step 6: Determine the ecological impact
The ecological impact associated with the predicted downstream water chemistry is determined using Figure 12. The pathway to determine the potential impact associated with the hypothetical proposed mine using Figure 12 is shown in Figure 38.
Figure 38. Modified Figure 12 showing the pathway to determine the potential ecological outcome for the proposed mine.
The description of outcome 1 is ‘The most severe impact on stream ecosystems occurs when water is highly acidic (pH < 4) and has a high concentration of metals. No New Zealand fish can survive for long in such water. Few macroinvertebrates, of very limited diversity, will be found. Algae and microbes, however, may be present, and even in high abundance in some cases. These communities tend to be dominated by a few taxa that are able to tolerate the stressful conditions. Remedial and treatment strategies will be essential.’
9.1.8 Step 7: Consider whether impacts are acceptable
This framework does not establish explicit ‘acceptable’ water quality criteria, but rather provides a robust scientific basis for this decision to be made by relevant parties. Section 8.1 provides an overview of the factors to be considered in determining acceptability of a given impact such as:
- Presence of any iconic or endangered species;
- Current stream state;
- Downstream water use; and
- Any stream quality criteria specified in regional plans.
There may also be broader social and economic considerations that determine whether the impact is acceptable or not.
This step requires discussion and consultation with interested parties, consenting authorities, iwi, and DOC to agree on the acceptable level of impact for the site.
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