Summary: Groundwater Assessment
Matthew Currell, Program Manager, Environmental Engineering; RMIT University
- Due to the nature and size of this project and the fact that it is a “greenfield site” and therefore lacking good baseline data, it is difficult to accurately predict water quality and quantity risks. There are areas where significantly more data collection and critical analysis could and should have been conducted to obtain a better understanding of the likely impacts of the project on ground water and surrounding water bodies.
- The predicted water quality impacts are potentially under-estimated due to the assumptions made and uncertainties in the modelling conducted (Groundwater Assessment (Appendix 4) and Site-Wide Load Balance (Appendix 6b)). This is particularly relevant to predictions of the likely contaminant loads – e.g., copper and other metal concentrations in mine pit water, the proposed tailings storage facility, and downstream surface water bodies.
- The models presented in the EIS can not be considered conservative. Impacts – such as heavy metal concentrations in surface water downstream of the site – are likely to be considerably different to mean or median scenarios predicted in the EIS. See below for specifics on how the assumptions should not be considered conservative.
 This site has no existing experience of mine related groundwater drawdown, inflows of groundwater to mine pits, or water level recovery with which to draw on when determining impacts.
Mine Waste and the Integrated Storage Facility (ISF)
- It has been assumed that the storage of mine waste underwater (‘sub-aqueous’) in the ISF will prevent any oxidation of mine tailings from occurring. This is then assumed to prevent the generation of highly contaminated water. The EIS does not consider a range of scenarios that could result in higher oxidation rates than have been allowed for, and does not provide and experimental basis or data from similar tailings storage facilities worldwide to support the assumption. This is significant, as it underpins all predictions of water quality downstream of the ISF.
Scenarios which would result in significant additional metal loads in waste water and have not been incorporated into the modelling, including mixing events in the ISF caused by extreme weather, oxidation of waste rock occurring more rapidly during transit and disposal than assumed, wall stability issues (e.g. partial landslide of tailings deposits), or formation of significantly higher quantities of acid rock within mine open pits post closure.
- The proponent has concluded that severe and catastrophic events that would result in significant additional water quality and quantity impacts are of “very low probability”. Minimal analysis of the impacts resulting from such events is contained in the EIS, which is not appropriate, even if their probability is considered low. The EIS should present a reasonable analysis of possible ‘worst case scenarios’ and their consequences as well as the prediction of impacts under standard operating conditions.
- Limiting the time between excavation and disposal of waste rock is proposed in the EIS as a major mitigation strategy for preventing poor water quality developing in the ISF. Using an average value of exposure time (12 weeks) and release rate of sulphate/metals over this time may not give the full potential range of metal contamination levels. The surface area of crushed or blasted rock and effect of exposure on contaminant release for typical waste rock particles is assumed, not based on experimental data. It is not clear how applicable these assumptions are, as the oxidation pathway(s) involved and trace/heavy metal element contents in the waste rock may be variable.
- Notwithstanding the above, the base model already predicts that dissolved copper, aluminium, cadmium, chromium and zinc concentrations downstream will exceed Australia/New Zealand guidelines (ANZECC 95% freshwater ecosystem protection guideline).
- The predicted surface water flow volumes at 13 nominated monitoring points vary significantly depending on climate conditions. This presents significant challenges for managing a range of flows and creates significant uncertainty in the water balance and the corresponding heavy metal loads predicted in surface water downstream of the ISF.
Mine Pits/ISF Groundwater Contamination
- The model that has been created doesn’t take the complex geology of the site into consideration. Groundwater flow is likely to be dominated by the highly fractured and complex geology. Water is likely to preferentially follow fracture lines rather than flowing at steady, averaged rates throughout the rock mass. This may result in higher and faster flow volumes than have been predicted by the EIS in particular regions (and lower rates in others). Therefore, the volume and quality of water seeping into mine pits (which will require ongoing treatment and management throughout the mine’s operational and post-closure phase) are not well understood and could be highly variable.
- Metal concentrations in water contacting the mine pit walls are calculated assuming they are limited by the rate that oxygen can diffuse into the exposed rock. It is acknowledged in the EIS that these estimates are uncertain because factors such as depth of fracturing of rock, porosity and saturation levels of the walls are unknown. Greater degrees of oxidation, and therefore higher levels of copper and other heavy metals in the water cannot be ruled out.
- The risk that groundwater flow from below the ISF and/or the abandoned mine pits could transport contaminants downstream into lower lying areas, or towards the surface (i.e. the Frieda River) has not been analysed in any detail. This is a long-term risk that may lead to significant mobilisation of copper and other heavy metals throughout the groundwater-surface water system, particularly following mine closure.
- The models have been generated using present and past climatic conditions. There is no consideration of the possible impacts of the likely future climatic conditions due to climate change. As a result, more extreme climatic conditions (e.g. periods of significantly higher or lower rainfall), which are likely to occur over the life of the project, are not factored into water balance or water quality modelling.
- The background water monitoring that was carried out to establish a baseline with which to compare future impacts was conducted at only four times per year. This does not provide a comprehensive idea of the full variability in the baseline water quality, which is likely to be highly dependent on rainfall (which fluctuates substantially throughout most years). Monthly averages would have provided more robust data for analysis but was not available due to low sampling effort. Alternatively, strategic timing to capture particularly low or high rainfall conditions, and the resulting effects on water quality would have given a more comprehensive baseline.
- No formal uncertainty analysis has been conducted in the groundwater modelling, which limits the ability to understand the level of confidence in predictions of key water quality risks during and post mining.
All predicted impacts assume “standard operating conditions”, assuming no infrastructure failure throughout the life of the mine. Given the operating environment (high rainfall and seismic activity) there is considerable risk that standard operating will be compromised at some point in the life of the mine. This has not been modelled.
The risks to water quality will exist for up to 2000 years; posing an ongoing hazard for downstream communities and the environment. It is unclear how these risks will be managed post mine closure.
Water collected from the open cut pits is proposed to be treated and then discharged into the Ubai Creek prior to flowing into the ISF. It is not clear what treatments will be conducted or how the plant will be constructed, operated or managed.
The Groundwater Assessment does not propose any monitoring of groundwater quality immediately surrounding the mine pits; instead relying on sampling of seepage/inflow into the pits.
With regards to background levels of metals (copper and aluminium) observed within the Frieda Base Camp monitoring site, the data indicates that the existing water quality may have already been impacted by anthropogenic disturbances in the region e.g. during previous mining exploration where drilling of the core has occurred. The Groundwater Assessment states that there are exploration bores with groundwater freely discharging to the surface already. The influence of these and other pre-existing disturbances which have likely led to the current water quality impacts should have been carefully analysed and discussed in the EIS (they are not).
This is also an indication that mobilisation of metals during mining construction is probable in the short term, which would require additional runoff control and management measures.