4.6 Groundwater

There are many references in the sections above to the need to monitor groundwater. What surface water and groundwater monitoring have in common is the need for good baseline (pre-mining), operational and closure data. The type of monitoring required (water level, water quality, aquifer yield, macro- invertebrates) and the locations of monitoring systems (close to the mine, and both upstream and at downstream compliance points) invariably need to evolve through time to address the different requirements at each of these stages in the mine’s life. In all cases, the key receptors (surface ecosystems, drinking or irrigation water, groundwater ecosystems) need to first be identified to ensure that the monitoring program is well targeted both to detect changes that may affect the receptors and to meet compliance targets.

The occurrence of macro-invertebrate organisms (stygofauna) in groundwater is increasingly being recognised and, depending on location, the monitoring of them should be included in site environmental monitoring and management plans. The strongest and earliest push for this in Australia was in karst and paleo-channel environments in Western Australia, but leading practice mining operations take stygofauna into account more broadly. Leading practice guidance on identifying and monitoring groundwater ecosystems is provided by WAEPA (2013) and Richardson et al. (2011.)

In contrast to surface water monitoring, a groundwater monitoring program needs to address multilayered hydrogeology created by changes in geology as a function of depth (Sundaram et al 2009; OOW 2014). Several aquifer formations may need to be monitored to assess both environmental impacts (such as groundwater drawdown and contamination of aquifers) and mine safety aspects (such as the stability of open-pit walls and underground workings). Commonly, there is a need to place monitoring bores at different depths at the same location so that the extent of vertical connection between aquifer systems can be determined. Another point of difference from surface water monitoring programs is that access to sampling points requires the installation of bores, which are usually substantially more costly to establish than a surface water monitoring site. Hence, the cost of access is more likely to limit the coverage of the groundwater sampling site network. That cost must be balanced against the need to undertake sufficient monitoring of groundwater to address both environmental and site safety performance aspects.

Detailed consideration of near-field hydrogeology is needed to produce a design specification for a monitoring borefield that will provide timely data cost-effectively. If monitoring systems are located too far away, or in hydraulically inappropriate locations, a developing cone of depression or contaminant plume may go undetected for so long that it will be too late for mitigation of the source to be effective in preventing the impact. A leading practice groundwater monitoring program needs the spatial and temporal resolution necessary to detect change so that management action can be taken before the extent of that change causes irreversible damage.

In summary, the groundwater monitoring network must be sufficient to:

  • identify groundwater yields in the exploration stage
  • establish the extent of hydraulic interactions between aquifers and surface water sources
  • detect impacts on water quality and quantity
  • measure or allow the prediction of impacts on identified sensitive receptors
  • enable the safe construction and ongoing operation of open-pit and underground workings and waste storages.

The number and location of groundwater monitoring bores is determined on a case-by-case basis, as every site is unique in its combination of geology and environmental receptors. Open-hole monitoring bores allow both the measurement of groundwater levels and the sampling and analysis of groundwater for quality (Sundaram et al. 2009). When only water level is needed, appropriately installed and calibrated pneumatic or vibrating wire piezometers can be an alternative to manual dipping of conventional open bore holes for monitoring groundwater levels within a geological formation.

One issue of key practical relevance is that the near-field monitoring bores installed at the beginning of a mine’s operating life may be subsequently destroyed as a result of the expansion of open pits, WRDs, or both. The consequences can be the loss of continuity of the baseline water-quality record needed to monitor performance and develop site closure criteria for groundwater. If one or more bores are lost, new bores should be installed far enough in advance that a period of parallel monitoring can be done to establish continuity of the monitoring record. In the case of bores that are encroached on by expanding WRDs, it may be possible with care to vertically extend the bore casing.

Specialised drilling, well construction and monitoring techniques and water sampling methods are needed to ensure a high degree of reliability in the monitoring data that is produced (Sundaram et al. 2009). The reliable measurement of groundwater quality requires special care and differs from surface water monitoring in several aspects. The installation of a monitoring bore, and the process used for the retrieval of water samples from various depths within the bore, need to be optimised to minimise the risk of sample contamination or chemical changes (such as those caused by oxygenation). Commonly, multiple, specially cleaned samplers need to be prepared for each bore sampling round. This is necessary both to minimise the potential for sample contamination and to account for the fact that different sampling methods may be needed, depending on the depth or recharge rate of each bore. The requirement for several different types of sampling device or multiple cleaned devices increases sample collection costs compared with surface water monitoring.

The groundwater monitoring program developed for a site should describe the location and depth interval of all monitoring bores and the frequency required for groundwater level and quality measurement (OOW 2014). In dynamic situations, the use of automatic data loggers should be considered the standard for groundwater level measurement. Instances where loggers may be required are at the groundwater – surface water interface, or when assessing the shorter term impacts of groundwater pumping on nearby bore users. Predictive numerical groundwater models are regularly used to assess the likely future impacts of mining on groundwater, and are increasingly being required by regulators as part of the performance assessment process (Barnett et al. 2012; OOW 2014). Model calibration requires groundwater-level data to be collected on a monthly basis as a minimum.

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