How could coal resource development result in hydrological changes?

The assessment identified potential hazards (Dataset 6) associated with coal mines that could result in hydrological changes, such as aquifer depressuriation due to groundwater extraction. In the Hunter subregion, waste rock blasting, excavation and storage, subsidence and subsurface fracturing from longwall mining and mine dewatering were identified as key hazards. Hazards in scope were further assessed by first estimating relevant hydrological changes through surface water and groundwater modelling and then identifying potential impacts on, and risks to, water-dependent ecosystems and assets (described in the following sections).

After the potential hazards were identified, the chain of events that commonly arise from coal resource development activities were analysed and categorised into four causal pathway groups (letters correspond to those in Figure 7):

‘Subsurface depressurisation and dewatering’ is triggered by extraction of groundwater to enable CSG extraction and dewatering of open-cut mine pits. This potentially directly affects the groundwater system, and indirectly affects surface water – groundwater interactions. Potential effects are likely to be in the short term (less than 5 years) for groundwater pressure changes, to long term (10 to 100s of years) for changes in groundwater movement or quality.
‘Subsurface physical flow paths’ are initiated by activities that cause physical changes to the rock mass or geological layers, resulting in new physical paths that water may potentially gain access to and flow along. Potential effects are in the medium (5 to 10 years) to long term and are likely to be restricted to aquifer or aquifer outcrop areas, but can also affect connected watercourses within and downstream of mines.
‘Surface water drainage’ starts with activities that physically disrupt the surface and near-surface materials (vegetation, topsoil, weathered rock). Medium- to long-term cumulative effects are possible for watercourses within and downstream of development. Activities may include construction of diversion walls and drains, interception of runoff, realignment of streams, and groundwater extraction for underground coal mining leading to subsidence of land surface.
‘Operational water management’ is triggered by modification of surface water systems to allow storage, disposal, processing and use of extracted water. Potential effects are likely to be in the medium to long term and include impacts on watercourses within and downstream of operations.

Many activities related to coal resource development may cause local or on-site changes to surface water or groundwater. These are not considered explicitly in bioregional assessments because they are assumed to be adequately managed by site-based risk management and mitigation procedures, and are unlikely to result in cumulative impacts.

Mine water use in NSW is regulated. Mines are required to prepare mine water management plans that identify environmental impacts and provide options for minimising impacts, hold licences for water extractions and discharges, and adhere to licence conditions intended to protect the environment on site and off site (see Section 2.3.4 in the conceptual modelling (Dawes et al., 2018)).

Although most of the regulatory framework is geared towards site-specific controls, the Hunter River Salinity Trading Scheme was introduced to manage the cumulative impacts on river salinity from discharges of water to the river from coal mines and power generators. Coal resource development can lead to changes in other water quality parameters in streams and aquifers, but these changes were not part of the scope of the bioregional assessments and their impacts have not been considered.