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2.6.2 Groundwater numerical modelling for the Gloucester subregion

Executive summary

View of the Gloucester valley NSW with the Barrington River and associated riparian vegetation in the foreground and the township Gloucester in the distance looking south from the Kia Ora Lookout, 2013 Credit: Heinz Buettikofer, CSIRO

Coal and coal seam gas (CSG) development can potentially affect water-dependent assets (either negatively or positively) through a direct impact on groundwater hydrology. Presented in this product are the modelled hydrological changes in response to likely coal resource development in the Gloucester subregion after December 2012.

First, the methods are summarised and existing models are reviewed, followed by details regarding the development and calibration of the model. The product concludes with probabilistic predictions of hydrological change (using hydrological response variables), including uncertainty analysis and a discussion of model limitations, opportunities and conclusions.

The groundwater numerical modelling in this product has a very specific objective; to probabilistically evaluate the impacts of a single development pathway at specified locations in the landscape to inform the impact and risk analysis (product 3-4). The modelling will therefore not produce a single best estimate of the impact, but will provide an ensemble of predictions.

Groundwater modelling for the Gloucester subregion follows the companion submethodology M07 (as listed in Table 1) for groundwater modelling. Numerical simulation of the impact of the coal resource development pathway (CRDP) on the identified receptors requires a model or model sequence that can simulate the impact on the regional groundwater system, the alluvial groundwater system and the stream network. In the bioregional assessment for the Gloucester subregion (the Assessment) this consists of a pragmatic coupling of three models. Firstly, an analytic element model (referred to as GW AEM) for regional groundwater. This model is designed to simulate the change in drawdown at the receptors associated with the groundwater bores in the Gloucester geological basin weathered zone, and to provide the change in groundwater level underneath the Avon and Karuah alluvium. Secondly, MODFLOW models (referred to as GW ALV) for both the Avon and Karuah alluvium, used to simulate the change in drawdown on receptors associated with the alluvium and the change in surface water - groundwater flux. Thirdly, the Australian Water Resources Assessment (AWRA) landscape (AWRA-L) surface water model to generate streamflow, taking into account surface water rainfall-runoff and surface water - groundwater flux.

Three existing groundwater models are identified that cover various parts of the Gloucester preliminary assessment extent (PAE). All three models are aimed at quantifying the impact of open-cut coal mines on groundwater systems and groundwater levels in alluvial aquifers. All three models are considered unsuitable for direct use in the Assessment, mainly because their spatial extent is too limited to simulate the cumulative hydrological change due the existing and proposed coal resource development.

The Gloucester Basin is considered to be a geologically closed basin with three main hydrogeological units:

  • surface alluvium up to 15 m thick, a semi-confined to unconfined aquifer
  • shallow weathered and fractured rocks up to 150 m thick, a confined to semi-confined aquifer
  • interburden units alternating with coal seams to a maximum depth of about 2500 m, only considered to be water-bearing strata.

The shallow weathered and fractured rock layer underlies the alluvium entirely, and outcrops extensively across the rest of the surface of the Gloucester subregion. Both the Avon and Karuah rivers are unregulated streams, connected with local groundwater. The river system is mostly gaining, with baseflow estimated to be about one-tenth of total streamflow. The alluvial aquifer only receives water from the river system during high flow and flood events.

The CRDP for the Gloucester subregion includes the Duralie and Stratford mines and their expansions and the Rocky Hill Coal Project. AGL’s proposed CSG development in the Gloucester Gas Project, stage 1 gas field development area is also included. The CRDP was confirmed in October 2015. There may be further stages (beyond Stage 1) of AGL’s proposed CSG development in the Gloucester Gas Project but there is no publicly available documentation of these as at October 2015. In December 2015 AGL withdrew from its proposed Gloucester Gas Project and, according to the companion submethodology M04 (as listed in Table 1) for developing a coal resource development pathway, once the CRDP is finalised (October 2015) it is not revisited.

The regional GW AE model, the alluvial MODFLOW models and the AWRA-L model evaluated 10,000 parameter combinations, generated through a Latin Hypercube sampling to simulate the drawdown due to the additional coal resource development. These model results are used to train emulators, statistical models that replace the actual model chain in the uncertainty analysis.

Normal prior distributions are specified for most parameters, either in native or log space, with a mean equal to the centre of the range sampled in the design of experiment. The variance is chosen such that 99% of the probability mass is within the sampled range. A covariance is specified between the parameters controlling hydraulic conductivity and storage and between parameters controlling horizontal and vertical fault hydraulic properties. For the parameters controlling the decrease with depth of hydraulic properties and the fault hydraulic conductivity, the mean is not chosen to be equal to the centre of the sampled range to ensure the hydrologic change is over- rather than underestimated.

The prior parameter distributions of the analytic element model are constrained with the maximum CSG water production rate which resulted in posterior probability distributions for maximum difference in drawdown for one realisation within an ensemble of groundwater modelling runs, obtained by choosing the maximum of the time series of differences between two futures (dmax) and year of maximum change (tmax) indicating that: (i) the effect on dmax is highly localized around the mine pits; (ii) it is unlikely to have drawdowns due to additional coal resource development in excess of 1 m, except in proximity of the mine pits; (iii) the largest dmax values are attained within or shortly after the production life of the coal mines and CSG field. Smaller drawdowns due to additional coal resource development, further away from the centre of the development activity, are realized at later times, where the smallest noticeable drawdowns due to additional coal resource development are not fully realized within the simulation timeframe; and (iv) in the modelled drawdowns, the effect of CSG production and the presence of faults and fractures cannot be distinguished from the effect of coal mining in the Gloucester subregion.

The prior parameter distributions for both alluvial MODFLOW models are constrained with historical estimates of the water balance. The resulting posterior ensembles of predictions indicate that (i) hydrological change is limited to the immediate vicinity of coal mines and (ii) it is very unlikely to have drawdown due to additional coal resource development in excess of 1 m in the alluvial aquifers and that (iii) it is very unlikely that the drawdown will cause the groundwater levels to drop below the drainage base of the stream network.

The qualitative uncertainty analysis lists the main model assumptions and choices and discusses their potential effect on the predictions. The model choices with the greatest perceived potential impact on the predictions are related to the implementation of the CRDP. Other model assumptions, such as the hybrid modelling approach, the choice for drainage boundary to represent the stream network and the length of the simulation period are shown to be conservative choices (i.e. the hydrological change is overestimated rather than underestimated).

The modelling framework is tailored to the specific CRDP and receptors and therefore should not be used for any other purpose without a rigorous reassessment of the validity of the model assumptions. The modelling did highlight that improved characterisation of hydraulic properties of the surface weathered and fractured rock layer and more detailed information of local geology around development have the most potential to reduce predictive uncertainty.

The results of this groundwater numerical modelling are used to inform the impact and risk analysis (product 3-4).

 

 
Last updated:
13 August 2018
Thumbnail of the Gloucester subregion
PRODUCT FINALISATION DATE
2018
PRODUCT CONTENTS