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2.3.2 Summary of key system components, processes and interactions

Summary

The geological setting has a major influence on many hydrological and ecological processes, and is one of the main drivers that form the landscape. It controls key processes such as groundwater recharge and discharge as well as inter-aquifer interaction. It also significantly influences the interaction between the surface water and groundwater systems.

The aim of this section is to describe the conceptual understanding of how key system components such as geology, hydrogeology and surface hydrology operate and interact in the Clarence-Moreton bioregion. In addition to drawing on existing literature and information, new analyses conducted as part of the Clarence-Moreton Bioregional Assessment have greatly enhanced the knowledge of geology, hydrogeology and surface water in the bioregion.

For example, three-dimensional geological models were developed to improve the understanding of geology and hydrogeology in the Clarence-Moreton bioregion, and explain spatial hydrological processes. These were supplemented by the use of existing pictorial conceptual models developed by the Queensland Department of Environment and Heritage Protection. The two-dimensional and three-dimensional representations of geology facilitated the identification of pathways between different system components of the hydrological cycle, which explained the spatial relationships between different stratigraphic units and the Walloon Coal Measures (the main target of coal seam gas (CSG) exploration in the Clarence-Moreton bioregion).

Based on available information (as of 25 February 2016), the assessment of the baseline and the coal resource development pathway (CRDP) outlined in Section 2.3.4 of this product suggested that only one coal mine (Jeebropilly Mine) exists in the baseline coal resource development (baseline) with no future plans for new coal mine development in the Clarence-Moreton bioregion. Furthermore, the assessment suggested that only one CSG development may potentially proceed (West Casino Gas Project), and is therefore considered as an additional coal resource development (ACRD) in the Richmond river basin. A recent decision by Metgasco Limited (Metgasco) (16 December 2015) to sell back their petroleum exploration licenses (PELs) and petroleum production license application (PPLA) to the NSW Government effectively means that future development of any CSG resources in the Clarence-Moreton bioregion is highly uncertain. However, as per the CRDP methodology (companion submethodology M04 (as listed in Table 1) for developing a coal resource development pathway (Lewis, 2014)), once the CRDP is determined, it is not changed for BA purposes, even in cases such as this where Metgasco have now discontinued their operations in the Clarence-Moreton bioregion.

The basin-wide three-dimensional geological model demonstrated that a basement high separates the Richmond river basin and the Bremer river basin (where the Jeebropilly Mine is located). This lack of hydraulic connection between the Bremer and Richmond river basins led the Assessment team to only focus on modelling impacts for the West Casino Gas Project, thus excluding the Jeebropilly Mine during the development of the groundwater model. Consequently, Section 2.3.2 focuses on the geological, hydrogeological and hydrological characteristics of the Richmond river basin to support the development of the groundwater model. However, geological similarities within many parts of the Clarence-Moreton bioregion mean that the conceptual models of hydrological processes in the Richmond river basin could be adopted from other parts of the Clarence-Moreton bioregion such as the adjoining Logan-Albert river basin, the Lockyer Valley and the Brisbane river basin.

Conceptual hydrogeological models that integrate information from geology, hydrogeology and surface water hydrology are subject to uncertainties, particularly in data-scarce areas or in regions with complex geology. The uncertainties resulting from data gaps, data-quality issues and complex geological and hydrogeological settings are discussed in Section 2.3.2.6 . This section highlights the fact that there are significant conceptual uncertainties regarding the role of faults as potential pathways linking deeper stratigraphic units such as the Walloon Coal Measures to shallow aquifers and surface water features in the Richmond river basin. Furthermore, there continues to be considerable uncertainty on the connectivity between deep and shallow aquifers in general. Due to the lack of nested bore sites where different aquifers are monitored simultaneously to assess vertical fluxes, other auxiliary data such as hydrochemistry were used to infer connectivity. The hydrochemical data suggest that there is likely to be aquifer connectivity in some areas within the Richmond river basin.

In contrast to the uncertainties regarding the role of faults and connectivity between shallow and deep aquifers, the role of the Lamington Volcanics as the major preferential recharge area within the Clarence-Moreton bioregion and in particular the Richmond river basin is well understood and supported by multiple lines of evidence. Recharge rates to these volcanic aquifers are at least one order of magnitude higher than recharge rates of sedimentary bedrock units such as the Walloon Coal Measures (the main target of CSG exploration). However, a large proportion of recharge to the Lamington Volcanics discharges into the stream locally with short lag times and following short flow paths; only a small proportion of this recharge percolates to deeper aquifers. The assessment of the spatial distribution of median streamflow rates further underpins the significance of the Lamington Volcanics as a major hydrological feature in the Richmond river basin, where most of the surface runoff is generated. The recharge assessment also confirmed that there are significant spatial variations in recharge rates to the alluvium in the headwaters (upper catchment), mid and lower catchment of the Richmond river basin. Hydrochemical data and analogues from data-rich parts of the Clarence-Moreton bioregion such as the Lockyer Valley or Bremer river basin indicate that sedimentary bedrock discharge to shallower aquifers or surface water is likely to be overwhelmed by the contribution from the Lamington Volcanics. However, in some areas and during periods of low flow (i.e. droughts), the contribution from the sedimentary bedrock may become more significant at the local scale.

Though various data sources representing multiple lines of evidence underpinned the current conceptual hydrogeological model, this model may not be unique. That is, there may be alternative models that adequately describe the geology, hydrogeology and hydrology of the Richmond river basin, and still honour the currently available data.

The detailed conceptual understanding presented in this section also underpins many activities including the landscape classification (Section 2.3.3) and the identification of causal pathways (Section 2.3.5), and thus provides the framework for the numerical groundwater model presented in companion product 2.6.2 for the Clarence-Moreton bioregion (Cui et al., 2016b).

Last updated:
11 July 2017
Thumbnail images of the Clarence-Moreton bioregion

Product Finalisation date

19 January 2017