A major data gap in the Galilee subregion is the lack of reliable bore screen information for determining the source aquifer. The majority of groundwater bores in the Galilee subregion do not have screen information to determine from which aquifer a particular bore is drawing water from. Therefore this analysis has been limited to only those bores that have sufficient information. Accurately determining a particular bores source aquifer is vital for interpreting both hydrochemistry and water level data. Hydrochemistry

The hydrochemistry data used in characterising the groundwater of the Galilee subregion are archival data collected over several decades and therefore there are considerable uncertainties associated with it. Many samples from the initial sample set had to be excluded from the analysis due to insufficient stratigraphic data. Without stratigraphic information, any other information associated with a groundwater sample is meaningless.

Additionally, the maximum uncertainty selected for use in filtering the data was relatively large: ± 10%. Commonly for regional hydrochemical studies ± 5% or better is considered best practice. An error of ± 10% was deemed acceptable to ensure a large number of samples were available for all stratigraphic units, and that there was sufficient spatial coverage in each unit to build a regional picture of hydrochemistry. However, it is possible that this level of uncertainty has obscured some relationships between different analytes in the dataset.

Trace element data are highly variable in the dataset, with no data for many important trace elements in several of the hydrogeologic units. Most trace elements are analysed in only a small number of hydrogeologic units, usually the Hutton-Precipice grouping and Westbourne-Birkhead grouping.

The GAB is a very large and complex groundwater system. A regional overview of the major aquifers in the Galilee subregion has been presented, but further targeted data collection and analysis would refine the conceptual understanding of groundwater hydrodynamics in the subregion, and assist in a more rigorous assessment of aquitard integrity. Further statistical analysis (e.g. cluster analysis of multiple hydrogeologic units) would assist in identifying where inter-aquifer mixing could be occurring, and differentiate regions based on the dominant chemical processes. Additional to major ion chemistry, groundwater isotopic data would greatly improve our understanding of hydrologic processes in the subregion.

In the case of aquifers with similar major ion chemistry, it is difficult to determine whether similarities are the result of aquifer connectivity, or similarities in aquifer composition or chemical processes. Isotopic analyses would help distinguish where similar chemical trends are the result of hydraulic connectivity, or similarities in aquifer material and/or recharge processes. Isotopic data may also help to identify water sources to aquifers or surface features (e.g. provide stronger evidence for which aquifer feeds a spring system), as well as further constrain residence times and flow rates. Potential isotopic systems are 87Sr:86Sr, 2H and 18O, 13C, 36Cl, and 4He:3He. Currently, there are limited isotopic data available for the Eromanga Basin sequence (see Moya et al. (2015) for a summary), and almost none available for the Galilee Basin sequence. Samples have been collected from both Eromanga and Galilee Basin aquifers by Queensland University of Technology but remain unanalysed. Analysis of these samples is warranted, but is beyond the current scope of the BA Programme.

In addition to uncertainties in the major ion data, very limited information on reduction/oxidation (redox) conditions is available in the dataset. For this reason redox chemistry was omitted from this regional overview, however, a sound understanding of redox conditions is necessary to understand the chemical changes that may occur if different groundwater bodies mix, making it essential information to comment on the potential impacts of inter-aquifer leakage. Water level mapping

Uneven spatial distribution of measurement points across the subregion introduces uncertainty to the interpreted water level mapping. In some cases mapping has relied on sparsely distributed measurement points, particularly in the western, deeper portions of the Galilee and Eromanga units. In the case of the Betts Creek beds (upper Permian coal measures), water level mapping for the central and western portion of these units is based solely upon drill stem pressure tests which are inherently less reliable than actual water level measurements.

While the most recent available water level data were utilised to produce the water level maps, the extent of recent measurements (i.e. over the last ten years) is limited. In many cases historical water level measurements, which may have changed, have been used to approximate current levels. Water level trends

Some anomalous hydrographs introduce uncertainty to the data. One bore in the Hooray Sandstone showing a 40+ m rise over six years is not located nearby any bores sealed in the GABSI program, yet there seems no other explanation for the rapid increase in water level than nearby artesian bores being sealed. If there are sealed bores missing from the records available to us, interpretations of trends observed in other bores may be incorrect.

Record lengths are highly variable between bores, even after removing bores with less than two years of data and bores with records finishing before 1997. This makes it difficult to compare the trends observed in different bores; a trend which is statistically significant over a 5 year time frame may seem less clear over 20 years. The magnitude in change of water level is also affected by the length of the record. Bores with very long records (back to the 1950s or 1960s) tend to show changes in water levels of several metres, while records beginning in the last 20 years show much smaller changes.

There are only a small number of nested piezometers meaning comparisons of water level trends between different aquifers are limited. This is unfortunate as comparing time series data for different aquifers at the same location is one of the best ways to investigate potential connectivity between hydrostratigraphic units. Additionally, there are no nested bores in the artesian area of the subregion, meaning connectivity can only be investigated this way in the eastern part of the subregion.

Last updated:
6 December 2018
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