2.1.3.3.3 Water chemistry and quality

Spatial variability of groundwater chemistry

Groundwater chemistry can provide useful insights into a range of important groundwater processes, including:

This section describes the results of analyses of groundwater chemistry from the records in the Queensland and NSW groundwater databases. Multivariate statistical analyses of groundwater chemistry records (hierarchical cluster analysis; HCA) were conducted to make inferences about the likely processes that control spatial and temporal variability of water chemistry. These analyses are useful in identifying areas where deep aquifers (bedrock) might interact with shallow aquifers (alluvia) and where aquifers interact with streams. It will support the development of a conceptual model of causal pathways and a numerical groundwater model in the context of coal mining and potential CSG development.

For different river basins within the Clarence-Moreton bioregion, a hydrochemical assessment of different subsets of the groundwater chemistry dataset was conducted to account for region-specific characteristics. The analysis focused on areas with similar geology to avoid ‘dilution’ of the hydrochemical signal. For example, in the Bremer river basin, Walloon Coal Measures and Main Range Volcanics form the bedrock underneath much of the alluvium, whereas in the Lockyer Valley, the Walloon Coal Measures are almost absent. The different subsets of hydrochemistry for which a multivariate statistical analysis was conducted are as follows:

The results of multivariate statistical analyses should not be considered in isolation, as it is possible to misinterpret them if the results are not placed within the geological and hydrogeological context. Therefore, it is important that these results are interpreted in combination with additional lines of evidence such as:

Only major findings are presented for the results of the multivariate statistical analysis. The median concentrations of the identified hydrochemical clusters are also shown on Piper plots to compare the major differences between clusters. More detail will be provided as part of companion product 2.3 for the Clarence-Moreton bioregion (Raiber et al., 2016), where the hydrochemical patterns will be assessed for specific areas within the Clarence-Moreton bioregion to determine the degree of hydraulic connectivity between bedrock and shallow aquifers and between groundwater and surface water. Where sufficient data exist, this more detailed assessment will also include the differentiation of hydrochemical patterns during dry and wet periods.

Bedrock hydrochemical assessment

HCA of the bedrock water chemistry records in the Clarence-Moreton bioregion demonstrates that there is considerable hydrochemical variability, which is likely to be linked to the geological characteristics of the different bedrock formations. Following the identification and removal of outliers, seven distinct clusters were identified. The median values for the different parameters were calculated to identify the major characteristic of each cluster (Table 16). Cluster membership and source aquifer of the water chemistry sample (Figure 55) were compared to show how many water chemistry records from each stratigraphic unit were assigned to the different clusters. The spatial variability of the clusters is shown in Figure 54 .

Figure 54 Spatial variability of hierarchical cluster analysis membership of bedrock groundwater in the Clarence-Moreton bioregion

Data: Bioregional Assessment Programme (Dataset 17)

Hydrochemical characteristics of major bedrock stratigraphic units

Woogaroo Subgroup. The Woogaroo Subgroup is the oldest bedrock stratigraphic unit in the Clarence-Moreton Basin. Most groundwater samples collected from the Woogaroo Subgroup were assigned during the HCA to clusters 1, 4 and 5 (Figure 55, Table 15, Table 16).

Clusters 1 and 5 generally contain fresh groundwater with median EC of 820 µS/cm and 210 µS/cm, respectively, and a dominance of HCO₃ over Cl (Figure 55). Cluster 4 contains samples that are brackish with a median EC of 3600 µS/cm. Overall, the Woogaroo Subgroup is the bedrock formation that contains the freshest groundwater in the Clarence-Moreton bioregion. This is likely attributed to the dominance of coarse-grained and clean quartz-rich sandstone as well as quartz-rich granule conglomerate over minor mudstones or siltstones (Rassam et al., 2014), and the lithological nature of this aquifer, which facilitates rapid groundwater recharge.

Some samples from the Woogaroo Subgroup, however, are assigned to cluster 2, which contains substantially more saline groundwater (median EC 12,000 µS/cm). The reasons are not clear, but could be explained in a few ways: (i) local differences in the composition of the Woogaroo Subgroup that impede groundwater recharge and movement, (ii) interactions with over- or underlying aquifer or (iii) the documented screened intervals of the bores were incorrectly assigned to the Woogaroo Subgroup.

Table 15 Median values for hydrochemical parameters of identified clusters in bedrock groundwater in the Clarence-Moreton bioregion

Data: Bureau of Meteorology (Dataset 1), Bioregional Assessment Programme (Dataset 17)

Gatton Sandstone. The Gatton Sandstone is the second oldest bedrock stratigraphic unit of the Clarence-Moreton bioregion in this multivariate statistical analysis. Most hydrochemical records of the Gatton Sandstone are assigned to clusters 2, 3 and 4 (Figure 55, Table 16).

Groundwater assigned to cluster 2 is generally saline (median EC 12,000 µS/cm) with very low HCO₃:Cl ratios (Figure 55), whereas groundwater grouped into clusters 3 and 4 is brackish and has higher HCO₃:Cl ratios. Nearly 60% of all groundwater hydrochemical records in cluster 2 originate from the Gatton Sandstone, highlighting that the Gatton Sandstone appears to contain the most saline groundwater in the Clarence-Moreton bioregion. This distinct difference, when compared to the underlying Woogaroo Subgroup, also suggests that the hydraulic properties of the Gatton Sandstone are very different. While the Gatton Sandstone is also dominated by sandstones, these are likely to contain substantially more clay and cement (i.e. a matrix such as calcite, dolomite or silicate minerals that bind together the grains of the rocks) than the clean sandstones of the Woogaroo Subgroup (Rassam et al., 2014). As a result of this and also the commonly thick weathering profiles, groundwater recharge to the Gatton Sandstone is likely to be very slow and the potential for evapotranspiration to occur during or prior to groundwater recharge is high. All groundwater samples assigned to cluster 4 are sourced from the Gatton Sandstone and the Woogaroo Subgroup in the western part of the Lockyer Valley near Helidon. These groundwater sources have very high HCO₃:Cl ratios (Figure 55) and elevated pH.

Figure 55 Piper diagram showing the major ion relationships of different bedrock hydrochemical clusters in the Clarence-Moreton bioregion (based on the median concentrations shown in Table 15)

Table 16 Cluster membership of bedrock groundwater chemistry sampling records in the Clarence-Moreton bioregion

Data: Bureau of Meteorology (Dataset 1), Bioregional Assessment Programme (Dataset 17)

Koukandowie Formation. The hydrochemistry of groundwater collected from the Koukandowie Formation in the Clarence-Moreton bioregion shows similarities to the Gatton Sandstone, as indicated by the assignment of all samples to clusters 1, 2 and 3 and the dominance of brackish to saline groundwater. This indicates that the composition of the Koukandowie Formation is similar to the Gatton Sandstone, and hydraulic properties are likely similar.

Walloon Coal Measures. Most Walloon Coal Measures groundwater chemistry records contained within the Queensland and NSW groundwater databases for the Clarence-Moreton bioregion were collected from the Bremer river basin and the Logan-Albert river basin. Most of the outliers removed from the entire groundwater chemistry dataset prior to HCA are from the Walloon Coal Measures in the Bremer river basin (Figure 54). The characteristic properties that differentiate the outliers from other samples are their very high pH and very low HCO₃ concentrations (pH is outside the range where HCO₃ is the dominant carbonate species).

During the HCA, the hydrochemical records of the Walloon Coal Measures were mostly assigned to clusters 1, 2 and 3. A further subdivision of cluster 2 into clusters 2a and 2b ( Table 15 and Figure 55) shows that there are two distinct groups with a median of 9,580 µS/cm and 19,500 µS/cm, respectively. Most of the Walloon Coal Measures groundwater samples were assigned to the more saline cluster 2b. This is attributed to very low recharge rates and long retention times in the unsaturated zone prior to groundwater recharge, causing a high degree of evapotranspiration.

Walloon Coal Measures groundwater that has interacted with coal is assigned to cluster 6. These groundwater samples show the typical characteristics of CSG groundwater (Van Voast, 2003), such as elevated pH and very high Na and HCO₃ concentrations, and simultaneously low Ca, Mg and SO₄ concentrations as shown in the Piper diagram (Figure 55). This was independently confirmed by the analysis of dissolved methane (Raiber, unpublished data) which showed high concentrations for these groundwater samples. However, rather than being controlled exclusively by methanogenesis, some of these characteristics are also due to the influence of ion exchange: Ca and Mg are exchanged for Na. This is indicated by the large number of samples from the Main Range Volcanics and Lamington Volcanics contained in the same cluster (Table 16 and Figure 54). A further subdivision of this cluster would likely result in a separation of groundwater from the Walloon Coal Measure and the volcanic aquifers.

Bungawalbin Member (Orara Formation). Only four groundwater chemistry records assigned to the Bungawalbin Member were included in the HCA. These were assigned to clusters 3, 5 and 7 (Figure 55, Table 16), but due to the small number of samples, it is not possible to make any inferences about groundwater chemistry patterns within the Bungawalbin Member.

Piora Member (Grafton Formation). Groundwater chemistry samples from the Piora Member, which is only present in the NSW part of the Clarence-Moreton Basin, were assigned to clusters 1, 2, 5 and 7 (Figure 55, Table 16). Most of the samples assigned to these clusters contain comparatively fresh groundwater, and this freshness suggests that the Piora Member is likely to be a good aquifer (i.e. relatively high yields and relatively fresh groundwater) with comparatively high recharge rates. The similarity of groundwater of the Piora Member and groundwater sourced from the Lamington Volcanics suggests that the Piora Member is recharged through the overlying basalts.

Rapville Member (Grafton Formation). Similarly to the Piora Member, the Rapville Member is characterised by variable groundwater chemistry, as documented by the assignment of samples to clusters 1, 2, 3, 5 and 7 (Figure 55, Table 16). This might reflect a variable composition of this formation, but possibly also variable recharge processes. For example, the fresh groundwater of the Rapville Member assigned to cluster 5 might be related to recharge through the Lamington Volcanics, which overlie the formation in the north-eastern part of NSW, whereas the brackish groundwater might be related to recharge through outcrops of the formation. In contrast to the Piora Member, groundwater samples of the Rapville Member are also assigned to cluster 3, which contains more saline groundwater.

Main Range Volcanics and Lamington Volcanics. Most groundwater samples from the Main Range Volcanics are assigned to cluster 1, which contains samples that are characterised by their freshness (median 820 µS/cm) and high HCO₃:Cl ratios (Figure 55). Interestingly, the Lamington Volcanics show more hydrochemical variability, as documented by assignment of most samples to clusters 1, 5 and 7. Cluster 7 samples are fresher than cluster 1 groundwater samples and have higher HCO₃:Cl ratios (Figure 55). Furthermore, they were collected from bores that are shallower (median depth of 35 m below ground level) than the bores from which the cluster 1 samples originate (median depth of 62 m). This suggests that within the Lamington Volcanics, there appears to be a depth-related resolution that results in an increase of EC with depth. This indicates that there are shallow flow paths where groundwater is recharged rapidly and probably has only short residence times within the aquifers before it discharges at springs (discussed in more detail in companion product 2.3 for the Clarence-Moreton bioregion (Raiber et al., 2016)).

Lockyer Valley hydrochemical assessment

Seven clusters were identified during the HCA of groundwater chemistry in the Lockyer Valley. The main focus of this catchment-scale hydrochemical assessment was to identify areas where the alluvial aquifers interact with the bedrock or with surface water systems. While a very comprehensive evaluation of the spatial and temporal patterns is beyond the scope of the Assessment (but will be studied in more detail in subsequent parts of this Assessment), some key characteristics are:

Figure 56 Piper diagram showing the major ion relationships of hydrochemical clusters in the Lockyer Valley (based on the median concentrations shown in Table 17)