|This paper uses major ion chemistry and a variety of different isotopes to determine that significant recharge occurred to an alluvial aquifer in eastern Australia during a major recent flood event, particularly in the upper parts of the catchment. The authors point out that climate change in this area will probably cause increased severity of floods, so the results of this study have important implications for groundwater management in the area. Therefore this paper could be a significant addition to the literature on this topic, and I think that the overall conclusions of this paper are likely to be correct, but there are several problems that need to be addressed. |
1. The authors relate the stable isotope composition of a recent flood event (Jan 2011) to the stable isotope composition of surface water samples collected in June 2011. However, the stream hydrograph presented (Fig. 4) shows that the flood peak had passed by the end of January. The authors argue that the surface water samples represent floodwater, because the upstream dam was filled during the flood and continued to flow into the creek up to and during the sampling period. This argument needs more detail. The dam water will be a mixture of pre-flood evaporated water and new rainwater; what are the likely relative proportions, and what will be the resulting stable isotope composition of the mixed water? This can then be related to the stable isotope composition of the surface water in more detail.
2. A more serious problem concerns the groundwater samples collected in June-September 2011; the bore hydrograph (Fig. 4) suggests that the flood peak had passed by the end of January, and the tritium data (Fig. 9) show that none of the groundwater samples have a tritium composition indicating very recent recharge (~3 TU). Therefore the groundwater samples cannot represent pure recharge during the Jan 2011 flood. The best that the authors can say is that the groundwater contains a component that was recharged during the Jan 2011 flood; to make their main case, they need to calculate (approximately) this component.
3. A single bore hydrograph from the lower catchment is not enough hydrogeological information to gauge the extent of flood recharge. Hydrographs need to be presented from each part of the catchment, and the maximum increase in the water table at each bore should be shown as a map.
4. The authors state that the stream was losing during the drought that preceded the flood, and became gaining during the flood. Some bore hydrographs are needed to verify this, particularly as this evidence is crucial to the conceptual recharge model (Fig. 10).
5. More hydrogeological data on the alluvial aquifer would be useful, e.g. porosity and permeability measurements from pumping tests on the upper and lower parts of the aquifer.
6. In the introduction, the authors present three mechanisms by which flooding can increase recharge. In the discussion, they should discuss which of these played the major role in their catchment (I suspect it was the second).
7. In the introduction, the authors state that poor quality groundwater is often associated with bedrock aquifers. While this is evidently the case in their study area, it is not the situation world-wide, e.g. bedrock groundwater is of excellent quality across much of Europe, North American and north Africa.
8. With regard to the stable isotope data, it is unclear whether the authors collected and analysed any samples themselves, or entirely used pre-existing data; this should be made clear in the methods section, where the Hughes and Crawford data also needs to be mentioned.
9. The authors need to explain why “heavy rainfall events are often more depleted than average rainfalls”, as this is crucial to the paper.
10. With regard to the hydrochemical facies, it is uncertain how these relate to the “multivariate statistical analysis of hydrochemical data” carried out in King et al 2014. The relevance of this previous paper (which I have not checked) needs to be made clearer.
11. Plotting the different hydrochemical facies as Stiff diagrams on the catchment map would make this section clearer.
12. The explanation about the difference in Sr isotope signature between the Esk Formation groundwater and soil relies on the Sr isotopic signature of the plagioclase being less radiogenic than that of the other minerals in the basalt; this needs to be made clear, and potential mechanisms for this discussed (e.g. fractional crystallisation of the magma).
13. The discussion on the hydrochemical evolution of the alluvial groundwaters is overly simplistic. Comparing rainfall and groundwater compositions standardised to chloride would make it clear what species are enriched in the groundwater and by how much, this can then be related to the weathering reactions proposed.
14. One mechanism that is likely to have affected the groundwater composition has been completely overlooked: cation exchange. Given the prevalence of smectites, this process is very likely to be active, and must be discussed.
15. In the discussion on hydrological process in the mid-lower catchment, the authors state that “fresh groundwaters…are probably recharged by surface waters”, i.e. the stream is losing, but that “groundwater-surface water interactions also appear to affect surface water compositions”, i.e. the stream is gaining. The contradiction between these statements needs to be resolved.
16. The authors demonstrate on hydrochemical grounds that groundwater from the bedrock is likely to be seeping into the alluvial aquifer, but present no hydrogeological evidence for this. They need to demonstrate that in these areas there is an upwards hydraulic gradient. They also need to calculate the relative contribution of the bedrock groundwater; I suspect that it is minor.