the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 30 Jun 2025
Crop salinization by intense pumping in regional discharge areas of an inland aquifer system (Cenozoic Duero basin aquifer, Spain)
Abstract. Salinization of crops irrigated with groundwaters in the Tordesillas area has been investigated to determine its cause. Hydrogeological, geophysical, and geochemical techniques reveal that regional saline groundwater flows through the Cenozoic aquifer system of the Duero Basin discharge into the Tordesillas area. Groundwater salinity increases below 150–200 depth. TDEM profiles indicate that salinity distribution is influenced by local and regional flow mixing, as well as by fault structures affecting the Cenozoic succession. Isotopic analyses (δ18O, δD, δ34S) suggest multiple sources of dissolved sulphate and evidence that regional groundwaters recharged at higher altitudes and/or lower temperatures.
Irrigation return flows do not noticeably contribute to salinization, as δ18O and δD data from boreholes in the Duero Floodplain do not show an evaporation trend. Instead, intensive groundwater pumping (from boreholes in the Duero River floodplain), particularly during the irrigation season, induces upwelling of saline groundwater. Piezometric records indicate that hydraulic potential at intermediate depths (about 100 m depth) decreases during pumping (summer), facilitating upwelling of deeper saline groundwaters. Salinity profiles confirm this process, demonstrating a shift from fresher to more saline conditions over time.
Groundwater management authorities must address this issue to prevent further salinization. These findings provide crucial insights for optimizing well design and identifying depths where groundwater is unsuitable for irrigation, ensuring sustainable water use in the region.
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RC1: 'Comment on egusphere-2025-2395', Anonymous Referee #1, 03 Jul 2025
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Unfortunately, this paper is not suitable for publication in an international journal such as HESS. It has no novel aspects and would not be of interest to the broader academic community. The conclusions are reasonable, but the main finding that groundwater pumping causes changes to aquifer salinities is not unexpected and not novel. In its present form it is more suited to a regional journal. Case studies are certainly acceptable in the international literature, but they need to add to our general understanding; so, indications of what new ideas come out of a study and what researchers working on similar projects elsewhere can take from it are needed.
The limited and parochial scope of the paper is evident in the Introduction, which is focused on the local issue of crop yields being impacted by salinization. Salinization of water resources is a major problem globally, but there is no effort here to review the global understanding or to put this study into a broader context. The Aims are also very specific to understanding the local hydrogeology and the Conclusions are just a restatement of the specific findings of the research with no indication of how or why this research is of general interest. Even within the local context, the end of the paper is underwhelming with a general suggestion that these data should help management (without specifying how).
There are several issues with the data and its interpretation.
- Data limitations. The geochemical interpretations are based on a standard set of parameters (major ions, water stable isotopes, sulfate stable isotopes) from a small number of groundwater samples (12 in total). This is a very limited dataset that is unlikely to yield much insight into processes. The groundwater samples are also poorly characterized – the text and Table A1 lists samples as being <40 m and >40 m, but what are the exact depths? It is much more straightforward to interpret geochemistry data from wells that have short screens and which sample water from a specific aquifer than from wells with long screens that integrate water from several layers. Section 5.2.1 divides the sequence up into several GU’s and it would be good to be able to link the geochemical data with those units, which is not possible with the current reporting.
- Major ions. The interpretation of the major ion geochemistry does not really add much. Section 5.3 is descriptive and mainly defines groundwater types and their distribution. Section 6 mainly makes use of the overall groundwater TDS and salinity but not really the major ion geochemistry. Mixing is discussed in Section 6.2, but there is little attempt to quantify it or justify the conclusions. Even if quantification is not possible, identifying the end-members and showing the mixing on the Piper diagram or other plots would help. However, it is difficult to use small datasets to produce robust conclusions about processes such as mixing, which makes this discussion speculative. Some of the changes in water chemistry may reflect processes such as mineral dissolution and precipitation, which can produce systematic changes in water chemistry with salinity (e.g., Herczeg et al., 2001. Origin of dissolved salts in a large, semi-arid groundwater system: Murray Basin, Australia. Marine and Freshwater Resources, 52, 41-52, https://doi.org/ 10.1071/MF00040); again, this needs consideration.
- Water stable isotopes. The comparison of the groundwater stable isotopes with those of the weighted mean precipitation (Section 5.4) implicitly assumes that the groundwater is recharged by precipitation with that isotopic composition and any deviations occurs due to recharge at elevation (Section 6.4). However, it is likely that recharge occurs preferentially from high rainfall events or during winter when evapotranspiration rates are low and the comparison needs to consider this. Is it possible that the isotopic composition of the rainfall that causes recharge is different from the annual mean – for example, recharge mainly from winter rainfall, which commonly has lower stable isotope values than the mean, may explain the observations.
- Water stable isotopes. The observation that the stable isotopes do not define an evaporation trend may not rule out irrigation returns. Open-system evaporation in surface water bodies (pools, lakes etc) does produce distinctive isotopic trends. However, transpiration does not and it is not clear whether evaporation from within the soils where the relative humidity is higher fractionates stable isotopes to a large degree. There are plenty of examples of saline groundwater caused by evapotranspiration where the stable isotopes lie close to the MWL. This would include the deep groundwater in this region – looking at the major ion geochemistry and description of the aquifers, the high salinities are probably the result of evapotranspiration (there are no evaporites reported and halite dissolution produces a distinct NaCl-type geochemistry that is different to what is shown in Fig. 6). Yet the stable isotopes lie very close to the MWL. Again, the interpretation of data needs to be better justified.
- Sulfur isotopes. The discussion of the sulfur stable isotopes (section 6.4) is also general and not well justified. The isotopic values are interpreted as solely representing gypsum dissolution in the aquifers without consideration of whether other sources of sulfur (e.g., pyrite) might be present or whether fractionation due to processes such as bacterial sulfate reduction (which is common in groundwater globally) may have occurred. The conclusion that the Tordesillas groundwaters have sulfate derived from several sources is untested (is that consistent with the other data and the hydrology?). Similar comments apply to the conclusion that the isotopes show mixing in the river. As with the other datasets, you need to justify potential interesting conclusions such as this rather than just making assertions.
- Integration with the geophysics data. Partially due to the lack of detail regarding sample depth, it is difficult to link the geochemistry with the geophysics data. The geophysics results (discussed in Section 6.1) are presented separately to the geochemistry. Integrating both halves of the work would help the study.
I have not gone through the paper in more detail, as I cannot see that the data can be woven into a story that is of sufficient interest for this journal. It is never pleasant to receive negative reviews, but I would encourage the authors to see if they can bring more rigor to the study and consider how generally interesting / novel this work is, which will dictate where it should be submitted.
Citation: https://doi.org/10.5194/egusphere-2025-2395-RC1
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