Crop salinization by intense pumping in regional discharge areas of an inland aquifer system (Cenozoic Duero basin aquifer, Spain)

Rodríguez-Jiménez, Esther; Huerta, Pedro; Llera, Laura; Carrasco, Pedro; Recio, Clemente

doi:https://doi.org/10.5194/egusphere-2025-2395

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https://doi.org/10.5194/egusphere-2025-2395

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30 Jun 2025

| 30 Jun 2025

Status: this preprint is open for discussion and under review for Hydrology and Earth System Sciences (HESS).

Crop salinization by intense pumping in regional discharge areas of an inland aquifer system (Cenozoic Duero basin aquifer, Spain)

Esther Rodríguez-Jiménez, Pedro Huerta, Laura Llera, Pedro Carrasco, and Clemente Recio

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.

Received: 21 May 2025 – Discussion started: 30 Jun 2025

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.

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Esther Rodríguez-Jiménez, Pedro Huerta, Laura Llera, Pedro Carrasco, and Clemente Recio

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RC1:
'Comment on egusphere-2025-2395', Anonymous Referee #1, 03 Jul 2025 reply
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.

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Citation: https://doi.org/10.5194/egusphere-2025-2395-RC1
- AC1: 'Reply on RC1', Pedro Huerta, 26 Jul 2025 reply
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-2395/egusphere-2025-2395-AC1-supplement.pdf
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  Citation: https://doi.org/10.5194/egusphere-2025-2395-AC1
RC2: 'Comment on egusphere-2025-2395', Anonymous Referee #2, 10 Oct 2025 reply

The manuscript focuses on the crop salinization event that occurred in the Tordesillas region. Based on hydraulic head observations, TDEM geophysical surveys, chemical analyses, and stable isotope data, the authors proposed that summer irrigation pumping induced the upwelling of deep saline water, leading to salinization as the main cause. The research topic has certain significance for regional water resources management. However, the current manuscript lacks sufficient evidence in several key aspects (especially the quantitative relationship between pumping volume and salinity variation, geochemical end-member identification, and validation between TDEM and well data) to support the conclusion proposed by the authors. The paper requires substantial additional work (including more data, quantitative modeling, and end-member analysis) and major rewriting. Therefore, my recommendation is Reject. The main issues and revision suggestions are listed below.

1. The authors mainly infer that pumping-induced upwelling occurs based on the temporal correlation between the observed seasonal decline in water level and the simultaneous increase of EC in the middle layer (100–150 m). However, no detailed extraction records—such as pumping volume, well location, and pumping schedule—are provided. In addition, no numerical hydrodynamic or solute transport modeling is conducted to quantify the relationship between upwelling intensity and salinity response, which weakens the persuasiveness of the key conclusion.
It is recommended to supplement and present detailed pumping records of the main irrigation wells (including well locations, pumping volumes, pumping time series, borehole structure, and screen intervals). A simple hydrodynamic and solute transport simulation, or a simplified 1D/2D model, should be performed to test whether the observed water-level decline and EC variations can be reproduced under the recorded pumping intensity, thereby quantifying the upwelling flow rate, salt flux, and EC variation timescale.

2.The authors use δ¹⁸O/δD, δ³⁴S, and δ¹⁸O(SO₄) data to discuss water and sulfate sources, but they have not adequately sampled or characterized the potential “end-members” (e.g., local Paleogene/Miocene gypsum, agricultural sulfate fertilizers, river water, or near-surface return flow) in terms of isotopic and chemical composition. The conclusion that “irrigation return flow is insignificant” is based merely on the absence of an evaporation line in δ¹⁸O/δD, which is not sufficient.
It is suggested to supplement the analysis of potential source contributions, perform end-member mixing analysis (EMMA, SIAR, or a simple mass-balance approach) to quantitatively estimate the contributions of different end-members, and assess the associated uncertainties.

3. The authors conducted TDEM surveys and presented several geoelectric units (GU-1 to GU-4), but the calibration between resistivity and salinity, the discussion of inversion uncertainty, and the evaluation of model non-uniqueness are insufficient. Moreover, no validation was carried out using borehole lithology, downhole resistivity, or in-well conductivity data. As a result, the geophysical (TDEM) interpretation lacks strong quantitative support.

4. The conclusion emphasizes that “pumping-induced upwelling leads to salinization,” but no salt flux estimation (such as upward salt flux, irrigation-area salt input/output, or accumulation rate) is provided. It is recommended to estimate salt fluxes and perform simplified salt mass-balance calculations based on observed water levels, salinity, and pumping data, and to model long-term salinity evolution under different pumping strategies. This would enable quantitative management recommendations (e.g., allowable extraction thresholds or well-screening depth strategies).

5. The Results and Discussion sections overlap considerably, and the current section division is fragmented, resembling a technical report rather than a scientific article. The Introduction lacks the background and research progress on issues such as crop salinization induced by intensive pumping, and does not sufficiently review recent literature. Most references are outdated. It is recommended to add recent studies from the past five years concerning groundwater-extraction-induced salinization in inland basins, TDEM interpretation, and isotopic tracing, to strengthen the arguments and improve the logical focus of the paper.

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Citation: https://doi.org/10.5194/egusphere-2025-2395-RC2

Esther Rodríguez-Jiménez, Pedro Huerta, Laura Llera, Pedro Carrasco, and Clemente Recio

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https://doi.org/10.5194/egusphere-2025-2395-supplement

Esther Rodríguez-Jiménez, Pedro Huerta, Laura Llera, Pedro Carrasco, and Clemente Recio

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Short summary

This study investigates why crops in the Tordesillas area are died during summer 2020 and finds that deep, saline groundwater rises by irrigation wells during heavy use. This happens because pumping lowers hydraulic head at mid-levels in the aquifer, allowing saltier water from deeper parts to rise. The findings highlight the need for better groundwater management to protect crops and suggest how well design and pumping depth can help avoid using poor-quality water.


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