Anthropogenic activities and salinization are increasingly threatening groundwater resources, especially in arid and semi-arid regions. Unlike previous studies focusing on isotopes or hydrochemistry separately, this study investigates the first application of stable isotopes (water (δ18OH₂O and δ2HH₂O), dissolved nitrate (δ15NNO₃ and δ18ONO₃) and sulfate (δ34SSO₄ and δ18OSO₄)), hydrochemical and statistical integrated approach to identify salinity and pollution sources in the Zeuss-Koutine (ZK) and Mio-Plio-Quaternary (MPQ) aquifers, Southeast of Tunisia. These techniques were combined to determine the water quality, recharge origin, salinity sources and potential sources of contamination, using 46 water samples collected from 40 exploitation wells and 6 wadis. The findings reveal that groundwater salinity and pollution is mainly controlled by evaporite dissolution and anthropogenic activities. Therefore, the electrical conductivity ranging from 1875 to 7090 µS/cm at 25°C. All samples are above the WHO-recommended tolerable limits for sulfate, chloride, sodium, and fluoride. The hazard quotients (HQ) for non-carcinogenic health risk assessment indicate that children are the most vulnerable to fluoride exposure. Nitrate concentrations ranged from 1 mg/L to 120.4 mg/L and 35% of samples surpassed the WHO standard for drinking water. δ15NNO₃ (varied from + 6.2 to + 12.3‰) and δ18ONO₃ indicates an origin from nitrogen fertilizers in rural areas and from wastewater in some samples located near urban and industrial areas and organic nitrate (manure) affected by denitrification process. Based on δ18OH₂O and δ2HH₂O, most of the ZK and MPQ samples have a local meteoric origin with an evaporation process. The relationship between high nitrate concentrations and isotopic composition confirms the irrigation return flow process. δ34SSO₄ (ranged from + 11.7 to + 14.4‰) and δ18OSO₄ indicates that sulfate is derived mainly from natural interaction with geogenic Triassic evaporites. Some samples show an origin from inorganic fertilizers and sewage. Statistical analysis confirms results and indicates the existence of two major groups of samples affected by natural evaporite dissolution and anthropogenic inputs such as agricultural fertilizers and wastewater. The integrated approach provides valuable insights in distinguishing natural from anthropogenic influences in semi-arid aquifers, emphasizes the urgent need to develop sustainable groundwater management in southeastern Tunisia and highlights the importance of long-term monitoring of groundwater quality. Graphical Abstract This graphical abstract provides a visual summary of the anthropogenic impacts on groundwater quality using hydrochemistry, isotopes (water (δ18OH₂O and δ2HH₂O), dissolved nitrate (δ15NNO₃ and δ18ONO₃) and sulfate (δ34SSO₄ and δ18OSO₄)) and statistical methods in the Zeuss-Koutine (ZK) and Mio-Plio-Quaternary (MPQ) aquifers, Southeastern Tunisia. The geographical map shows the ZK, MPQ and surface samples repartition across the study area (46 water samples). Physicochemical parameters and isotopic analysis are performed. Piper diagrams reveal the existence of three major water groups: Na-Ca/SO4-Cl for ZK samples, Na-Ca-Mg/SO4-Cl wand Ca-Mg/SO4 for MPQ samples. δ18OH₂O vs. δ2HH₂O diagram indicates the meteoric origin of groundwater recharge. δ15NNO₃ vs. δ18ONO₃ shows the existence of a denitrification process and identifies fertilizers and wastewater as main source of nitrate pollution. Based on δ34SSO₄ vs. δ18OSO₄ diagram, the Triassic evaporites are the main source of dissolved sulfate on groundwater. Correlation heat maps and PCA reveal that the groundwater salinity is governed by natural processes such as seawater intrusion, evaporation, and evaporite dissolution, while isotopic signatures and nitrate-related parameters reveal significant anthropogenic contributions, particularly from agricultural and urban sources.

Assessing Anthropogenic Impacts on Groundwater Quality Using Environmental Isotopes in Semi-Arid Regions: Evidence from Southeastern Tunisia

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发表日期：

2025年12月1日

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Hanen Jarray,
Mounira Zammouri,
Mohamed Ouessar,
Manuela Barbieri,
Raul Carrey,
Albert Soler

摘要

Anthropogenic activities and salinization are increasingly threatening groundwater resources, especially in arid and semi-arid regions. Unlike previous studies focusing on isotopes or hydrochemistry separately, this study investigates the first application of stable isotopes (water (δ¹⁸O_H₂O and δ²H_H₂O), dissolved nitrate (δ¹⁵N_NO₃ and δ¹⁸O_NO₃) and sulfate (δ³⁴S_SO₄ and δ¹⁸O_SO₄)), hydrochemical and statistical integrated approach to identify salinity and pollution sources in the Zeuss-Koutine (ZK) and Mio-Plio-Quaternary (MPQ) aquifers, Southeast of Tunisia. These techniques were combined to determine the water quality, recharge origin, salinity sources and potential sources of contamination, using 46 water samples collected from 40 exploitation wells and 6 wadis. The findings reveal that groundwater salinity and pollution is mainly controlled by evaporite dissolution and anthropogenic activities. Therefore, the electrical conductivity ranging from 1875 to 7090 µS/cm at 25^°C. All samples are above the WHO-recommended tolerable limits for sulfate, chloride, sodium, and fluoride. The hazard quotients (HQ) for non-carcinogenic health risk assessment indicate that children are the most vulnerable to fluoride exposure. Nitrate concentrations ranged from 1 mg/L to 120.4 mg/L and 35% of samples surpassed the WHO standard for drinking water. δ¹⁵N_NO₃ (varied from + 6.2 to + 12.3‰) and δ¹⁸O_NO₃ indicates an origin from nitrogen fertilizers in rural areas and from wastewater in some samples located near urban and industrial areas and organic nitrate (manure) affected by denitrification process. Based on δ¹⁸O_H₂O and δ²H_H₂O, most of the ZK and MPQ samples have a local meteoric origin with an evaporation process. The relationship between high nitrate concentrations and isotopic composition confirms the irrigation return flow process. δ³⁴S_SO₄ (ranged from + 11.7 to + 14.4‰) and δ¹⁸O_SO₄ indicates that sulfate is derived mainly from natural interaction with geogenic Triassic evaporites. Some samples show an origin from inorganic fertilizers and sewage. Statistical analysis confirms results and indicates the existence of two major groups of samples affected by natural evaporite dissolution and anthropogenic inputs such as agricultural fertilizers and wastewater. The integrated approach provides valuable insights in distinguishing natural from anthropogenic influences in semi-arid aquifers, emphasizes the urgent need to develop sustainable groundwater management in southeastern Tunisia and highlights the importance of long-term monitoring of groundwater quality.

Graphical Abstract

This graphical abstract provides a visual summary of the anthropogenic impacts on groundwater quality using hydrochemistry, isotopes (water (δ¹⁸O_H₂O and δ²H_H₂O), dissolved nitrate (δ¹⁵N_NO₃ and δ¹⁸O_NO₃) and sulfate (δ³⁴S_SO₄ and δ¹⁸O_SO₄)) and statistical methods in the Zeuss-Koutine (ZK) and Mio-Plio-Quaternary (MPQ) aquifers, Southeastern Tunisia. The geographical map shows the ZK, MPQ and surface samples repartition across the study area (46 water samples). Physicochemical parameters and isotopic analysis are performed. Piper diagrams reveal the existence of three major water groups: Na-Ca/SO₄-Cl for ZK samples, Na-Ca-Mg/SO₄-Cl wand Ca-Mg/SO₄ for MPQ samples. δ¹⁸O_H₂O vs. δ²H_H₂O diagram indicates the meteoric origin of groundwater recharge. δ¹⁵N_NO₃ vs. δ¹⁸O_NO₃ shows the existence of a denitrification process and identifies fertilizers and wastewater as main source of nitrate pollution. Based on δ³⁴S_SO₄ vs. δ¹⁸O_SO₄ diagram, the Triassic evaporites are the main source of dissolved sulfate on groundwater. Correlation heat maps and PCA reveal that the groundwater salinity is governed by natural processes such as seawater intrusion, evaporation, and evaporite dissolution, while isotopic signatures and nitrate-related parameters reveal significant anthropogenic contributions, particularly from agricultural and urban sources.