Hydrochemical characteristics, spatial distribution, and sources of major ions in groundwater
The spatial distribution of major ions (HCO₃⁻, Cl⁻, SO₄²⁻, and NO₃⁻) in the study area reflects a complex interaction between natural hydrogeochemical processes and anthropogenic activities. Bicarbonate (HCO₃⁻) shows relatively high and consistent concentrations across the study area, indicating its dominance in groundwater chemistry and suggesting widespread carbonate weathering as a primary geochemical control. In contrast, chloride (Cl⁻) and sulfate (SO₄²⁻) exhibit noticeable spatial variability, with relatively elevated concentrations observed in zones influenced by human activities. These variations highlight localized contamination sources and differences in recharge and flow conditions within the aquifer. Nitrate (NO₃⁻) concentrations, although generally low, show distinct spatial fluctuations, suggesting point-source inputs rather than uniform distribution. The geochemical behavior of these ions is governed by dissolution, ion exchange, and transport processes within the aquifer system. Carbonate dissolution contributes significantly to HCO₃⁻ levels, while sulfate is partly derived from the dissolution of evaporitic minerals such as gypsum. Chloride behaves conservatively in groundwater and is commonly used as an indicator of contamination sources. Nitrate, being highly mobile and soluble, is particularly sensitive to surface-derived inputs and rapidly transported through the vadose zone into groundwater. These processes collectively define the hydrochemical facies and influence groundwater suitability for different uses. From an environmental perspective, the observed ion composition can be attributed to three main sources. First, agricultural activities play a significant role, where excessive application of fertilizers leads to nitrate enrichment, and irrigation return flow contributes to the accumulation of dissolved salts such as chloride and sulfate. Second, wastewater infiltration, including leakage from septic systems and unregulated disposal of domestic effluents, introduces additional loads of chloride and nitrate into the aquifer. Third, natural geochemical processes, particularly the dissolution of carbonate and evaporite minerals, are responsible for the baseline concentrations of bicarbonate and sulfate in groundwater. These findings are consistent with regional hydrogeochemical studies and emphasize the combined impact of anthropogenic pressures and natural controls on groundwater quality.
A comparison between the current Qalyubia groundwater study and similar global research, synthesized from the search results
Water Quality Indices (WQI): The current study’s WQI range (22–36, “Good”) aligns with assessments in Kerala, India (Satish Kumar et al. 2016), where 70% of samples were suitable for drinking. However, it contrasts sharply with Linfen Basin, China, where 33% of samples exceeded safety limits due to Pb, F⁻, and SO₄²⁻ pollution38,39. Heavy Metal Indices: Qalyubia’s low Heavy Metal Pollution Index (HPI ≤ 15) resembles “minimal pollution” in Telangana, India. Conversely, its Metal Index (MI: 0.81–3.29, “moderately affected”) mirrors Coimbatore, India, where industrial zones showed MI > 2.5 due to geogenic/anthropogenic fluoride23,39.
Contamination profiles and sources
Anthropogenic vs. Geogenic Dominance: Qalyubia’s marginal Fe/Mn exceedances stem from natural aquifer leaching, similar to fluoride in Shanxi, China38. In contrast, studies near industrial belts (e.g., Coimbatore, India) attributed 60% of nitrate/fluoride pollution to textiles and smelting23. Microbiological Threats: Unlike Qalyubia (which focused on chemical parameters), Egyptian drains like the Bahr El-Baqar exhibited severe microbial contamination (fecal coliforms in 43% of wells), highlighting regional neglect of sewage management40,41. The significantly higher QH value for Manganese (Mn) compared to other PTEs, as shown in Figs. 6B and 7B, indicates it is a primary pollutant of concern in the study area. This is likely attributed to the geogenic conditions of the alluvial aquifer in the Nile Delta, which are naturally anoxic and favor the reduction of Mn-oxides, leading to the mobilization of Mn²⁺ into the groundwater. Furthermore, anthropogenic activities such as the infiltration of irrigation return flows containing organic matter can enhance these reductive dissolution processes, exacerbating Mn concentrations.
Health risk assessment variations
Non-Carcinogenic Risks: The absence of significant risks in Qalyubia diverges from Linfen Basin, where 80% of children faced fluoride/nitrate hazards39. Children’s heightened susceptibility (2–3× higher than adults) was consistent globally, driven by greater water intake/kg body weight and developmental vulnerabilities42,43. Carcinogenic Threats: Qalyubia reported no carcinogenic concerns, whereas Linfen Basin identified Cr⁶⁺ and Cd as key carcinogens, with 100% of samples posing risks38,39.
Irrigation suitability challenges
Salinity vs. Sodicity: Qalyubia’s irrigation indices indicated “good” quality (low Na⁺), akin to Santuri, India. However, the Bahr El-Baqar drain (Egypt) fell into “high salinity” due to agricultural runoff, restricting use in poorly drained soils41,43. Heavy Metal Transfer: Unlike Xinzhou, China, where Cd accumulation in crops exceeded safety limits, Qalyubia did not evaluate soil-crop transfer as a critical gap for agricultural planning39.
Climate variability and its effects on groundwater quality and recharge
Climate variability plays a significant role in controlling groundwater recharge and quality, particularly in arid and semi-arid regions such as Egypt. Variations in precipitation patterns, coupled with increasing temperatures, directly influence recharge rates to aquifer systems. Reduced rainfall and higher evapotranspiration lead to limited natural recharge, which in turn increases the concentration of dissolved ions in groundwater. This process enhances salinization and deteriorates water quality, especially in shallow alluvial aquifers of the Nile Delta44,45,46. Furthermore, prolonged dry periods reduce dilution capacity, allowing contaminants such as nitrates and chlorides to accumulate. Similar findings were reported by Abd-Elaty et al.47, who highlighted that climate-induced stress on groundwater systems in Egypt contributes to declining water quality and increased vulnerability of aquifers to pollution. In addition to recharge reduction, climate variability also alters groundwater flow dynamics and geochemical processes within the aquifer. Rising temperatures and changing hydraulic gradients can accelerate mineral dissolution and ion exchange reactions, thereby modifying groundwater chemistry over time. In coastal areas, sea level rise associated with climate change further exacerbates groundwater salinization through seawater intrusion, increasing concentrations of chloride and sulfate. Eltarabily et al.48 emphasized that the combined effects of climate variability and anthropogenic activities, such as over-abstraction and agricultural practices, significantly impact groundwater sustainability in the Nile Delta region. These findings underscore the importance of integrating climate considerations into groundwater management strategies, including continuous monitoring, adaptive recharge enhancement, and protection of vulnerable aquifer zones.
Socioeconomic and policy implications
Monitoring Gaps: Similar to rural Arizona wells42, Qalyubia’s reliance on homeowner testing risks under-detection of contaminants like Mn. India’s COVID lockdown underscored how industrial regulation improves water quality (33% nitrate decline)23, reinforcing Qalyubia’s call for stricter oversight. Remediation Strategies: Qalyubia’s “immediate remediation” aligns with WHO’s push for water safety plans. However, Egypt’s El-Qalyubia drains show policy fragmentation, as agricultural reuse continues despite known pollutants41,49.
Regional vulnerability and climate interactions
Qalyubia’s location within the Nile Delta provides relativily favorable water availability compared with the semiarid regions, reducing the severity of groundwater scarcity pressures.For example, in Xinzhou (China), approximatly 70% dependence on groundwater has been reported to increase vulnerability to pollution expousure and water quality deterioration 39. In contrast, groundwater in Qalyubia benefits from continuous recharge associated with the Nile Delta irrigation system, which helps maintain generally good water quality, as refelected by the WQI (Fig. 5) and irrigation indices (Table 7). Nevertheless, the moderate Metal Index (MI) values observed in this study indecate the presence of potentiallty toxic elements that require attention. Climate variability, including rising temperatures and increased evapotranspiration, may futher concentrate dissolved constituents and trace metals through reduced diluation and recharge. Therefore, continuous monitoring and adaptive groundwater manegement are essential to prevent future deterioration of groundwater quality and protect public health.
Synthesis and research gaps
The Qalyubia study reflects a “moderately affected” groundwater system with lower risks than industrial zones but higher vulnerability than pristine aquifers. Critically, it shares limitations with similar Egyptian studies (e.g., unassessed microbial/mixture toxicity40. Future work must: (i) expand monitoring: Include emerging contaminants (e.g., pesticides) and seasonal dynamics, as demonstrated in monsoon-sensitive regions23,49, (ii) Cross-media transfer: Evaluate groundwater-soil-crop pathways, modeled after Xinzhou’s integrated assessments39, and (iii) Policy integration: Adopt WHO’s water safety plans, leveraging Coimbatore’s success in pollution reduction via industrial controls23,49. This comparison underscores that while Qalyubia’s groundwater is currently secure, its moderate metal pollution and Egypt’s rapid urbanization necessitate preemptive management, a lesson from global counterparts where delayed action escalated risks.
Final outcomes of the study and how it will reach society?
Validation of current suitability: Groundwater in Qalyubia is currently suitable for drinking (WQI = 22–36, “Good”) and irrigation based on tested parameters and non-carcinogenic risk assessment.
Identification of emerging threat: The Metal Index (MI = 0.81–3.29) provides a clear warning of moderate cumulative metal pollution (Fe, Mn, Cd, As, Cr, Pb, Hg, Ni, Zn), even though individual metals (except minor Fe/Mn exceedances) and HPI suggest low immediate risk.
Vulnerability assessment: Confirmation that children face higher susceptibility to potential contaminants than adults.
Call to action: The study concludes that proactive monitoring and remediation are urgently needed to prevent further deterioration and protect public health and agricultural sustainability.
How these outcomes reach society & create impact:
Informing policy & regulation: government agencies: Findings are presented to the Egyptian Ministry of Water Resources and Irrigation, the Ministry of Health, and the Environmental Affairs Agency. This provides scientific evidence to:
Strengthen monitoring programs: Mandate regular, targeted testing of PTEs in Qalyubia’s groundwater wells, especially where MI was highest.
Develop remediation plans: Allocate resources for treating contaminated wells (e.g., filtration for Fe/Mn) or identifying alternative safe water sources. The remediation of contaminated groundwater in the study area requires the implementation of integrated and sustainable approaches that address both point and diffuse pollution sources. In-situ remediation techniques have proven to be particularly effective under hydrogeological conditions similar to those of the Nile Delta. Among these, permeable reactive barriers (PRBs) represent a promising solution, as they enable passive treatment of groundwater through natural flow processes. PRBs filled with reactive materials such as zero-valent iron, activated carbon, or natural media can effectively remove nitrates, iron, and manganese through mechanisms including adsorption, reduction, and precipitation. Recent work by Meky et al.50,51,52 demonstrated that PRBs are cost-effective and environmentally sustainable systems suitable for large-scale application in alluvial aquifers. In addition, reactive well systems, as highlighted by Meky50,51, can enhance in-situ remediation by promoting groundwater circulation and facilitating contaminant degradation through geochemical reactions. These systems are particularly useful in areas with localized contamination and limited accessibility. In addition to in-situ methods, ex-situ and preventive measures are essential for ensuring safe water supply and long-term groundwater protection. Point-of-use treatment systems, such as aeration and filtration units, can effectively remove iron and manganese from contaminated wells, making groundwater suitable for domestic use. Furthermore, managed aquifer recharge (MAR) techniques can improve groundwater quality by diluting contaminant concentrations and enhancing natural attenuation processes, provided that recharge water is adequately treated. Controlling pollution at the source remains a key priority; this includes optimizing fertilizer application, adopting precision agriculture practices, and improving wastewater management to reduce nitrate and salinity inputs into the aquifer. The identification of alternative safe water sources, along with the implementation of continuous monitoring systems supported by GIS tools, is also crucial for sustainable groundwater management. These combined approaches align with recent research emphasizing the importance of integrating remediation technologies with proactive management strategies to mitigate groundwater contamination under increasing environmental and climatic pressures.
Update standards/guidelines: Inform potential revisions to national water quality standards based on cumulative risk (MI concept).
Local authorities (Qalyubia Governorate): Enable targeted local action plans and budget allocation for well maintenance and pollution source control.
Guiding water management & infrastructure:
Water utilities/providers: Results guide decisions on well selection, water blending strategies, and potential infrastructure upgrades (e.g., installing specific treatment technologies in affected areas).
Agricultural cooperatives: Irrigation suitability confirmation supports sustainable farming, while the metal warning prompts awareness about potential long-term soil accumulation risks.
Raising public awareness & protection:
Public health campaigns: Health authorities and NGOs use the findings to:
Reassure communities about current safety (mitigating unwarranted fear).
Highlight the specific risk to children, promoting protective measures (e.g., continued use of treated water for infants where available, awareness of symptoms related to metal exposure).
Educate the public on the early warning nature of the moderate pollution finding and the importance of remediation efforts.
Community engagement: Local leaders and NGOs disseminate simplified findings, explaining why monitoring and potential future actions (like well treatment) are necessary for long-term safety.
Scientific foundation for future action:
Source identification: The study prompts further research to identify why metals are accumulating (e.g., industrial discharge, geological leaching, sewage infiltration).
Long-term tracking: Establishes a baseline for future studies to track pollution trends and measure the effectiveness of interventions.
Broader application: The methodology (especially using WQI, MI, and HPI together) serves as a model for assessing groundwater in similar urban, densely populated delta regions globally.
In essence, the study translates scientific data into actionable intelligence: It provides reassurance about current use, sounds an early alarm about a growing problem (cumulative metal pollution), identifies the most vulnerable group (children), and provides the scientific justification for authorities and communities to invest in monitoring and remediation before the water becomes unsafe. This directly contributes to protecting public health, ensuring sustainable agriculture, and safeguarding a critical water resource for a heavily populated region.
