# Fate of heavy metals in the water-scarce Nile Delta lagoons: a post-Aswan Dam perspective

Said A. Shetaia, Yanna Wang, Nick Marriner, Maotian Li, Xiaoshuang Zhao, Jing Chen, Alaa Salem, Zhongyuan Chen

► To cite this version:

Said A. Shetaia, Yanna Wang, Nick Marriner, Maotian Li, Xiaoshuang Zhao, et al.. Fate of heavy metals in the water-scarce Nile Delta lagoons: a post-Aswan Dam perspective. Environmental Pollution, 2025, 382, pp.126754. 10.1016/j.envpol.2025.126754. hal-05456845

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Shetaia, S. A., Wang, Y., Marriner, N., Li, M., Zhao, X., Chen, J., Salem, A., & Chen, Z. (2025). Fate of heavy metals in the water-scarce Nile Delta lagoons : A post-Aswan Dam perspective. *Environmental Pollution, 382, 126754*. https://doi.org/10.1016/j.envpol.2025.126754

## Review article

# Fate of heavy metals in the water-scarce Nile Delta lagoons: a post-Aswan dam perspective

Said A. Shetaia¹,²), Yanna Wang¹)*, Nick Marriner³), Maotian Li¹), Xiaoshuang Zhao¹), Jing Chen¹), Alaa Salem⁴), Zhongyuan Chen¹)

1) The State Key Laboratory of Estuarine and Coastal Research, East China Normal University, China

2) Geology Department, Faculty of Science, Al-Azhar University, Cairo, Egypt

3) CNRS, ThéMA, Université de Franche-Comté, UMR 6049, MSHE Ledoux, 32 rue Mégevand, 25030, Besançon, Cedex, France

4) Faculty of Science, Kafrelsheikh University, Kafrelsheikh, Egypt

## Abstract

Deltas around the world are increasingly threatened by metal pollution, caused by human activity, endangering the health of vital ecosystems. This study presents a comprehensive review of the evolution of heavy metals in the arid Nile Delta, aiming to establish a model for the metals’ source-sink dynamics and their underlying drivers. Based on an overall assessment of heavy metal distribution across 639 sites interpreted by numerous previous studies, we synthesized individual datasets into an integrated delta-scale model, to reassess the source-sink pollution dynamics. Using

<footnote>*Corresponding author: ynwang@sklec.ecnu.edu.cn</footnote>
pre-Aswan dam data (prior to 1964) as a reference point, we defined major heavy metals (Pb, Cu, Zn, Cd, Cr, Mn, Sr, Ni) in deltaic surface sediments as being of anthropogenic origin, through statistical analyses. We found a latitudinal (south-to-north) increasing trend in anthropogenic metal concentrations, which were progressively transported from the delta plain to coastal lagoons over the past half-century. Integrated pollution indices revealed that metal concentrations in the lagoons were 3 to 4 times higher than those in various delta sectors, i.e. river branches, canals, drains, soils and coastal beaches, indicating a cumulative process-consequence following dam construction. The increasing enrichment factor (EF) values of lagoonal metals are closely correlated with proxies of social development, including wastewater discharge, fertilizer application, GDP, population, etc., among which the water stress level (WSL) plays a determinant role in raising EF value. In-depth study revealed a non-linear relationship between EF and WSL, indicating that lagoon sediments will lose their capacity to absorb anthropogenic metals by the end of 2060, implying an irreversible metal contamination crisis in the water-scarce Nile Delta. Our study calls for an urgent policy reform and implementation to improve coastal management.

Keywords: Anthropogenic metals; Irreversible metal sink; Wastewater reuse; Aswan High Dam; Capability of absorption; Water scarcity

# 1. Introduction

The world's deltas are among the most critical ecological habitats for human societies, particularly in terms of productivity and economic value. Deltas host intensive human activities such as fisheries, fossil energy production, agriculture, navigation and manufacturing that contribute to some of the highest values of ecosystem goods and services, globally (Day et al., 2016; 2019). Today, deltas face severe challenges from intensive human impacts, such as dam construction, coastal erosion, sea-level rise and water pollution, which is intensifying under pressure of global climate warming (Syvitski et al., 2009; Vörösmarty et al., 2009; Renaud et al., 2013; Giosan et al., 2014).

The Nile Delta is one of the most vulnerable ecological settings (Fig. 1A), given its arid climate setting and limited inhabitable land, i.e. only 2% of Egypt's territory that hosts 63% of agriculture and 40% of industries (Fig. S1) (Negm et al., 2017). Four large coastal lagoons
(Manzala, Burullus, Edku and Mariout, from east to west) are the sole conduit connecting the densely-populated delta plain and the Mediterranean Sea, serving as a vital hub for Egyptian livelihoods. The Aswan High Dam (AHD), ca. 900 km south of Cairo (Fig. 1B), and completed in 1964, has significantly promoted the development of agriculture and industry in the delta (Mohamed, 2018). This, in turn, has triggered rapid population growth in the delta (Hamza, 2010), which now accounts for 41% of the country's population with a density of 1360 inhabitants per km² (Negm et al., 2017). However, rapid socioeconomic development and dam construction have compromised the healthy the deltaic ecosystem, leading to freshwater shortages, water pollution, relative sea-level rise, coastal erosion and overall environmental degradation (Stanley and Warne, 1998; Hamza, 2006; Barceló and Kostianoy, 2017; Day et al., 2021).

Agricultural expansion in the Nile Delta over the past decades (Fig. S1) has significantly worsened Egypt's water availability, shifting from a surplus of $20×10^9 m^3 a^{-1}$ to a total deficit of $40×10^9 m^3 a^{-1}$ between the 1960s and 2000s, making the country one of the most water deficient per capita globally (Nikiel and Eltahir, 2021). Nevertheless, the water deficit has been partially compensated by the reuse of agricultural drainage water, with limited and/or even on treatment, since the 1980s (Tawfik et al., 2021; Abotalib et al., 2023). In addition, the construction of the Grand Ethiopian Renaissance Dam (GERD) (Fig. 1B) has prompted the expansion of water reuse in the Nile Delta as an initial step for toward to addressing water shortages and meeting the growing demand for irrigation and urbanization (Omran and Negm, 2018). Recently, the reuse of agricultural drainage water has reached $9.5×10^9 m^3 a^{-1}$, of which only about one third undergoes treatment (Tawfik et al., 2021). On the other hand, the construction of the Aswan Dam has resulted in the retention of ca.95% of the sediment load ($50×10^6 t a^{-1}$) from the upper basin, as these sediments carry non-replenishable sources of potassium and phosphorus (Badawy et al., 2017). This has led to a significant increase in the use of agricultural fertilizers, from ca. $730×10^3 t a^{-1}$ in 1952 to $6,240×10^3 t a^{-1}$ in 1990 and pesticides from $2.1×10^2 t a^{-1}$ to $17.2×10^3 t a^{-1}$ during the same period, in order to offset the loss of nutrient-rich sediments entrapped in the Nasser reservoir (Wahaab and Badawy, 2004; Oczkowski and Nixon, 2010). Water rich in fertilizers and pesticides is released from agricultural fields into the large drains of the delta plain, eventually flowing directly into the coastal lagoons (Wahaab and Badawy, 2004). Moreover, Egyptian industry consumes ca. $2.9×10^9 m^3 a^{-1}$ of Nile freshwater, of which $2.5×10^9 m^3 a^{-1}$ is added into the drainage system (Nikiel and Eltahir, 2021). The industries of Alexandria and the Greater Cairo region
account for 40% of this consumption (Wahaab and Badawy, 2004). The municipal wastewater is discharged, after partial or no treatment into the water branches and canals, totaling ca. $4.2×10^9$ m³ in 2021 (CAPMAS, 2022; Abotalib et al., 2023). Thus, Egypt ranks among the top countries globally in both per capita wastewater production (i.e., >100 m³ a⁻¹) and wastewater reuse (i.e., 10-25%) (Abotalib et al., 2023).

Many studies have shown that the recycling of water and the use of phosphate-rich sewage water for irrigation in the Nile Delta have led to the accumulation of heavy metals (Abdel-Azim and Allam, 2005; Mostafa et al., 2005; Abdelrazek, 2019; Abu Khatita et al., 2020; Abotalib et al., 2023). This has largely degraded sediment quality of the Nile branches (Damietta and Rosetta, Fig. 1A) (Abotalib et al., 2023). Notibally, the Nile lagoons have become the final reservoir for most sewage, and agricultural and industrial wastewater, due to extremely reduced fluvial dynamics following the construction of the AHD. Burullus lagoon on the central delta-coast, receives approximately $4.1×10^9$ a⁻¹ of wastewater from agriculture and megacities (e.g., Tanta, Kafr El-Shaikh and El-Mahalla) (Chen et al., 2010; Shetaia et al., 2022). Manzala lagoon receives a large amount of municipal, industrial and agricultural effluents from local urban areas and Greater Cairo, while Mariout lagoon mainly collects the effluents for Alexandria's industries (Wahaab and Badawy, 2004; Gu et al., 2013). Unlike other lagoons, Edku lagoon receives less drainage water, since no major cities or industrial zones are located nearby (Oczkowski and Nixon, 2010).

As has been widely documented, heavy metals have become a global concern due to their toxicity and negative impacts on biodiversity and human health (Shi et al., 2022; Biswas et al., 2023; Nasr et al., 2023; Shetaia et al., 2023). Heavy metals originate from both natural and anthropogenic sources, with their relative contributions varying significantly by region. Natural sources primarily include geological weathering of metal-bearing rocks and minerals (Pejman et al., 2015; Ye et al., 2020), while anthropogenic sources have become dominant in many areas, particularly influenced by industrial discharge, agriculture (through fertilizers and pesticides), and urban waste (Ali et al., 2016; Chen et al., 2017; 2020; Dong et al., 2021). Heavy metals are absorbed onto sediment, which act as carriers of those pollutants (Ranjbar Jafarabadi et al., 2020; Varol et al., 2020). Accumulated metals in water bodies affect living organisms and are transferred to humans via the food chain, leading to serious health problems (Abdelhady et al., 2019; Yin et al., 2019; Sheikhzadeh and Hamidian, 2021). Over exposure to heavy metals is associated with a range of negative health impacts, including cancer, kidney damage, decrease in intelligence
quotient, behavioral disorders, and, in rare cases, mortality due to excessive levels (Soliman et al., 2006; Alomary and Belhadj, 2007; Ashayeri et al., 2023).

The accumulation of heavy metals on the Nile Delta has become a growing concern, especially in relation to water recycling and wastewater reuse. There is an urgent need to understand the processes of metals’ transport, redistribution and underling driving forces. Over the past decades, many studies investigated heavy metals in the Nile Delta, including sediments of lagoons (Gu et al., 2013; Abu Khatita et al., 2015; Elkady et al., 2015; Zahran et al., 2015; Abdel-Satar et al., 2017; El-Amier et al., 2017; Okbah et al., 2018; Abdelhady et al., 2019; EL-Bady, 2020; El-Alfy et al., 2021; Aly-Eldeen et al., 2023; Shaheen et al., 2024), of the Nile branches (El Bouraie et al., 2010; Redwan and Elhaddad, 2016, 2020; Al-Afify and Abdel-Satar, 2020; Badawy et al., 2020; Goher et al., 2021; Al-Afify and Abdel-Satar, 2022; Abotalib et al., 2023), the coast (El-Serehy et al., 2012; Badawy et al., 2020, 2022; El-Alfy and Elnaggar, 2023), the drains and canals (Khallaf et al., 2018; Badawy et al., 2020; El-Alfy et al., 2020; El-Amier et al., 2020, 2021; Nour et al., 2022; Sayed et al., 2022; El-Amier et al., 2023; Aly-Eldeen et al., 2023), and the delta plain (Abu Khatita, 2011; Shokr et al., 2016; Khalifa and Gad, 2018; Badawy et al., 2020; Guda et al., 2020; El-Zeiny and Abd El-Hamid, 2022). Most of these studies focus on heavy metals concentrated in the surface sediments, highlighting the processes and consequence of elevated concentrations of heavy metals. Pb and Cd are commonly used as tracers of pollution sources, such as industrial discharges, agricultural wastewaters, and untreated urban sewage (Elkady et al., 2015; El-Amier et al., 2020; Nour et al., 2022). Gu et al. (2013) and Elkady et al. (2015) reported high levels of Mn, Pb, Zn and Cd pollution in Manzala Lagoon, correlating with industrial activities in Cairo. Similarly, El-Metwally et al., (2021) and Shetaia et al., (2022) identified high Cu and Zn levels in Burullus Lagoon, associated with agricultural runoff. Notably, almost none of the previous studies have thoroughly investigated the processes of heavy metals accumulation prior to of the pre-Aswan dam, except for Gu et al., (2013), who systematically measured the post-dam evolution of heavy metals in the Nile Delta lagoons. This study applied radionuclide ($^{210}$Pb/$^{137}$Cs) dating to short sediment cores from the lagoons, providing reliable insights into the post-dam fate of heavy metals.

While recognizing their valuable contributions, these studies were inevitably constrained by their limited spatial scales and differing objectives, making them inadequate for fully addressing the origin, transport and accumulation of metals at a delta scale. Within this context, the goal of
present study is to synthesize heavy metal data from the past decades, not only to clarify their
spatiotemporal distribution and related mechanism across the entire delta, but also to quantify the
fate of these metals in relation to wastewater reuse that has prevailed since the construction of
Aswan Dam construction in 1964. In particular, how heavy metals will evolve in relation to future
wastewater reuse is a fundamental question that must be addressed. Undoubtedly, addressing task
will help mitigate the hazards associated with heavy metals through government policy
modification and integrated management strategies.

**2. Data sources and methods**

**2.1 Surface-sediment heavy metals and measurements**

The concentrations of 9 heavy metals (Cu, Pb, Cd, Sr, Zn, Cr, Ni, Mn, Fe) in surface deltaic sediments were collated from numerous previous studies (Table S1), covering the entire Nile Delta, including lagoons, river branches (Damietta and Rosetta), canals, drains, agricultural soil and coastal beaches from Port Said to Alexandria. Keywords were used to search for relevant information from previously published publications via bibliographic search engines (e.g., Scopus, Google Scholar Pubmed, and Web of Science). In total, 119 research papers were identified, and after rigorous bibliographic review, 35 articles were selected for this study. The following criteria had to be met in the literature (shown in Table S1): concentrations of 9 heavy metals (with Fe mandatory for normalization), the latitude and longitude coordinates for each sampling site and adherence to rigorous scientific procedures for analytical methods.

The different techniques used to measure heavy metal in these studies (Table S1) included
Atomic Absorption Spectrophotometer (AAS), Inductively Coupled Plasma Atomic Emission
Spectroscopy (ICP-OES), Inductively Coupled Plasma Mass Spectrometer (ICP-MS), laser
ablation inductively coupled plasma mass spectrometry (LAICP-MS), Instrumental Neutron
Activation Analysis (INAA) and X-ray Fluorescence (XRF) spectrometer. Although
methodological differences may affect the accuracy of the results, these studies' adherence to strict
laboratory protocols (such as calibration, repeated measurements, use of validated reference
materials and maintaining measurement errors consistently below 10%) helps mitigate variability
and enhance comparability between measurments and datasets. Furthermore, we reasoned that
measurments obtained using different methods would not significantly deviate from the main trend of metal source-sink patterns in this study, especially after all heavy metals were normalized to Fe (metal×10⁴/Fe) by the authors, which helps to eliminate the grain size effect.

In total, 639 sampling sites from 35 articles were documented to represent the spatial distribution of heavy metals (Fig. 1A). The concentration of heavy metals in the surface sediments of the Nile Delta is presented in the distribution map (Fig. S2, in ppm), while their averaged values are listed in Table S2.

## 2.2 Short-core sediment heavy metals and measurements

The concentrations of 9 heavy metals (the same as the surface metals) were collected from 6 dated short-cores from the Manzala, Burullus and Edku lagoons (Gu et al., 2013), of which 3 representative core were used in this study (Table S3). Notably, these short cores were dated using radioisotopes ²¹⁰Pb/¹³⁷Cs, from which the sedimentation rates were determined for the pre- and post-dam chronology. The short-cores of Gu et al. (2013) was chosen because it is based on subsamples taken every 1.0 cm, and covers the pre- and post-Aswan Dam time periods. Also, the high-resolution sedimentation rates provide greater accuracy than those in other studies when comparing core sediment data with socioeconomic indicators. Due to the lack of dated heavy metal records in Mariout lagoon, it was excluded from the temporal analyses in this study. Dating results of short cores allowed us to reconstruct trends in heavy metal accumulation through time. The marked increases in dated heavy metals above 10-15 cm core depth were attributed to post-dam anthropogenic contributions (Chen et al., 2010; Gu et al., 2013).

## 2.3 Assessment indices

Individual pollution indices, i.e. *EF* and the modified geo-accumulation index (*mI*<sub>geo</sub>) were used to discriminate anthropogenic metals and to evaluate metal pollution at specific sites (Xu et al., 2014; Birch et al., 2020; Birch, 2023). Complex pollution indices, including the Modified Nemerow Pollution Index (*mPI*) and Modified Degree of Contamination (*mCd*) were used to assess pollution from multiple heavy metals at individual sites (seeing Supplementary Materials) (Abraham and Parker, 2008; Brady et al., 2015; Birch, 2023). The formulae and descriptions of the indices used in this study are detailed in Tables S4-S7.

## 2.4 Pre-dam metal background values
The pre-dam heavy metal concentrations (Table S8) were used as “background values”, representing non-anthropogenic influence, selected from 6 dated short cores in the Manzala, Burullus and Edku lagoons (Gu et al., 2013). The metal concentrations from the deeper core sections (below core depth 10-15 cm) indicate pre-industrizatipon background values (Chen et al., 2010; Gu et al., 2013). Thus, using local background values allowed for the Enrichment Factor (EF, see Supplementary Materials for details) and other indices for all surface metals used in this study.

## 2.5 Socio-economic indicators

Based on a comprehensive review of previous research, 10 factors affecting heavy metal distribution in the Nile delta were compiled (Table S9). These factors were selected based on the following criteria: (1) the indicators reflect human activities (e.g., demographic, economic, and industrial development) and their well documented impacts on heavy metal accumulation (e.g., El-Alfy et al., 2020, 2021; Mandour et al., 2021; Fouad et al., 2022; Abotalib et al., 2023; Abdel-Raheem et al., 2024); and (2) the temporal resolution of the data is consistent with that of other heavy metal datasets, ensuring uniform analysis and ease of data access. The 10 factors used in this study are: fertilizer use per hectare (sourced from FAO, 2024a), GDP per capita (sourced from FAO, 2024d), urban area of the Nile Delta (sourced from Abd-Elmabod et al., 2019; Fouad et al., 2022), water resource per capita (sourced from Fleifle and Allam, 2016; FAO, 2024c), wastewater discharge to the lagoons (sourced from MWRI, 2002; Abdel-Azim and Allam, 2005; Molle et al., 2016), rice cultivation area of the Nile Delta (sourced from Abdel-Raheem et al., 2024), level of water stress (sourced from FAO, 2024b, refer Supplementary Materials), manufacturing value added (sourced from World Bank Group, 2024), drainage water reuse of the Nile Delta (sourced from Abdel-Azim and Allam, 2005; Mostafa et al., 2005; Ghaffar and Shaban, 2014), and the total population of Egypt (sourced from FAO, 2022).

## 2.6 Statistical analysis

The Inverse Distance Weight (IDW) method was used in ArcGIS (version 10.8) for the spatial interpolations of different variables (Nasr et al., 2023; Shetaia et al., 2023). The ClustVis web application, Hierarchical Cluster Analysis (HCA) was used to identify the link between the sites based on different variables using Canberra distance and Ward's method (Metsalu and Vilo, 2015). The coefficient of variation (CV%; a ratio of the standard deviation to the mean) and the Pearson
correlation analysis were obtained using Microsoft Office Excel. Furthermore, Redundancy
Analysis (RDA) in CANOCO 5.0 was applied to explore the potential major drivers of metal
pollution (ter Braak and Smilauer, 2002).

RDA is a multivariate statistical method that incorporates multiple linear regression to
examine the relationships between response variables (e.g., heavy metal contamination) and
explanatory variables (e.g., influencing factors) (Wang et al., 2024). The total variance of the
dataset reflects the extent to which variation in the response variables overlaps with variation
explained by explanatory variables (Angelidis et al., 2021). Furthermore, the length and direction
of RDA vectors provide insights into the strength and significance of correlations with the
ordination axes or other variables (Gabarrón et al., 2018). Based on the RDA results, the
relationship between the major driver (level of water stress) and the cumulative EF was established
to project the overall EF capacity of the lagoon sediments. The EF assessment offers more reliable
and accurate determinations of enrichment compared to other indices (Birch, 2023), and is
therefore used in this study to better understand the driving forces behind metal enrichment.

**3. Heavy metals: distribution, sources, and forcings**

**3.1 Defining anthropogenic metals**

Naturally, heavy metals are essential sediment components transported from the up-basin provenances to feed geo-ecological habitats of delta-coast. However, the recent human activities have added a large amount of anthropogenic metals that mixed with natural metal sources, causing difficulties to evaluate environmental degradation and to quantify source-sink processes. Of note, most of all former studies of the Nile Delta lack of pre-dam (pre-industry) heavy metals as a background reference for non-human-influenced evaluation, thus to be inadequate to address the origins and issues of heavy metal. The pre-dam metals of the Nile lagoons are insightful to solve this referential barrier (Table S8) (Gu et al., 2013).

When considering only the absolute concentrations of metals
(Mn>Cr>Zn>Cu>Pb>Sr>Cd>Ni, in ppm) in surface delta sediments reported by previous studies,
one may be misled in their interpretation for environmental assessment (Fig. S2). This can hinder
the ability to track metal process-response relationships and the underlying driving forces,
potentially leading to misinterpretation and misguided conclusions for effective environmental management. In addition, using environmental indices that do not incorporate a reference element can lead to inconsistent results, particularly across large geographic areas influenced by variations in mineral composition and grain size (Birch, 2023). This issue is particularly evident in small-scale studies that relied on absolute values or global background values for assessment (e.g., Zahran et al., 2015; El-Amier et al., 2017; Shaheen et al., 2019; El-Alfy et al., 2021; Aly-Eldeen et al., 2023; Badawi and Magdy, 2023), which are clearly insufficient for accurately quantifying metals from anthropogenic sources (Kowalska et al., 2018; Birch, 2023). We also acknowledge that the surface metal samples collected in this study were subjected to different chemical digestion methods in addition to different sediment grain size can affect heavy metal concentrations. Therefore, this study used normalized heavy metal values (individual metal/Fe) to minimize the effects of grain size effect (refer Supplementary Materials) and to distinguish anthropogenic metals from natural sources (Loring and Rantala, 1992; Kowalska et al., 2018; Karaouzas et al., 2021; Birch, 2023). Moreover, the indices used in this study (i.e., EF, $mI_{geo}$, mPI, and mCd) are calculated using a reference element (Fe in this case) and local background values (pre-dam values; Table S8) to account for textural variations in sediments.

Based on the synthesized delta-scale dataset, we reordered the sequence of normalized heavy metals as follows: Mn>Zn>Sr>Cr>Pb>Cu>Ni>Cd (Fig. 2). Clearly, there are significant differences between the absolute concentrations of heavy metals and their normalized values (Fig. S2; Fig. 2). The former shows a random distribution of heavy metals, reflecting a mixture of natural and anthropogenic sources, while the latter reveals more regular pattern. Compared to the uneven distribution of absolute heavy metal concentrations across the delta plain (Fig. S2), the normalized values exhibited a more clustered pattern, with concentrations centered around the coastal lagoons (Fig. 2). The normalized Pb, Cu and Zn shared similar distribution patterns, showing higher values (Zn >2000), (Pb >900) and (Cu >590) in the Manzala, Burullus and Mariout lagoons (Fig. 2). Cr (>1000) and Cd (>200) were both present in the Manzala and Burullus lagoons (Fig. 2). Mn (>4000) occurred in Burullus and Mariout lagoons. Sr and Ni each exhibited distinct distribution patterns, with higher Sr concentrations (>1300) along the Alexandrian coast, and elevated Ni levels (>220) primarily in the eastern delta plain near the Damietta Branch, covering the Greater Cairo area (Fig. 2).
The *EF* analysis in this study further reordered the metals as follows: Cd>Pb>Zn>Cu>Cr>Mn>Ni>Sr (Fig. 3), reflecting the relative pollution intensity of heavy metals from various sources. Apparently, the higher *EF* of most metals occurred on the northern delta coast, except for Ni and Sr, which were mainly concentrated on the eastern delta plain and to the west of Alexandria (Fig. 3). In northern lagoons, the *EF* of Cd (746) and Pb (695) were highest, followed by Zn (201) and Cu (96). Cr (79) and Mn (50) were classified as sub-level (Fig. 3). Ni and Sr (both 31) showed the lowest enrichment levels, with slight presence observed in the eastern delta plain and lagoons. The *EF* values of Edku Lagoon exhibited the lowest level of metal pollution.

Thus, the *EF* analysis in the present study, which uses pre-dam metal concentrations as “background values”, effectively discriminates anthropogenic metals and supports the conclusion that metal pollution in the Nile Delta intensified following the completion of the Aswan Dam in 1964. The similarities and discrepancies in the spatial distribution of anthropogenic metals are attributed to their own distinctive sources and associated delivery dynamics (Figs. 2, 3).

## 3.2 Diagnostic heavy metals: tracking source-to-sink dynamics

Burullus Lagoon, located in the central Nile Delta, contains the highest levels of heavy metal pollution, characterized by a suite of Cd, Pb, Zn, Cu, Cr and Mn (Figs. 2, 3). Agricultural wastewaters from farming and domestic sewage are believed to be the main pollution sources for these metals (Chen et al., 2010; Gu et al., 2013; El-Amier et al., 2017; El-Alfy et al., 2021). Cu and Zn were diagnostic elements enriched in the lagoon, suggesting a distinct source along the central delta coast. Enrichment of Cu and Zn is linked to a mixture of municipal and agricultural wastewater from the central delta region along with intensive fish farms, which have expanded in recent decades in the southern part of the lagoon (Khalil and El-Gharabawy, 2016; El-Amier et al., 2017; El-Alfy et al., 2021; El-Metwally et al., 2021; Aly-Eldeen et al., 2023). The accumulation of Cu and Zn was clearly associated with fish feeding practices, antifouling agents and fishery medicines used in aquaculture. This has also been widely reported in related studies from the eastern Mediterranean, Canada, Spain, Greece and China, showing that sediments in aquaculture zones exhibit higher Cu and Zn concentrations than those in non-aquaculture areas (Belias et al., 2003; Mendiguchía et al., 2006; Yu et al., 2011; Farmaki et al., 2014). In Sansha Bay, for example, the use of chilled trash fish (containing ~1.000 mg/kg Cu) and formulated feed introduces metals
that accumulate in sediments through uneaten feed and fecal waste (Song et al., 2023). Excessive use of Cu-based medicines (e.g., CuCl₂ and CuSO₄) often results in leaching into sediments via animal feces (Brooks and Mahnken, 2003; Seiler and Berendonk, 2012; Liu et al., 2017), while Egypt's aquaculture policies mandate reliance on metal-laden drainage water (Egyptian Law No. 124/1983) (Abumourad et al., 2013; Ahmed et al., 2023) that further exacerbates contamination. Additionally, high amounts of Cr can be attributed to the application of ammonium fertilizer (El-Amier et al., 2020). Cd and Pb are mostly associated with phosphate fertilizer (Redwan and Elhaddad, 2022; Aly-Eldeen et al., 2023). The consumption of fertilizers in Egypt has increased more than six-fold (FAO, 2024a), along with intensified pesticide application following the construction of the AHD (Abdel-Raheem et al., 2024). More than 950 thousand feddan in the Nile Delta (about 74.3% of the Nile Delta's total area) discharge agricultural wastewater of 4.1×10⁶ m³ a⁻¹ into Burullus Lagoon (Shetaia et al., 2022). Consequently, the polluted metals in Burullus are chiefly related to agriculture, fisheries and urban expansion in the central-north delta plain (El-Amier et al., 2017; El-Alfy et al., 2021; Shetaia et al., 2022).

Rice cultivation in the central delta contributes to the transport and redistribution of heavy metals due to pH fluctuations (Abuzaid and Bassouny, 2020; Abotalib et al., 2023). The central part of the delta (Kafr El-Sheikh and Gharbia governorates) accounts for 30.7% of rice cultivation in the delta (Tolba et al., 2020). This region discharges high-pH wastewater (El-Agha et al., 2020) into Burullus Lagoon (Shetaia et al., 2022) and even into the Rosetta Branch (Abotalib et al., 2023), contributing to Cd enrichment (El-Amier et al., 2017). Since Burullus Lagoon receives agricultural wastewater from the entire central delta through drainage networks, amounting to 4×10⁶ m³ d⁻¹ (Brown et al., 2003), this has contributed significantly to metal enrichment in the lagoon (El-Amier et al., 2020; El-Alfy et al., 2021).

Manzala lagoon is characterized by elevated concentrations of a suite of metals, including Cd, Pb, Cu and Cr (Figs. 2, 3). The spatial distribution of these metals clearly shows different pollution sources. Higher concentration of metals in this area is linked to pollution from the megacities of Port Said, Damietta and Matariya (Fig. 1A) (Arnous and Hassan, 2015; Ahmed El Badry, 2016), where local industrial effluents derive primarily from petrol-refineries and petrochemical plants (Gu et al., 2013). Industrial wastewater from the Greater Cairo area is a major source of pollution in the lagoon (Elkady et al., 2015; Zahran et al., 2015). Manzala Lagoon receives about 65% of
Cairo's wastewater (industrial and municipal), which is discharged directly into the lagoon through the anoxic Bahr El-Baqar drain (Oczkowski and Nixon, 2010) (Fig. 1A). Industrial waste from Greater Cairo accounts for 23% of Egypt's total industrial waste, with chemical industries responsible for >60% of heavy metal discharges (Harris et al., 1995; Wahaab and Badawy, 2004). The notorious Bahr El-Baqar drain, the main source of pollution for Manzala Lagoon (i.e. 3 BCM/year), collects $64×10^3 m^3 d^{-1}$ of industrial waste, mostly from the Greater Cairo, while the El-Gebel El-Asfar and Baraka treatment plants only treat $1.5×10^6 m^3 d^{-1}$ and $0.6×10^6 m^3 d^{-1}$, respectively (Brown et al., 2003; Elbana et al., 2017).

Mariout Lagoon is also characterized by a similar suite of heavy metals, i.e., Pb, Zn, Cu and Mn, but with different pollution sources (Figs. 2, 3). The lagoon was heavily impacted by industrial effluents from approximately 25% of Egypt's industries located in the Alexandria area, primarily petrol refining, petrochemical, paper mill, textile, plastic, pharmaceutical, soap and food processing sectors (Wahaab and Badawy, 2004; Oczkowski and Nixon, 2010). More than 1250 manufacturing plants in Alexandria are responsible for the metal contamination of Mariout (Abdel-Shafy and Aly, 2002). Furthermore, >60 factories discharge industrial effluent directly into the lagoon through pipelines or drains (Fig. 1) (Wahaab and Badawy, 2004). This inevitably led to elevated concentrations of heavy metals, rendering the lagoon a “dead lake” along the northwestern Nile coast (Oczkowski and Nixon, 2010; Abu El-Magd et al., 2021).

The eastern delta plain (east of the Damietta Branch) is notably characterized by widespread enrichment of Pb, Zn, Cu and Ni, though at lower concentrations than those observed in the lagoon areas (Figs. 2, 3). The enrichment of these metals in this area is mainly attributed to the reuse of wastewater for irrigation (e.g., Hadous and Bahr El-Baqar drains) (Omran, 2016). The growing issue of water scarcity in Egypt has increased the reliance on drainage water reuse, particularly in the eastern delta plain (Abdel-Azim and Allam, 2005; Barnes, 2014). Notably, most drains in the eastern delta remain in poor to very poor condition, with high concentrations of heavy metals (Brown et al., 2003; El-Alfy et al., 2020; Ramadan et al., 2021, 2021; Conference, 2022). Among the suite of polluted metals, Ni showed a distinctly high concentration in this region (Figs. 2, 3). The high anomaly of Ni may be influenced by the physicochemical condition of the eastern delta plain, where the soil is more alkaline (Omran, 2016). The majority of Ni discharged into alkaline soils is likely to be readily adsorbed, rendering it relatively immobile (Wuana and Okieimen,
2011). Given the higher pH (7.58–8.82) and CaCO₃ content (29.2 %), the soil in eastern plain is particularly prone to metal adsorption, especially for Ni (Omran, 2016). On the other hand, the highest enrichment of Sr along the western coast of Edku can be attributed to industrial waste, although overall pollution levels in this area remain the lowest among all regions. There are 22 different factories, including food production, canning, paper mill, fertilizer and textile industries, in addition to Abu Qir Fertilizer Company, Abu Qir Power Plant and gas production activities, resulting in discharge of 15,000 cubic meters of industrial waste daily (Abdel Ghani et al., 2013; El-Alfy and Elnaggar, 2023).

### 3.3. Cumulative impact and ecological threats

The cumulative concentration of anthropogenic metals in the Nile Delta is shown in Fig. 4A, with higher concentrations observed in the lagoons, except for Edku as previously explained. The cumulative *EF* and *mI<sub>geo</sub>* for Burullus, Manzala and Mariout displayed higher contamination levels in sediments compared to that of canals, drains, soil and coasts (Fig. 4B, C). While *mCd* and *mPI* generally correspond to *EF* and *mI<sub>geo</sub>*, in reflecting cumulative metal pollution levels, they further classified the intensity of metal pollution in the following order: Burullus > Manzala > Mariout > drains > Nile Branch > canals > coast > soil > Edku (Fig. 4D). The metal pollution in Burullus, Manzala and Mariout has reached extremely high levels (i.e., *mCd* >32, Table S6), and/or is classified as severely polluted (i.e., *mPI* > 10, Table S7) (Fig. 4E, F).

Recently, there has been concern that living organisms in lagoons are increasingly affected by metal pollution, with potential implications for human health (El-Hak et al., 2021; Qaysi et al., 2022). Previous research has identified a causal relationship between metal pollution and the development of organisms in the northern lagoons. It is manifested by a decrease in the diversity of molluscan communities in Manzala (Abdelhady et al., 2019), as well as a reduction in the species richness and increased deformities of benthic foraminifera in Burullus and Manzala (El Baz, 2017; El Baz and Khalil, 2018; Badr-El Din et al., 2019). Moreover, high concentrations of Zn, Cd and Pb have been detected in the soft tissues of the shellfish *Arca noae* from the northwestern Nile coast (Said et al., 2022). Additionally, high bioaccumulation of Pb and Cu has been observed in *Mugil cephalus* from Manzala, considered as one of the most commonly consumed fish by the Egyptian population (El-Hak et al., 2021).
In fact, certain health conditions in the Nile Delta have been linked to heavy contamination (Osfor et al., 1998; Soliman et al., 2002, 2006; Kriegel et al., 2006; Abdel Wahab et al., 2018). Cholangiocarcinoma cancer in the Nile Delta is linked with elevated serum levels of metal pollutants, particularly Cd and Pb (Abdel Wahab et al., 2018). Pancreatic cancer in the northeastern delta is associated with Cd contamination (Kriegel et al., 2006; Soliman et al., 2006), where people have an unusually high rate of early-onset pancreatic cancer (Soliman et al., 2002). The significantly higher incidence of urinary, musculoskeletal and respiratory symptoms among fishermen in Manzala lagoon, along with a marked decrease in neutrophils and an increase in lymphocytes and eosinophils, has been considered potentially linked to heavy metal pollution (Osfor et al., 1998).

**4. Controlling factor: anthropogenic forcings**

The Nile Delta is a complex ecosystem influenced by both natural and anthropogenic factors. Human impact has intensified, particularly under the increasing stress of water scarcity following the construction of the AHD. The historical trend of cumulative *EF* for metals in the Nile lagoons reveals a marked increase beginning after the 1960s (Fig. 5). Since excessive heavy metal pollution in the delta today is primarily caused by artificial wastewater discharge, the key contributing factors are likely the rapid growth in population, urbanization, GDP, agricultural practices and manufacturing over the past decades (Fig. S3). Analytical results provide a comprehensive picture, showing that all related factors have been intercorrelated with the increasing *EF* values in the Nile lagoons since the 1960s (Fig. 5).

Over past 70 years, Egypt's population has increased fivefold, from 21 million in 1950 to more than 110 million in 2022 (FAO, 2022) (Fig. 5), with 41% residing in the delta (Negm et al., 2017). This rapid population growth has driven intensive agricultural and industrial activities, accompanied by increasing urbanization and economic development (Fig. 5) (Nikiel and Eltahir, 2021). As a result of this urban expansion encroaching on agricultural lands (Fig. S1), soil nutrients have declined since the 1960s, requiring increased use of agricultural fertilizers to compensate for the loss of nutrient-rich sediments entrapped behind the AHD (Oczkowski and Nixon, 2010; Abd-Elmabod et al., 2019). Consequently, the consumption of fertilizers in Egypt has increased more than sixfold over the past six decades (Fig. 5) (FAO, 2024a). The concentrations of metals in fertilizers used in Egypt have reached 21 ppm for Pb, 278.7 ppm for Ni and 42 ppm for Cd
(Abdelhafez et al., 2012). Fertilizer usage per hectare increased from 96.95 kg/hectare in 1961 to 542.6 kg/hectare in 2021 (Fig. 5) (OWD, 2024). Given that rice cultivation in the Nile Delta accounts for about 32.7% of the total cultivated area (Tolba et al., 2020), the increase in rice farming and irrigation following the construction of the AHD may also contribute to the transport and redistribution of heavy metals through water reuse cycles (Fig. 5) (Abuzaid and Bassouny, 2020; Abotalib et al., 2023; Abdel-Raheem et al., 2024).

Industrial and agricultural development following the construction of the AHD has been accompanied by a growing water deficit in recent decades (Nikiel and Eltahir, 2021). This has led Egypt to begin reusing wastewater since the 1970s through policy stipulations and adjustments, moving towards an official approach by the 1980s (El Gammal and Ali, 2011; Barnes, 2014). Since then, the Ministry of Water Resources and Irrigation has constructed large reuse pumping stations on the main drains in the delta (Fig. 1A), where water reuse increased from 2.9×10⁹ m³ in 1984 to 7.5×10⁹ m³ in 2011, of which only 30% undergoes treatment (Tawfik et al., 2021). Due to the lack of comprehensive wastewater collection and treatment systems in the delta, the extensive drainage system primarily carries untreated domestic and industrial wastewater, laden with high concentrations of heavy metals that are redistributed through irrigation drain networks (Gad et al., 2020; Ramadan et al., 2021; Tawfik et al., 2021).

To further assess the influence of key factors on *EF*, RDA analysis was used to evaluate their contribution to heavy metal pollution (Fig. 6). Water stress Level (WSL) data sourced from FAO (2024b) were also included in the RDA analysis. The level of water stress increased from 87.8% (high stress) in 1975 to 128.3% (critical stress) in 2008 (Fig. 5), resulting in greater reliance on wastewater reuse. This increased dependency, in turn, has elevated the risk of heavy metal accumulation in water systems due to the high concentration of pollutants in recycled water. Notably, since the first RDA axis accounts for 64.36% of the total variation and shows positive loadings on axis 1, the key factors collectively contribute to promoting heavy metal accumulation (Fig. 6). The influence of freshwater shortage was significant, as attested by a strong negative loading (-0.9 ~ -1.0) on axis 1 (Fig. 6). Among the factors, the role of WSL on *EF* had the highest (positive) loading on axis 1, but was nearly zero on axis 2, indicating its strong correlation with heavy metal pollution (Fig. 6).
## 5. Nile Delta lagoons: an irreversible sink for heavy metals

The spatial distribution of normalized heavy metals shows distinctly higher concentrations moving from the southern delta plain towards the coastal lagoons, with the exception of the coastal beach sediments (Fig. 7A). Moreover, the hierarchical cluster analysis (HCA) classified the metal-concentrated sediments into two main groups (I and II) (Fig. 7B): Group I) Burullus, Manzala, and Mariout, and Group II) Edku, Nile Branch, canals, coast, drains and soil. The first cluster is highly contaminated with heavy metals, while the second primarily consists of non-lagoonal sediments, where concentrations of anthropogenic metals are lower. In general, concentrations of most anthropogenic metals in lagoons sediments were 3 to 4 times higher than those in non-lagoon sediments (Fig. 7A).

Therefore, the present Nile Delta lagoons have become a sink for heavy metals originating from pollution across the entire delta system following the completion of the AHD. Given that the coefficient of variation (CV%) for metals exceeds 30%, indicating significant anthropogenic impacts (Fig. 7C) (Qu et al., 2013; Shetaia et al., 2023; Abu Khatita, 2024), the CV% values for Pb, Cu, Zn and Cd in the lagoon sediments are predominantly attributed to anthropogenic sources. The cumulative CV% for all metals indicates a high anthropogenic influence in the following order: Manzala > Burullus > Mariout.

The arid climate setting of the Nile Delta, combined with the construction of the AHD, has exerted a “dual pressure” on environmental degradation (Chen et al., 2021), significantly weakening freshwater dynamics responsible for removing anthropogenic metals from the delta-coast to the offshore area (Fig. 8) (Gu et al., 2013). Compared to wet-climate deltas of the world, e.g., those of Asia, where anthropogenic metals are also intensively produced (Fig. 8) (Xu et al., 2014; Chen et al., 2017; Wang et al., 2017; Koukina and Lobus, 2020; Perumal et al., 2021; Zhuang and Zhou, 2021; Liang et al., 2023; Naik et al., 2023; Xiang et al., 2023; Cai et al., 2024), the high monsoon precipitation in these regions has effectively diluted metal pollutants along the delta coast (Chen et al., 2004). The Yangtze River in eastern China, one of the world’s mega-rivers with >400 million people living in its basin, has contributed remarkably high levels of heavy metals to its delta-coast area. However, the strong monsoon climate, with an average annual precipitation of 1100 mm washes anthropogenic metals seawards, resulting in high metal concentrations offshore rather along the delta coast (referring Fig. 2 of Chen et al., 2004; Gu et al., 2013). Also, the EF of
the arid Nile Delta is much higher than that of the wet delta-estuaries of Asia (Fig. 8). Evidently, EF is primarily influenced by regional precipitation, in addition to many other factors such as population, industrial and agricultural intensity and local management practices.

The cumulative anthropogenic metal load in the Nile Delta lagoons is linked to water scarcity and poor management. It is well understood that Egypt’s water future is shaped by a combination of growing social demands, environmental pressures and geopolitical challenges (Nikiel and Eltahir, 2021; Esraa et al., 2023). Using WSL as the dominant factor to correlate EFs in lagoonal sediments (selected from those listed in the RDA result of Fig. 6) reveals a non-linear trajectory. This suggests that EF may approach its maximal value before 2040, during a period when WSL increases at a relatively low rate. It further implies that EF could reach its peak around 2060 as the rate of WSL continues to rise (Fig. 9). This suggests that anthropogenic metals in lagoon sediments could reach saturation by the end of 2060. In another words, anthropogenic metals delivered into lagoon as dissolved chemical elements would remain in the lagoon water bodies, indicating that riverine sediments will no longer sever as the primary carrier for metal transport. Consequently, lagoon-based aquaculture could become a potentially high-risk source of contamination, posing threats to human health.

**6. Policy implications**

Clearly, the increasing heavy metal pollution of the Nile Delta has raised concern amongst Egyptian communities about the sustainability of current practices. Currently, Egypt is reclaiming land outside the Nile Delta to meet growing socioeconomic demands in response to rapid population growth, with the population projected to reach 147 million by 2037 (Esraa et al., 2023). Water from the Nile River will continue to be allocated to the development of commercial agriculture on Egypt’s desert margins (Barnes, 2012; Tawfik et al., 2023). Therefore, one of the proposed mitigation countermeasures to address the anticipated freshwater deficit is to increase the reuse of partially treated wastewater to 16.26 BCM in order to meet the water demand by 2037 (Tawfik et al., 2021). Despite its potential to reduce the gap between irrigation water supply and demand, the reuse of wastewater presents significant challenges, including ecological risks from increased accumulation of heavy metals. Most water treatment plants in the Nile Delta have limited capacity to remove micropollutants, including heavy metals, due to their reliance on conventional
filtration processes such as coagulation, sedimentation, filtration and chlorination (Tawfik et al., 2021). To address this, the Egyptian National Development Plan (2017-2037) was launched in April 2019, with a special focus on the challenges of freshwater deficits (Mostafa et al., 2021). The plan allocates 50 billion USD to rationalize water use and improve water quality, in addition to enhancing cooperation between government agencies (Fouad et al., 2022). This includes the construction of 52 new treatment plants in the Nile Delta with a total capacity of 418 million m³/year (Kwasi et al., 2022). In addition, authorities have begun lining irrigation canals (20,000 km out of 55,000 km of canals) to reduce water loss from leakage, improve water quality and encourage farmers to adopt modern irrigation systems. This countermeasure also supports basic research aimed at developing water-saving crops (Fouad et al., 2022).

Therefore, it represents a promising effort to manage the challenges of water reuse in the Nile Delta and, in turn, to mitigate risks to human health, the environment and biodiversity associated with these pressures. Investments in wastewater treatment (Hafez et al., 2009; El-Awady et al., 2014; Mahmoud et al., 2018; Mohamed, 2020) and modern irrigation methods (see Mostafa and Fujimoto, 2014; ElGamal et al., 2019; Abdelhaleem et al., 2021; Fouad et al., 2022) are important steps toward protecting and preserving water resources. However, ensuring long-term water security will also require diplomatic initiatives with Nile Basin countries. To secure its future and effectively manage its water resources, Egypt should integrate technological advancements, pollution mitigation strategies, legislative reforms and regional collaboration.

## 7. Future perspectives

Given the metal-related pressures driven by multiple factors and their potential consequences (Figs. 5-7, 9), this study proposes a management strategy (Fig. S3) to help mitigate the anticipated heavy metals crisis in the coming decades. Intensifying anthropogenic activities in the world's mega-deltas have significantly impacted the natural flow of sediment, water and nutrients, reshaping land use through urban development, coastal structures and altered ecosystems. Whether wittingly or unwittingly, the consequences will have far-reaching implications for deltaic environments, particularly concerning heavy metal pollution. Strong governance through policy modification and effective implementation should be pursued within the context of social awareness, fundamental education, legislation and the application of new technologies. Urgent
communication regarding dam-associated gaining and losing processes should be urgently
encouraged among government and scientific communities to harmonize and sustainably allocate
water resources at a cross-basin scale (Chen et al., 2024). Does the non-linear response of elevated
metals to wastewater reuse in the Nile lagoons offer a viable way to mitigate metal pollution in the
coming decades by reasonably demobilizing polluted lake bottom sediments?

In the near further, cutting-edge research is necessary to fully understand the impact of climate
fluctuations on the mobility, accumulation and bioavailability of heavy metals in delta ecosystems,
particularly given the changes in post-dam sediment dynamics and hydrodynamic conditions. The
complexity of heavy metal sources in the Nile Delta highlights the need to address gaps in the
application of isotope-based methods (e.g., Pb, Cd, Hg, Zn isotopes) that hold great promise for
uncovering the migration pathways and transformation mechanisms of heavy metals in ecosystems.
Long-term research that integrates ecotoxicological evaluations with risk assessments for human
health is crucial, especially for communities in delta regions that are particularly vulnerable.
Effective pollution control should depend on creating sustainable remediation techniques that take
into consideration the distinct hydrological and geochemical features of deltas and encouraging
international cooperation to pinpoint regional issues and challenges as well as global trends (e.g.,
Mega-Delta Programme; https://ocean decade.org/). In addition, this study calls for effective river
basin management by modeling the interactions between climate change, regional precipitation
patterns, sediment transport to deltas and governance strategies to help mitigate these risks.
Addressing these interconnected factors through future research will advance our understanding
of heavy metal dynamics and support policies that ensure the resilience and sustainability of deltaic
regions worldwide.

8. Concluding remarks

The unique anthropogenic setting of the Nile Delta, characterized by water scarcity and
intensifying human activities, is transforming its lagoons into an irreversible sink for
anthropogenic metals, posing a severe challenge to societal sustainability. In fact, the heavy metal
and related water issues in the Nile Delta are complex and longstanding resulting from increasing
social demands, environmental pressures and geo-political competition (Nikiel and Eltahir, 2021;
Esraa et al., 2023). As the country continues to rely heavily on Nile-derived water resources while
facing population growth, climate warming and transboundary water challenges, such as the
construction of Grand Ethiopian Renaissance Reservoir in the upper Nile River basin (Fig. 1B),
we must apply wisdom to overcome any potential barriers on the path ahead.

To mitigate anthropogenic metal risk, this study proposes 3 levels of strategy. 1) Technical level: urgent improvements are needed in water treatment for reuse processes, including increasing expanding the capacity of treatment plants along sewerage drains and upgrading technologies for heavy metal removal. 2) Scientific level: broader regional and international collaboration is essentioal. Scientists from both physical and social disciplines should work effectively together with a focus on theoretical and practical approaches to understanding metal processes, consequences and hazard mitigation. 3) Administrative level: an administrative committee should be established consisting of basin-scale stakeholders, including scientists, government officials, decision-makers and residents. This committee should focus on problem-solving, sharing databases and monitoring systems, applying advanced technologies and securing funding. More importantly, the committee should strengthen communication between two geographic communities, i.e., reservoir community and delta-coast community, to facilitate the sharing of “gains and losses” of dam-related water issues. This would ideally represent a robust approach to combatting anthropogenic-metal hazards in the Nile Delta, and one that could be shared with communities worldwide.

## Acknowledgements

This study was supported by the International Joint Laboratory of Estuarine and Coastal Research, Shanghai (Grant No. 21230750600); postdoctoral funding was from the State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai; National Natural Science Foundation of China (Grant No. 42261144741); and Science and Technology Commission of Shanghai Municipality (23590780200).

## Individual contributions

S.A. Shetaia, Y. Wang, M. Li, A. Salem and Z. Chen designed the project; S.A. Shetaia, Y. Wang, A. Salem and Z. Chen collected database; S.A. Shetaia and Y. Wang made artworks and
computation; S.A. Shetaia, Y. Wang, and Z. Chen wrote manuscript; all co-authors were
participated discussion and revision.

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<figure><img src="image_2.png" /><figcaption>Figure 1. A) The Nile Delta, where heavy metal samples were collected for this study; B) The Nile River Basin, showing the Aswan High Dam and Grand Ethiopian Renaissance Dam (Modified after Woodward et al., 2015).</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>Figure 2. Distribution of normalized heavy metals in the Nile Delta (displayed according to normalized concentration values).</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Figure 3. Distribution of enrichment factors (EF) of heavy metals in the Nile delta (displayed from high to low values)</figcaption></figure>
<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /><figcaption>Figure 4. Distributions of heavy metals and pollution indices in the Nile Delta, A) heavy metal concentrations, B) modified geo-accumulation index (mIgeo); C) enrichment factor (EF); D) modified degree of contamination (mCd) and modified Nemerow pollution index (mPI); E) spatial distribution of modified degree of contamination (mCd); and F) spatial distribution of modified Nemerow pollution index (mPI)</figcaption></figure>
<figure><img src="image_11.png" /></figure>

Figure 5. Trending analysis, including the temporal distribution of cumulative enrichment factor (EF) (based on data integrated from dated core sediments of Manzala, Burullus and Edku lagoons; Gu et al., 2013), along with water resources per capita (sourced from Fleifle and Allam, 2016; FAO, 2024c), wastewater discharge to the lagoons (modified after MWRI, 2002; Abdel-Azim and Allam, 2005; Molle et al., 2016), fertilizer use per hectare and total fertilizer use (modified after FAO, 2024a), rice cultivation area of the Nile delta (modified after Abdel-Raheem et al., 2024), GDP per capita (modified after FAO, 2024d), level of water stress (modified after FAO, 2024b, refer Supplementary Materials), manufacturing value added (sourced from World Bank Group, 2024), urban area in the Nile Delta (modified after Abd-Elmabod et al., 2019; Fouad et al., 2022), drainage water reuse of the Nile Delta (sourced from Abdel-Azim and Allam, 2005; Mostafa et al., 2005; Ghaffar and Shaban, 2014), and the total population of Egypt (FAO, 2022).
<figure><img src="image_12.png" /><figcaption>Figure 6. RDA analysis of potential drivers of heavy metal pollution (the elemental data used are normalized metals).</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>Figure 7. A) Distributions of normalized heavy metal concentrations in the Nile Delta along a latitudinal gradient (South to North), presented as unitless values using the Min-Max normalization method (Ribeiro et al., 2018); B) Dendrogram based on hierarchical cluster analysis (HCA), showing relationships between different variables and locations in the Nile Delta; C) Coefficient of variation (CV%) of heavy metals in the Nile lagoons.</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>Figure 8. Comparison of enrichment factor (EF, methodology detailed in Supplementary Materials) between the arid Nile lagoons and wet estuaries and lagoons worldwide - <sup>a</sup>(Pradit et al., 2024); <sup>b</sup>(Naik et al., 2023); <sup>c</sup>(Perumal et al., 2021); <sup>d</sup>(Liang et al., 2023); <sup>e</sup>(Koukina and Lobus, 2020); <sup>f</sup>(Zhuang and Zhou, 2021); <sup>g</sup>(Chen et al., 2017); <sup>h</sup>(Wang et al., 2017); <sup>i</sup>(Xiang et al., 2023); <sup>j</sup>(Wang et al., 2022); <sup>k</sup>(Xu et al., 2014). Water resources per capita and precipitation for those regions were obtained from the FAO (2024c).</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>Figure 9. Projection of heavy metal pollution vs. level of water stress, identified as the primary driver influencing anthropogenic metal concentrations (based on RDA results; see Fig. 6).</figcaption></figure>