Heavy metals in the sediments of the Nile Delta: Anthropogenic-induced hydrologic changes

Aquatic ecosystems act directly or indirectly as sinks for heavy metals, which persists for longer times in the biogeochemical cycle and poses high ecological risks and thus, it represents a major worldwide concern. To evaluate the metal contamination in the Nile Delta, twenty sites along both Rosetta and Damietta branches were investigated. Most heavy metals concentrations are above the sediments quality guidelines. The Contamination Factor (CF), Pollution Load index (PLI), and geo-accumulation index (I geo) indicated that the sediments are moderately to extremely polluted by Ni, Cd, Cr, Cu, Pb, and Zn. A trend of increasing metals concentration towards the Mediterranean was observed and attributed to historical sinking of heavy metals in the sediment, where many agricultural drains bringing mixed wastewater. Cluster analyses support the latter and indicated that the heavy metals, which clustered together, may have the same source (i.e. anthropogenic). Non-metric Multidimensional Scaling, Analysis of Similarity (ANOSIM), and Partial Least Square (PLS) regression conrmed that the distance along the river is the major predictor of the observed metal concentrations, while sediment grain–size has no or very minor effect. The lower hydrodynamics associated with the construction of Aswan High-Dam in 1964 and the developed agricultural system in addition to arid climate with scarce rainfall, all leave no chance for natural annual ooding. Therefore, freshening up of the sediments was prohibited. The marinating of well-controlled ooding may erode the upper contaminated portion of river sediments, dilute the metal concentrations, and improve the sediments quality.


Introduction
The heavy metal contamination persists for longer times in the biogeochemical cycle and poses high ecological risks and thus, it represents a major worldwide concern. Anthropogenic activities (agricultural, domestic, and industrial) are the main source for higher concentrations of these metals (Dean et al., 2007, Kalantzi et al. 2014, which usually discharged into aquatic environments (Abdelhady et al. 2019), suspended in water column, sinking in the sediment, and entering the food chain (benthic organisms, sh and humans, Rainbow andPhillips 1993, Rainbow 1995). Polluted sediments are good indicators for water pollution (Förstner and Wittmann 1981). Although water quality may be improved due to current global polices, underlying sediments may release these metals again into the water column due to any physicochemical environmental change (e.g., temperature, pH, and hydrodynamics). Therefore, metal concentrations in the sediments have to be regularly monitored at different spatiotemporal scales (Branica et al. 1985, Schropp et al. 1990).
The Nile runs for more than 6000 km from Ethiopia in the south to the Mediterranean Sea in the North. For thousands years ago, it has enabled settlement of the old Egyptian, where one of the most ancient civilizations initiated (Dumont 2009). The annual ooding of the world's longest river has been delivered both water and good soil required for agriculture. The Nile is fundamental source of freshwater in Egypt, which share about 55.5 BCM (billion cubic meters)/year (MWRI, 2010). The Nile Delta is situated in the middle of the Egyptian Mediterranean coastline of approximately 1,000 km (Fig. 1). The Nile Delta was occupied for at least the ve thousand years, where it has been intensively farmed (Dumont, 2009). The Nile Delta represents only 2% of Egypt area but it hosts 41% of the country's population (one of the top 5 densely populated agricultural areas in the world, Hereher, 2010). Intensive agricultural practices in the past decades, occasionally after the construction of the Aswan High-Dam, were resulted in a gap between water supply and demand. To overcome this problem, drainage water was reused since the 1980s (Khater, et al. 2014). The usage of phosphate-rich sewage water with partial or without any treatment in agriculture has increased the metals concentration in sediments of the Nile Delta (Karatas et al. 2006). During the past Century, the regulation of the Nile ooding by constructing dams, development of complicated agricultural systems, and increasing water exploitation due to rapid population growth, may have left dramatic changes in the hydrologic cycles. Oczkowski and Nixon (2008) indicated that the Nile Delta lakes (Mariut, Edku, Burullus, and Manzala) are polluted at different levels by heavy metals. A result, which has been clearly demonstrated by many others in the past decade (e.g., Chen et al. 2010, Gu et al. 2013, Abdelhady et al. 2018, 2019a. The closed or semi-closed system of these lakes may be the main cause for such higher concentrations of heavy metals. To test if the hydrologic/hydrodynamics is a main source for heavy metal sinking in the sediments, the heavy metals in the sediments of the two branches, Rosetta and Damietta, of the Nile Delta were analyzed. Herein, we aim to assess the distributions of the both metals in the sediments along Nile Delta branches to quantify their sensitivity to location of the pollution source, water energy (i.e. hydrodynamics), and other physicochemical variables (i.e. sediment grain-size). Therefore, we propose to address the following research questions: 1) What are the main sources of the heavy metals in the Nile delta? 2) Is there any gradient of heavy metal pollution in the sediments? 3) Why the metals (if any present, concentrated in the sediments? 4) is there any proper method to overcome/reduce the pollution intensity? The spatial distribution of these metals in the river can enable tracing their sources and assess the pollutants mobility for better management in the future.

Study Area
The Nile delta is suitable for intensive agriculture and it supports 63% of the country's agricultural and represents more than 80% of the shing industry. The Nile Delta is part of one of the world's most important migration routes for birds. Every year, millions of birds pass between Europe and Africa along the East Africa Flyway, and the wetland areas of Egypt are especially critical stopover sites. The Nile Delta is subjected also to shoreline changes resulting from erosion and accretion, subsidence, and sea level rise (SLR) resulting from climate change (Fishar 2018). The Nile has two branches starts at Cairo and ends at two promontories (estuaries) at Damietta on the east or Rosetta in the west (Fig. 1a).
Damietta branch, as shown in Fig. 1, located at the eastern part of the Nile Delta. Its length is about 241 km, average width 280 m with an average depth 11 m and average elevation 2 m. The Rosetta branch extends for more than 239 with shallow depths (2 to 4 m) and average of 180 m width (Negm et al. 2017). Mixed domestic, industrial, and agricultural wastes disposed to the Rosetta branch through a drainage network starting from Cairo and extends to the north with only partial or without any treatments. El- Barbary et al. (2008) indicated that 900 MCM of wastes discharges monthly in the Rosetta branch.
The water of the two branches is negatively affected and therefore, the environmental conditions have become extremely harsh for fauna (Appendix A, Fig. 2).

Sampling and granulometry
Samples of sediments were collected in two replicates from twenty sites in the Nile Delta from February to April of 2014 using 20*20 cm Ekman grab (Fig. 1a, Table 1). In the laboratory, sediment samples were air-dried and then sediment grainsize analysis was done using an electric shaker for 100 gram of the sediments (sieves openings are 2, 1, 0.5, 0.25, 0.125, and 0.063 mm). The percentages of the different size classes were calculated (Quintino et al. 1989). Percentage of ne sediments (silt and clay) obtained by wet sieving method. The mm calculation transformed to phi scale (φ) and the main features of the sediments grain-size (mean, standard deviation, skewness, and kurtosis) were used for statistical analyses (Table 1).

Geochemical analyses
The rest of samples were crushed with a ceramic pestle and mortar to pass through a 0.063 mm mesh screen. About 0.1 g of dried sediments and 1 ml of water aliquot was taken in a 50 ml beaker and then 3 ml of a concentrated mixture of HNO 3 and HCl (1:1) were added. Subsequently, all the treated samples were diluted to 10 ml with ultra-pure water (Milli-Q). Then the samples were ltrated for 5 minutes through a centrifuge to split the solids from the solution. We focused on the dissolved ltrate fraction, which is more likely to have measurable biological effects on aquatic organisms (Di Toro et al. 2000). Eight metals including Zn, Cd, Cr, Pb, Cu, Fe, Mn, and Ni) were analyzed using the Atomic absorption spectroscopy (Perkin Elmer 400) in Nationalinstitue of oceanography and shery (NIOF), Cairo, Egypt. In order to ensure the quality of the analyses, strict laboratory regulations and procedures regarding cleaning, calibration, and duplicates measurements were followed. The applied analytical method was continuously checked using a certi ed reference material to ensure the quality of the performed analyses. The certi ed reference materials were analyzed every 10 samples. The errors in all measurements were less than 5 %. In addition, analysis of heavy metals in many samples was repeated. The Relative Percent Differences (RPD) between the duplicate and the sample ranged from 0.1 to 3 %. The practical quantitation limits (PQLs) of the metals analyses were in the range of 0.01-0.05 ppm.

Data analyses
Contamination Factor (CF), the concentration of metal in the sediments divided by background base value, was used to evaluate the sediments quality, where pollution intensity can be estimated as follows, CF < 1, low pollution, 1 < CF < 3, moderate pollution, 3 < CF < 6 is considerable pollution, and CF > 6 as high pollution. Pollution load index (PLI) was also used to comment on the overall pollution intensity by different metals, where PLI = (CF1 x CF2 x CF3 x …x CFn)1/n, Where (n) is the number of metals and CF is the contamination factors. In addition, the geo-accumulation index (I geo = log2 (cn/1.5 * bn), where (cn) is the measured concentration of examined element (n) in the sediment sample and (bn) is the geochemical background for the element (n), was also implemented, where pollution intensity is quanti ed based on as scale from 0 to 6.
The grain-size and metals concentrations data were normalized using Euclidean transformation to give the same importance to all variables included in the statistical analyses and to comment on the co-linearity (Theodoridis andKoutroumbas 2008, Abdelhady 2019). Pearson correlation, RMA (reduced major axis) linear/exponential regression models, and hierarchial clustering were used for assessing the relationships. We tested the signi cance for all correlations at signi cance level (p < 0.01). Sequential Bonferroni correction was applied to multiple correlations. In addition, metal concentrations were compiled for non-metric Multidimensional scaling (NMDS, Zhang et al. 2009, Gu et al. 2013. The non-parametric Analysis of the Similarity (ANOSIM) test was used to test the signi cance of the relationships among the examined metals. Furthermore, Partial Least Squares (PLS) regression was implanted, where the strength of the tted model (Q2) and the loadings or the VIPs (Variable Importance for the Projection) were used to quantify the effect of each physicochemical parameter on the metals concentrations (Carrascal et al. 2009, Zelditch et al. 2012).

Sediment grain-size
Grain-size analyses indicated a gentle gradient of the river toward the Mediterranean Sea, where Skewness, kurtosis, and standard deviation decreases in both branches of the Nile (Table 1, Appendix A, Fig. 2A-B). Mean grain-size also gradually decreases in both branches (Appendix A, Fig. 2C-D). The gradual decrease in coarse-grain proportion of the sediments and the decrease in variation may attribute to gentle sloping, where the elevation of northern parts of the Delta is about 2 m (see Fig. 1b). Fin-grain nature and high variance in the sediments of southern sites in addition to the platykurtic curve indicate illsorting fabric and point to the dominance of the suspended mode of transportation (immature) and/or different sources of the sediments (i.e. mainstream, erosion of the river banks, agricultural drains). Reduced Major Axis (RMA) regression revealed non-signi cant relation between distance along the branches and the sediment grain-size (Appendix A, Fig. 2C-D). The nonsigni cant decreasing trend in grain size may be attributed to random discharge from agricultural drains that carry usually muddy water. Most sites are composed mainly of sand size sediments (Table 1), while mean grain-size increases northward and reaches gravel size at station 8 on Damietta branch (Appendix A, Fig. 3D). Most of the samples are slightly skewed with polymodal distribution and platykurtic distribution, which re ects the immaturity of these sediments and that they were transformed for only short-distance/time and thus, may be related to agricultural drains and their source is not the main course of the river.

Metal concentrations
The average concentrations within the samples arrange the metals in the sediments of both branches in the following descending order (Fe > Mn > Zn > Cr > Cu > Pb > Cd, Table 2). The metal concentrations show a gradual increase from the south to the north (Table 2, Fig. 4) and this is true for most metals of Rosetta branch (Appendix A, Fig. 3A-H). In contrast, the Damietta branch only a similar trend was found for Mn, Cu, and Zn but not Ni, and Pb (Appendix A, Fig. 4). There is a strong signi cant exponential correlation between the distance to capital city (Cairo) and most of the heavy metals concentrations in sediments of Rosetta branch include Ni, Cu, Pb, and Zn (Appendix A, Fig. 3, R2 > 0.27, P < 0.001). Fe, Mn, Cd, and Cr show chaotic distribution and have no clear trends and thus, they may have generated from other sources (see below). Table 2 Metal concentrations in core samples at 10 cm intervals. Effective Range Low (ERL) and Effective Range Medium (ERM) according to Long and Morgan (1990) Long and Morgan, 1990). In addition, concentrations of Cd, Pb, Zn, and Cu, are also above ERM at northern stations in both branches (Table 2). According to CF, most stations are moderately to very highly polluted (Appendix A;  Fig. 2A), which is not the case in Damietta branch (Fig. 2B). Similarly, and according to I geo, pollution intensity increases also northward in Rosetta stations (Fig. 2C). A trend which is not observed in the stations of the Damietta branch (Fig. 2D). PLI indicated a strong positive exponential correlation along the Rosetta branch (stations 1:10, Fig. 2E). In contrast, PLI show negative correlation (Fig. 2F). In general, the metal concentrations were consistently higher at northern stations of Rosetta branch (Appendix A, Fig. 3). Furthermore, most of the examined metals in Rosetta branch showed a consistent increasing trend, which re ect a steady increase in the anthropogenic input.

Source of pollution
To identify the source of the different metals in the sediments of the Nile Delta, Pearson correlation and hierarchical clustering were applied. A signi cant positive correlation was found between Ni and Cr (r = 0.63, p < 0.001, Appendix A; Table 2) and also between Cd and Pb (r = 0.63, p < 0.001, Appendix A; Table 2), which may indicate that they originated from the same source. To better characterize their relationships, cluster analysis was applied to the concentration data matrix. The dendrogram showed that Cd is slightly behaved differently from the other heavy metals (See also Appendix A, Fig. 3) and in the same time is clustered with Fe, which may point to same source or may be their accumulation is governed by different physicochemical factors. In contrast, other heavy metals (Ni, Cr, Cu, Zn, and Pb) were clustered together (Fig. 3A). A slight difference between Mn and Zn is seen from metal concentrations in the Damietta branch (Fig. 3B). However both still clustered with the rest of the heavy metals, which may point to another possible source for both Mn and Zn.
The stations of both branches divided into two groups, north and south stations. Differences between the two groups of stations are no signi cant (ANOSIM test, P < 0.001). The latter may result from the fact that metals concentrations are gradual without sharp change in the concentration between northern and southern stations. The variation between both groups of the metal concentrations is well characterized on the 2D NMDS plot, where both groups of stations are well overlapped (Fig. 3C). Moreover, the variation in the metal concentrations is highly variable among the measured metals (Fig. 3D). Ni and Cd have the lowest concentrations with narrower range of variation among the studied stations. In contrast, Cr and Zn show wider range of variation (Fig. 3D).
To test the effect of sediment grain-size on the metal concentration, PLS regression was applied to the dataset including both metal concentrations as observations and sediment grain-size parameters in addition to distance from Cairo as predictors.
The rst three components comment on 99% of the variation (Fig. 4A). The VIPs loading showed that the distance from the stations on the two Nile Delta branches to Cairo is the main predictor (0.99). In contrast sediment grain size parameters (mean grain size, standard deviation, skewness, and kurtosis) have no or very minor effect (Fig. 4B).

Pollution levels and source evaluation
The PLI indicates progressive deterioration of most stations, occasionally those to the north, where metals concentrations are usually exceed their baseline levels many times (Tables 2 and 3). For the Damietta branch, the results indicated a negative exponential correlation, where the pollution levels decreased with increasing distance from the source (Cairo and megacities at the mouth of the Nile delta). In contrast, a positive correlation was found and attributed to a steady discharge from ve main agricultural drains and not only a single drain as in case of Rosetta branch. The source of Cd may be differing from other metals (Appendix A, Fig. 4). Probably, Cd is originated from industrial activities as well as from lithogenic sources as do Fe. Fe may have generated from lithogenic origin such as the volcanic rocks in the rst distributaries of the Nile in Ethiopia. The later explain why both metals were clustered together ( Fig. 2A-B). Pb, Cr, and Cu may have mixed sources (i.e. tra c and industrial practices).
The combined domestic, industrial, and agricultural wastes brought by ve main drains in Rosetta branch (i.e. El-Rahawy, Sabal, El-Tahadi, El-Tahrir, and Tala) have had signi cant in uences on the distribution of the metal concentrations and are the main source for pollution of the sediments in the Nile delta. As the topography is of very gentle slope (Fig. 1b), river is no longer strong enough to carry out these metals to the sea and hence, they sink to the ground. The reduction of the water quantity that discharges to the sea after the construction of Aswan high-Dam in 1964 exaggerated the heavy metal sinking (Gu et al. 2013). The sediments recorded higher values may be more than in polluted water itself as it represent historical value for metal that have accumulated for years ago (Abdelhady et al., 2019a, Fig. 5A). It is known that sediments close to the drains enriched with metals by two main mechanisms, the high content of the metals from industrial, domestic, and agricultural sources in addition the modi cation of physicochemical conditions include but not limited to high pH, high organic-rich mud, and associated dysoxia (Dean et al. 2007, Kalantzi et al. 2013. A possible solution for the heavy metal accumulation in the sediments may be the maintaining of (controlled) high-intensity oods, which may erode the upper portion of the bottom sediments, where heavy metals accumulated, and leaching them to the sea (Fig. 5B). The Hydrodynamic changes associated with the construction of Aswan high-Dam, the development of complex agricultural system, reduction in water owing to the sea, all have attributed to the accumulation of the heavy metals in the sediments of the Nile branches in the Nile Delta.

Temporal changes and ecological risk
Cu and Zn are essential micronutrients in all aqueous habitats (Simkiss and Taylor 1989). Anthropogenic activities, related to agriculture are signi cantly altered the biogeochemical cycle of the trace metals and enhance their bioavailability (Garrels et al. 1975). Elevated concentrations of certain trace metals (e.g., Cd, Cr, Cu, Hg, Ni, Pb) may be extremely toxic, and harm biota (Radix et al. 2000). Contamination entering in the aquatic ecosystem also arrived to the human food chain. Abbassy et al. (2018) found that water of the Rosetta estuary contain persistent organic pollutants. Heavy metals may be absorbed in the suspended clay particles and deposited in the sediments, and thus, their concentrations in the water may be decreased (Keshta et al. 2020). Pollutants in both water and sediments will be enter the geochemical cycle, absorbed by the benthic fauna and hence, reached shes and humans (Dean et al. 2007, Ryu et al. 2011, O'Brien and Keough 2013, Chariton et al. 2015. Pollution of aquatic environment possess a high risk to the biodiversity (Rodrigues et al. 2017, Abdelhady et al. 2019a, Mosbahi et al. 2019 A fundamental problem of the trace metals is their resistance to biodegradation and once entered the aquatic environment they are redistributed throughout the water column, accumulated in sediments and consumed by biota (Abdelhady et al. 2019b, Long et al. 1996, Fichet et al. 1998. Spatial distribution of the trace metals is critical for differentiating natural concentrations from introduced anthropogenically (Galloway 1979, Long et al. 1996, Hatje et al. 2001, Korfali and Davies 2003.

Conclusion
Analysis of the grain-size and the concentrations of eight metals in 20 sediment samples from the two branches of the Nile Delta revealed the following: The sediments are mainly of coarse grain sands, and grain size increases northward, while the concentrations of most of the heavy metals increases in northward (sea) direction.
The concentrations of most heavy metals are above the ERM and exceeding their base level values many times. The sediments quality indices (i.e. CF, PLI, and I geo) indicated that the sediments are highly polluted by Ni, Cu, Pb, and Zn and the pollution intensity, occasionally in the Rosetta branch increasing exponentially in northward direction.
The spatial variation in the metals concentrations was interpreted to be a consequence of the source (drains) location and intensity/quantity of wastewaters discharged to both branches. The main agricultural drains located along both branches (i.e. El-Rahawy, El-Tahadi, El-Tahrir, Sabal, Tala, and Omar-bek) discharge huge amount of heavy metals-rich wastewaters.
The PLS regression revealed that the distance from Cairo to the stations in the two Nile Delta branches is the main predictor (0.99). In contrast sediment grain-size (mean grain size, standard deviation, skewness, and kurtosis) have no or very minor effect.
The hydrodynamic changes associated with the construction of Aswan High-Dam, the development of complex agricultural system, reduction in water owing to the sea, all have contributed to the accumulation of the heavy metals in the sediments of the Nile branches in the Nile Delta.
The sediments recorded higher concentrations of heavy metals, which may exceed their concentration in the polluted water itself, as it represents historical accumulated of the metals. A controlled high intensity oods may erode the upper portion of the bottom sediments, where heavy metals accumulated, and leaching them to the sea.