Risk assessment of heavy metal pollution in water, sediment and plants in the Nile River in the Cairo region, Egypt

Egypt faces a rapidly increasing deterioration of its surface and groundwater due to the discharges of industrial effluents and domestic wastes into its waterways, which contain harmful or poisonous substances. The excessive application of fertilizers and pesticides in agriculture also causes water pollution problems. Contamination of water caused by different effluent discharges could probably be hazardous to human health (Wahaab & Badawy 2004; Al-Afify et al. 2018).

The Nile River constitutes the main water resource of Egypt. The Nile River in the Cairo region supplies water to a population of approximately 15 million people and supports many commercial and industrial activities. Heavy industry is located around North and South Cairo, in addition to some heavy and small industries located randomly throughout the city.

Water pollution is one of the serious environmental problems resulting from urbanization, overpopulation and industrialization, in addition to ignorance (Narain et al. 2011). A large discharge of heavy metals into the aquatic environment eventually accumulates in water, sediments and dependent biotic components like fish and aquatic plants (Rybak et al. 2013). Heavy metals are of particular concern because of their characteristics such as toxicity, abundance, ubiquity, bioaccumulation capacity and resistance to decomposition. These compounds do not degrade and therefore accumulate in bodies of organisms or sediments, and can pose a significant threat to the health of humans and other plants, animals and ecosystems (Alahabadi & Malvandi 2018). Water contamination with heavy metals is therefore a serious concern in today’s world (Miretzky et al. 2004).

Rivers receive sediment from several diffuse and point sources, which is deposited at the bottom and acts as potential sources of metal accumulation in the aquatic food chain through the biomagnification process (Singh et al. 2017). Sediments are considered a sink and reservoir of many toxic contaminants, including heavy metals, and have been used to assess the historical pollution status (Thevenon et al. 2011). Heavy metals can be stored in sediment for a short period of time, where some of these fixed heavy metals may be released into the overlying water and taken up by the aquatic biota (Singh et al. 2017). Many factors, such as temperature, pH and dissolved oxygen levels in water, control the fate of metals through sorption, precipitation and dissolution processes (Duncan et al. 2018).

Aquatic plants play an important role in sequestering large quantities of nutrients and metals from the environment by storing them in the roots and/or shoots. Aquatic plants have high remediation potential for macronutrients due to their general fast growth and high biomass production (Shaltout et al. 2009). Macrophytes are important in the biological monitoring of aquatic ecosystems, because changes in the composition of aquatic vegetation are considered a reliable biological indicator of the water quality. Many studies have researched the use of macrophytes as indicators of bioaccumulation of metals (Pajević et al. 2002; Prasad & Freitas 2003; Vardanyan & Ingole 2006). However, while macrophytes are useful biomonitors, the bioconcentration of metals in macrophytes may result from exposure to metals in both water and/or sediments, making it difficult to directly compare between the concentrations measured in plants and in the environment (i.e. water or sediments).

Unusually high inputs of metals into the aquatic environment have resulted in great financial losses, affected commercial fisheries and in some cases have been hazardous to human health (Banerjee 2003). Research on heavy metals in aquatic environments has become very important due to concerns over accumulation and toxic effects in aquatic organisms and humans through the food chain (Alahabadi & Malvandi 2018).

The most important objectives of this study include: (a) determination of the distribution and concentration of Fe, Mn, Ni, Co, Zn, Cu, Cr Pb and Cd in the water, sediment and macrophytes (Eichhornia crassipes and Ceratophyllum demersum) in the Nile River in the Cairo region, (b) assessment of the degree of contamination with the studied metals in the river using the heavy metal pollution index and the contamination index in the case of water, and the geoaccumulation index (I_geo), the pollution load index (PLI), the enrichment factor (EF), the potential ecological risk factor (Eⁱ_r) and the risk index (RE) in the case of sediment.

Materials and methods

Study area

The Nile is the main source of fresh water in Egypt; its flow rate relies on the available water stored in Lake Nasser to meet the needs defined under the annual water budget of Egypt (Agricultural Policy Reform Program 2002). The Nile River enters Egypt at its southern border with Sudan and runs through a narrow valley (1000 km long) whose width varies from 2 to 20 km. Nile pollutants are derived from different uncontrolled sources such as agricultural drainage, industrial wastewater and municipal wastewater. The river in the Cairo region receives heavy polluted, treated or partially/untreated industrial drainage water, containing waste from the iron, cement, and sugar industry in the Helwan area. These types of industry tend to pollute the Nile system with heavy metals, suspended matter and organic micropollutants. The main characteristics of the Nile water in the Cairo region are presented in Table 1 and the sampling sites are presented in Figure 1.

Map of the Nile River showing the sampling sites (after Al-Afify et al. 2018)

Table 1

Main characteristics* of the Nile River water in the Cairo region

Variable	Range	Mean ± SD
Transparency (cm)	35–150	86.97 ± 25.63
Depth (m)	1–10	4.02 ± 0.82
Dissolved oxygen (DO) (mg l⁻¹)	4.0–10.26	8.02 ± 1.36
pH	7.32–8.81	8.02 ± 0.30
Alkalinity (mg l⁻¹)	92.0–186.5	135.8 ± 25.98
Chloride ion (mg l⁻¹)	11.0–88.97	48.51 ± 21.35

^*cited after Al-Afify et al. (2018)

Collection of water samples

Subsurface (about 30 cm) water samples were collected using a polyvinyl Van Dorn plastic bottle. Samples were kept in clean stoppered plastic bottles and preserved with 65% HNO₃ to pH <2. Finally, water samples were digested using 65% HNO₃ according to APHA (2005).

Collection of sediment samples

Sediment samples were collected using an Eckman sampling device from the top 20 cm layer of the bottom at eight sampling sites in the Nile River in the Cairo region. Samples were air-dried and stone fragments were removed by passing the dried samples through a 2 mm sieve. The sieved samples were powdered and 0.5 g of finely ground samples was digested according to the method described by Kouadia and Trefry (1987).

Collection of plant samples

Representative macrophytes were sampled at each site according to the type of plant (submerged or floating). The floating macrophyte (Eichhornia crassipes) was collected from a 0.5 × 0.5 m quadrant. While the submerged plant (Ceratophyllum demersum) was collected using a grab with a known volume.

Plants were rinsed thoroughly with distilled water (four times) and dried at 60°C until completely dry. The dried plant parts were ground and precise weigh (0.50 gram) of each sample was digested using 65% HNO₃, 98% H₂SO₄ and 35% H₂O₂ as prescribed by Saison et al. (2004).

Chemical analysis

Samples (water, sediment and macrophytes) were analyzed for Fe, Mn, Ni, Co, Zn, Cu, Cr Pb and Cd concentrations using an atomic absorption reader (Savant AA-AAS with graphite furnace; GF 5000). The precision of metal analysis was controlled by triplicate readings and the mean value was determined with relative standard deviations below 5%.

Risk assessment of heavy metals in water

Heavy Metal Pollution Index

The suitability of Nile water for the drinking purpose with respect to metals was determined using the heavy metal pollution index (HPI). It is based on the weighted arithmetic quality mean method as presented in the following equation (Prasad & Bose 2001; Mohan et al. 1996):

H P I = \frac{\sum_{i = 1}^{n} Q_{i} W_{i}}{\sum_{i = 1}^{n} W_{i}}

$$HPI=\frac{\sum\limits_{i=1}^{n}{{{Q}_{i}}{{W}_{i}}}}{\sum\limits_{i=1}^{n}{{{W}_{i}}}}$$

where Q_i is the individual quality rating of the i^th metal and calculated as follows:

Q_{i} = \frac{C_{i}}{C_{S}} \times 100

$${{Q}_{i}}=\frac{{{C}_{i}}}{{{C}_{S}}}\times 100$$

While W _i is calculated as 1/C_s, where C_s is the recommended standard of the relevant metal and n is the number of metals estimated. C_i is the estimated value of the metals in μgl⁻¹. The permissible standard value (C_s) for each metal was taken from WHO (2011) and Egyptian drinking water quality standards (EWQS 2007). For drinking water, the critical heavy metal pollution index score is 100, above which the overall pollution level should be considered unacceptable for drinking water (Prasad & Mondal 2008).

The Contamination Index (C_d)

C_d indicates the relative contamination of different metals separately and manifests the combined effects of all metals. It is calculated using the following equation (Backman et al. 1997):

C_{d} = \sum_{i = 1}^{n} C_{f i}

$${{C}_{d}}=\sum\limits_{i=1}^{n}{{{C}_{fi}}}$$

where C_fi is calculated according to the following equation:

C_{f i} = \frac{C_{i}}{C_{S}} - 1

$${{C}_{fi}}=\frac{{{C}_{i}}}{{{C}_{S}}}-1$$

C_fi is the contamination factor for the i^th metal, C_i is the measured value for the i^th metal and C_s is the upper permissible value of the i^th metal. The resultant C_d values are grouped into three classes: low (C_d < 1), medium (C_d = 1–3) and high (C_d > 3). C_s is formerly introduced in the HPI calculation.

Risk assessment of heavy metals in sediment

The geoaccumulation index I-geo, the pollution load index (PLI), the modified contamination degree (mC_d), the enrichment factor (EF) and the potential ecological risk index (ER) were used to assess the metal pollution status.

Geoaccumulation index

The I-geo was computed as follows (Muller 1969):

I - g e o = l o g_{2} (\frac{C_{n}}{1.5 B_{n}})

$$I-geo=lo{{g}_{2}}\left( \frac{{{C}_{n}}}{1.5{{B}_{n}}} \right)$$

where C_n is the total metal concentration in the sediment sample; B_n is the metal background value, and 1.5 is the factor for the background matrix correction. Concentrations of heavy metals as defined in the freshwater sediment benchmarks (EPA 2006) were used as background. The I-geo consists of seven classes (Table 2).

Table 2

Geoaccumulation index (I-geo), enrichment factor (EF) and modified contamination factor for elements and contamination levels

Index
Geoaccumulation index (I-geo)			Enrichment factor (EF)		Modified contamination degree (mC_d)
Class no.	I-geo value	Contamination level	EF value	Contamination level	mC_d value	Contamination level
0	I-geo ≤ 0	practically unpolluted	1 < EF < 3	minor enrichment	mC_d < 1.5	nil pollution
1	0 < I-geo < 1	unpolluted to moderately polluted	3 < EF < 5	moderate enrichment	1.5 ≤ mC_d < 2	low pollution
2	1 < I-geo < 2	moderately polluted	5 < EF < 10	moderately severe enrichment	2 ≤ mC_d < 4	moderate pollution
3	2 < I-geo < 3	moderately to heavily polluted	10 < EF < 25	severe enrichment	4 ≤ mC_d < 8	high pollution
4	3 < I-geo < 4	heavily polluted	25 < EF < 50	very severe enrichment	8 ≤ mC_d < 16	very high pollution
5	4 < I-geo < 5	heavily to extremely polluted	EF ≥ 50	ultra-high	16 ≤ mC_d < 32	extremely high pollution
6	I-geo ≥ 5	extremely polluted			mC_d ≥32	ultra-high pollution

Enrichment factor

The EF of a single trace element in the sediments was calculated as follows (Yahaya et al. 2012):

E F = \frac{{(\frac{M}{F e})}_{s a m p l e}}{{(\frac{M}{F e})}_{b a c k g r o u n d}}

$$EF=\frac{{{\left( \frac{M}{Fe} \right)}_{sample}}}{{{\left( \frac{M}{Fe} \right)}_{background}}}$$

Fe was used as a conservative tracer to differentiate natural from anthropogenic components. (M/Fe)_sample is the ratio of a metal and Fe concentration in a sample in the examined environment and (M/Fe)_background is the ratio of a metal and Fe concentration of the background (Yahaya et al. 2012). The classification of EF values is presented in Table 2.

Pollution Load Index

PLI was evaluated as follows (Tomlinson et al. 1980):

P L I = {(C f_{1} \times C f_{2} \times C f_{3} \times \dots \times C f_{n})}^{\frac{1}{n}}

$$PLI={{\left( C{{f}_{1}}\times C{{f}_{2}}\times C{{f}_{3}}\times \ldots \times C{{f}_{n}} \right)}^{\frac{1}{n}}}$$

where n is the number of elements and Cf is the contamination factor, which is the ratio between the element level (C_i) in sediment samples and its background concentration (B_i). Background concentrations of heavy metals were taken from freshwater sediment benchmarks (EPA 2006):

C f_{i} = \frac{C_{i}}{B_{i}}

$$C{{f}_{i}}=\frac{{{C}_{i}}}{{{B}_{i}}}$$

where a PLI value > 1 indicates a contaminated site, while a PLI value <1 indicates no contamination.

Modified contamination degree (mC_d)

The mC_d was introduced to estimate the overall degree of contamination at a given site according to the formula (Abrahim & Parker 2008):

m C_{d} = \frac{\sum_{i = 1}^{n} C f}{n}

$$m{{C}_{d}}=\frac{\sum{_{i=1}^{n}}Cf}{n}$$

where n = the number of analyzed elements, i = the i^th element and Cf = the contamination factor. The classifications of mC_d are presented in Table 2.

Potential Ecological Risk Index (ER)

The potential ecological risk index (ER) was introduced to assess the degree of heavy metal pollution in sediments according to the toxicity of heavy metals and the response of the environment, where ER is calculated as the sum of all risk factors (Eⁱ_r) for heavy metals in sediments (Hakanson 1980; 1988):

E R = \sum_{i = 1}^{n} E_{r}^{i}

$$ER=\sum{_{i=1}^{n}}E_{r}^{i}$$

and

E_{r}^{i} = C_{f} \times T_{r}^{i}

$$E_{r}^{i}={{C}_{f}}\times T_{r}^{i}$$

where Eⁱ _ris the monomial potential ecological risk factor, Cf is the contamination factor, and T ⁱ r is the toxicity response factor of a heavy metal. The factor scores on each heavy metal according to Hakanson’s (1980) approach were as follows: Mn (1), Zn (1), Cu (5), Ni (5), Co (2), Pb (5), Cd (30) and Cr (2). The pollution levels according to ER and Eⁱ_r values are presented in Table 3.

Risk assessment of heavy metals in plants

The ability of plants to absorb and accumulate metals from the aqueous growth media was assessed using the bioconcentration factor (BCF). The BCF value is calculated as the ratio of metal concentrations in the plant and the associated water, where higher BCF values reveal high accumulation ability of the plant:

B C F = \frac{{[m e t a l]}_{p l a n t}}{{[m e t a l]}_{w a t e r}}

$$BCF=\frac{{{\left[ metal \right]}_{plant}}}{{{\left[ metal \right]}_{water}}}$$

Statistical analysis

Correlations among metal concentrations in water, plants and sediment samples were estimated using Pearson correlation coefficients (p<0.01). In addition, data obtained from water and sediment samples were examined for significant differences among different seasons and sites using the ANOVA test.

Table 3

Pollution levels according to ER and $E_{r}^{i}$ $\,E_{r}^{i}$values

$E_{r}^{i}$ $\,E_{r}^{i}$scope	Ecological risk levels for single factor pollution	ER scope	Potential ecological risk levels
$E_{r}^{i}$ $\,E_{r}^{i}$<40	Low	ER<150	Low grade
40 ≤ $E_{r}^{i}$ $\,E_{r}^{i}$<80	Moderate	150 ≤ER < 300	Moderate
80 ≤ $E_{r}^{i}$ $\,E_{r}^{i}$<160	Considerable	300≤ ER< 600	Severe
160 ≤ $E_{r}^{i}$ $\,E_{r}^{i}$<160	High	600 ≤ ER	Serious
320 ≤ $E_{r}^{i}$ $\,E_{r}^{i}$	Serious	-	-

Results and discussion

Table 4 summarizes the range, mean concentrations and standard deviation of Fe, Mn, Ni, Co, Zn, Cu, Cr, Pb, and Cd in river water in different seasons. At most sites, the concentrations of Fe, Cu, Zn, Pb, Co and Cd in different seasons were higher than the CCME (2007) threshold limits established to protect water quality for aquatic life. Pb and Cd concentrations in the Nile water were several times higher than the limits recommended by CCME (2007), where cadmium has lethal effects on aquatic biota such as crustaceans (Effendi et al. 2016). Ni is the only metal whose concentration in water is still at a safe level for aquatic life according to CCME (2007) limits.

Table 4

Seasonal variation of heavy metals (μg l⁻¹) compared with different criteria and HPI values in the Nile water in the Cairo region

		Autumn	Winter	Spring	Summer	CCME (2007)	WHO (2011)	EWQS (2007)
Fe	Range	642.0–1090.0	775.0–1300.0	403.0–862.0	196.0–690.0	300	1000	300
Fe	Mean±SD	869.0±169.6	1005.8±166.4	566.6±142.7	433.25±165.13	300	1000	300
Mn	Range	97.81–234.81	96.46–287.68	90.79–161.5	87.33–121.24	-	400	100
Mn	Mean±SD	152.2±44.7	191.7±61.9	124.9±25.0	97.9±10.5	-	400	100
Ni	Range	9.54–20.54	11.89–26.47	9.04–25.41	8.45–17.68	65	70	20
Ni	Mean±SD	12.79±3.87	16.51±5.32	16.48±5.78	11.94±3.19	65	70	20
Co	Range	4.85–20.54	6.95–22.63	9.51–24.35	8.45–21.35	1	-	-
Co	Mean±SD	13.45±5.30	14.75±5.30	16.39±5.85	13.99±4.83	1	-	-
Zn	Range	22.56–75.48	28.91–98.56	24.35–36.55	18.34–42.18	30	4000	3000
Zn	Mean±SD	38.97±17.34	38.97±17.34	31.74±4.58	29.87±9.49	30	4000	3000
Cu	Range	6.45–18.40	6.15–19.42	7.05–14.12	5.12–9.45	2	2000	2000
Cu	Mean±SD	11.71±4.14	10.44±4.48	10.65±3.10	7.47±1.38	2	2000	2000
Cr	Range	9.47–17.96	8.79–20.68	10.69–21.34	9.75–16.58	-	50	50
Cr	Mean±SD	12.28±3.03	13.99±3.72	14.24±3.84	12.42±2.12	-	50	50
Pb	Range	5.75–12.45	4.15–12.15	4.01–9.85	3.15–18.47	2	10	10
Pb	Mean±SD	9.18±2.18	7.26±2.72	6.13±1.89	9.65±5.60	2	10	10
Cd	Range	1.98–2.98	1.45–2.98	1.75–3.15	1.98–3.84	0.18 according to CCME (2014)	3	3
Cd	Mean±SD	2.35±0.37	2.26±0.54	2.40±0.55	2.79±0.69	0.18 according to CCME (2014)	3	3
HPI according to WHO (2011);	Range	61.3–96.3	46.8–88.7	56.6–96.7	64.3–104.2
HPI according to EWQS (2007);	Range	64.3–99.0	53.3–90.4	64.3–96.7	66.4–104.1

The calculated ranges of HPI values in different seasons are presented in Table 4 in relation to the standards endorsed by EWQS (2007) and WHO (2011) for drinking water. The HPI measurements at sites 4 and 8 in the summer season exceeded the critical metal pollution index of 100 proposed by Prasad & Mondal (2008) for drinking water, identifying potential dangerous effects on the Nile water environment. However, the HPI for most studied sites does not exceed the critical limit indicating that the Nile water in the Cairo region is not critically polluted with respect to metals (Abdel-Satar et al. 2017). No significant differences (p > 0.05) were found among seasons or among sites for the metal pollution index.

Concerning the metal pollution, the Nile water in Cairo can be categorized as having a low level of metal pollution (Cd < 1), except site 4 in the winter season (low flow) when water is moderately polluted by metals (Cd ≤ 3) according to EWQS (2007) limits (Fig. 2). Therefore, severe precautions at the anthropogenic input sites should be taken to control the influx of elements.

Contamination index values (Cd) of the Nile River according to a) WHO and b) EWQS criteria

The concentrations of heavy metals in sediment from different sites at the Nile River are presented in Table 5. As shown in Tables 4 and 5, the levels of heavy metals in the Nile sediment were much higher than those in water and showed enrichment with metals in the Nile sediment.

Table 5

Heavy metals distribution (μg g⁻¹) compared with EPA (2006), contamination factor (Cf) and PLI values in the Nile sediment in the Cairo region.

Metal		Autumn	Winter	Spring	Summer	EPA (2006)	Cf
Fe	Range	27154–53468	18238–25014	17548–26481	17185–25678	20000	0.86–2.67
Fe	Mean±SD	38858 ± 9784	22774 ± 2166	21121 ± 2878	19217 ± 2862	20000	1.27±0.47
Mn	Range	283.0–1334.0	339.0–988.0	273.0–456.0	512.0–984.0	460	0.59–2.90
Mn	Mean±SD	849.3 ± 354.4	708.0 ± 215.2	326.8 ± 58.5	754.4 ± 174.4	460	1.43±0.64
Zn	Range	45.00–122.45	58.70–120.00	46.32–70.00	62.40–97.50	121	0.37–1.01
Zn	Mean±SD	84.44 ± 27.82	77.34 ± 19.69	60.06 ± 8.54	83.66 ± 12.05	121	0.63±0.17
Cu	Range	18.70–30.45	19.50–27.89	19.85–25.91	10.80–20.15	31.6	0.34–0.96
Cu	Mean±SD	25.49 ± 4.24	23.61 ± 2.77	22.58 ± 2.20	14.31 ± 3.22	31.6	0.68±0.17
Ni	Range	28.38–87.12	27.95–80.52	24.75–72.85	32.52–67.85	22.7	1.09–3.84
Ni	Mean±SD	50.44 ± 20.19	50.94 ± 19.39	47.94 ± 16.75	45.06 ± 13.23	22.7	2.14±0.74
Co	Range	7.19–19.50	7.40–20.45	5.40–17.85	7.56–17.64	50	0.11–0.41
Co	Mean±SD	13.06 ± 4.11	12.95 ± 4.92	11.20 ± 4.00	11.27 ± 3.65	50	0.24±0.08
Cr	Range	16.85–54.65	18.96–52.56	13.67–48.97	14.52–47.52	43.4	0.31–1.26
Cr	Mean±SD	25.27 ± 12.83	28.11 ± 12.93	26.85 ± 11.37	25.44 ± 11.23	43.4	0.61±0.27
Pb	Range	6.52–18.41	9.50–25.12	10.00–30.33	5.86–11.50	35.8	0.16–0.85
Pb	Mean±SD	12.12 ± 4.23	16.82 ± 4.47	18.83 ± 8.18	7.85 ± 1.83	35.8	0.39±0.18
Cd	Range	2.05–3.19	1.74–3.21	1.86–3.42	2.14–3.24	0.99	1.76–3.45
Cd	Mean±SD	2.75 ± 0.45	2.51 ± 0.53	2.74 ± 0.50	2.62 ± 0.40	0.99	2.68±0.46
PLI	Range	0.76–1.08	0.72–1.05	0.73–0.97	0.56–0.73

There is a significant difference between the seasons (p<0.05) for Fe, Mn, Zn, Cu, Pb concentrations, where metal concentrations were higher in the cold seasons than in the summer due to the lower water flow during the cold seasons, which could facilitate the accumulation of the metals in the sediment (Islam et al. 2015; Abdel-Satar et al. 2017).

The average concentrations of heavy metals in sediments were in descending order: Fe > Mn > Zn > Ni > Cr > Cu > Pb ≈ Co > Cd. The high level of Cr recorded at site 4 (54.65, 52.56, 48.97 and 47.52 mgkg⁻¹ in autumn, winter, spring and summer, respectively), exceeding the freshwater sediment EPA benchmarks, indicates its higher input, which may originate from urban and industrial waste (Mohiuddin et al. 2012).

For most sites, especially during the low flow seasons, the concentrations of Mn, and Ni were higher than the freshwater sediment benchmarks of EPA (2006). However, the levels of Cd were significantly higher than the benchmarks for all sites in different seasons.

Pearson’s correlation matrix for the analyzed sediment elements showed a significant positive correlation between Ni, Co and Cr (Table 6), suggesting similar sources of input (human or natural) for these elements in the Nile River sediment. High correlations between the same elements (Ni, Co and Cr) in water (Table 6) may reflect similar levels of contamination or/and release from the same sources of pollution, identical behavior during their transport in the Nile system and mutual interactions (Ali et al. 2016; Abdel-Satar et al. 2017).

Table 6

Correlation between the elements in water and sediments of the Nile River in the Cairo region

Water
	Fe	Mn	Zn	Cu	Ni	Co	Pb	Cd	Cr
Fe	1.00
Mn	−0.02	1.00
Zn	−0.30	−0.24	1.00
Cu	0.20	0.05	0.19	1.00
Ni	−0.13	0.23	0.07	−0.20	1.00
Co	−0.29	0.13	0.27	−0.09	0.75 *Correlation is significant at p < 0.01; n = 32	1.00
Pb	0.17	0.07	0.14	0.66 *Correlation is significant at p < 0.01; n = 32	−0.46	−0.40	1.00
Cd	−0.06	−0.06	0.09	−0.01	−0.02	0.12	−0.11	1.00
Cr	−0.20	0.15	0.19	−0.21	0.83 *Correlation is significant at p < 0.01; n = 32	0.73 *Correlation is significant at p < 0.01; n = 32	−0.41	0.15	1.00

Sediment
Fe	1.00
Mn	0.52	1.00
Zn	0.14	0.34	1.00
Cu	0.53 *Correlation is significant at p < 0.01; n = 32	0.16	0.06	1.00
Ni	0.18	−0.08	−0.20	−0.23	1.00
Co	0.10	−0.15	−0.09	−0.20	0.84 *Correlation is significant at p < 0.01; n = 32	1.00
Pb	−0.04	−0.18	−0.32	0.54 *Correlation is significant at p < 0.01; n = 32	−0.35	−0.47	1.00
Cd	0.01	−0.24	−0.03	−0.05	0.29	0.48 *Correlation is significant at p < 0.01; n = 32	−0.24	1.00
Cr	−0.01	−0.13	−0.16	−0.20	0.85 *Correlation is significant at p < 0.01; n = 32	0.81 *Correlation is significant at p < 0.01; n = 32	−0.36	0.33	1.00

The results of the I-geo index are shown in Figure 3. According to the classification provided by Müller (1969), the calculated I-geo values showed that most of the elements studied (Fe, Zn, Cu, Co, Pb and Cr) at all sites were included in the zero class (i.e. practically uncontaminated), while Mn was included in class 0 for all sites except sites 2 and 3, where it was included in class 1 (uncontaminated to moderately contaminated). Ni and Cd had the highest I-geo values and were included in classes 1 and 2.

I-geo values for metals in the Nile sediment

The enrichment factor (EF) is an appropriate index to distinguish between natural and anthropogenic sources of elements. Accordingly, EF values of less than 1.5 represent the natural origin and EF values greater than 1.5 represent the anthropogenic origin of an element (Chen et al. 2015; Alahabadi & Malvandi 2018). The EF results showed that the mean values of the index were > 1.5 for Ni and Cd at all sites in different seasons (Table 7), indicating that these elements originated primarily from anthropogenic sources. On the other hand, Zn, Cu, Co, Cr and Pb at all sites originated from natural sources (EF values < 1.5). The lowest EF values were recorded for Mn in the summer season (EF < 1.5), while in the autumn, winter and spring the EF values at some sites were greater than 1.5. According to the EF, most of the studied samples were categorized as class 1 with minor enrichment with metals (1 < EF < 3).

Table 7

Enrichment factor for the studied elements in the Nile River in the Cairo region

		Mn	Zn	Cu	Ni	Co	Pb	Cr	Cd
Autumn	Range	0.35–1.54	0.17–0.70	0.33–0.65	0.78–1.96	0.06–0.22	0.10–0.38	0.15–0.57	0.87–2.21
Autumn	Mean±SD	0.95±0.34	0.38±0.17	0.43±0.11	1.22±0.47	0.14±0.06	0.18±0.09	0.31±0.14	1.54±0.57
Winter	Range	0.66–1.82	0.44–0.88	0.55–0.79	1.15–3.03	0.13–0.34	0.23–0.76	0.34–0.99	1.76–2.81
Winter	Mean±SD	1.34±0.36	0.56±0.15	0.65±0.09	2.03±0.74	0.23±0.08	0.46±0.20	0.56±0.23	2.22±0.42
Spring	Range	1.16–2.21	0.49–0.91	0.53–0.81	1.18–2.95	0.11–0.35	0.23–0.73	0.32–0.97	2.02–3.35
Spring	Mean±SD	1.57±0.38	0.66±0.16	0.68±0.11	2.06±0.68	0.21±0.08	0.46±0.18	0.58±0.22	2.64±0.57
Summer	Range	0.58–1.14	0.43–0.66	0.30–0.73	1.44–3.16	0.15–0.37	0.13–0.37	0.29–1.10	2.05–3.42
Summer	Mean±SD	0.75±0.16	0.52±0.07	0.48±0.15	2.15±0.61	0.24±0.08	0.23±0.07	0.61±0.27	2.79±0.47

Another index to assess the contamination of elements in the Nile River sediments in the Cairo region is the pollution load index. The resulting PLI values are presented in Table 5 and range from 0.56 to 1.08. The values of the PLI index were < 1 at all sites except sites 4 and 5 in the winter season, which showed that the concentration of the studied elements was lower than the background values and therefore were categorized as being uncontaminated. Consequently, there is no serious concern about contamination with these elements. The contamination factor (Cf) values for Fe, Mn, Zn, Cu, Co, Pb and Cr showed a moderate degree of contamination (1 ≤ Cf < 3) at different sites and in different seasons (Table 5), whereas the Cf values for Ni and Cd showed a slightly high degree of contamination (Cf > 3) for some sites in different seasons. The range of mC_d values (0.78–1.48) determined for the analyzed elements indicates very low levels of contamination at all sites (Fig. 4).

mCd values for different sites in the Nile sediment

The average potential ecological risk (ER) was categorized as low-grade risk (ER < 150), viewed from the overall perspective of the study (Fig. 5). The contribution of Cd to the ecological risk assessment was about 80%, while the contribution of Ni was about 10%. The contribution of the other six investigated metals was as follows: Mn – 1.5%, Zn – 0.6%, Cu – 3.5%, Co – 0.5%, Pb – 2.0% and Cr – 1.2%. Consequently, Cd contained in the Nile sediment surface may have a significant ecological effect. Based on the monomial ecological risk index (Eⁱ_r), the Eⁱ_r values of the eight investigated heavy metals indicated that all values were burdened with low ecological risk (< 40) except Cd values, which showed a moderate ecological risk ranging from 52.7 to 103.6 (Table 8). No significant differences (p > 0.05) were observed between the ER values in different seasons, while high significant differences were recorded between the sites (p<0.01), with site 5 showing the highest values in different seasons.

ER values for different sites in the Nile sediment

Table 8

$E_{r}^{i}$ $E_{r}^{i}$ranges and the average contribution (%) to the ecological risk

	Mn	Zn	Cu	Ni	Co	Pb	Cd	Cr
Min.	0.59	0.37	1.71	5.45	0.22	0.82	52.73	0.63
Max	2.90	1.01	4.82	19.19	0.82	4.24	103.64	2.52
Contribution (%)	1.5	0.6	3.5	10.7	0.5	2.0	80.0	1.2

Concentrations of the metals in the two plants from the Nile River are listed in Figure 6. The distribution of most studied elements between water, sediment and plants at different sites showed a similar trend: sediment >plants >water. Fe was the most frequently accumulated metal in the two plants, followed by Mn, Zn, Cu, Ni, Co, Cr and Pb, whereas Cd was the least frequently accumulated one. The results show a significant increase in the level of most of the studied metals in E. crassipes and C. demersum compared to water samples. BCF values for metals were ranked as follows: Cu > Mn > Fe > Ni > Co > Zn > Pb ≈ Cr > Cd and Mn > Cu > Fe > Ni > Pb ≈ Zn > Cr ≈ Co > Cd in E. crassipes and C. demersum, respectively (Fig. 7). The bioaccumulation of Cu and Mn was several times higher than the accumulation of the other studied elements in E. crassipes and C. demersum.

Average distribution of metals in E. crassipes and C. demersum in the Nile River in the Cairo region

Average BCF in E. crassipes and C. demersum in the Nile River in the Cairo region

There were differences in the sequences of the studied metal levels in the macrophytes compared to the sequences of their bioaccumulation ability. These differences indicate a different capacity of macrophytes for different metals (Kastratovi et al. 2014). Factors involved in the identification of such differences include ionic exchange, the rate of chelation, translocation of element ions, chemical precipitation and precipitation induced by microorganisms or by root exudates (Pajević et al. 2008).

Conclusions

The distribution of most studied metals between water, sediment and plants at different sites showed a similar trend: sediment > plants > water. Pb and Cd concentrations in the Nile water were several times higher than CCME recommended limits, while Ni is the only metal whose concentration in water is still at a safe level for aquatic life. The Nile water in Cairo is not critically polluted by the studied metals and the HPI and the contamination index for most sites do not exceed the critical limit according to WHO and EWQS.

The geoaccumulation index (I-geo), the enrichment factor (EF), the pollution load index (PLI) and the contamination factor (Cf) showed that the Nile sediments are contaminated with Cd, Ni and Mn, and the Cd levels in the Nile sediments are significantly higher for all sites than the EPA freshwater sediment benchmarks, while Mn and Ni levels exceeded the limits during the low flow seasons. The contribution of Cd to the ecological risk assessment was about 80%, while the contribution of Ni was about 10%, reflecting that these elements originated primarily from anthropogenic sources.

The BCF values for metals were ranked as follows: Cu > Mn > Fe > Ni > Co > Zn > Pb ≈ Cr > Cd and Mn > Cu > Fe > Ni > Pb ≈ Zn > Cr ≈ Co > Cd in E. crassipes and C. demersum, respectively. The bioaccumulation of Cu and Mn was several times higher than the accumulation of the other studied elements in E. crassipes and C. demersum.

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Risk assessment of heavy metal pollution in water, sediment and plants in the Nile River in the Cairo region, Egypt

Article Category: Research Article

Pubblicato online: 06 mar 2020

Pagine: 1 - 12

Ricevuto: 17 giu 2019

Accettato: 28 ago 2019

DOI: https://doi.org/10.1515/ohs-2020-0001

© 2020 Faculty of Oceanography and Geography, University of Gdańsk, Poland

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.

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