Eco-Toxicological Risk Assessment of Heavy Metals and Polycyclic Aromatic Hydrocarbons in Sediments of Tigris River, Iraq

Abstract


Introduction
River sediments provide excellent repositories for various organic and inorganic contaminants (Eker, 2020).Sediment contaminants may decrease or eradicate any kind which has a commercial, recreational, or environmental significance, either via direct impacts or by influencing the food supply required by inhabitants.Thus, polluted sediment may bring about fatal and sub-fatal impacts on benthic (sediment-residents) and other sediment-related organisms (Spellman, 2017).The Contaminant concentrations presented in sediments are controlled by natural activities represented in their abundance in the rocks and soils and by anthropogenic activities by adding excess amounts of chemical compounds (Perry and Taylor, 2007).
Heavy metals are exceedingly linked with ecological deterioration and the quality-of-life organisms, and therefore have evoked concern throughout the world (Ustaoğlu and Islam, 2020;Al-Obeidi and Al-Jumaily, 2020).Many trace elements are of worry due to their contaminated characteristics (e.g.Cd and Pb) and some of them are also essential (e.g.Cu and Zn) for human and animal health (Sarkar, 2002).Heavy metals concentration data are frequently used in observing and evaluating the grade of contamination of freshwater environments utilizing sediment quality indicators (Benson et al., 2018).Literature points out that trace elements in sediments cause substantial damaging impacts on aquatic ecology due to their bioaccumulation prospect, non-biodegradability, and toxicity (Abed et al., 2022;Abed, 2020;Abreu et al., 2016).
Polycyclic aromatic hydrocarbons (PAHs) could pose a serious threat to aquatic environments, for that reason, they are listed in the US EPA and European Union priority pollutant listings (Chen and Chen, 2011).They persist in the environment, with the potentiality of mutagenesis and carcinogenesis and are widely present in the aquatic systems, including sediments, and other environmental compartments (i.e.air and soil) (Abed et al., 2015;Singare, 2015;Bin et al., 2007).Because most polycyclic aromatic hydrocarbons are hydrophobic, they deposit on the river bed sediments due to the strong sorption on the organic matter present, and under suitable circumstances, they may release to the water as a continuous source and threatening the aquatic ecosystem via bioaccumulation in the food chain (Chen and Chen, 2011).The main sources of PAHs could be natural and anthropogenic activities (e.g., oil spills, forest fires, volcanic activity, petrochemical industrial effluents, and combustion processes) (Abed, 2015).
As the Tigris River is an important source of drinking water, irrigation, fishing, etc., for millions of people, assessing the river environment, including sediments, is an important issue in evaluating the river's health.The towns on the river discharge their wastes (i.e., municipal, agricultural, and industrial) into the river, degrading the river environment.Since river sediments are an accretion depository for pollutants, analysis of those sediments and assessing inorganic and organic contaminants using ecotoxicological indices are critical for observing the freshwater environment.
The geological formations exposed along the banks of the Tigris River within the study area, starting from the city of Al Qayyarah to Baiji area, are each of the Fatha Formation (Middle Miocene) and the Injana Formation (Upper Miocene).The Fatha Formation consists of successive layers of red clay, followed by green limestone and clay stones, successively with gypsum and carbonate.The Injana Formation consists of successions of sandstones and alluvial mudstones (Zarraq, 2012).The Mukdadiyah Formation (Upper Miocene -Pleistocene) appears on the banks of the Tigris River between Baiji and Tikrit (Abdulhafidh, 2019), which consists of sandstone, covered from time to time with pebbles, alternating with mudstone and siltstone.Quaternary sediments (Pliocene -Holocene), which form the surface cover of the area extending from Al Fatha to Samarra city, are gypseferous soils of different thicknesses.These deposits also consist of a heterogeneous mixture of sediments of different sizes ranging from silts to large boulders (Zarraq, 2012).The aims of this study are: (1) to determine the concentrations and distribution of heavy metals (i.e.Cu, Cd, Cr, Pb, Zn, Ni, and As) and Polycyclic Aromatic Hydrocarbons in Tigris river sediments, and (2) to assess the environmental quality of sediments using multivariate statistical methods.

Sampling and Analysis
In order to assess the environmental quality of river sediments, seven heavy metals and 16 polycyclic aromatic hydrocarbons were analyzed in nine sediment samples collected in April 2022.Northing and easting coordinates with location map are shown in Table 1 and Fig. 1.
For chemical analyses of both inorganic and organic contaminants, river sediment samples were collected two to three meters away from the riverbank toward the riverbed.Samples intended for heavy metals analysis were placed in polyethylene bags and labeled.In the lab, those samples were dried at room temperature, homogenized, and sieved through a 2 mm screen.Determining metal concentrations was done using the digestion method and an atomic absorption spectrophotometer (AAS) in Tikrit University labs.
Samples collected for PAHs analysis were kept in dark glass vessels to protect them from light, and frozen until examination.100 grams of sediment were mixed with 1 L of chloroform: methanol (50:50, v/v).The mix is agitated for 1 hour and kept to settle down for 3 hours, then the extract is decanted and replicated once, and collect the two extracts.Then the mixture is concentrated by vaporizing the solvent with a stream of fluid N2 until reaches almost 0.5 ml, then add some of the mobile phases to reach 5 ml.The mixture was passed through 2.5 µm filters, and then 5 µl was injected into the HPLC column.The concentration for each compound was quantitatively defined by comparison of the peak area of the standard with that of the samples.Determining 16 PAHs using HPLC and UV detection is consistent with the USEPA method 8310.PAHs concentrations were determined using High Performance Liquid Chromatography HPLC in Tikrit University labs.

Ecological Risk Assessment of Heavy Metals
To give a comprehensive picture of the degree of contamination of the Tigris river sediments by Cu, Cd, Cr, Pb, Zn, Ni, and As, eco-toxicological assessment indices were used as follows:

Sediment pollution index (SPI)
The grade of toxicity in the sediments can be assessed using the SPI, developed by Singh et al. )2002(.It is a useful approach for assessing the aquatic sediment quality depending on the element concentration and toxicity (Gopal et al., 2018;Godson et al., 2018;Krishnakumar et al., 2017;Cheng et al., 2014).SPI can be calculated using the two next equations: where CFi is the contamination factor of a metal which is the proportion between the examined metal concentration (Cm) and the background concentration of the same metal (Cb) and Wi is the weight of metal toxicity i.According to Hakanson (1980), the toxic response factor of metals under consideration for Cu, Cd, Cr, Pb, Zn, Ni, and As are 5, 30, 2, 5, 1, 5 and 10 respectively.The SPI is accompanied by five quality categories: ≥ 20 (risky sediments), 10 ≤ SPI ˂ 20 (highly polluted sediments), 5 ≤ SPI ˂ 10 (moderately polluted sediments), 2 ≤ SPI ˂ 5 (low polluted sediments), 0 ≤ SPI ˂ 2 (natural sediments) (Benabdelkader et al., 2018).

Toxic risk index (TRI)
The TRI is a new contamination indicator developed by Zhang et al. (2016).This index takes into consideration the critical toxicity of heavy metals in river sediments with their relative impact and can be calculated using the equation below (Christophoridis et al., 2019): Where TRIi indicates the toxic risk index of element i, the Ci is the concentration of element i; n is the number of studied elements.TELi and PELi refer to threshold effects level and probable effects level of a specific element respectively.TELi and PELi values for heavy metals in freshwater sediments were obtained (MacDonald et al., 2000).TRI values can be classified into five categories of toxicity risks.TRI ≤ 5 refers to no toxic risk, 5 < TRI ≤ 10 indicates a low toxic risk, 10 < TRI ≤ 15 suggests a moderate toxic risk, while 15 < TRI ≤ 20 means a considerable toxic risk, and TRI > 20 is a very high toxic risk (Kükrer et al., 2020;Zhang et al., 2016).

Mean effects range median quotient (MERMQ)
The MERMQ is involved as a useful means for decreasing a considerable number of contaminants into only one number.This indicator can be utilized to identify and prioritize areas of possible risks concerning the quality of sediments (Mondal et al., 2018).
Where Ci is the concentration of the measured elements in the river sediment; n indicates the number of elements; ERMi values refer to the effect range median for a given element in freshwater sediments (MacDonald et al., 2000).The rating scale of the toxicity probability for MERMQ values is: MERMQ ˂ 0.1, has a 9% possibility to be toxic and display no damaging effects; 0.11 -0.5, has a 21% possibility to be toxic and suggests probable damaging effects; 0.51 -1.5, has a 49% possibility to be toxic and exhibits intermediate damaging effects; and > 1.5, has a 76% possibility to be toxic and shows considerable damaging effect (Chen et al., 2018;Zhou et al., 2014).

Mean effects range median quotient (MERMQ)
The MERMQ is used for investigating the ecological hazard of numerous harmful chemicals that surpass their ERM (effect range median) guidelines (Nourian et al., 2021).
Where Ci is the concentration of any PAHs in the river sediment; n indicates the number of PAHs; ERMi value refers to the effect range median for a given any PAH in sediments (Fakhradini et al., 2019).Sediment samples that contain MERMQ values of < 0.1 have a toxicity probability of 9% and refer to no harmful biota effect; MERMQ of 0.11-0.5 have a toxicity probability of 21% and indicates potential adverse effect; MERMQ of 0.51-1.5 have a toxicity probability 49% and suggest a moderate adverse effect and MERMQ of > 1.5 have a toxicity probability of 76% and refer to a significant harmful effect (Aghadadashi et al., 2019;Ashayeri et al., 2018).

Sediment quality guideline quotient (SQGQ)
The SQGQ is an index used to assess the possible impacts of sediment-associated pollutants on aquatic life and to evaluate the possible biologic impacts at every site influenced by pollutant mixes (Basavaiah et al., 2017;Liu et al., 2016;Singare, 2015).This index can be calculated using the formula: Where ci refers to the measured concentration of a pollutant i, n indicates the numbers of contaminants, and PELi is the probable effect level for each PAH.Depending on SQGQ values, three categories of sediment contamination are given: (a) an SQGQ value ˂ 0.1 suggests unaffected sediment, (b) 0.1 ≤ SQGQ ˂ 1 reveals a moderately affected sediment, (c) an SQGQ value ≥ 1 refers to highly affected sediment having possible damaging biologic effects (Liu et al., 2016;Singare, 2015).

Risk quotient (RQ)
The RQ is an index utilized for evaluating the unwanted effects caused by contaminants in an environmental system.For assessing the ecosystem hazard due to PAHs in the Tigris River, the RQ of the negligible concentration (RQ(NCs)) and maximum permissible concentration (RQ(MPCs)) for each a PAH and for the sum of PAHs were calculated and compared with their reference quality values.In Iraq, no data concerning reference values are available for organic pollutants, so the values documented by (Hezhong et al., 2016;Cao et al., 2010) were applied.An RQ(NCs)<1.0indicate that the single PAH may be of insignificant worry, while an RQ(MPCs)>1.0 suggest that the contamination of the single PAH poses an intense hazard, and an RQ(NCs)>1.0and RQ(MPCs)<1.0 denote that the contamination of the single PAH is of moderate hazard (Aziz et al., 2014;Feng et al., 2013).In a state of RQ∑PAHs, values are interpreted as RQNCs ≥ 800 and RQMPCs ≥ 1 high risk, 1 ≤ RQNCs < 800 and RQMPCs < 1 low to moderate risk, and RQNCs < 1 very low risk (Mohammed et al., 2021).RQNCs and RQMPCs can be calculated as follows: where CQV(NCs) and CQV(MPCs) are the quality values of the NCs and the MPCs of PAHs in the medium.

Heavy Metals
Table 2 shows the values of heavy metal concentrations in sediment samples.The concentrations of Cd and Pb, As, Cr, Ni in some locations have exceeded their background concentrations in the earth's crust.According to the SPI values (Table 2) sites S1 and S3 fall within low-polluted sediments, while S2, S4, S5, S6, S7, S8, and S9 can be considered natural sediments.The TRI value is based on a threshold of a toxic element derived through the eco-toxicological laboratory tests of some aquatic organisms living in sediments.The toxicity hazard of metals calculated by the TRI index is shown in Table 2.The eco-toxicological hazard status of the river sediments ranged from a moderate-risk level at sites S1 and S3 to no risk at the remaining sites.The values of MERMQ ranged from 0.12 at Al Tarmiyah (S9) to 0.69 at Al Qayyarah (S1) (Table 2), indicating a 21% -49% possibility of river sediments being toxic, and this reflects low -medium priority to medium-high priority of risk levels respectively (Mondal et al., 2018).
In general, the composition manner of polycyclic aromatic hydrocarbons supplies useful knowledge on the origin responsible for the environmental contribution of PAHs.In the environment, PAHs are either from a pyrolytic source (anthropogenic burning of biomass and fossil fuel) or from a petrogenic source (production, usage, and transporting of petroleum and its refined produces) (Aghadadashi et al., 2019).The ratios of the low molecular weight (LMWPAHs, which contain 2 -3 rings as Nap, Any, Ane, Fle, Phe, and Ant) to the high molecular weight (HMWPAHs, which contain 4 -6 rings as Flt, Pyr, BaA, Chr, Bbf, Bkf, BaP, DBA, Bpe, and Ipy) could be useful in defining the source of PAHs. the LMW/HMW ratio ˂ 1 refers to pyrogenic sources, while > 1 indicates petrogenic sources (Pohl and Kostecki., 2020;Kadhim et al., 2019).It is evident from the LMW/HMW values (Table 3) that PAHs in sites S1, S3, S4, and S6 can be considered from a petrogenic source, while in sites S2, S5, S7, S8, and S9 come from a pyrogenic source.The MERMQ values of PAHs ranged from 0.031 at S9 to 1.262 at S3 (Figure 2).There is no harmful biota effect with a toxicity probability of 9% at S5, S6, S7, S8, and S9.The MERMQ value at sites S2 and S4 reflects a potential adverse effect with a toxicity probability of 21%.There is a moderate adverse effect with a toxicity probability of 49% at sites S1 and S3 (Figure 2).The reason behind the high values of MERMQ in S1 and S3 is the industrial activity, specifically the oil industries.

Fig. 2. Distribution of MERMQ values of PAHs with ecological effects in sediment samples
The distribution of SQGS values in sediment samples is shown in Fig. 3.The lowest value was 0.13 at S9 while the higher one was 5.03 at S3. Based on the SQGQ criteria, the results revealed that the PAH pollutants would give rise to moderate effects at sites S2, S4, S5, S6, S7, S8, and S9.The largest values of SQGQ were noted at sites S1 and S3, implying a high effect of PAH contaminants on sediments.This may be due to the disparity in human activity along the river course.The risk quotients (RQ) in sediments of the study area were presented in Table 4.For individual PAHs, RQNCs and RQMPCs values were higher than 1 for Any, Ane, Ant, and Pyr compounds, suggesting that the contamination by these PAHs poses an intense hazard to the aquatic environment.The RQNCs values of Flt, Chr, Bkf, Bpe, and Ipy compounds were less than one, indicating low ecological risk of these compounds on river sediments.The Nap, Fle, Phe, BaA, Bbf, Bap, and DBA have RQNCs>1.0,but RQMPCs<1.0,indicating that the aquatic environment undergoes moderate hazard.Based on ∑RQNCs and ∑RQMPCs values of total PAHs, the environmental risk of sediments ranged from low at S2, S4, S5, S6, S7, S8, and S9 to moderate at S1 and S3(Fig.4).

Conclusions
This research evaluated the level and potential risks of heavy metals and polycyclic aromatic hydrocarbon contamination in 9 sediment sites on the Tigris River.The average values of the heavy metals concentrations in river sediments were in the order Cr > Ni > Zn > Cu > Pb > As > Cd.Analyzing environmental risk employing contamination indices revealed the following: The toxic risk levels of river sediment according to SPI values ranged from low polluted sediments at S1 and S3, to natural sediments at other sites.The integrated TRI values ranged from no toxic risk at S2, S5, S6, S7, S8, S9, low toxic risk at S4, and moderate toxic risk at S1 and S3.According to the mean-ERM-quotient values, the probability of toxicity of S2, S4, S5, S6, S7, S8, and S9 is 21%, while S1 and S3 had a probability of toxicity of 49%.The MERMQ results of PAHs showed that sites S5, S6, S7, S8, and S9 were characterized by no harmful biota effect and the toxicity probability is 9%.The probability of toxicity at S2 and S4 is 21% with potentially adverse effects, while at S1 and S3 there was 49% of toxicity probability with moderate adverse effects.The SQGS values have revealed that the sites S1 and S3 indicated highly affected sediments, while the remaining sites showed moderately affected sediments by PAHs contaminants.The high RQNCs and RQMPCs values for Any, Ane, Ant, and Pyr compounds demonstrated their accumulated risk in the river sediments.According to RQ∑PAHs values, the environmental risk of the river sediments ranged from low to moderate.

Fig. 1 .
Fig. 1.Sampling sites of the river sediments in the study area

Fig. 3 .
Fig. 3. Distribution of SQGS values of PAHs with ecological effects in sediment samples

Table 1 .
Coordinates and location name of river sediment samples

Table 2 .
Concentrations of heavy metals (mg/kg) and SPI, TRI, and MERMQ values

Table 3 .
Concentrations (μg/kg) of the 16 PAHs compounds in sediment samples from Tigris River, with sediment quality guidelines (SQGs) values (ERM and PEL) for PAH (µg/kg dry weight)

Table 4 .
NCs and MPCs of individual PAHs with mean values of RQ(NCs) and RQ(MPCs) in the sediment of the Tigris River