Geochemistry pollution status and ecotoxicological risk assessment of heavy metals in the Pahang River sediment after the high magnitude of flood event


 The present work is aimed at assessing the aftermath effects of the 2014 flood tragedy on the distribution, pollution status and ecological risks of the heavy metals deposited in the surface river sediment. A series of environmental pollution indexes, specifically the enrichment factor (EF), geo-accumulation index (Igeo), contamination factor (CF), modified degree of contamination (mCd), pollution load index (PLI), potential ecological risk index (PERI) and sediment quality guidelines (SQGs) have been adopted. Results revealed that the freshly deposited sediments collected soon after the flood event were dominated by Cu, Fe, Pb, Ni, Zn, Cr and Cd, with the average concentrations of 38.74, 16,892, 17.71, 4.65, 29.22, 42.36 and 0.29 mg/kg, respectively. According to the heavy metal pollution indexes, Pahang River sediments were moderately to severely contaminated with Pb, Ni, Cu, Zn and Cr, while Cd with the highest risk of 91.09 was the predominant element that illustrated an aesthetic ecological risk to the water body after the tragic flood event. The findings highlighted a critical deterioration of the heavy metals content, driven by the catastrophic flood event, which has drastically altered their geochemical cycles, sedimentary pollution status and biochemical balance of the river's environment.


INTRODUCTION
As the partitioning and distribution patterns of these heavy metals are of critical importance to the potential toxicity, mobility and route of contamination, the unique understanding on the metal speciation in the sedimentary environment, rather than the concentration is essential for the hazard identification, and the residual implications to the food chain equilibrium. Although heavy metal pollution in the aquatic environment continues to be the aesthetic concern and focused attention at the global level, the potential ecological risk after the devastating flood events has not been widely explored. In this sense, this study has been carried out with the key objectives to determine the metals distribution of the sediments along Pahang River after the 2014 historical flood event. A series of environmental assessment indexes, specifically the enrichment factor (EF), geo-accumulation index (I geo ), contamination factor (CF), modified degree of contamination (mC d ), pollution load index (PLI), potential ecological risk index (PERI) and consensus-based sediments quality guidelines (SQGs) for the detected metals were evaluated as reliable sedimentological datasets on the mobilization, origins of metal species and the potential availability of metals to biota within different environmental settings. This useful information could serve as a benchmark and useful tool for the establishment of an up-to-date status of the sediment contamination along Pahang River and highlight the probable adverse impacts on the natural ecosystem, for river pollution control, environmental management and ecosystem conservation in the near future.

Environmental settling
Pahang River, located between the longitudes of E 101 16 0 31″-E 103 29 0 34″, and between the latitudes of N 2 48 0 45″-N 3 40 0 24″, is the longest river in the Peninsular Malaysia, with the maximum length and breadth catchment of 459 and 236 km, respectively. This area has a humid tropical climate and characterized by a bimodal pattern of Southwest and Northeast monsoons, with the annual rainfall ranging from 1,488 to 3,071 mm/year. It is one of the most important water resources for domestic water supply, fishing, agricultural and industrial activities and recreational purposes to the local community. The monsoon climate-con- The superficial sediment samples were collected from each sampling site using a stainless steel Ponar-typed box corer sampler. To avoid metal contamination from the grab's wall, the outer fraction of these sediment samples was removed, while the inner fraction was refrigerated at 4 C for further analysis. The samples were oven-dried and grounded with clean mortar and pestle and sieved through a 63-μm stainless steel aperture for extraction purpose. The granulometric analysis was conducted according to the standard dry and wet sieving techniques for the specific quantification of clay (<4 μm), silt (4-63 μm) and sand (>63 μm) fractions. As the samples contain a high percentage of clay, the samples were subjected to pipette analysis as proposed by Krumbein & Pettijohn (), and each fraction of sucking pipette was dried and weighted to the nearest of 0.0001 g.

Heavy metals analysis
The wet acid digestion, also known as the aqua regiaþ hydrofluoric acid (HF) digestion method was adopted. The laboratory apparatus and glassware were soaked in 10% of concentrated nitric acid, HNO 3 (v/v) for overnight, rinsed with deionized water and oven-dried before being used to minimize potential contamination as part of the Quality assurance and Quality controls (QA/QC). The heavy metals (Cu, Fe, Pb, Ni, Zn, Cr and Cd) were analysed according to the United States Environmental Protection Agency (USEPA) 3052 guideline, by digesting 50 mg of dried and homogenized sediment under the grain size of <63 μm using a microwave oven digestion system, with 1.5 mL of mixed concentrated acids (HF: HNO 3 : HCl) in the ratio of 2: 3: 3. The qualitative and quantitative analyses were carried out using an Inductively Coupled Plasma-Mass Spectrometer model 7500 Perkin Elmer Ltd.) in triplicates, and the findings were presented in mg per kg (mg/kg) of dry mass of sediments. The differences (p < 0.05) in heavy metals content before and after the 2014 flood were analysed by one-way analysis of variance (ANOVA) using the Statistical Package for Social Sciences (SPSS) version 22.0.

Quality assurance and quality control
The quality assurance and quality control (QA/QC) for the strong acid digestion method, including reagent blanks, triplicate samples and standard reference materials (SRM1646a) were prepared and analysed using the same procedure and reagents to ascertain the precision of the analytical method using ICP-MS. The analytical results were reliable with the mean errors below 5%, and the analytical precision for the replicated samples was within ±10%.

Geo-accumulation index
The I geo approach, which was firstly proposed by Müller (), has been widely applied for the effective measurement of sediment quality and served as a reference to the heavy metal pollution, with regards to the pre-industrial levels of the bottom sediment and the background matrix correction factor of 1.5, which is specifically applied to counteract the variations of the background values, driven by the lithogenic effects.

CF and mC d
The CF is an indicator of sediment contamination, commonly applied to evaluate the pollution of the environment by a given toxic substance over a period of time (Hákanson ). This environmental pollution assessment requires at least five surficial sediment samples to produce a mean pollutant concentration, to be compared with the baseline pristine reference level. Hákanson's study has proposed a sedimentological approach using a diagnostic tool,

Pollution load index
PLI is a standardized system that provides simple and comparative means to assess the overall toxicity status, and contribution of several heavy metals in the sediment samples, to allow the comparison of pollution levels between different locations and at different timeframes (Tomlinson et al. ). The average shales were adopted as the undisturbed background values for those metals in the same way for the computation of CF.

Potential ecological risk index
PERI is a precise measurement, to ascertain the synergetic ecological risk of heavy metals according to the sedimentary theory (Hákanson ). This assessment takes into the account of the geological background conditions, environmental chemistry, biological toxicology and ecological variability. In the PERI concept, the standardized biological toxic-response factors (T i f ) for the single pollutant in the surficial sediment are adopted from the Hákanson () theory, where Cu, Pb, Ni, Zn, Cr and Cd are 5, 5, 5, 1, 2 and 30, respectively.

Sediment quality assessment guidelines
Numerical SQGs has been developed using a variety of approaches, typically involves statistical comparisons of chemical concentrations, and measurement of adverse biological effects during the exposure to sediment, with the primary purpose to protect the aquatic biota against the deleterious effects associated with sediment-bound heavy metals, to rank or to prioritize contaminated areas or chemicals of concern for further investigation (Long & MacDonald ). The mean quotient of these guidelines is a useful tool to summarize multiple datasets or figures  Table 1.

Sediment profile
Granulometric composition is a useful tool in the particle search and analysis techniques, commonly applied in the forensic evaluation of sediment, soil and a wide range of related particulate materials. This analysis is a very  Field studies along with granulometric analysis of the collected sediment samples showed that the surface sediments of Pahang River were characterized by a high percentage of sand (6-100 wt%, mean ¼ 58.9 wt%), and the spatial distribution was more uniform along the river during the ambient condition (Figure 2(a)). The sand content increased significantly towards the river mouth and reached the higher range of 85-100 wt% at S1-S9 after

Seasonal variation of metal load in sediment
The seasonal variation of heavy metals in the sediment samples before and after the flood tragedy is illustrated in Figure 3. Generally, the heavy metals deposited in the   which suggested that these metals were originated mainly from the land-based runoff and river discharge at the study area.

Enrichment factor
Measurement of the total concentration of heavy metals, as the only criterion for the assessment of heavy metal contamination in the sediment environment, is not satisfactory in tracing the natural or anthropogenic origins according to their grain sizes and mineralogy. EF technique is a powerful tool to differentiate between the natural and anthropogenic sources of metal enrichment in sediment, and to discern the status of environmental contamination.
Within this framework, the normalization of element contents is usually adopted, in which the metals concentration is normalized to a textural or compositional characteristics of the tested water sediment (Hornung et al. ).
In the present work, Fe was applied for the determi-

Geo-accumulation index
The (I geo ), originally introduced by Müller (), is applied to quantitatively evaluate the heavy metal contamination status in the sediment samples in comparison with the preindustrial concentrations. The factor 1.5 is adopted for the correction of regional background difference, and according to the theory of I geo , the contamination levels could be classified into seven major grades, given by: practically uncontaminated (I geo 0), uncontaminated to moderately contaminated (0 < I geo 1); moderately contaminated (1 < I geo 2); moderately to heavily contaminated (2 < I geo 3); heavily contaminated (3 < I geo 4); heavily to extremely contaminated (4 < I geo 5); or extremely contaminated (I geo > 5) (Müller ).
The   Table 2 as: CF < 1 indicates low degree of sediment contamination; 1 CF < 3 is referred to a moderate degree of sediment contamination; 3 CF < 6 illustrates a considerable degree of sediment contamination; and CF ! 6 represents a very high degree of sediment contamination, whereas the contamination level of mC d was classified, ranging from: mC d < 1.5 ¼ nil to a very low degree of contamination; 1.5 mC d < 2 ¼ a low degree of  and mC d 32 ¼ an ultra-high degree of contamination.
The variation of CF for different heavy metals detected in the deposited sediments along Pahang River is illustrated in Figure 6. From the presented results, the calculated CF for Pb (CF ¼ 5.74), which has been defined as the metal of concern, has shown a great reduction after the flood event. which has normalized the overall contamination behaviour.
The acquired PLI values indicated a moderate degree of pollution to the river system. This integrated index is coded by the advantage of easily understood by the researchers but is criticized by the weakness of the ecotoxic potential of individual metal that has not been taken into account.

Potential ecological risk index
The potential ecological hazard index was originally pro-  Table 3. Value of E i f < 40 indicates a low potential ecological risk; 40 E i f < 80 is a moderate ecological risk; 80 E i f < 160 is a considerable ecological risk; 160 E i f < 320 is a high ecological risk; and E i f ! 320 is a very high ecological risk, while PERI < 150 indicates a low potential However, the average E i f value of Pb in the sediment samples, that reached the peak value of 46.95, was likely to show moderate risk than other metals, as it is manifested with a relatively broad dispersion, which may be attributed to the anthropogenic intrusions. On the whole, the mean PERI value before the flood event, which was found at 116.03 (PERI < 150), indicated a low ecological risk of the aquatic biota along Pahang River.
In contrast, the PERI acquired after the flood event was 158.96. The ecological risk factors for Pb, Zn, Cu, Ni and Cr that fall under the low ecological risk (E i f < 40) were identified to be 26. 18, 1.31, 27.41, 5.45 and 7.52, respectively. Nevertheless, Cd, which has recorded the highest average

Sediment quality assessment guidelines
Hazardous waste site evaluation involves the collection of substantial quantities of sediment chemistry data, and these data would serve as the backbone to support the screening-level of ecological risk assessments (USEPA ). To effectively predict the heavy metal pollution, a comparative study was conducted according to the SQGs as proposed by the USEPA standards; and the analytical results were compared with SQGs, notably probable effect limit (PEL) to ascertain the true extent of sediment contamination, and to predict the potential biological effects to the natural environment (MacDonald et al. ). Generally, it is a powerful tool to be applied in numerous applications, including monitoring plans design, historical data interpretation, remedial investigations and developing sediment quality remediation objectives.  Accordingly, the mean-PEL-quotient of <0.1 has a 9% probability of being toxic; a mean-PEL-quotient of 0.11-0.5 has a 21% probability of toxicity; a mean-PEL-quotient of 0.51-1.5 has a 49% probability of being toxic; and a mean-PEL-quotient of >1.5 has a 76% probability of toxicity (Long & MacDonald ). In the present study, the mean- Among all, heavy metals are of considerable environmental concern, primarily due to their tetra-toxicity, persistent and bio-accumulative behaviours. These heavy metals would undergo numerous changes in the natural media during their transport, assisted by the dissolution, precipitation and sorption phenomena. In this sense, the identification and quantification of the heavy metal sources, mobility, toxicity, bioavailability, as well as the fate of the individual metal, remains a critical challenge to be highlighted. The additivity, antagonism or synergism of multiple heavy metal contaminants, may demonstrate the overall toxicity rather than an individual implication. In the present work, the geochemical studies with respect to the EF, geoaccumulation index (I geo ) and CF have been analysed to ascertain the magnitude of single metal pollution, and to estimate the impact of anthropogenic activities at the polluted sites; while the modified degree of contamination (mC d ) and PLI were applied for the evaluation of the overall risk of multiple heavy metals in the river sediment. These geochemical approaches, however, involve a different level of subjectivity, and the possible weighting role, combined antagonism as well as the ecological risk on the river sediment have been neglected (Ahamad et al. ).
The complementary approaches of PERI and sediment quality assessment guidelines (SQGs), which integrates sediment standard criteria, grain size effect, toxic-response factor, EF, I geo and CF indexes are therefore necessary for an accurate assessment of heavy metals accumulation from various anthropogenic sources, and evaluation of the resultant environmental and ecological risks in the sedimentary environment, with the possible adverse effects to the aquatic organisms, for the interpretation of sediment quality (Kulbat & Sokołowska ).
In the present study, the analysis with respect to EF, I geo and CF illustrated that the river sediments were severely accumulated by Pb and Cu, and moderately contaminated    The variations of EF, I geo , CF and E i f of the examined heavy metals summarized in Table 4

CONCLUSION
In this work, the complex relationships and contamination status between the sediment texture and heavy metal concentrations of the surficial sediment samples along Pahang River before and after the heavy flood event were analysed with respect to the distribution characteristics, geochemistry pollution features and ecotoxicological risks, by adopting the chemometric analysis, sediment quality criteria, regional sedimentary reference data and environmental hazard assessment indexes. Seven major heavy metals, specifically Fe, Cu, Pb, Ni, Zn, Cr and Cd were detected in the deposited sediments, with the highest concentrations of 20,451, 54.20, 28.60, 7.40, 38.00, 55.30 and 0.65 mg/kg, respectively, which was 4-5-folds higher than the corresponding local natural background levels. The environmental assessment indexes revealed a polymetallic contamination of the river sediments, with: (1) the magnitude of single-element indexes, notably EF, I geo and CF illustrated that the river sediments were severely accumulated by Pb and Cu, but moderately contaminated by Cr, Cd, Zn and Ni; (2) the combination of mC d and PLI values was greater than 1; (3) while the PERI value acquired after the flood event was observed at 158.96, with Cd had hit the highest of single ecological risk factor, E i f of 91.09, indicating the possible long-term adverse biological implications to the ecosystem. The newly organized baseline data could prevail to be a valuable reference for the reliable assessment of heavy metal pollution status under different environmental settings, which are necessary for the aquatic system protection, and future restoration or rehabilitation of the river basin after the flood tragedy.