Comprehensive Evaluation of Some Toxic Metals in the Surface Water of Louhajang River, Bangladesh

34 Louhajang River, Bangladesh, crosses Tangail, which is a densely industrialized and urbanized 35 city. Louhajang River is an essential water source for domestic, irrigation, and urbanization 36 purposes. This study reports the levels of pH, electrical conductivity (EC), and some toxic heavy 37 metals in 40 water samples collected during summer and winter seasons from Louhajang River. 38 The levels were found to be in the ranges of pH 6.22-7.43 and EC 345-798 mS/cm, Cr 0.18-13.2, 39 Ni 0.02-19.04, Cu 0.96-15.92, As 2.18-12.51, Cd 0.02-2.42, and Pb 0.49-15.74 µg/L. The winter 40 season reported higher levels of the examined parameters than the summer season with 41 significant variation ( p < 0.05) for all parameters, with the exception of Cd. The metal contents 42 were assessed against local and international standards for drinking, irrigation and aquatic life 43 purposes where different trends were observed. The heavy metal evaluation index (HEI) and the 44 ecological risk index (ERI) reported low to moderate risks. The spatial distribution of metal 45 contents assigned hot spots in some sites along the riverbed, which were attributed to specific 46 manmade sources. The health risk assessment for three population categories, i.e., adult male, 47 adult female, and children, were examined in terms of hazard index (HI) and total cancer risk 48 (TCR) for oral and dermal pathways during both seasons. Cr and Cd recorded HI more than unit 49 in all cases, indicating possible non-cancer risk. TCR values of As for the three examined 50 population categories during both seasons were > 1.0×10 -6 , indicating possible cancer risk, while 51 that of Pb were < 1.0×10 -6 . For Ni, about 10-25% of the sampled sites recorded TCR > 1.0×10

). In addition, exposure to small concentrations of Cd may affect the 85 4 physiology and health of wildlife. Pb is considered highly toxic and causes several health 86 problems like damage to the nervous system and immune function, blood pressure, abdominal 87 pain, kidney damage, gliomas, lung cancer, and stomach cancer (Mortada et al. 2001;Järup 88 2003; Kormoker, Proshad, Islam, Tusher, et al. 2020). Moreover, the contamination of surface 89 water by toxic metals may affect other environmental counterparts as these metals may enter into 90 food chain through the consumption of fish and other aquatic plants (Loska & Wiechuła 2003). 91 Furthermore, surface water contamination by metals can be a serious threat to aquatic organisms. 92 Toxic metals contamination is currently a serious concern in developing countries such as continually raises metal contents and degrades the quality of water ). 98 The current study focused on the assessment of some toxic heavy metal contents in the surface 99 water from Louhajang River, Tangail district, central Bangladesh. Due to urbanization and 100 industrialization in nearby areas, Louhajang River has received considerable amounts of urban 101 wastes and hence it may be at risk from exposure to toxic metals ).

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However, so far, no scientific research has been reported concerning toxic metal contamination 103 in the surface water of Louhajang river. The main objective of the present study was therefore to 104 evaluate the contents of some toxic heavy metals (Cr, Ni, Cu, As, Cd, and Pb) in the surface 105 water of Louhajang River. The study demonstrates a comprehensive assessment including 106 comparison with standards, spatial distribution, seasonal variation, and statistical analysis, 107 besides the use of ecological and health risk indices.

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Study area 110 Louhajang River (Fig. 1) has a length of 85 km and a width of 78 m. Louhajang River crosses 111 Tangail city, which is considered an industrial growing area in Bangladesh. The city includes 112 several industries producing a wide range of materials such as metallic workshop tools, batteries, 113 packing materials, leathers, garments, dyes, bricks, and food (Fig. 1). Despite it has a limited 114 5 area of about 29.04 km 2 , Tangail city is considered a densely populated area with population of 115 750,000 residents in 2017. The residents have also practiced agricultural activities in some areas 116 in Tangail city and along the banks of Louhajang River. Unfortunately, there is no controllable 117 treatment for wastes dumped from industrial, municipal, and household activities. As a result, 118 Louhajang River may be susceptible to environmental pollution from these discharges. It was 119 reported that ) wastes were mixed with sediment and water of Louhajang 120 River, resulting in possible river pollution.  The pH and the EC of the collected water samples were measured on-site and before 131 acidification using portable appropriate meters. Standard protocols were applied for the meter 132 calibration. For metal analysis, all chemicals and reagents used in this study were of analytical 133 grade. The water used in this study was Milli-Q grade, which was purified by Elix® Essential 5 134 UV Water Purification System, US. The Teflon vessels and high-density polyethylene bottles 135 were cleaned, soaked in 5 % (v/v) HNO3 for more than 24 h, rinsed with water, and allowed to 136 dry at room temperature. Water sample (20 mL) was treated with 5 mL of 69% HNO3 and 2 mL 137 of 30% H2O2 in a closed Teflon vessel, which was afterward digested in a microwave digestion 138 system. Blank samples were prepared in triplicate following the same procedure. Analysis of      The ecological risk index (ERI) from the river water consumption was also computed to assess  Table 1. where the RfD is the reference dose. The values of RfD of each metal is described in Table 1.

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The RfDdermal values were calculated by multiplication of the RfDing by the dermal absorption 235 fraction (ABS) as described in   CSF is the cancer slop factor as described in Table 1 Table 2. The raw data of the 267 same parameters in each site in the two examined seasons are shown in Table S1. Primarily, were in the following decreasing order: Cr > Cu > As > Pb > Ni > Cd.

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The mean concentration of Cr in the summer season was 4.35 that was lower than 5 μg/L as the    Table 2 shows that the mean Cd concentrations in both seasons were lower, as for As, than all 360 standards recommended for drinking, aquatic life, and irrigation purposes. However, Table S1 the hot spot sites (Fig. 2 and 3). For Ni distribution, the upstream showed more hot spots than the  Table 5. The raw data before and after rotation in 427 the summer and winter seasons are shown in Table S2 and Table S3, respectively. Table 5 shows examined sites were at low risk (ERI < 150), while 20% were at moderate risk (ERI 150-300).

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For the winter season, 25% of the examined sites were at low risk, while 75% were at moderate 468 risk.

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The spatial distributions of ERI during the summer and the winter seasons are depicted in Fig. 6.

470
With reference to Fig. 1, it may suggest that the effluents from the industrial area and the metal 471 workshop put the river water in the upstream at moderate risk in both seasons. With reference to 472 Fig. 2, the moderate risk in both seasons was caused by high levels of Cr, Ni, and Cd.

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Additionally, the agricultural activity and/or aviation activities in the vicinity of the midstream 474 may also put the river water at moderate risk, which was caused by high levels of Cr, Ni, Co, and 475 Pb.

486
In contrast, the minimum HI values (Table 6) Tables S21-S29. 500 Among the examined elements in the current study, the carcinogens were Ni, As, and Pb.

501
The minimum values of TCR from Ni exposure (Table 6) for the three examined population 502 categories were less than 1.0×10 -6 , indicating acceptable range, while the maximum values were 503 more than 1.0×10 -6 , indicating possible cancer risk from exposure through oral ingestion and 504 dermal contact pathways. In particular, four, four, and five sampled sites recorded TCR more 505 than 1.0×10 -6 during the summer season for adult male, adult female, and children, respectively, 506 while seven, eight, and 12 sampled sites recorded TCR more than 1.0×10 -6 during the winter 507 season for adult male, adult female, and children, respectively. Unfortunately, the TCR values of 508 As (Table 6) for the three examined population categories during both seasons summer and 509 winter were more than 1.0×10 -6 , indicating possible cancer risk from exposure to As in water 510 samples from all sampled sites along Louhajang River. Fortunately, the TCR values of Pb (Table   511 6) in all sampled sites during both seasons were < 1.0×10 -6 , indicating no cancer risk.  The spatial distribution of pH reported that most spots in the riverbed were alkaline in winter, 531 whereas few spots in the riverbed area were alkaline in summer. In contrast, EC reported similar 532 spatial distribution during the two seasons. In general, the EC in the downstream in both seasons 533 was higher than that in the upstream. This result indicates that the downstream was more 534 influenced by discharge flux. The spatial distributions of Cr, Cu, and Pb recorded few hot spots 535 in almost the same sites in both seasons, indicating permanent sources. Moreover, hot spots were 536 observed for Cr and Ni, which were attributed to the release from dying industry and metallic 537 workshops.

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The HEI and ERI, as ecological indices used for overall assessment of collection of heavy 539 metals, were also applied in this study. Based on the WHO classification of HEI, the water was 540 at low risk from exposure to heavy metals. Nevertheless, the ERI reported low to moderate risks.

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The HI, as a health risk assessment index, reported possible non-cancerogenic risk from The authors declare that they do not have any competing interests that could have appeared to 579 influence the work reported in this paper. territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.

Figure 2
Spatial distribution of pH, EC, and the examined toxic metal concentrations in the surface water samples collected during the summer season from Louhajang River, Bangladesh. Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.

Figure 3
Spatial distribution of pH, EC, and the examined toxic metal concentrations in the surface water samples collected during the winter season from Louhajang River, Bangladesh. Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.

Figure 4
Dendrograms obtained from cluster analysis of pH, EC, and the examined toxic metal concentrations in the surface water samples from Louhajang River in the summer and the winter seasons Spatial distribution of heavy metal evaluation index (HEI) in the surface water samples collected during the summer and the winter seasons from Louhajang River, Bangladesh. Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.

Figure 6
Spatial distribution of ecological risk index (ERI) in the surface water samples collected during the summer and the winter seasons from Louhajang River, Bangladesh. Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.

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