3.1 Heavy Metals in Water
Metals concentrations in both summer and winter seasons in lake water were found in the following order: Zn > Cu > Pb > Cd (Tables 1, A1). The mean value of Zn was higher in summer season (0.512 ± 0.873 mg/L) than winter season (0.293 ± 0.551 mg/L) and was highest among all metals in both seasons. Nevertheless, there were no significant differences between Zn and Cu. The average concentrations of these metals in both seasons were lower than the permissible level given by Nepal Drinking Water Quality Standard (CBS, 2019) and WHO (2017).
Table 1
Heavy metal concentration (mg/L) in water sample of Ghodaghodi Lake and maximum permitted concentration in water (mg/L)
Heavy metals | Summer | Winter | NDWQS (CBS, 2019) | WHO (2017) | t-test value | p-value |
---|
Mean ± SD | Mean ± SD |
---|
Zn | 0.512 ± 0.873 | 0.293 ± 0.551 | 3 | 0.01-3 | 0.7617 | 0.454 |
Cu | 0.005 ± 0.01 | 0.007 ± 0.003 | 1 | 2 | -1.2081 | 0.239 |
Cd | 0.0015 ± 0.00 | 0.0015 ± 0.00 | 0.003 | 0.003 | NA | NA |
Pb | 0.005 ± 0.00 | 0.005 ± 0.00 | 0.01 | 0.01 | NA | NA |
Note: SD, standard deviation; NDWQS, National Drinking Water Quality Standard (CBS, 2019); WHO, World Health Organization (2017) |
Zn had the highest concentration in water among four metals analyzed in the present study Table 1); this is probably the reason of the highest concentration in plants and sediments (Núñez et al., 2011; Flefel et al., 2020).
3.2 Heavy Metals in Sediments
Concentrations of heavy metal in sediments are summarized in Table 2. Lake sediment metal concentrations in both seasons were found in the order Zn > Cu > Pb > Cd (Tables 2, A2). Additionally, the mean concentrations of all four metals in sediments were higher in winter than in summer season. For winter season, the maximum values of Zn, Cu, Cd and Pb are 76.23, 16.95, 1.98 and 21.59 µg/g, respectively. And for summer season, the maximum values of Zn, Cu, Cd and Pb are 73.02, 16.41 ,0.01 and 23.94 µg/g, respectively (Table 2).
Heavy metal concentrations of sediments in Ghodaghodi Lake differ among seasons for Cu, Cd and Pb but for Zn it showed insignificant value (Table 2). Mean concentrations for Zn, Cu and Pb was lower than the Average shale value (ASV), Toxicity reference value (TRV), Lowest effect level (LEL) and Severe effect level (SEL) in both seasons except for Cd in winter seasons.
Table 2
Heavy metal concentration (µg/g) in sediment sample of Ghodaghodi Lake and maximum permitted concentration in sediment(µg/g)
Parameters | Period of year | Mean ± SD | Max | Min | t-test | p-value | ASVa (Turekian & Wedepohl, 1961) | TRVb (USEPA, 1999) | LELc (Persaud et al., 1993) | SELd (Persaud et al., 1993) |
---|
Zn | Winter | 56.29 ± 13.13 | 76.23 | 0.01 | 1.78 | 0.09 | 95 | 110 | 120 | 820 |
Summer | 44.00 ± 21.08 | 73.02 | 33.9 |
Cu | Winter | 12.99 ± 3.16 | 16.95 | 5.98 | 2.28 | 0.03* | 45 | 16 | 16 | 110 |
Summer | 9.13 ± 5.21 | 16.41 | 0.01 |
Cd | Winter | 0.79 ± 0.71 | 1.98 | 0.01 | 3.95 | 0.001* | 0.3 | 0.6 | 0.6 | 10 |
Summer | 0.01 ± 0.00 | 0.01 | 0.01 |
Pb | Winter | 11.39 ± 6.88 | 21.59 | 0.01 | 3.48 | 0.002* | 20 | 31 | 31 | 250 |
Summer | 2.16 ± 6.64 | 23.94 | 0.01 |
*: Significant at 0.05, ASVa, average shale value ; TRVb, toxicity reference value; LELc, lowest effect level; SELd, severe effect level |
Sediments consist of very high metal concentrations compared with water. This is facilitated as sediment particles attract the metals which accumulate in the sediment over time (Sancer & Tekin-Özan, 2016). Metals concentrations in sediment were higher in winter than summer due to the lower water flow during winter which could assist to accumulate the heavy metals in sediment, particularly in non-residua phase such as Cd and Pb (Islam et al., 2015). Cd, Cu, Pb, and Zn concentration in sediment, when compared with the average shale values given by Turekian and Wedepohl (1961), showed that they are primarily derived from pollution sources except for Zn (Korfali & Davies, 2005). The major sources are probably the atmospheric dispersion, vehicular combustion from the nearby roads, and other anthropogenic activities. Additionally, formation of smog in the Gangetic Plain (study area is a part of GP) during winter contributes to the scavenging of pollutants that may increase their concentrations in the sediments (Mishra & Kulshrestha, 2020).
3.3 Geo-accumulation index
The mean values of geo-accumulation index in sediment were greater in winter than summer season (Fig. 2). Among four metals, Cd in winter season has the highest Igeo value (0.53 ± 0.47). When compared to geo-accumulation index (Igeo) value given by Muller (1969), mean value of all metals falls under uncontaminated to moderately contaminated category with Class 1 although the Cd maximum value in winter season showed the moderately contaminated category value with Class 2 of Igeo class.
The increase in concentration for Cd in winter season could be due to industrial processing and intensive agricultural practices resulting in the contamination of forage feed and water which are sources of Cd exposure for farmed ruminants (Lane et al., 2015). Furthermore, eventual mixing of feces with water source might have increased the Cd concentration in winter season which was prominent in the study area but was absent in summer season. Some other research found low impact of the highway traffic on the content of other heavy metals in soils and find out moderately polluted category of geo-accumulation index of cadmium near highway (Różański et al., 2017). These findings are also consistent with our results, as some sampling sites were near highway roads with higher traffic concentrations and Nepal's lowlands are blanketed in smog during the winter season causing atmospheric deposition of these pollutants could have escalated Cd concentrations along the sampling sites.
3.4 Heavy Metals in Macrophytes
The concentrations of heavy metals are compared in two dominant submerged macrophytes in both seasons (Tables 3, A3, A4). Heavy metal concentrations were found in the following order: Zn > Pb > Cu > Cd for Nymphaea sp. and Ludwigia sp. in winter and for Nelumbo sp. and Ludwigia sp. in summer season.
Significant difference for Ludwigia sp. was found only in Cd and Pb (p < 0.05; Table 3). The maximum value of Zn, Cu, Cd and Pb concentrations in winter season were 87.48, 14.53. 1.31 and 53.9 µg/g, respectively.
Table 3
Heavy metal concentration (µg/g) in macrophytes sample of Ghodaghodi Lake
Parameters | Period of year | Species | Mean ± SD | Max | Min | t-test value | p-value |
---|
Zn | Winter | Ludwigia sp. | 59.18 ± 24.8 | 87.48 | 20.25 | 1.21 | 0.2978 |
Summer | 40.37 ± 6.86 | 49.84 | 30 | | |
Cu | Winter | 6.58 ± 5.5 | 14.53 | 0.77 | -0.001 | 0.99 |
Summer | 6.10 ± 2.72 | 10.6 | 2.4 | | |
Cd | Winter | 1.01 ± 0.3 | 1.31 | 0.63 | 5.27 | 0.003* |
Summer | 1.89 ± 0.98 | 3.39 | 0.01 | | |
Pb | Winter | 16.70 ± 21.0 | 53.9 | 3.77 | 6.97 | 0.0006* |
Summer | 24.08 ± 9.11 | 33.96 | 3.99 | | |
Zn | Winter | Nymphaea sp. | 58.34 ± 21.6 | 89.81 | 37.82 | | |
Cu | 4.00 ± 1.4 | 5.22 | 1.73 | | |
Cd | 0.90 ± 0.5 | 1.38 | 0.2 | | |
Pb | 4.22 ± 4.2 | 10.08 | 0.01 | | |
Zn | Summer | Nelumbo sp. | 32.94 ± 14.82 | 62.87 | 14.89 | | |
Cu | 8.51 ± 4.50 | 16.19 | 4.29 | | |
Cd | 2.17 ± 0.43 | 2.79 | 1.6 | | |
Pb | 27.96 ± 5.72 | 36 | 17.99 | | |
Note: Ludwigia sps. is dominant in both seasons
*Correlation is significant at the 0.05 level.
Similarly, t-test for Ludwigia sp. for both seasons showed that Cd and Pb concentrations were significantly different (t = 5.2719, p = 0.003 and t = 6.9722, p = 0.0006, respectively). However, Zn and Cu were not significantly different in Ludwigia sp. seasonally. Ludwigia sp. could be proposed as Cu and Zn phytoremediators in Lake Ghodaghodi as has also been suggested by Núñez et al. (2011). Ludwigia sp. showed high metal removal efficiencies from water where Pb was accumulated fundamentally by roots, while Zn was accumulated more in the leaves (San Juan et al., 2018); and the present study also showed similar pattern of result (Zn > Pb > Cu > Cd) in both seasons.
In a study, the roots and shoots of Nelumbo nucifera showed the pattern of Pb > Zn > Cu > Cd and Zn > Pb > Cu > Cd. respectively (Chabukdhara & Nema, 2012); and our study has shown the same order. Also, another study by Verma et al. (2018) in Dudh Talai, Udaipur, Rajasthan shows the accumulation of the heavy metals of N. nucifera in order Fe > Zn > Pb > Cd that was also similar to our study for common metals. The abundance patterns of heavy metals in leaf, petiole and root were Cd > Cu > Pb > Zn (Kabeer et al., 2014) in Nymphea sp. which is contrast to our study. Whereas, both roots and stem based on preliminary study of heavy metals in water lily plants around Kota Samarahan, Malaysia shown the Zn accumulation capacity higher of all (Rahim, 2014) which matches our findings.
No seasonal differences in Zn and Cu concentrations in two seasons in the present study might be because of high bioavailability rate for Zn and Cu (Perin et al., 1997) around all seasons. However, significant seasonal differences of Cd and Pb between winter and summer may be due to vehicular engine combustion as its source, other human activities because of nearby roadway and atmosphere as means of dispersion. This may cause the difference in accumulation of heavy metals (Cd and Pb) seasonally in macrophytes based on the sources and its availability.
3.5 Relation between Macrophytes and Sediments
Table 4 shows a correlation between heavy metals in macrophytes (Ludwigia sp.) and sediments for winter and summer seasons. Overall, the heavy metal concentrations are negatively correlated for Zn and Cd while positively correlated for Cu and Pb in winter season. Meanwhile, Zn and Cu were positively correlated and Cd and Pb were negatively correlated in summer season.
Table 4
Relation between Macrophytes and Sediments
Winter | Zn | Cu | Cd | Pb |
---|
Zn | -0.244 | -0.100 | 0.152 | -0.731 |
Cu | 0.162 | 0.386 | 0.453 | -0.356 |
Cd | -0.507 | -0.252 | 0.479 | -0.567 |
Pb | -0.280 | -0.022 | 0.879* | 0.019 |
Summer | Zn | Cu | Cd | Pb |
Zn | 0.292* | 0.331* | NA | -0.238 |
Cu | 0.286* | 0.299* | NA | -0.014 |
Cd | -0.517* | -0.537* | NA | -0.104 |
Pb | -0.492* | -0.518* | NA | -0.003 |
The uncommon correlations of metal concentration were observed between sediment and plant tissues. Such results indicate that metal concentrations in plants depend on several factors rather than on sediment concentration only. Moreover, when focused on correlation between Cu and Zn in sediment showed increment in metal concentrations of Ludwigia sp. probably because they have a higher tolerance and metal accumulation capacity than the other species. Therefore, Ludwigia sp. could be proposed as Cu and Zn bio-monitors (Núñez et al., 2011).
3.6 Bioaccumulation factor
Bio-accumulation factor (macrophytes/sediment) followed the order of Cd > Cu > Pb > Zn for Ludwigia sp. and Cd > Zn > Cu > Pb for Nymphaea sp (Figs. 3 & 4). in winter season. Cd by Ludwigia sp. shows the highest bioaccumulation factor among all metals in winter season (Fig. 3).
Similarly, bioaccumulation factor (macrophytes/sediment) for summer season followed the order of Pb > Zn > Cd > Cu for Ludwigia sp. and Pb > Cd > Zn > Cu for Nelumbo sp. in summer season (Figs. 5 & 6). Pb showed the greatest accumulation factor among all four metals in Ludwigia sp. in summer (Fig. 5).
Plants with bioaccumulation factor (BAF) greater than 100 have potential to act as hyperaccumulator and indicator of pollution (Wilson & Pyatt, 2007). Ludwigia sp. and Nelumbo sp. in summer were of hyper-accumulator macrophyte species indicating high efficiency of plants to accumulate these metals from sediment and water. The present study is in line with earlier reports indicating the immobile nature of lead from soil and sediments to aerial parts (Siedlecka et al., 2001). Some hyper accumulator plants are able of accumulating metals in higher concentration and evaluate the persistent and acute toxicity from the aquatic ecosystem (McGeer et al., 2003).