Evaluation of the Level of Mercury Pollution in the Sediments of the Rivers Draining the Gold Panning Sites in the Territory of Fizi, Eastern Democratic Republic of Congo

The sediments collected respectively from the Etó, Kacumvi, Kimbi, Lubichako, Makungu, Kuwa, Mandje, Misisi and Kimuti Rivers draining the gold panning sites in the Fizi territory were studied during a 16-month cycle (Au-gust and December 2016 to August and December 2017) in order to assess their degree of mercury pollution in the dry season as well as in the rainy season. The assessment of the degree of pollution of the said sediments focused on six parameters including the total mercury content (THg) and the indices of mercury pollution such as the mercury enrichment factor (EF), the mercury contamination factor (CF), the mercury geoaccumulation index (I geo ), the mercury potential ecological risk factor (PERF) and the mercury ecological risk index (ERI). Total mercury was determined by atomic absorption spec-trophotometry (AAS) while the mercury pollution indices were successively calculated using the appropriate formulas. The results thus obtained revealed that all the sediments of the rivers studied are considerably polluted by mercury according to the values relative to their total mercury content and mercury pollution indices, including the

their degree of mercury pollution in the dry season as well as in the rainy season. The assessment of the degree of pollution of the said sediments focused on six parameters including the total mercury content (THg) and the indices of mercury pollution such as the mercury enrichment factor (EF), the mercury contamination factor (CF), the mercury geoaccumulation index (I geo ), the mercury potential ecological risk factor (PERF) and the mercury ecological risk index (ERI). Total mercury was determined by atomic absorption spectrophotometry (AAS) while the mercury pollution indices were successively calculated using the appropriate formulas. The results thus obtained revealed that all the sediments of the rivers studied are considerably polluted by mercury according to the values relative to their total mercury content and mercury pollution indices, including the mercury enrichment factor (EF), the mercury contamination factor (CF), the mercury geoaccumulation index (I geo ), the mercury potential ecological risk factor (PERF) and the mercury ecological risk index (ERI), which greatly exceed the standards recommended by the Canadian Council of Ministers of the Environment. In particular, the sediments of the Kimbi River are highly polluted by mercury compared to those of other rivers studied. This reported pollution is the result of anthropogenic gold panning activities that generate effluents and elemental mercury that pollute the streams.

Introduction
Almost the entire surface of planet earth is covered by aquatic ecosystems. In recent decades, these aquatic ecosystems have been seriously threatened by pollution, often of anthropogenic origin. In fact, effluents containing chemical pollutants from artisanal and industrial activities (iron and steel industry, metallurgy, tannery, etc.) are discharged into all environmental compartments, particularly in aquatic environments (Pestana et al., 2010).
Mercury is an integral part of the major metallic pollutants commonly found in aquatic ecosystems. This toxic global pollutant can enter watercourses either by surface runoff and/or directly from locally polluted wastewater or by wet atmospheric deposition. It has proven toxicity even at low levels, as do other trace metallic elements such as lead and cadmium (Wang et al., 2009).
In the aquatic environment, mercury and other trace metal elements (TMEs) accumulate in sediments to the point where they become reservoirs for contaminants. As a result of physical, biological or chemical transformations, sediments can also be a significant source of mercury for the water column, benthic organisms, fish, etc. (Tessier, 2012).
Contaminants stored in sediment in this way can continue to disrupt the aquatic ecosystem, even after measures are taken to stop or regulate their inputs. It is therefore necessary to determine the mercury content in sediments, which constitute a "memory" of the life of the river, including episodes of pollution (Kennish, 2002).
Nowadays, studies relating to the assessment of mercury pollution levels in river sediments occupy the hearts of many researchers. Some of them have reported that the sediments of the Toulon roadstead are polluted by mercury (Tessier, 2012), the sediments of the Porto-Novo lagoon (South Benin) are heavily polluted by the mercury contained in the effluents that are regularly discharged there (Chouti et al., 2010), the mercury content is worrying in the sediments of the Tambopata, Malinowski and Heath rivers draining the Peruvian artisanal gold mining sites, etc. (Gerardo et al., 2018).
At present, there is no information on the level of mercury pollution in the sediments of the rivers draining the gold panning sites in the province of South Kivu in general and those in the territory of Fizi in particular. However, the quality of the sediments in the rivers of the gold panning sites in the territory of Fizi is said to be deteriorating, all the more so as, in this region, the effluents resulting from artisanal gold mining are discharged into the rivers without having been recycled in advance and the gold-mercury amalgam is generally incinerated in the open air on the said sites. Thus, this study aims to fill the above-mentioned gaps. Its main purpose is to evaluate the degree of mercury pollution in the sediments of the rivers draining the gold panning sites in the Fizi territory using the total mercury content and the indices of mercury pollution, including the mercury enrichment factor (EF), the mercury contamination factor (CF), the mercury geoaccumulation index (I geo ), the mercury potential ecological risk factor (PERF) and the mercury ecological risk index (ERI).

Methods of Sampling and Analysis of Sediment Samples
Samples of the studied sediments were collected during 4 Campaigns (August The sampling in question took place between 9:00 and 10:00 for all target locations but on different days in order to minimize the risk of daily variability and to take into account as much as possible the timing of the largest releases. It should be noted that these sediment samples were taken 0.5 m from the bays by driving the PVC core barrel into the river. This sampling operation was repeated twice and the samples were mixed to form a composite sample to reduce the variation in results and the uncertainty surrounding the mean value obtained in the laboratory analysis of the parameters sought. The resulting composite sediment core was packaged in a clean, pre-labeled 500 mL plastic box. All boxes containing the composite sediment core were stored in situ at 4˚C in coolers before being sent to the laboratory for analysis.  Six variables were measured to assess the degree of mercury pollution in sediments collected from the rivers studied, including total mercury content and mercury pollution or contamination indices such as the mercury enrichment factor, the mercury contamination factor, the mercury geoaccumulation index, the mercury potential ecological risk factor and the mercury ecological risk index. Total mercury was determined according to standard analytical methods using the Atomic Absorption Spectrophotometer (AAS) (AA500). Mercury standard solutions were prepared in advance to determine the mercury content in different sediment samples. These solutions were kept in clean plastic bottles that were pre-washed with concentrated nitric acid and distilled water. In addition, the enrichment factor was calculated using the following formula: [Al] mes = The concentration of aluminum in the sediment sample collected from sites subject to anthropogenic activities.
[Me] Th = The concentration of the analyte (Hg in this case) in the soil sample from the reference site, natural site or site with no anthropogenic activities. This is the reference concentration of the analyte (Hg in this case). This is generally referred to as the natural background level.
[Al] Th = The concentration of aluminum in the soil sample from the reference site, natural site or no human activity site.
For the case of the present study, the reference site chosen is indeed the field of the village of Lulimba where no gold panning activity is carried out. Hence, total mercury and aluminum concentrations in topsoil from this field were considered as reference concentrations to determine the mercury enrichment factor in all river sediment samples from the artisanal gold mining sites in Fizi.
In addition, it should be pointed out that the element aluminum has been used for geochemical standardization as usual for the following reasons: aluminum, generally measured, is poorly soluble and so far unaffected by pollution.
The geoaccumulation index was calculated using the following formula proposed by Müller in 1979: The coefficient 1.5 is a correction factor that takes into account natural fluctuations in the grade of a given metal that can be attributed to mineralogical changes in the sediment. This geoaccumulation index is associated with a pollution scale of seven classes below: I geo ≤ 0 (Unpolluted sediments or soils or Background); 0 < I geo < 1 (Unpolluted to moderately polluted sediments); 1 < I geo < 2 (Moderately polluted sediments); 2 < I geo < 3 (Moderately to heavily polluted sediments); 3 < I geo < 4 (Heavily polluted sediments); 4 < I geo < 5 (Heavily to extremely polluted sediments); I geo > 5 (Extremely polluted sediments) (Manjunatha et al., 2001).
The Ecological Potential Risk Factor was determined by multiplying the mercury toxic response factor values by the mercury contamination factor values.
where Tf Hg = Mercury toxic response factor = 40; CF = Mercury contamination factor. It should be noted that the potential ecological risk factor of mercury is related to a pollution scale of the following five classes: EPRF < 40 (contamination/pollution with low ecological risk potential); 40 ≤ EPRF < 80 (contamination/pollution with moderate ecological risk potential); 80 ≤ EPRF < 160 (contamination/pollution with considerable ecological risk potential); 160 ≤ EPRF < 320 (contamination/pollution with high ecological risk potential); EPRF > 320 (contamination/pollution with very high ecological risk potential) (Bhuiyan et al., 2010).

Statistical Treatment of Data
The results obtained have been encoded and processed by Microsoft Excel 2010.
The software R version 2.15.1 was used for the descriptive analyses of these results while XLSTAT 2016 was used to separate the means of different factors at the 5% significance level.

Total Mercury Levels in the Sediments Studied
Results for total mercury levels in various sediments studied are presented in Table 1.
N. M. Pascal et al. The results presented in Table 1 show that the mean values of total mercury levels in the sediments of all the rivers of the gold panning sites in the Fizi territory were of the order of 34.275 ± 0.364 mg/Kg. However, all of the sediments collected from the rivers studied were found to be contaminated with mercury because their total mercury concentrations significantly exceeded the sediment quality standards that require tolerable levels of total mercury in sediment to be no more than 0.4 mg/Kg. In addition, it was found that total mercury concentrations in the sediments of all the rivers studied did not vary significantly between study Campaigns (P-value = 0.743) although they did vary highly significantly from river to river (P-value = 0.000

Mercury Enrichment Factor for the Sediments Studied
Results that are consistent with the mercury enrichment factors in the sediments studied are presented in Table 2. The results shown in Table 2 show that the mean value of mercury enrichment factors in the sediments of all these rivers was of the order of 24.545 ± 0.266. However, these mercury enrichment factors in the sediments of the rivers studied were found to vary highly significantly from one river to another (P-value = 0.000). In fact, the Kimbi River sediments had the highest mean mercury enrich- Based on the mercury enrichment factor, Niane (2014) had also found that anthropogenic mercury enrichment was very high in the sediments of the Gambia River, which drains gold mining sites in the Kédougou region (eastern Senegal).
In addition to this, it also found that mercury enrichment factors in the sediments of the Gambia River varied very significantly between campaigns (January 2011 and November 2011).

Mercury Geoaccumulation Index for the Sediments Studied
Results that are consistent with the mercury geoaccumulation index are shown in Table 3.
Based on the results in Table 3, it appears that the mean values of the mercury geoacumulation index in the sediments of the rivers studied were 4.771 ± 0.384.
Moreover, these values varied highly significantly from one river to another

Mercury Contamination Factors for the Sediments Studied
Results related to mercury contamination factors in different sediments studied are highlighted in Table 4.
The results reported in Table 4 show that the mean mercury contamination factor values in the sediments of the Etó, Misisi, Lubichako, Kimbi, Mandje, Kacumvi, Kimuti, Kuwa and Makungu Rivers were equal to 9.340 ± 0.304. In addition, the values of mercury contamination factors in the sediments of the rivers studied varied highly significantly from one site to another (P-value = 0.000).

Potential Ecological Risk Factor (PERF) for Mercury in the Sediments Studied
The results for the potential ecological risk factors for mercury in the sediments studied are highlighted in Table 5.
In view of the results highlighted in Table 5, it can be seen that the average values of potential ecological risk factor related to mercury pollution in the sediments of the above-mentioned rivers were 194.900 ± 0.371. However, the potential ecological risk factor values related to mercurial pollution in the sediments of the rivers studied varied highly significantly from one river to another (P-value = 0.000 In addition, the level of potential ecological risk from mercury pollution of sediments in all the rivers studied was high (160 ≤ PERF < 320). Similarly, the average values of the potential ecological risk factors relating to mercury pollution of the sediments of the rivers studied also varied in a highly significant manner from one investigation Campaign to another (P-value = 0.000) while undergoing considerable increases from Campaign 1 (August 2016) to Campaign 4 (December 2017). Indeed, Campaign 4 (December 2017) presented the highest average values of potential ecological risk factors relating to mercury pollution for all the sites studied (375.639 ± 0.314), followed successively by Campaign 3 (August 2017) for which these values were of the order of 223.336 ± 0.362; Campaign 2 (December 2016) for which the mean PERF values were 130.921 ± 0.413 and finally Campaign 1 (August 2016) for which the mean FREP values were of the order of 108.283 ± 0.395. Based on the level of potential ecological risk of mercury pollution, Banu et al. (2013) and Banerjee et al. (2016) also noted that the level of potential ecological risk of mercury pollution of sediments in the Turag River (Bangladesh) and Subarnarekha River (India) was high. Moreover, these researchers also found that potential ecological risk factors related to mercury pollution of sediments in the rivers studied varied very significantly depending on the study campaigns.

Ecological Risk Index for Mercury in the Sediments Studied
Results that are consistent with the Ecological Risk Index for mercury in the sediments studied are reported in Table 6.
N. M. Pascal et al. Based on the results presented in Table 6, it can be seen that the average values of the ecological risk index in relation to mercury pollution in the sediments of the rivers studied were of the order of 539.524 ± 0.503. Nevertheless, the various values of the ecological risk index (ERI) related to mercury pollution in the sediments of the rivers studied varied in a highly significant way from one river to another (P-value = 0.000).  Banu et al. (2013) and Leila et al. (2014) also found that the level of ecological risk related to mercury pollution of the sediments of the Turag River (Bangladesh) and those of the Boumerzoug Basin (Algeria) was considerable. In addition to this, these researchers have also observed that the ecological risk index related to mercury pollution of sediments in the rivers studied vary very significantly depending on the study campaigns.

Conclusion
The sediments of the rivers that drain the gold panning sites in the Fizi territory are amply polluted by mercury in relation to their total mercury content and the values of their mercury pollution index, in this case the mercury enrichment factor, the mercury contamination factor, the mercury geoaccumulation Index, the mercury potential ecological risk factor and the ecological risk index that deviate from the standards recommended by the Canadian Council of Ministers of the Environment. Faced with this high level of mercury pollution in the sediments studied, it is necessary to consider the best solutions likely to protect the aquatic ecosystems of the gold panning sites in the Fizi territory. Thus, it would be advisable to regularly reinforce the capacity of gold panners by organizing workshops for their attention, focusing mainly on the mercury pollution of aquatic ecosystems, the importance of recycling gold panning effluents before discharging them into waterways, etc. Further research on this topic will focus respectively on the identification of mercury-resistant bacteria communities in the sediments studied and on the in vitro study of the effects of these sediments on fish.