Toxic metal concentration and ecotoxicity test of sediments from dense populated areas of Congo River, Kinshasa, Democratic Republic of the Congo

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Article

Reference

Toxic metal concentration and ecotoxicity test of sediments from dense populated areas of Congo River, Kinshasa, Democratic

Republic of the Congo

MATA, Henry K., et al .

Abstract

The objective of the present research was to evaluate the contamination level and potential risks at a part of a large and important river in Central Africa, the Congo River. Surface sediments were collected from seven stations located in Maluku-Kinsuka section, at the vicinity of Kinshasa, capital city of Democratic Republic of the Congo. The concentration of toxic metals (Cr, Co, Ni, Cu, Zn, As, Cd and Pb) in sediment samples were determined using Inductive Coupled Plasma-Mass Spectroscopy (ICP-MS). The Hg analysis was carried out using Advanced Mercury Analyzer (AMA). The evaluation of the pollution degree was based on Sediment Quality Guidelines (SQGs), Enrichment Factor (EF), Geoaccumulation Index (Igeo), and using toxicity test based on exposing Ostracods to the sediment samples. The result revealed high metal concentrations in sediments from the 4 densely populated areas (Kinkole, Kauka fishermen, Demoulin, Chanic), reaching the values (mg kg-1) of of 95.5, 14.3, 37.1, 139.9, 281.5, 4.8, 6.6, 200.9 and 4.9 mg kg-1 for Cr, Co, Ni, Cu, Zn, As, Cd, Pb and Hg, respectively. These values are above SQGs and probable [...]

MATA, Henry K., et al . Toxic metal concentration and ecotoxicity test of sediments from dense populated areas of Congo River, Kinshasa, Democratic Republic of the Congo. Environmental Chemistry and Ecotoxicology , 2020, vol. 2, p. 83-90

DOI : 10.1016/j.enceco.2020.07.001

Available at:

http://archive-ouverte.unige.ch/unige:141193

Disclaimer: layout of this document may differ from the published version.

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Toxic metal concentration and ecotoxicity test of sediments from dense populated areas of Congo River, Kinshasa, Democratic Republic of the Congo

Henry K. Mata

^a

, Dhafer Mohammed M. Al Salah

^b,c

, Georgette N. Ngweme

^a

, Joel N. Konde

^a

, Crispin K. Mulaji

^d

, Guillaume M. Kiyombo

^a

, John W. Poté

^b,d,

⁎

aUniversity of Kinshasa (UNIKIN). Faculty of Medicine, School of Public Health, B.P. 11850. Kinshasa I, Democratic Republic of the Congo

bDepartment F.-A, Forel for Environmental and Aquatic Sciences and Institute of Environmental Sciences, Earth and Environmental Science Section, Faculty of science, University of Geneva, 66 Boulevard Carl-Vogt, CH-1211 Geneva 4, Switzerland

cKing Abdulaziz City for Science and Technology, Joint Centers of Excellence Program, Prince Turki the 1^stst, Riyadh 11442, Saudi Arabia

dUniversity of Kinshasa (UNIKIN), Faculty of Sciences, Department of Chemistry, B.P. 190. Kinshasa XI, Democratic Republic of the Congo

A B S T R A C T A R T I C L E I N F O

Article history:

Received 16 May 2020

Received in revised form 2 July 2020 Accepted 3 July 2020

Available online xxxx

The objective of the present research was to evaluate the contamination level and potential risks at a part of a large and important river in Central Africa, the Congo River. Surface sediments were collected from seven stations located in Maluku-Kinsuka section, at the vicinity of Kinshasa, capital city of Democratic Republic of the Congo. The concentration of toxic metals (Cr, Co, Ni, Cu, Zn, As, Cd and Pb) in sediment samples were determined using Inductive Coupled Plasma-Mass Spectroscopy (ICP-MS). The Hg analysis was carried out using Advanced Mercury Analyzer (AMA). The evaluation of the pollution degree was based on Sediment Quality Guidelines (SQGs), Enrichment Factor (EF), Geoaccumulation Index (Igeo), and using toxicity test based on exposing Ostracods to the sediment samples. The result revealed high metal concentrations in sediments from the 4 densely populated areas (Kinkole, Kaukaﬁshermen, Demoulin, Chanic), reaching the values (mg kg⁻¹) of of 95.5, 14.3, 37.1, 139.9, 281.5, 4.8, 6.6, 200.9 and 4.9 mg kg⁻¹for Cr, Co, Ni, Cu, Zn, As, Cd, Pb and Hg, respectively. These values are above SQGs and probable effect levels. In these sites, Igeo and EF values showed heavily to extremely polluted and severe enrichment to extremely severe enrichment for analysed metals. Additionally, Ostracods exposed to sediments resulted in 100% mortality rates after 6 days of incubation, demonstrating the sediment toxicity as well as potential risks for aquatic living organisms.

The pollution in these sites may be explained by local intense human activities with various commercial, presence of uncontrolled landﬁlls in riverbank and industrial settlements, as well as by the construction of boats for the regular navigation along the river.

This research presents useful tools for evaluating sediments quality and risk, which can be applied to similar environments.

Keywords:

River pollution Sediments Toxic metals Toxicity test

Anthropogenic activity and risk Congo River

1. Introduction

Due to its persistence, ecological and human-associated health risks, toxic metal pollution are intensively investigated globally in different environmental compartments. The pollution of aquatic environment by organic and inorganic micro-pollutants such as persistent organic pollutants (POPs), pharmaceutical drugs and toxic metals can cause the deterioration of water quality and present substantial human and

agricultural usage concern [1–4]. Surface water (including rivers, lakes and oceans) is considered as a major public resource and the ﬁnal destination of untreated and treated urban, hospital and industrial efﬂuent waters. Therefore, urban rivers are heavily polluted by varieties of contaminants including POPs, toxic metals and pathogenic organisms in many parts of the world [2,5–10]. The situation is considered alarming in developing countries such as in Sub-Saharan Africa. Anthro- pogenic pollutants due to intensive and uncontrolled urbanization affect

–

⁎Corresponding author at: University of Geneva, Faculty of Sciences, Earth and Environmental Sciences, Department F.-A. Forel for environmental and aquatic sciences, Bd Carl-Vogt 66, CH- 1211 Geneva 4, Switzerland.

E-mail address:john.pote@unige.ch. (J.W. Poté).

http://dx.doi.org/10.1016/j.enceco.2020.07.001

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Contents lists available atScienceDirect

Environmental Chemistry and Ecotoxicology

j o u r n a l h o m e p a g e :w w w . k e a i p u b l i s h i n g . c o m / e n / j o u r n a l s / e n v i r o n m e n t a l - c h e m i s t r y - a n d - e c o t o x i c o l o g y /

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regions, where most urban rivers are receiving industrial, hospital and communal effluent waters without treatment, and urban storm water runoff [1,9,11–15]. Many studies have shown that the urban rivers in sub-Saharan African countries are of major concern because of hospital and industrial effluents, urban runoff, uncontrolled landfill leachates, and domestic wastewaters lack treatment [16–18]. Some of main actions are still to be performed for the reduction of the potential environmental and human risks caused by toxic metals include the iden- tification of their potential sources, the development of reliable surveil- lance and risk assessment procedures, andfinally, the implementation of technological solutions that can prevent environmental contamination [8,9,19]. Sediments can act as a reservoir for organic and inorganic pollutants for an extended period. It has been demonstrated that sediments may accumulate 100 to 1000 times as many heavy metals, as the overlying water and offer the opportunity for reconstructing the pollution history and evaluating the impacts [2,3,20]. In this context, the assessment of heavy metals and POPs in sediments can help to validate the ecological and human-associated risks [3,20,21].

The Congo River plays a vital role in the economic development of the country, but also for other countries of central Africa such as Demo- cratic Republic of the Congo, Congo-Brazzaville, Central Africa Republic and Sudan through various activities such asﬁshing and trade, hydro- electric production, navigation, domestic exploitation, irrigation and recreational activities. Nevertheless, only few studies have been performed to assess its environmental conditions and the ecological quality of this crucial ecosystem for central African states [14,15,22,23]. These studies recommended further research work to evaluate the quality of this important river. The evaluation of Congo River quality is therefore

very important to identify ecological and potential human risks to the population using these waters for diverse purposes. Thus, the purpose of this research is to assess the quality of sediments from Congo River at the main important sites located close to the city of Kinshasa. The assessment was based on the sediment physicochemical characterization including sediment grain-size, total organic matter, and metals (Cr, Co, Ni, Zn, Ag, Cd, Sn, Sb, Pb and Hg). Sediment toxicity test was performed by exposing Ostracods to the sediment samples.

2. Material and methods

2.1. Study site description and sampling procedure

This research was conducted in the Congo River, Maluku-Kinsuka section in the vicinity of Kinshasa, the capital city of Democratic Repub- lic of the Congo (DRC) (Fig. 1). The Congo River is the most important river of DRC, with an overall length of 4700 km and a drainage basin covering 4,014,500 km². The river basin is the second largest drainage basin in the world after the Amazon and has a great economic impor- tance [22]. Due to the degradation of the main road network and to the rare railways in the country, the Congo River constitutes the princi- pal transportation artery of supply for goods and services between the large towns of DRC such as Kinshasa (10 million inhabitants), Kisangani (nearly 700,000 inhabitants), Bumba (about 110,000 inhabitants), Mbandaka (about 345,000 inhabitants) and Matadi (about 310,000 inhabitants). Consequently, there is an intensive river navigation along the Congo River. In the vicinity of large towns, the river and its

Fig. 1.Satellite map adapted from Google Earth indicating; (a) Africa continental map showing Democratic Republic of the Congo location, (b) Map showing the location of Kinshasa City in Democratic Republic of the Congo, and (c) Sampling sites in Congo River at the vicinity of Kinshasa.

H.K. Mata et al. Environmental Chemistry and Ecotoxicology 2 (2020) 83–90

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tributaries receive untreated industrial and hospital efﬂuents, urban storm water runoff and urban liquid and solid waste.

Sediment sampling took place in March/April 2018. Seven stations namely Maluku (MA), Ngamanzo (NG), Kinkole (KI), Kaukaﬁshermen (KA), Demoulin (DE), Chanic (CH) and Kinsuka Mimosa (KIM) were chosen to collect surface sediments (0–6 cm). These stations were selected according to the recommendation formulated in our previous studies [14,15], and based on their accessibility, the proximity of pol- lutant releases points and/or recreational activities. Approximately 300–350 g of sediments were collected in triplicates from each site, at a distance of about 3 m from the shore and at less than 1.5 m water depth. The sampling distance between each replicate was about 2 m. The sediment samples were frozen, freeze-dried and ground into aﬁne homogenized powder for further chemical analysis. After this preliminary treatment, the samples were transported to the department F.A. Forel of the University of Geneva (Switzerland) for further analyses.

2.2. Sediment grain size and organic matter content

The sediment particle grain size was measured using a Laser Coulter®

LS-100 diffractometer (Beckman Coulter, Fullerton, CA, USA), following 5 min ultrasonic dispersal in deionized water according to the method described by Loizeau et al. [24]. The sediment total organic matter (TOM) and carbonates (CaCO3) content were estimated by loss on ignition at 550 °C and 1000 °C respectively, for 1 h in a Salvis oven (Salvis AG, Emmenbrücke, Lucerne, Switzerland).

2.3. Toxic metal analysis

Before analysis, the sediment samples were lyophilized, grounded into a fine and homogenized powder and sieved through a 63μm mesh size sieve and digested according to the previous method as described by Poté et al. [2] and Loizeau et al. [24]. The digested samples were subjected to the analysis of toxic metals including Sr, Cr, Co, Ni, Cu, Zn, As, Cd, and Pb by Inductive Coupled Plasma-Mass Spectros- copy (Agilent 7700 x series ICP-MS developed for complex matrix analysis, Santa Clara, CA, USA). A collision/reaction cell (helium mode) and interference equations were utilized to remove spectral in- terferences that might otherwise bias results. This is sufficient for many routine applications. Multi-element standard solutions at different concentrations (0, 0.2, 1, 5, 20, 100 and 200μg L⁻¹) were used for calibration. The certified sediment reference material LKSD-4 was used for river sediment analysis in order to verify the sensibility of the instrument and the reliability of the results. Concentrations are in mg kg⁻¹(ppm) dry weight (dw). The standard deviations of 3 replicate measurements were below 5%, and chemical blanks for the procedure were less than 1% of the sample signal.

Total Hg analysis in sediment samples was carried out using atomic absorption spectrophotometry for mercury determination (Advanced Mer- cury Analyzer; AMA 254, Altec s.r.l., Czech Rep.) following the procedure described by Bravo et al. [25]. This method is based on sample combustion, gold amalgamation and atomic absorption spectrometry.

2.4. Geoaccumulation index and enrichment factor

The enrichment factor (EF) and geo accumulation index (Igeo) in sediment samples were calculated as described by [14,16,26,27]. The Igeo accumulation index is deﬁned as in Eq.(1).

Igeo¼Log2ð ÞCn

1:5ð ÞBn ð1Þ

Cn: concentration of metals (n) examined in the sediment samples.

Bn: concentration of the metal (n) geochemical background.

1.5: Lithospheric effect background correlation matrix factor

Enrichment factor is a useful tool to determine the degree of anthropogenic heavy metal pollution. EF is calculated using Eq.(2), and according to our previous study, Scandium (Sc) was used for the geochemical normaliza- tion [11,14,16].

EF¼ metal Sc

Sample= metal Sc

background ð2Þ

2.5. Sediment toxicity test

The toxicity test of sediment samples were performed using the TK36- Ostracodtox kit (MicroBioTests Inc., Belgium) following the manufacturer's recommendations, and as described by Mwanamoki et al. [15]. Brieﬂy, the benthic ostracod crustaceanHeterocypris incongruenscysts were hatched in standard fresh water (provided with the kit) at 25 °C and with permanent illumination (3000–4000 lx approximately), 54 h prior to the tests. The ne- onates were then measured for length and immediately placed in test wells.

Each test well consists of 1 mL of test sediment, 2 mL of standard fresh water, 2 mL of algal food suspension (provided with the kit) and 10 living ostracods. The test plates containing six wells were sealed with paraﬁlm, covered with lid and incubated at 25 °C in dark for 6 d. The mortality rate (%) of the organisms was determined using the following formula:

Mortality (%) = B/A*100; where B is the total number of dead Ostracods and A is the total number of Ostracods added in the test plate. Finally, the length of the surviving Ostracods was measured using a micrometric strip placed at the bottom of the glass microscope plate. Growth Inhibition was calculated using Eq.(3).

Growth inhibitionð Þ%

¼100–½ðgrowth in test sediment=growth in reference sedimentÞ 100 ð3Þ

2.6. Data analysis

Triplicate measurements were performed for all the analyses. Statistical treatment of data (Pearson product moment correlation) was obtained using SigmaStat 11.0 (Systat Software, Inc., USA). Statistical analysis data were also done with Rstudio version 3.4.4. with a statistical signiﬁcance de- ﬁned asp< .05.

3. Results and discussion

3.1. Sediment physicochemical characteristics

Total organic matter (TOM) values in sediment samples varied substan- tially with sampling station and replicate (p< .05). TOM values (in %) ranged from 1.1–6.0 (MA), 7.1–9.5 (NG), 5.5–7.1 (KI), 8.7–10.8 (KA), 3.5–8.9 (DE), 3.3–5.6 (CHA) and 0.6–2.0 (KIM). These results tendency was similar to that previously reported in many sites of Congo River basins [14,15], with maximum values of 16% in the site of pool Malebo (Mongole). In the Kinshasa urban rivers, tributaries of Congo Rivers, the sediment TOM content reaches the values of 30–45% [12]. According to the results of our previous studies [1,2,9], the organic matter in non- contaminated freshwater sediments varied from 0.1–6.0%. The results of this study indicated that the sediment from studied sites can be considered as polluted/moderately polluted by organic matter. There was no signiﬁ- cant difference in CaCO3among all samples (triplicate) from the same site (p>.05).

The percentage of silt, clay and sand also vary sensibly depending on the sampling area/sites. In general, the investigated sediment samples can be classiﬁed as silty-sandy sediments except for the Maluku area. The percentage of clay remains low with values below 10%, except for the Maluku area with the values ranged between 13.9 and 29.2%. The coarser sandy sediments were found in the area of Kinsuka Mimosa, with the highest mean grain-size values (200μm) and proportion of sand (91%).

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3.2. Metal concentrations in the surface sediments: comparison with standard values and potential biological effects

The concentration of toxic metals (Cr, Co, Ni, Cu, Zn, As, Cd, Pb and Hg) in the sediment samples are shown inTables 1 and 2, where they are compared to the Sediment Quality Guidelines for the Protection of Aquatic Life (SQGs). In general, metal concentrations varied signiﬁcantly with sampling area (p< .05). There was no signiﬁcant difference among all samples (triplicate) from the same site (p>.05). For example, in the sediments from the

area Chanic (CH1, CH2, CH3), the range of concentrations is: 63.7–69.2, 2.5–3.9, 6.3–8.5, 62.6–66.3, 80.5–85.0, 0.9–1.1, 0.20–0.24, 130.5–138.9 and 2.3–3.2 mg kg⁻¹for Cr, Co, Ni, Cu, Zn, As, Cd, Pb and Hg, respectively.

For the sediments from Demoulin (DE1, DE2, DE3), the range of concentrations is: 22.0–25.4, 4.5–5.6, 10.4–12.5, 13.5–19.7, 263.1–285.0, 0.18–0.54, 61.5–66.6, 0.25–0.35 mg kg⁻¹for Cr, Co, Ni, Cu, Zn, As, Cd, Pb and Hg, respectively.

The maximum concentration for all analysed metals is found in the area of Kinkole with the values of 95.5, 14.3, 37.1, 139.9, 281.5, 4.8, 6.6, 200.9 and 4.9 mg kg⁻¹for Cr, Co, Ni, Cu, Zn, As, Cd, Pb and Hg, respectively. The pollution in this site may be explained by local intense human activities with various commercial and industrial settlements, as well as by the construction of boats for the regular navigation along the river [14]. Addition- ally, this site is characterized by the presence of several uncontrolled landfills, which are within a close proximity to the river, with the presence of different waste materials such as batteries, plastics, mobile phones, scrapped vehicles and metals with old paints, as well as intensive urban ag- riculture [11,12]. The sediments from Kauka and Demoulin zones present high concentration of toxic metals, mainly for Cr, Zn, Pb and Hg. These areas are characterized with several potential sources of river contamination, including the industrial effluent discharge, the presence of uncontrolled landfills and urban agricultural and storm runoff [28]. Other investigated areas such as Maluku, Ngamanzo and Kinsuka Mimosa present lower toxic metal concentrations. Maluku and Ngamanzo areas are located upstream of Kinkole and about 80 km east of the Kinshasa City Centre. No commercial or industrial activities are performed in these three areas. In comparison with the results from some urban rivers in Kinshasa, the obtained concentration of toxic metals are equivalent or 2 to 3 times less than the values reported by several studies; e.g., the values of 154.2, 186.0, 1105.3, 3.69, 548.0 and 5.5 mg kg⁻¹for Cr, Cu, Zn, Cd, Pb and Hg, respectively were found in sediments from Funa River [12]. The values (in mg kg⁻¹) of 47.9 (Cr), 213.6 (Cu), 1434.4 (Zn), 2.6 (Cd), 281.5 (Pb), and 13.6 (Hg) were observed in the sediments from Makelele river [9], while [16] reported the values (mg kg⁻¹) of 325 (Cu), 549 (Zn) and 165 Table 1

Sediment physicochemical characteristics.

Station/coordinate Sample TOM (%)

CaCO3 (%)

Clay (%)

Silt (%)

Sand (%)

Mean grain size (μm)

Maluku MA1 1.48 0.86 20.65 72.53 6.82 11.05

X: 4°2′54.13″Y: 15°33′

25.74″

MA2 6.03 0.86 13.95 74.14 11.91 14.89 MA3 1.08 0.18 29.22 69.56 1.22 7.13

Ngamanzo NG1 7.43 0.46 3.54 48.39 48.07 42.40

X: 4°10′25.43″Y: 15°32′

7.15″

NG2 7.14 0.78 6.14 69.04 24.82 23.20 NG3 9.52 1.02 7.22 68.01 24.77 25.10

Kinkole KI1 6.37 1.00 3.41 37.20 59.39 61.90

X: 4°19′0.80″Y: 15°30′

41″

KI2 5.53 0.91 3.41 42.72 53.87 44.05 KI3 7.14 0.71 9.00 71.36 19.64 21.63 Kaukaﬁshermen KA1 10.27 1.40 5.89 73.00 21.11 24.69 X: 4°19′6.56″Y: 15°20′

16.33″

KA2 10.76 1.42 4.37 59.11 36.52 33.03 KA3 8.71 0.76 3.92 53.09 42.99 37.59

Demoulin DE1 8.87 0.59 3.28 34.47 62.25 59.14

X: 4°19′20.50″Y: 15°20′

46.07″

DE2 8.65 1.09 4.05 51.03 44.92 39.98 DE3 3.54 0.40 0.698 89.02 90.40 149.40

Chanic CHA1 5.62 1.67 3.13 35.9 60.97 63.18

X: 4°19′46.74″Y: 15°15′

24.9″

CHA2 3.25 0.46 2.68 14.54 82.78 119.9 CHA3 3.44 0.56 3.63 38.31 58.06 95.09 Kinsuka Mimosa KIM1 0.67 0.23 1.28 17.37 81.35 137.3 X: 4°19′41.59″Y: 15°13′

31.6″

KIM2 1.96 0.73 1.31 15.91 82.78 137.0 KIM3 0.62 0.28 0.614 80.36 91.35 200.00

Table 2

Metal contents (mg kg⁻¹dw) of surface sediment samples from Congo River analysed by ICP-MS^aand AMA for Hg.

Station Sample Sc Cr Co Ni Cu Zn As Cd Pb Hg

Maluku MA1 1.53 13.36 1.12 3.84 4.63 12.69 0.15 0.02 5.08 0.04

MA2 3.53 33.42 2.56 8.04 12.47 27.21 0.46 0.08 8.85 0.08

MA3 1.04 9.17 0.33 1.40 2.16 7.41 0.64 0.02 2.36 0.06

Ngamanzo NG1 2.33 22.54 6.65 13.04 6.94 26.94 1.34 0.11 6.87 0.04

NG2 3.56 33.46 9.73 19.37 9.50 36.94 1.49 0.14 9.22 0.06

NG3 6.90 55.01 16.88 29.63 15.85 61.66 3.63 0.24 14.95 0.07

Kinkole KI1 1.86 95.49 14.12 37.10 139.91 164.62 0.56 6.14 200.89 4.92

KI2 1.63 85.33 14.25 37.06 138.20 153.75 1.58 6.19 150.42 3.62

KI3 1.59 85.76 12.11 35.95 128.01 281.53 4.80 6.55 148.04 3.79

Kaukaﬁshermen KA1 4.85 87.43 11.65 19.97 51.44 148.91 1.46 0.23 65.64 0.94

KA2 4.41 65.41 10.65 19.17 61.18 146.78 1.30 0.22 60.93 0.92

KA3 4.10 81.81 8.27 17.83 50.35 144.00 0.96 0.21 60.24 0.79

Demoulin DE1 1.30 24.46 4.59 13.81 19.67 278.21 1.10 0.54 64.88 0.35

DE2 2.64 22.03 5.60 10.02 15.36 263.09 1.12 0.30 66.58 0.25

DE3 0.58 25.36 4.54 12.49 13.52 285.03 0.32 0.18 61.52 0.26

Chanic CHA1 2.16 66.89 3.50 10.43 62.60 84.59 1.08 0.24 130.48 3.18

CHA2 2.02 63.86 2.53 11.28 62.64 85.00 0.89 0.20 131.56 2.38

CHA3 2.29 69.15 3.94 8.45 66.28 80.45 0.94 0.23 138.94 2.25

Kinsuka Mimosa KIM1 0.35 3.08 0.72 1.32 1.77 11.29 0.17 0.03 3.48 0.02

KIM2 0.56 6.60 1.79 3.01 3.49 20.95 0.46 0.07 11.82 0.04

KIM3 0.30 9.58 2.81 3.50 24.46 69.19 1.19 0.09 43.76 0.12

LKSD4^b Ref. values 21 11 32 30 189 12 1.9 93 –

Rec. values 19.01 10.42 31.4 29.7 172.3 10.7 1.7 92.2 –

SQGs. Rec^c.max 37.30 35.70 123.00 5.90 0.60 35.00 0.17

The values in bold represent the concentration of the toxic metals above the recommended concentration according to the Sediment Quality Guidelines (SQG) for the Protection of Aquatic Life.

a Total variation coefﬁcients for triplicate measurements are smaller than 5% for ICP-MS analysis.

b The recovery values from the ICP-MS measurements for reference material (LKSD 4) was above 90% for all elements.

c Sediment Quality Guidelines for the Protection of Aquatic Life recommendation. In bold represent the concentration of the toxic metals above the recommended concentration according to the Quality Guidelines for the Protection of Aquatic Life recommendation.

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(Pb) in sediments from Nsanga river. In the sediments from the Congo River, area of Pool Malebo, the values of 107.2, 111.7, 88.6, 39.3, 15.4, 6.1 and 4.7 mg kg⁻¹for Cr, Ni, Zn, Cu, Pb, As and Hg, respectively were observed [14].

The results from this study were compared to Sediment Quality Guidelines for the Protection of Aquatic Life (SQGs). The evaluation of the potentially deleterious effects of the metals towards benthic fauna, which is based on consensus-based guidelines for the sediment quality [29,30], can give an estimate of the hazard that the sediments may represent for the local biota. According to the SQGs, the surface sediments from Kinkole are heavily contaminated with toxic metals including Cr, Cu, Zn, Cd, Pb and Hg. The sediments from Kauka, Demoulin and Chanic are heavily contaminated with Pb and Hg. These sediments can be considered to present adverse biological effects and potential environmental and human impacts.

3.3. Enrichment factor (EF) and geoaccumulation index (Igeo)

The exploitation of geo-accumulation index (Igeo) and enrichment factor (EF) enabled understanding and evaluation of the potential environmental risk by comparing metal background value to metal concentrations determined in the present study. EF and Igeo are essential to discriminate the source of metals and give a quantitative criterion for characterizing the sediment according to the degree of metal pollution [31]. The results of the calculation of Igeo and EF are presented in Table 3. The highest values of EF and Igeo for the analysed metals are found in the area of Kinkole, while non-negligible values are found in the other areas such as Demoulin, Kauka and Chanic. These results indicate that these zones of Congo River are severely enriched and polluted due to the anthropogenic activities and cannot be attributed to geochemical background. It has been proposed that when EF ≤0.5 is indicative of Table 3

Igeo and EF values for Cr, Ni, Cu, Zn and Pb in the sediment samples.

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bioturbation in the upper mixed-layers and dilution with less contaminated sediment components; 0.5≤EF≤1.5 is indicative of metals from crustal sources or natural sources; EF≥1.5 is indicative of specific metal inputs, e.g. from anthropogenic sources [32–34]. Rapid population growth and ad- jacent urbanization in Kinshasa City have been observed in recent years and are considered to be a potential source of contamination of the environment and the food chain [11,12,15]. Consequently, many areas of the investigated section of the Congo River receive the untreated industrial, hospital and communal effluent waters, and the river banks are used as uncontrolled landfills for solid wastes [9,11,12,16,17].

The Igeo and EF values in sediment samples from Maluku, Ngamanzo and Kinsuka Mimosa are very low except for the site KMI3. This site present the maximum metal concentrations of the KMI area, with the values (mg kg⁻¹) of 9.6, 2.8, 3.5, 24.5, 69.2, 1.2, 0.1, 43.8, and 0.1 for Cr, Co, Ni, Cu, Zn, As, Cd, Pb and Hg, respectively. However, all of these values are under sediment quality guidelines (SQGs).

3.4. Sediment toxicity test

The full evaluation of sediment toxicity needs to combine chemical (such as toxic metals) analysis with ecotoxicological aspects [1,17,35,36].

Ostracods (Heterocypris incongruens) and various other species such as the brine shrimp (Artemia franciscana) are usually included as useful

bioindicators in changing environmental conditions including contrasting soil and sediment receiving systems [8,37,38]. Accordingly, in this study we used Ostracods as a possible bioindicator. The results for percentage of growth inhibition and the mortality rate of Ostracods for the selected areas are presented inTable 4.

All sediment samples from Kinkole and Kaukaﬁshermen present a mortality rate of 100%, which is explained by the fact that besides metals there are many other compounds present, which could accumulate in the sediments [8]. Sediments from the areas of Maluku, Ngamanzo and Kinsuka Mimosa present the mortality rate ranging from 15.1–16.2, 22.8–82.8 and 26.2 to 100%, respectively. The sites NG2, NG3, KIM2 and KIM3, which present high mortality rates have metal values under SQGs and probable effect level. These results indicate that sediments could carry other sources of contaminants, which could affect the growth of the ostracods added to the test sediments [8,15]. In comparison with sediment samples from Kinkole, Demoulin, Ngamanzo, Chanic, Kaukaﬁshermen and Kinsuka Mimosa areas according to toxicity test, the sediments from the Maluku can be considered to present the smallest effect on benthic Ostracods with a mortality rate of approximately 16% and growth inhibition of 15%.

3.5. Correlation between parameters

The correlations between all the parameters are as displayed inTable 5.

The organic matter (TOM) content of the sediments is negatively propor- tional to the grain size at thep< .01 level. This correlation translates to sediments with smaller grain size will retain higher organic matter content as previously stated by many studies [5,8,9,39,40]. In addition, the TOM correlates positively with Cr, Zn, and As at thep< .05 level and with Co and Ni atp< .01 level. Cr correlates positively atp< .01 level with all the metals except Zn and As. Co correlates with Hg at thep< .05 level and with Ni, Cu, As, and Cd at thep< .01 level. Interestingly enough Ni correlates positively with all the metals at thep< .01 level with the excep- tion of Zn at thep< .05 level. This could be attributed to the common use of Ni in all industries in general and alloys speciﬁcally [40–44]. Cu correlates at thep< .01 level with Cd, Pb and Hg. Zn correlates positively with Pb. As correlates with Cd at thep< .05 level. Cd correlates very strongly with Pb and Hg and Pb and Hg correlate with each other at the p< .01 level. The strong correlations observed among the metals could be attributed to the same source of pollution or the same retention pathway since all the metals are positively charged and are expected to behave in somewhat a similar manner [39,45–47].

4. Conclusion

The results of this research show that Congo River basin is an impacted area of the Congo River in the section of Maluku-Kinsuka Mimosa, at the vicinity of Kinshasa, especially in the areas of Kinkole, Kaukaﬁshermen,

Table 5

Pearson correlation matrix for selected parameters analysed in the sediment samples.^a

OM Grain size Cr Co Ni Cu Zn As Cd Pb Hg

OM 1 −0.58⁎⁎ 0.54⁎ 0.70⁎⁎ 0.59⁎⁎ 0.24 0.47⁎ 0.46⁎ 0.13 0.15 0.10

grain size 1 −0.32 −0.40 −0.38 −0.13 −0.01 −0.32 −0.18 0.06 −0.07

Cr 1 0.71⁎⁎ 0.78⁎⁎ 0.85⁎⁎ 0.36 0.42 0.59⁎⁎ 0.77⁎⁎ 0.78⁎⁎

Co 1 0.93⁎⁎ 0.60⁎⁎ 0.36 0.65⁎⁎ 0.59⁎⁎ 0.39 0.44⁎

Ni 1 0.78⁎⁎ 0.47⁎ 0.64⁎⁎ 0.80⁎⁎ 0.60⁎⁎ 0.66⁎⁎

Cu 1 0.42 0.38 0.87⁎⁎ 0.91⁎⁎ 0.95⁎⁎

Zn 1 0.32 0.42 0.51⁎ 0.33

As 1 0.45⁎ 0.22 0.28

Cd 1 0.71⁎⁎ 0.81⁎⁎

Pb 1 0.95⁎⁎

Hg 1

a Parameters include toxic metals, median grain size and total organic matter (OM)n= 8.

* statistically signiﬁcant coefﬁcients at thep< .05 level.

** statistically signiﬁcant coefﬁcients at thep< .01 level.

Table 4

Percentages of mortality and growth inhibition of Ostracods (Heterocypris incongruens) exposed to the sediments.

Section Sample % Mortality % Growth inhibition

Maluku MA1 16.2 14.8 ± 4.0

MA2 15.4 14.4 ± 5.0

MA3 15.9 14.7 ± 3.0

Ngamanzo NG1 22.8 54.3 ± 4

NG2 76.7 n/d

NG3 84.8 n/d

Kinkole KI1 100 n/d

KI2 100 n/d

KI3 100 n/d

Kaukaﬁshermen KA1 100 n/d

KA2 100 n/d

KA3 100 n/d

Demoulin DE1 100 n/d

DE2 100 n/d

DE3 100 n/d

Chanic CHA1 100 n/d

CHA2 100 n/d

CHA3 100 n/d

Kinsuka Mimosa KIM1 93.2 n/d

KIM2 26.2 53.7 ± 8.0

KMI3 100 n/d

n/d: Growth Inhibition not determined if the mortality rate is more than 30% according to the manufacturer's recommendation.

88

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Demoulin and Chanic. Based on the results of sediment toxic metal contents, sediment quality guidelines, Enrichment factor and geoaccumulation index, it can be concluded that the sediments from these zones can be considered as highly polluted by toxic metals. Sediments present potential risks to the aquatic living organisms as illustrated by mortality and growth inhibition rate from the ecotoxicological test. Consequently, the sediments can present harmful effects on aquatic organisms and pose the serious human health risk through frequent exposure pathways. The main ecological and human risks are the remobilization of the toxic metals from sediment to water column, infiltration into the groundwater, their accumulation in aquatic living organisms or vegetables and their return to the human food chain. The pollution may be explained by several human activities including commercial and industrial settlements, presence of uncontrolled landfills on the riverbanks, as well as by the construction of boats for the regular navigation along the investigated section. The results from this study are in agreement with previous studies and pointed out toxic metal pollution in Congo Rivers and its tributaries [9,11,12,14,15]. Therefore, to limit the input sources and implement management strategies to reduce the heavy metal contamination in the river aquatic ecosystems would highly be recommended in DRC. This investigation provides not only a first baseline information on the contamination of Congo River in the investigated section, but also represents useful tools incorporated to evaluate sediment quality in the river receiving systems which can be applied to similar aquatic environments.

Compliance with ethical standards

We conﬁrm that theﬁeld studies did not involve endangered and protected species. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Credit author statement

GMK, CKM, JK and JWP conceived and designed research; HKM, DMM.

Al S and GNN performed research: sampling and laboratory analysis; All authors analysed data and wrote the paper; All authors have read, corrected and approved the manuscript before submission.

Declaration of Competing Interest The authors declare no conﬂict of interest.

Acknowledgments

We are grateful toﬁnancial support from the Swiss National Science Foundation (grant n° IZSEZO_188357 / 1) and the association of the market gardeners of Kinshasa forﬁeld collaboration. This research presents the collaboration between University of Geneva (Department F. A. Forel), Univer- sity of Kinshasa and Pedagogic National University of Congo (The Democratic Republic of the Congo).

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