The Assessment of Heavy Metals Pollution in the Nyabarongo River Sediments – Case Study: Mageragere Sector

GeoScience Engineering Vol. 69 (2023), No. 2 geoscience.cz pp. 147–159, ISSN 1802-5420 DOI 10.35180/gse-2023-0096 THE ASSESSMENT OF HEAVY METALS POLLUTION IN THE NYABARONGO RIVER SEDIMENTS – CASE STUDY: MAGERAGERE SECTOR Cedrick NSENGIYUMVA , Theophile MUGERWA , Anastase GASHABUKA, Enock BIKORIMANA, Digne Edmond RWABUHUNGU RWATANGABO University of Rwanda, College of Science and Technology, School of Mining and Geology, Department of Geology, 3900 Kigali, Rwanda E-mail: nsenga[email protected] ABSTRACT The accumulation of heavy metals in rivers’ sediments near industrial wastes and mining activities has been a serious issue in developing countries including Rwanda and it has caused different environmental impacts on aquatic and terrestrial lives. Environmental studies have reported heavy metals pollution (lead, manganese, copper, zinc, nickel, iron, and cadmium) in major Rwandese rivers such as Mpazi, Rusine, and Nyabugogo. The increase in heavy metal concentrations in the environment results in abnormal enrichment, which in turn, affects the surrounding environment. This study aimed to investigate the heavy metals in Nyabarongo sediments, Mageragere sector using both X-ray fluorescence (XRF) and Atomic Absorption Spectrometer (AAS) instruments. Sediments samples were collected systematically on four sampling stations and the results showed that the Cadmium (Cd), Lead (Pb), and Manganese (Mn) concentrations among the investigated heavy metals are above the Rwanda Standard Board (RSB) and World Health Organization (WHO) guidelines for soils and sediments, meaning that those metals are the measure pollutants in the sediments of Nyabarongo River and the study has recommended the people around the river to reinforce the erosion control measures and not to use Nyabarongo water for domestic use and animal farming activities because of the concentration of heavy metals it contains that are toxic to animals, plants and micro-organisms. Keywords: Heavy metals; Nyabarongo River; Pollution; River sediments; World Health Organization (WHO). 1 INTRODUCTION Heavy metal accumulation in river sediments originating from industry and mining activities has been a serious issue in developing countries including Rwanda and especially due to the toxicity effects of the metals mentioned [1]. The river sediments tend to accumulate heavy metals and other organic pollutants and therefore the sediment quality serves to be a good indicator of water pollution [2]. Unfortunately, concerning the subject of our study, researchers usually consider only sediments that do not mention the rivers of our concern. The presence of heavy metals in river sediments is of paramount importance because the aquatic environment provides living conditions for many marine creatures including fish and it affects human organs, like kidneys, digestive and nervous systems. Heavy metal sources are both natural and anthropogenic [3]. Mining activities such as milling and grinding operations, concentrating ores, and disposal of tailings from all mining scales (artisanal, small-scale, and large-scale mining) affect the rivers as well as the environment when they are not well managed [4]. There are two different pathways by which heavy metals enter the river sediments and those are point and nonpoint sources. A point source is where the waste containing heavy metals enters directly in rivers through pipes 148 GeoScience Engineering Vol. 69 (2023), No. 2 geoscience.cz pp. 147–159, ISSN 1802-5420 DOI 10.35180/gse-2023-0096 or canals from a known source such as industry, mining site or other known sources while a non-point source, also known as “Polluted runoff”, is a source of which the exact location cannot be easily qualified, like the pollution originating from entire landscape areas [5]. The Nyabarongo River and its tributaries are considered to be an important source of water for agriculture, human consumption, wildlife, and livestock. Even if it is highly useful, the Nyabarongo River water as well as the sediments are subject to different contamination. Environmental studies reported heavy metals pollution (manganese, copper, zinc, nickel, iron, lead, and cadmium) in major Rwandese rivers such as Mpazi, Rusine, and Nyabugogo in the Kigali section [6]. The increase in concentrations of heavy metals in the environment results in abnormal pollution of the soils with those heavy metals at the surface. Pollution of soil with those heavy metals may negatively affect community health concerning digestion, breathing as well as contact with the skin [7]. The Nyabarongo River and its tributaries in the Kigali section are presently becoming a major issue concerning uncontrolled growth of industrial waste as well as tailings from uncontrolled mineral processing in mining areas in and around waterways which increase water turbidity and concentration of heavy metals as is the case for many countries over the world [8], leading to instability of rivers physically, chemically and biologically [9]. Most of the reports and journals have focused on the assessment of water pollution levels in different catchment areas as well as the physicochemical quality of water and the associated health risks. Heavy metals are natural constituents of rivers’ sediments. Nevertheless, they become a problem when they go beyond the standards of being safe mostly due to their toxic effects on human beings, plants, and different animals [1]. The World Health Organization reported 143,000 deaths and 600,000 new cases of child intellectual disabilities [10], 3,000 people have diseases resulting from heavy metals concentration, and 600 of them died in Japan [11]. Most heavy metals present in river sediment come from anthropogenic activities such as mining which pollutes it in processes of milling, grinding, concentrating ores, tailings disposal, and industrial waste as well as geomorphic processes [4]. The study conducted by Omara, et al. [9] listed the heavy metals contained using water samples but they failed to consider sediment samples that are in direct contact with the river, although they provide useful information on toxicity levels. Even if there are no anthropogenic activities that can release heavy metals in rivers, there is also another way like the formation of geochemical anomalies where ground and surface water migrate and transport metallic ions away from ore deposits and sometimes move in rivers [12]. To contribute to environmental monitoring, this study assessed the heavy metal pollution in Mageragere, part of Nyabarongo River (Figure 1) sediments, their level of pollution, and their toxicological effects on people and then suggested possible solutions. Figure 1. Hydrology of the study area / Mageragere (Source: Author) 149 GeoScience Engineering Vol. 69 (2023), No. 2 geoscience.cz pp. 147–159, ISSN 1802-5420 DOI 10.35180/gse-2023-0096 All metals with high densities from 3500 to 7000 mg cm-3 which are toxic and/or poisonous even at low concentrations are referred to as “Heavy metals” [13]. The suitability of the water, which includes the Nyabarongo River and its tributaries, for different uses and its ability to support aquatic life depends on elements including heavy metals like manganese, zinc, copper, iron, nickel, lead, and cadmium [9]. Nyabarongo (Figure 2) is a major and the longest river entirely in Rwanda; it is 351 kilometres (218 mi) in length and covers an area of about 142.62 square kilometres. It is a tributary of the Nile River [14]. Nyabarongo starts its journey at the confluence of the Mwogo and Mbirurume rivers, which are also among the sources of the river Nile, the origin of these two rivers is in Nyungwe Forest. The river water is mainly used for agriculture, bathing as well as cooking. Therefore, it receives waste products containing heavy metals from industries and mining activities [15]. Figure 2. Nyabarongo River at administrative map of Rwanda (Source: Author, data source: RNRA) The study conducted by Mushtaha, et al. [16], on heavy metals contamination in Nile River sediments and its main tributaries including Nyabarongo River, concluded that Iron (Fe) metal with values of concentration ranging from 0.42 to 2.46 mg/l was of high concentration while Molybdenum (Mo) had the lowest one. As population growth is gradually increasing, industrialization, mining activities, and urbanization are also increasing which are responsible for increasing river sediment pollution. Therefore, there is always a gap in carrying out research activities for determining the increase in concentrations of heavy metals and their respective effects by analysing the samples collected from different locations in Kigali city [17]. 150 GeoScience Engineering Vol. 69 (2023), No. 2 geoscience.cz pp. 147–159, ISSN 1802-5420 DOI 10.35180/gse-2023-0096 Figure 3. Google Earth satellite image showing the Nyabarongo River in the study area 2 MATERIALS AND METHODS 2.1 Field method Mageragere area is characterized by elevated mountains, swamps, rivers, and valleys. Geological data were collected using different materials such as administrative and topographic maps, hammers, geological compasses, and GPS receivers. 2.2 Sampling Four river sediment samples were collected at 4 sampling stations (Figure 4) using a hand auger, with a depth range from 25 to 30 cm. Station 1 and Station 4 were placed at the sections where Nyabarongo River enters and ends its course in the Mageragere sector. Station No. 2 and No. 3 were placed before and after the confluence of the Akanyaru River in the Nyabarongo River/Mageragere sector. At every station, after making site descriptions (weather conditions and geomorphic process), the sample was kept in a labelled plastic bottle; geographic coordinates were taken and recorded in the field notebook. 151 GeoScience Engineering Vol. 69 (2023), No. 2 geoscience.cz pp. 147–159, ISSN 1802-5420 DOI 10.35180/gse-2023-0096 Figure 4. Sampling stations at the study area (Data source: RNRA) 2.3 Laboratory Analysis Methods 2.3.1 Sample Conservation After sampling, samples were conserved in a secured and cold place to reduce loss of volatiles and bacteria activities reduction as well as to avoid mixing with other chemicals following the procedures illustrated by Simpson & Batley [18]. 2.3.2 Sample Preparation and Element Analysis After sample conservation, samples were subjected to drying by sun and oven so that moisture as well as organic matter content would be reduced. The sample sizes were reduced to 2 mm in diameter by crushing using a jaw crusher of Ngororero Mining Company and pulverized in the University of Rwanda laboratory following the procedures illustrated by Simpson & Batley [18]. i. XRF Method The X-ray fluorescence (XRF) elements analysis was performed using the X-MET8000 instrument manufactured by Hitachi High-Tech. The sediment samples were dried before XRF scanning. Dry sediments of 50 g were analysed concerning each sample. ii. Flame Atomic Absorption Spectrometer (F AAS) Analysis Flame AAS developed by Agilent was used to analyse the concentration of the following heavy metals Cadmium (Cd), Iron (Fe), Lead (Pb), Manganese (Mn), and Zinc (Zn) present in samples by using Beer Lambert’s law detailed in Filip, et al. [19]. 152 GeoScience Engineering Vol. 69 (2023), No. 2 geoscience.cz pp. 147–159, ISSN 1802-5420 DOI 10.35180/gse-2023-0096 iii. Data Analysis The concentration of heavy metals identified in sediment samples was compared with World Health Organization (WHO) standards for soil and water as well as Rwanda Standard Board (RSB) guidelines. Heavy metals above allowable limits often lead to negative effects on living creatures and the environment. The enrichment factor (EF) defined by Muller [20] was calculated to assess sediment contamination in the Nyabarongo River. This factor can also be used to determine the metals of anthropogenic sources as opposed to those of natural sources [21]. EF is calculated as below: EF = 𝑪𝒏(𝒔𝒂𝒎𝒑𝒍𝒆)/𝑪𝑭𝒆(𝒔𝒂𝒎𝒑𝒍𝒆) 𝑪𝒏 (𝑼𝑪𝑪)/𝑪𝑭𝒆(𝑼𝑪𝑪) (1) where Cn/CFe for the sample is the ratio of the concentration of the element concerned to the reference element in the sediments sample, Cn/CFe for UCC (upper continental crust) background is the ratio of the concentration of the concerned element to that of the reference element in the geochemical background. Many studies have taken different reference elements for normalization based on their natural occurrences. The commonly used reference metals for normalization are found to be iron and aluminium due to their abundance in the earth’s crusts. This study used iron as a reference element to distinguish the sources, whether they were of natural or anthropogenic origin as it has been used by different successful studies [22–24]. Sutherland [25] has divided soil contamination into 5 categories based on EF values (see Table 1). Table 1. Contamination categories based on EF values as shown by Sutherland [25] EF value Contamination category < 2 Deficiency to minimal enrichment 2–5 Moderate enrichment 5–20 Significant enrichment 20–40 Very high enrichment > 40 Extremely high enrichment 3 RESULTS AND DISCUSSION The lithology of the Mageragere sector is generally made up of Quartzite, meta-sandstone (dominant lithology), schist, and laterite to a later degree. The results of heavy metal measurements in river sediment samples were discussed with the references of different previous studies and World Health Organization (WHO) standard guidelines. The results of the analysis are presented by tables and graphs to show linear interdependence between variables. i. XRF Results Four sediment samples were collected at four different stations along the Nyabarongo River. The results of heavy metals identified by XRF(X-MET8000) are in Table 2. Table 2. Heavy metal concentration results by XRF (Concentration in percentages (%)) Name of heavy metals Station1 Station2 Station3 Station4 Cr 0.0688 0.0459 0.0393 0.0452 Mn 0.0561 0.0448 0.0562 0.0443 Fe 4.609 3.7454 4.5015 3.889 Zn 0.0079 0.0089 0.0077 0.0055 153 GeoScience Engineering Vol. 69 (2023), No. 2 geoscience.cz pp. 147–159, ISSN 1802-5420 DOI 10.35180/gse-2023-0096 0,00 10000,00 20000,00 30000,00 40000,00 50000,00 60000,00 station 1 station 2 station 3 station 4 standard Variation of Fe concentration at different stations (mg.kg-1) As ND ND ND ND Pb 0.0054 0.0041 0.0051 0.0032 Th 0.0029 0.0032 0.0042 0.003 ii. Atomic Absorption Spectroscopy (AAS) Results Table 3. Heavy metals concentration results by AAS (Concentrations of heavy metals mg.kg-1) Name of heavy metal Average composition of the upper continental crust (UCC) Station1 Station2 Station3 Station4 WHO standards quality guidelines for sediments Fe 35,000 47,030 38,454 46,015 39,896 50,000 Pb 17 48 45 47 29 35 Cd 0.098 3.25 3.56 3.95 4.22 0.6 Zn 71 81 91 79 59 123 Mn 600 551 438 552 433 300 The results of heavy metal concentration at each station are presented in Tables 2 and 3. a. Iron (Fe) Iron is the second most abundant metal in the Earth’s crust making up 5 per cent which makes the average content of iron in the upper continental crust (UCC), 35,000 mg.kg-1. Elemental iron is rarely found in nature, as the iron ions Fe2+ and Fe3+ readily combine with oxygen and sulphur-containing compounds to form oxides, hydroxides, carbonates, and sulphides. Iron observed in Nyabarongo sediments ranged from 38,454 mg.kg-1 to 47,030 mg.kg-1 which is below the allowable limit of 50,000 mg.kg-1 by World Health Organization standards (see Table 3). The following graph (Figure 5) shows the variation of Iron concentration in sediments of the Nyabarongo River at different sampling stations. The iron concentration found in the analyzed samples shows normal variation as compared to the background. Figure 5. Variation of Iron concentration in sediments of Nyabarongo River 154 GeoScience Engineering Vol. 69 (2023), No. 2 geoscience.cz pp. 147–159, ISSN 1802-5420 DOI 10.35180/gse-2023-0096 b. Lead (Pb) Lead (Pb) is a minor element in the Earth’s continental crust. It is distributed at low concentrations in soils that are not contaminated, river sediments, and rocks. Pb is one of the highly poisonous metals that affect human soft tissues and organs, causing cancer in the kidneys, lungs, or brain, and acts synergistically with other carcinogens. The possible sources of lead include mining, lead-containing gasoline, smelting, and Pb paint [26]. In this study, Lead metal was analyzed using AAS (Atomic Absorption Spectrometer) and the results show the Lead metal in Nyabarongo river sediments ranging from 29.00 mg.kg-1 to 48.00 mg.kg-1 at all stations which lay the above values of WHO standards for soils and sediments. Below is the graph (Figure 6) showing the variation of Lead concentration in the sediments of the Nyabarongo River at all stations. Red colour indicates the values that are above the allowable limit, green indicates the concentrations values below the standards and blue indicates the standard. Figure 6. Variation of Lead concentration in sediments of Nyabarongo River c. Cadmium (Cd) Cadmium (Cd) is considered a toxic element, the results obtained from the Atomic Absorption Spectrometer (AAS) show a higher concentration of cadmium at all stations compared to the WHO standards of sediments and soils. The results range from 3.25 mg.kg-1 to 4.22 mg.kg-1 where 0.6 mg.kg-1 is the maximum allowable limit in sediments. 0 10 20 30 40 50 60 station 1 station 2 station 3 station 4 standard Variations of Pb concentration in sediments at different stations (mg.kg-1) 155 GeoScience Engineering Vol. 69 (2023), No. 2 geoscience.cz pp. 147–159, ISSN 1802-5420 DOI 10.35180/gse-2023-0096 Figure 7. Variation of Cd concentration in sediments of Nyabarongo River d. Zinc (Zn) Zinc naturally occurs in sulphides and as carbonates in many rocks and soils at low concentrations. Zinc gets in sediments in two ways rock weathering, erosion of minerals from rocks and soils, or anthropogenic activities. Zinc is slightly soluble in water. Almost all zinc in water comes from artificial sources like steel production, burning of waste materials, and fertilizer usage containing zinc which may leach into groundwater [27]. The results show that the concentrations of Zinc metal at all stations range from 59.00 mg.kg-1 to 91.00 mg.kg-1. The Nyabarongo sediments are not polluted with Zn because the values lie below the maximum allowable limits of 123 mg.kg-1 by the World Health Organization. Figure 8 shows the variations of Zinc concentrations at all sampling stations. Figure 8. Variation of zinc concentration in sediments of Nyabarongo River 0 0,5 1 1,5 2 2,5 3 3,5 4 4,5 station 1 station 2 station 3 station 4 standard Variations of Cd concentration in sediments at different sampling stations (mg.kg-1) 0 20 40 60 80 100 120 140 station 1 station 2 station 3 station 4 standard Variations of Zn concentration at different sampling stations (mg.kg-1)