Introduction

Environmental studies in Rwanda have reported that rivers: Mpazi, Nyabarongo, Rusine and Nyabugogo are being continuously polluted by anthropomorphic contributions [1]. Nyabugogo river pour its water into Lake Victoria and the pollution of this lake is now rated among the top ten worldwide [2]. The increasing load of contaminants has greatly deteriorated the quality of water and fish caught in Lake Victoria [3]. The presence of toxic heavy metals in water and fish poses health risks such as development of cancer, renal failure, liver damage, cardiovascular diseases and eventually death [4].

As a contribution to environmental monitoring and public health, the current study investigated the physicochemical profile of water, heavy metal content of Protopterus annectens and estimated the health risks associated with use of water and consumption of P. annectens from Nyabarongo and Nyabugogo rivers. The results were compared with reports from previous studies.

Main text

Method

The current study was done in Nyabarongo and Nyabugogo rivers of Rwanda. The apparatus were those previously used [5, 6]. Study approval was granted by Department of Chemistry, College of Science and Technology, University of Rwanda (Approval No. 213000076).

Sampling and analysis

Samples were taken from Ruliba station in Kigali (1° 58′ 37″ S and 30° 0′ 50″ E) and Kirinda bridge in Karongi district (204° 4″ S and 290° 20′ 46″ E) on Nyabarongo river. On Nyabugogo river, samples were taken from Giticyinyoni (10° 55′ 22″ S and 300° 2′ 52″ E). Water samples (n =1 for each site) were obtained in triplicate between April 2019 and May 2019 (10:00 to 11: 00 am, Central African Time) as described by Omara et al. [5]. Fish (6.2 to 8.1 cm; 700–903 g) were caught in triplicate from Nyabarongo river (n =3 for each site), identified and prepared for analysis as described before [7].

Temperature, pH and electrical conductivity of water samples were determined on-site [6]. Total, ammoniacal, nitrite and nitrate nitrogen, sulphate and phosphate contents of water samples were determined following APHA method [8]. Iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), chromium (Cr), cadmium and lead (Pb) in water samples were quantified using HACH DR/2500 spectrophotometer. Fish samples were analyzed for heavy metals using a Varian AA240 atomic absorption spectrometer and results in mg/L were converted to mg/kg [7].

Quality control was performed with spiked samples analyzed once for every 10 fish samples. Recovery percentages ranged from 97.6 to 102.5%. Blanks were determined throughout the analyses and used to correct concentrations obtained. Samples were analyzed in triplicate.

Human health risk assessment

Average daily doses (mg/L/day) were calculated for adults (as the general population) and children (as a sensitive group) to estimate human exposure through direct ingestion (ADDIng) and dermal contact (ADDderm) with water (Eqs. 1, 2). Estimated daily intake (EDI, mg/kg/day) for fish was calculated as described elsewhere (Eq. 3) [5, 7, 9].

$${\text{ADD}}_{\text{Ing}} = \frac{{C_{hm} \times W_{ir} \times E_{d} \times E_{f} }}{{W_{ab} \times T_{aet} }}$$
(1)
$${\text{ADD}}_{\text{derm}} = \frac{{C_{hm} \times S_{A} \times AF \times E_{d} \times E_{f} }}{{W_{ab} \times T_{aet} }}$$
(2)
$${\text{EDI = }}\frac{{E_{f} \times E_{d} \times F_{ir} \times C_{f} \times C_{hm} }}{{W_{ab} \times T_{aet} }}$$
(3)

where Chm= metal concentration in water or fish, SA is exposed area = 4350 and 2800 cm2 for adults and children [9], Wir is water ingestion rate = 21.0 and 1.8 L/day for adults and children [10], Ed is the exposure duration = 67 years [11], Ef is the exposure frequency = 365 days/year, AF is skin adherence factor = 0.7 and 0.2 mg/cm2/day for adults and children, Fir is fresh fish ingestion = 48 g/person/day, Cf is conversion factor for fresh to dry weight for fish = 0.208, Wab is average body weight = 15 kg and 60 kg for children and adults, Taet is the average exposure time = Ed × Ef [5, 12].

Health risk index, the total risk of a non-carcinogenic element was evaluated using target hazard quotient (THQ) (Eq. 4) [5, 7, 13].

$${\text{THQ}} = \frac{ADD}{{R_{f} D}}\,\,{\text{or }}\,{\text{THQ}} = \frac{EDI}{{R_{f} D}}$$
(4)

where RfD is the reference dose. Since exposure to two or more toxicants results in additive and/or interactive effects, the total THQ was treated as the sum of the individual metal THQs. Carcinogenic risk, which is the product of ADDIng, ADDderm or EDI and the ingestion cancer slope factor was calculated for Cr, Cd and Pb.

Statistical analysis

Analytical data were presented as means ± standard deviations. One way ANOVA was done followed by Tukey test (p < 0.05) using Sigma plot software (v14, Systat software Inc., USA).

Results

Results of water and fish analyses are given in Tables 1 and 2. Toxicity indices used for risk assessments are given in Table 3, Additional files 1, 2, 3 Tables S1, S2 and S3.

Table 1 Hydrochemical properties of water from Nyabarongo and Nyabugogo rivers, Rwanda
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Table 2 Heavy metal concentrations in P. annectens from Nyabarongo river in comparison with other global studies
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Table 3 Estimated daily doses through dermal contact and ingestion of water and consumption of P. annectens
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Discussion

Water quality

Nearly all the water quality parameters were within WHO permissible limits [14]. Temperatures were normal while the pH of samples were slightly alkaline, comparable to 7.8 reported by Usanzineza et al. [21] in Lake Muhazi. Nhapi et al. [1] reported a pH of 7.24 ± 0.18 at Rwesero, the point after which Nyabugogo river flows out of Lake Muhazi. Alkaline pH in Rwandese rivers were reported to be due to alkaline wastes from UTEXRWA industry in Kigali [22]. Overall, the pH values recorded were within WHO limits [14]. It should be noted that even within acceptable pH ranges, slightly high pH causes water to have a slippery feel whereas slightly low pH may cause water to have a bitter or metallic taste [6].

Conductivity and total dissolved solids recorded were lower than those previously reported for water from Nyabugogo marshland, Nyabugogo, Rwanzekuma and Ruganwa rivers [21]. High total dissolved solids affect the aesthetic quality of water, interferes with washing operations and can be corrosive to plumbing fixtures. Total Kjeldahl nitrogen were also low; only water from Giticyinyoni had total nitrogen higher than the maximum acceptable limit. Similarly, nitrite, nitrate and ammoniacal nitrogen levels were low. Insignificant differences in nitrite levels in water from Nyabugogo river stretch have been reported previously [1]. Presence of nitrites indicate oxidation which is influenced by environmental factors such as re-aeration, photosynthesis and presence of ammonium. Nitrate levels on the other hand were lower than those previously reported for Nyabugogo river [1]. Overall, total nitrogen in levels above acceptable limits in water may lead to low levels of dissolved oxygen affecting aquatic organisms. Thus, there is no potential contamination of water from sewage discharges and agronomic activities at the studied stations of Nyabarongo and Nyabugogo rivers.

High phosphate levels in a river indicates pollution from sewage discharges or agricultural activities [6]. In this study, low levels of phosphates and sulphates were recorded, corroborating a previous report [1] which speculated that high levels of sulphates in some sites of Nyabugogo river could be due to pollution by wastes from UTEXRWA factory.

For heavy metals, Fe, Mn and Pb were in concentrations higher than WHO limits. The high levels of Fe recorded agreed with Usanzineza et al. [21] who reported 0.756 ± 0.734 mg/L of Fe in Lake Muhazi. Nhapi et al. [1] speculated that high Fe levels in this area could be due to geological composition of its red soils and this is supported by a study [23] which reported 2896 mg/kg of Fe in soils from Nyabugogo downstream. For Pb, Nhapi et al. [1] hinted that the high levels could be due to alkaline chemicals from Nyabugogo tannery. Occurrence of Pb in the rivers could also be due to use of leaded petrol and dumping of dead lead accumulators into rivers [7]. Lead is a toxic non-essential metal that interferes with essential trace metals such as calcium and Zn. High Mn levels recorded in this study is corroborated by a preceding study which recorded 28.85 ± 23.53 mg/L of Mn in Nyabarongo stream [1]. Thus, the high Mn levels could be due to the surrounding geological formation and disturbance of soils which causes discharge of manganese-rich runoffs [1].

Heavy metal content of P. annectens muscles

Fish is migratory but heavy metal accumulation in fish is an evidence of exposure to a polluted aquatic environment. High levels of heavy metals were recorded in muscles of P. annectens and the chemical sequence followed was Pb > Mn > Fe > Cr > Zn > Cu > Cd at Ruliba station and Fe > Mn > Pb > Cr > Zn > Cu > Cd at Kirinda bridge (Table 2). All metal concentrations recorded except that of Cu and Cd were above FAO/WHO limits. Fish ingest heavy metals by direct uptake from water or by absorption through its organs [5, 7]. Chronic intake however depends on both external and inherent factors. Thus, the high levels of heavy metals recorded in P. annectens in this study could be because it is a piscivorous species [5]. The metal levels reported in P. annectens were higher than those previously reported except [17] which reported a concentration of 211.33 mg/kg for Zn (Table 2). Overall, differences in metal concentrations in P. annectens could be attributed to differences in the heavy metal concentrations of water in the studied parts of Nyabarongo river.

Human health risk assessment

In this study, the estimated average daily doses through ingestion and dermal contact with contaminated water ranged from 7.00 × \(10^{ - 9}\) to 5.86 × \(10^{ - 5}\) mg/L/day for both adults and children (Table 3). All the estimated doses were lower than corresponding reference doses for ingestion and contact with the heavy metals in water, thus no serious health risks can result from contact and consumption of water from the sampled stations of the rivers.

For consumption of P. annectens, EDIs ranged from 4.30 × \(10^{ - 2}\) to 2.88 × \(10^{0}\) mg/kg/day for both children and adults. Most EDIs exceeded individual metal reference doses, implying that there are possible health risks from consumption of P. annectens. For non-carcinogenic risks, THQs were all below 1 for exposure through contact and ingestion of water by both children and adults (Additional file 1: Table S1). However, THQs for Mn and Pb were above 1 for adults while only Zn and Cr had THQ < 1 for children. Thus, consumption of P. annectens from the studied parts of Nyabarongo river may have negative health impacts as TTHQs were greater than 1 in both adults and children.

Carcinogenic risks (CR), defined by US EPA as “the incremental probability of an individual to develop cancer, over a lifetime, as a result of exposure to a potential carcinogen” were estimated for Cr, Cd and Pb using ingestion cancer slope factor [24] (Additional file 2: Table S2). The range of risks borderline by US EPA is 1 × 10−4 to 1 × 10−6 and is unacceptable if the risks are surpassing 1 × 10−4 [25]. Considering intake of Cr, Cd and Pb through ingestion and dermal contact with water, the total cancer risks were below the safety level (1 × 10−4). Thus, there is no carcinogenic risk for both adults and children through ingestion and contact with water from the studied rivers. The CR for consumption of P. annectens contaminated with Cr, Cd and Pb ranged from 1.84 × 100 to 3.38 × 102 for both groups. These values were higher than 1 × 10−4, suggesting that there are potential cancer risks from consumption of P. annectens.

Limitations

In this study, (i) the body weights and daily intakes were not estimated for Rwandese, (ii) ingested dose was considered to be equal to the absorbed dose, (iii) probability variables used were from US EPA guidelines which may not apply to this population, (iv) CR was estimated for Cr, Cd and Pb only because there are no CSF values for the other heavy metals investigated, (v) CSF was considered as a constant for all individuals, but this is known to vary between individuals, and (vi) the health risks were only assessed using metal toxicity in the muscles, but water and fish contain other chemicals from possible exposure routes, and metabolically active organs of P. annectens may contain higher heavy metal concentrations. Thus, the level of health risks may be higher than estimated in this study.