Mercury and lead pollution in rivers in Ghana: geo-accumulation index, contamination factor, and water quality index

Water pollution, any change in the state of water that results in adverse consequences on living organisms when consumed, is a major global issue (Akubugwo et al. 2013; Wirnkor et al. 2018; Hasan et al. 2019). Water can be polluted through anthropogenic or natural means (Shukla et al. 2010; Machiwal & Jha 2014; Singh et al. 2019; Bhardwaj et al. 2020). Anthropogenic sources of pollution include the discharge of partially treated/untreated wastewater, the use of synthetic fertilizers, and improper waste management (Akpoveta et al. 2011; Hussain et al. 2019; Anjum et al. 2021). The determinants of water pollution vary extensively depending on the population growth rate, techniques of waste management, and the local development level (Olajuyigbe et al. 2012). Akubugwo et al. (2013) and Seiyaboh et al. (2016) reported that countries with poor waste management systems and high population growth rates generate more waste compared to those with proper waste management systems and relatively low population growth rates, which can cause water pollution, when contaminants leach from waste dumpsites.

The exponential growth rate and rapid industrialization and urbanization over the past decade in major cities in Ghana have contributed significantly to river quality deterioration in the country's cities. Causes of water pollution in Ghana vary depending on the prevailing economic activities in the various regions. In Kumasi (Ghana's second largest city), major pollution causes include the leaching of synthetic fertilizers, pesticides, dyes, and heavy metals from farms, tanning industry sites, and industrial/electronic waste dumpsites (Agyarko et al. 2010; Oduro et al. 2012). Industrial/electronic waste contains trace amounts of heavy metals such as arsenic, cadmium, cobalt, nickel, mercury, and lead (Singh et al. 2011). In mining communities, the main cause of water pollution includes heavy metal leaching from mine sites (Boateng 2018; Duncan 2020). The presence of heavy metals in water sources (sediments and aqueous phase) increases fish mortality, reduces biodiversity, and can cause carcinogenic and neurotoxic effects in humans when the water is ingested (Wang et al. 2012; Ansah et al. 2018; Opoku et al. 2020). Public health may be at risk when heavy metals bioaccumulate in livestock that are later consumed by humans. Ingestion of contaminated water can also cause typhoid, polio, hepatitis, dysentery, diarrhea, and cholera in humans (Levallois & Villanueva 2019; Lin et al. 2022).

Several studies have been made in the levels of heavy metals and nutrients in major rivers – e.g., the Pra, Ankobra, Tano, and Densu Rivers – in Ghana (Oduro et al. 2012; Awuah 2016; Ansah et al. 2018; Duncan et al. 2018). Awuah (2016) assessed the levels of mercury, lead, and arsenic in the Tano and Ankobra rivers and concluded that the concentration of heavy metals was in the order of mercury > arsenic > lead. In Duncan et al. (2018), the concentrations of zinc, arsenic, iron, and manganese in the Pra River showed low geo-accumulation indices (I_geo), indicating low to moderate contamination with these species. Bessah et al. (2021) assessed surface waters and pollution impacts in southern Ghana, mainly focusing on 25 rivers in the Pra River Basin. They reported that the concentration of lead exceeded the WHO recommendation in about 30% of the rivers assessed, and that concentrations of iron, mercury, copper, and arsenic were above the permissible levels for farm irrigation, particularly, in mining communities along the river. They recommended that a more efficient and effective land-use policy be formulated to minimize water quality degradation in the basin. Ansah et al. (2018) established the concentrations of iron, manganese, cadmium, copper, nickel, zinc, and lead in the Weija Reservoir. Obiri-Yeboah et al. (2021) assessed the potential health effect of mercury and lead and the influence of artisanal mining activities in the Bonsa River in Tarkwa. A comparative assessment of heavy metals in drinking water sources in northern Ghana was evaluated by Cobbina et al. (2015). They reported that the concentrations of mercury, zinc, lead, cadmium, and arsenic in water sources in the Nangodi and Tinga areas far exceeded WHO's recommended maximum, and the local water sources were not suitable for drinking. In effect, they have the potential to cause significant health risks among the local people. Kpan et al. (2014) investigated heavy metal pollution in soil and water in towns in Dunkwa-on-Offin District, central Ghana, due to small-scale gold mining. They reported that heavy metal contamination levels have spread beyond control since the concentrations of lead, copper, and mercury all exceed the WHO recommendation. Asiedu et al. (2013) conducted a non-cancer, human health risk assessment from exposure to copper, mercury, lead, and cadmium in surface water and groundwater in Konongo-Odumasi Municipality, Ghana, and found that children and adults were at risk of non-cancer health diseases if they consumed groundwater in the area. However, surface water consumption does not expose them to such agents, because the hazard quotient (defined as the ratio of the average daily dose (ADD) of exposure to a substance to the reference dose (RfD) which is the daily dose that enables an exposed individual to sustain some degree of exposure over a certain period of time without experiencing significant harmful effects (Asiedu et al. 2013)) in the area's groundwater was greater than 1, whereas in the surface water it was below 1.

The water quality of rivers in Kumasi has not been extensively investigated, and the collective impact of contaminants on river water quality there is not known. This makes it difficult to determine the extent to which the ingestion of contaminated water directly affects human health.

In this study, mercury and lead pollution in the Wiwi and Aboabo Rivers in Kumasi were evaluated through the determination of contamination factor (CF), I_geo, and water quality index (WQI). The Wiwi River was selected because of the prevalent use of synthetic fertilizers along it by farmers and its use to irrigate crops. Ahmad et al. (2019) and De Santiago-Martín et al. (2020) have reported that crops absorb contaminants from water used during irrigation, potentially posing risks to human health when consumed. The Aboabo River was selected because of the tanning and livestock farming activities carried out along its banks, as well as the electronic dumpsite about 10 m away from it. This study (1) determined the concentrations of mercury and lead in sediment and water in the rivers; (2) evaluated the CF and I_geo of those metals in the sediment; and (3) established the WQIs of the rivers with respect to lead and mercury concentrations.

Water (500 mL) and sediment samples were taken from communities along the Aboabo – e.g., Moshie Zongo, Dichemso, Anloga, Asokwa – and Wiwi Rivers – e.g., Wiwiso, KNUST, Gyinyase, and Atonsu. In each community, four samples were taken (sampling sites within communities were 1 km apart), and the average lead and mercury concentrations were determined. All 32 samples were collected over 3 months (June to August 2019). The samples were labeled and transported on ice (4 °C) to the KNUST Chemistry Laboratory.

Both water and sediment samples were subjected to wet digestion before atomic absorption spectrometry (AAS) (SHIMADZU AA7000 Series, Japan) to determine lead concentrations. Water sample preparation for lead analysis involved adding 10 mL of HNO₃ (5 M) to 50 mL of each sample, swirling gently and heating for about 30 min before cooling to room temperature (25 °C) (Azanu & Voegborlo 2014).

Sediment samples were dried for 7 days, then ground and sieved before 1 mL of distilled water was added to 1 g of sample, followed by 4 mL of each HNO₃ (5 M) and HClO₄ (4 M). The mixture was homogenized, and 5 mL of H₂SO₄ (6 M) was added before boiling at 200 °C for about 45 min (Mwegoha 2008).

The sediment and water digestion supernatants were filtered (0.45 μm) into 50 mL flasks for AAS lead analysis. A hollow cathode lamp (Akoto et al. 2016) was used to determine Pb concentrations. AAS calibration depended on a linear five-point calibration curve for which the calibration co-efficient was higher than 0.999 (Amankwaa et al. 2020).

A cold vapor AAS/hydride generator (Pyro-915⁺, Alumex, Canada) was used to determine sediment mercury concentrations; hence, the sediment samples for mercury analysis were not digested. The water samples for mercury analysis were digested, however, and determined by AAS. Wet digestion of the water samples for mercury analysis involved adding H₂SO₄ (5 mL, 6 M) and HNO₃ (2.5 mL, 5 M) to 50 mL of each sample in a 300 mL Erlenmeyer bottle, followed by 15 mL of KMnO₄ (4 M). The mixture was homogenized and allowed to stand for about 15 min (Azanu & Voegborlo 2014) before K₂S₂O₈ (8 mL, 5 M) was added and the mixture was heated for 2 h (95 °C) before cooling at ambient temperature. Excess KMnO₄ was removed by adding 5 mL of hydroxylamine. The water digestion supernatants were filtered (0.45 μm) into 50 mL flasks for AAS analysis. A blank solution was prepared for AAS analysis using the same process as wet digestion. For the sediment, the mercury concentration was determined using the hydride generator. Analyses were done in duplicate and the values were analyzed statistically.

The water quality classification based on the WQI values is shown in Table 3 (Yidana et al. 2008).

The validity of the results was obtained by replicating the treatments two times in order to obtain similar results. The mean of the replicated results was calculated in order to facilitate the interpretation of the results and comparison with the literature.

The higher concentration of mercury in the sediment found in the Aboabo River compared to that in water sampled from it could be attributed to adsorption (Akoto et al. 2021), and this is thought to be likely in relation to both rivers.

The calculated WQI values that depict the potability of the rivers are shown in Table 4. The water quality in the Gyinyase area can be attributed to the presence of water hyacinth in the Wiwi River in that area. The water hyacinth, in this case, serves as phytoremediation, absorbing the heavy metal content (Mwegoha 2008).

The degree of mercury and lead pollution in two major rivers in Kumasi was evaluated in this study. The evaluation was done by determining the mean concentrations of mercury and lead in water and sediment in the rivers. The I_geo, CF, and WQI were also determined to further investigate the extent of pollution of the rivers by mercury and lead. The mean concentrations of mercury were found to be low in both rivers as compared to lead. The assessment of the I_geo showed that mercury was more accumulated in the sediment samples as compared to lead. Moreover, the geo-accumulation of mercury was highest in the Aboabo River in the Anloga and Asokwa areas. The CFs of mercury and lead were also high in the Aboabo River, particularly, in the Anloga area. It was therefore concluded that the anthropogenic activities (especially dumping, collection, and processing of electronic waste) carried out along the Aboabo River largely contribute to its pollution. Overall, the Wiwi River was also found to be unsuitable for drinking with the exception of the water in the Gyinyase area (in terms of the heavy metals assessed) due to the presence of water hyacinth. However, it is indisputable that further assessment of water in the Gyinyase area in terms of biological components could show otherwise.

We acknowledge the support of Profs. Gideon Darko and Osei Akoto.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.