Mercury and Lead Contamination in Three Fish Species and Sediments from Lake Rukwa and Catchment Areas in Tanzania.

Background
Mining activity in the catchment area of Tanzania's Lake Rukwa is suspected of adding to the lake and connected rivers' heavy metal load. There has been no study done, however, on the levels of mercury (Hg) and lead (Pb) in lake sediment and fish muscle, and what the results could mean for human health.


Objectives
This study investigated the concentration of Hg and Pb in lake sediment and in the muscles of African sharptooth catfish (Clarias gariepinus), Lake Rukwa tilapia (Oreochromis rukwaensis) and Singida tilapia (Oreochromis esculentus) from Tanzania's Lake Rukwa and connected rivers.


Methods
Concentrations of Hg and Pb in fish muscle and lake sediment were measured using inductively coupled plasma atomic emission spectroscopy (ICP-AES) and mercury analyzers, respectively.


Results
Levels of Pb and Hg from C. gariepinus ranged between 0.01 to 1.9 μg/g and 0.03 to 0.33 μg/g, respectively. Pb and Hg in O. esculentus varied between 0.02 to 1.4 μg/g and <0.01 to 0.29 μg/g, respectively. Pb and Hg levels in O. rukwaensis ranged from 0.12 to 0.88 μg/g and 0.12 to 0.88 μg/g, respectively. On the other hand, concentrations of Pb and Hg in the sediment samples ranged between 0.02 to 16.23 μg/g and from 0.01 to 1.43 μg/g, respectively. Concentrations of Hg in the muscles of C. gariepinus and O. esculentus were above World Health Organization (WHO) permissible limits, indicating that they are not safe for human consumption. Concentrations of Pb in fish muscle samples were below WHO permissible limits and United States Environmental Protection Agency (USAEPA) provisional tolerable weekly intake (PTWI) standards. Furthermore, Hg and Pb in sediment were below the threshold value of Environment Canada and Florida's 'No effect level'.


Conclusions
Although levels of Pb in fish samples and Hg and Pb levels in sediment were below international standards, it is important to consider that fish forms an important source of animal protein for local inhabitants, who are likely to consume more fish than considered by these standards. The study recommends further research on the levels of mercury and lead in humans, especially children and pregnant women.


Introduction
Lake Rukwa is one of the principal freshwater resources of Tanzania, meeting demands for drinking water, transport and fishery activities. Contamination of fresh water with heavy metals and pesticides is a matter of great concern in this region, as it is globally. Pollution of Lake Rukwa and rivers in the catchment area may have devastating effects to public water supplies and aquatic life, and lead to effects to human health. Pollution by heavy metals such as mercury (Hg), lead (Pb) and chromium (Cr), in particular, tend to reduce the quality of ecosystems on which all humans depend for daily life. The lake and its catchment area is known as a biodiversity hotspot as it supports numerous fish species, crocodiles, water birds and hippopotamuses. The ecosystem also provides services like food, recreation, medicine, transport and domestic water to the surrounding communities and beyond. Fish species of commercial importance include Rukwa tilapia (Oreochromis rukwaensis), introduced Singida panning to more diverse methods, including amalgamation and cyanide leaching. The latter methods are thought to be increasing the levels of Hg in the rivers and lake because they involve the use of the metal to extract visible gold and the extraction process in our study area takes place in the vicinity of the Luika and Songwe rivers. Unfortunately, mining areas around Lake Rukwa lack waste treatment systems; untreated waste is discharged directly into the waterways or enters the lake through runoff during the rainy season via a network of streams and rivers. In addition, mining has attracted a large number of people to the area, consequently increasing anthropogenic wastes that contain harmful metals, such as Pb.
Following mining activities in the area, there has been public concern that fish and water from Lake Rukwa may no longer be safe for human or livestock consumption. Most of the surrounding communities get their domestic water supply and animal protein from the lake, rivers and streams. It is for this reason that key information on the concentration of Hg and Pb within the aquatic environment and the potential danger of fish contamination to the public health need to be assessed and quantified.
Hg occurs naturally as a mineral and is widely distributed throughout the environment as a result of both natural and human activities. Inorganic mercury is the most common form of the metal released into the environment by industrial activity. Once in the aquatic environment, part of inorganic mercury is microbiologically converted to methylmercury, which is rapidly absorbed through gills, skin and the digestive tract of fish. Worldwide, gold mining is commonly held to be responsible for much of the mercury contamination of aquatic ecosystems. 2 It has been reported that artisanal gold mining releases up to 1350 tons of Hg to the global environment annually. 3 Furthermore, research has documented that other heavy metals are also highly toxic and persistent in the environment causing serious health hazards in ecosystems and to people. 4-8 Hg, Pb and cadmium (Cd) are especially of concern because they are known to cause diseases globally. These include: Minamata disease (organic mercury poisoning), Itai-itai disease (cadmium poisoning), arsenous acid (H 3 AsO 3 ) poisoning, and air pollution-related asthma. 9 Further studies by Olaifa et al. have shown that fish are particularly affected by these pollutants, thus are widely used to assess the health of aquatic ecosystems. 10 It is clear that high concentrations of Hg in fish can lead to pathological alterations, with subsequent inhibition of metabolic processes, hematological changes and decline in fertility and survival. 11 In Nigeria, an elevated concentration of Hg and Pb in the Aba River was investigated as contamination of shellfish and other aquatic food was a matter of concern to the public health. 8 Also, Jonathan and Maina investigated the accumulation of heavy metal in mudfish (Clarias anguillaris) and African bonytongue (Heterotis niloticus) in Lake Geriyo, Nigeria. 12 Guimarães and Lacerda reported that most of the aquatic organisms, including fish and plants, accumulate metals up to 10 5 of the

Description of the Study Site
This study was conducted in the southern and northern part of Lake Rukwa, Tanzania ( Figure 1). The southern part of the lake includes study areas Songwe River, Luika River, Maleza, and Kambi Potea. The northern and western parts refer to Rungwa and Momba rivers, respectively. The lake is located in southwestern Tanzania, in eastern Africa. It has a length of about 165 km, width of 37 km on the north basin and a maximum width of 48 km near the middle of the lake. 16 The lake covers an area of about 1,000 square miles (2,600 square km) and lies midway between Lake Tanganyika and Lake Nyasa. It has an average depth of 10 feet (3 m). Its drainage basin has an area of 31,000 square miles (80,000 square km) in Tanzania. There is no outlet from the lake and fluctuations in size are caused by the varying inflow of streams and rivers.
There are several large tributaries to the lake. The Lupa, Luika, and Songwe rivers drain the Mbeya Range and flow into the lake from the south, the Rungwa feeds the lake from the north, and the Momba River flows in from the west. There is one rainy season, falling from October to April, and dry conditions are experienced from May to October. Average annual rainfall ranges from about 650 mm in the south of the basin, about 900 mm in the north, to about 2,500 mm in the Ufipa Highlands. 16 In the southern portion of the area, the mean annual temperature is 210°C. 17 The surface water temperature ranges between 200°C to 350°C. 16

Sampling Design
In the lake, 2 sites, namely Maleza and Kambi Potea, were selected. For the Luika, Songwe, Momba and Rungwa rivers, 3 sites each were selected for sample collection. The 1st sampling site was set at the river mouth and the 2nd sampling site was selected levels present in the environment. 13 A number of studies reported that the occurrence of heavy metals in rivers, lakes or any other aquatic environment could change both aquatic species diversity and ecosystem health due to toxicity and accumulative behavior. 14 Consequently, humans can be contaminated by heavy metals that are associated with an aquatic system by consumption of contaminated fish and other aquatic food. 15 Environmental and health impacts resulting from heavy metal require both coordinated action and global responses. However, these responses can only come after been enough data has been collected on the level of heavy metal resulting from anthropogenic activities. Because of the importance of Lake Rukwa to the fishery, tourism and domestic water supply of Tanzania, it is important to investigate and quantify Hg and Pb levels in the aquatic environment and its implication to human health.

Figure 1 -Map of Lake Rukwa Showing Sampling Sites A) Rungwa river; B) Momba river; C) Songwe river; D) Luika river; E) Maleza; F) Kambi Potea
Research 1.5 km upstream of the river mouth. The 3rd sampling site was located further upstream, near mining areas. The selection of the 3rd sampling site at Luika and Songwe rivers were based on the presence of mining in the catchment areas. For the Momba and Rungwa rivers, samples were collected 4.5 km from the 2nd sampling site. In the lake, Kambi Potea and Maleza were selected based on the potential of the areas for fishing and their proximity to the mouths of the Luika and Songwe rivers ( Figure 1) were collected using beach seine and gill nets. The fishes were dissected and care was taken to avoid external contamination of the samples. A sterilized rust-free stainless steel kit was used to dissect the fish. Nonedible parts (fins, scales, intestine) were removed and muscle tissue was stored before air-drying and then dried in an oven at 60°C until constant weight was obtained. The

Mercury and Lead Contamination in Tanzania
dried samples were crushed into fine powder. The resulting fine powder was stored in a dark unit for digestion. Fifty-six samples of sediment were collected using a sediment corer and placed in prelabeled transparent polythene bottles rinsed with 10.0% HNO 3 and kept in an ice chest at <4°C. Sediments were wet-sieved through a 100 μm mesh plastic screen, and the fraction of <100 μm were used for analysis. Samples were frozen at -200°C before elemental analysis. Samples were transported to SEAMIC Chemistry Laboratory in Tanzania for analysis.
In the laboratory, frozen samples of fish muscles were thawed at room temperature and separately oven-dried to constant weight and grounded to powder using mortar and pestle. About 20 g sub samples of muscles tissues were taken and digested in a Teflon cup with concentrated HNO 3 at 105 ± 20°C for 8 hours and frozen before analysis. The powdered samples were digested, and the digests kept in sterile plastic bottles prior to analysis. 18 The concentration of Pb and Hg were determined using ICP-AES and a mercury analyzer, respectively. International standards from the Community Bureau of Reference (CBR) were used to check for the precision and accuracy of analysis.

Data Analysis
Statistical analysis was conducted in a computer program known as GraphPad InStat and STATICA 7. Two-sample t-tests were used to assess variations in the concentration of Hg and Pb in sediment and fish muscle between the sampling sites (P<0.05). Pearson's correlation was used assess the relationship between the concentration of Hg and Pb in fish muscle and sediment. Tables were used for data presentation. In addition, to ascertain the safety and the degree of health risks associated with consuming contaminated fish caught from Lake Rukwa and its catchment areas, results were compared with the permissible limits as set by the World Health Organization (WHO), the United States Environmental Protection Agency (USEPA) provisional tolerable weekly intake (PTWI), and the Food and Agriculture Organization and the World Health Organization (FAO/ WHO Codex), among other national and international standards. 19

Results
The results of Pb and Hg concentrations (µg/g) measured in muscles of C. gariepinus, O. esculentus and O. rukwaensis are shown in Table 1.

Lead (Pb) contamination in C. gariepinus
The value of Pb in C. gariepinus ranged between 0.01 to 1.9 µg/g in both dry and rain seasons. The highest (1.88±0.009 µg/g) recorded mean value of Pb was recorded from the Luika River mouth during the dry season. The lowest (0.01±0.001 µg/g) mean values of Pb in the muscles of C. gariepinus were recorded from the mouth of the Rungwa River in the dry season. The lowest (0.5±0.02 µg/g) mean value of Pb from the sites in the lake were recorded from Maleza during the rainy season ( Table 1). The highest recorded value (1.44±0.01 µg/g) of Pb from the lake sites was measured from Kambi Potea. The value of Pb from all areas in the Songwe and Luika rivers were not different from the corresponding value measured from Kambi Potea and the Maleza (P>0.05). This is an indication of an even distribution in Research the levels of metal between the study areas. Further analysis revealed that values of Pb from areas including Maleza, Kambi Potea, Luika and Songwe rivers were significantly higher than values from the Rungwa and Momba Rivers (P<0.05), suggesting that C. gariepinus from the southern part of the lake are largely contaminated with Pb compared to the samples from the northern part. The study did not reveal differences in the value of Pb in the muscles of C. gariepinus between seasons. The estimated value of Pb in the muscles of C. gariepinus from all areas in both seasons was 0.7 6 µg/g.

Research Mercury and Lead Contamination in Tanzania
Permissible Heavy Metal Limit Metal esculentus from the lake and rivers showed a wide variation, between 0.02 to 0.9 µg/g and 0.04 to 1.4 µg/g, respectively. The lowest value of Pb (0.23±0.03 µg/g) in the muscle samples from the rivers was recorded in the rainy season at the middle of the River Songwe (

Lead (Pb) contamination in sediment
Pb concentrations in sediments from the lake and its catchment area showed notable variations ( Table  3). The concentration of Pb in river sediment samples ranged between 0.02 to 16.23 µg/g and from 5.12 to 14.8 µg/g in lake sediment samples. The lowest (0.01±0.0001) value of Pb in the river samples were obtained from the mouth of the Momba River.
The highest (14.53±1.06 µg/g) mean levels of Pb in river samples were measured at the mouth of Luika River ( Table 3). The concentration of Pb in sediment from the Luika

Table 3 -Concentration of Mercury (Hg) and Lead (Pb) in Sediment Collected from Lake Rukwa and Catchment Areas ND-Not Detected
Research and Songwe rivers was higher than the concentration of the same elements measured from the Momba and Rungwa rivers (P<0.05). Also, the lowest (9.83±0.76 µg/g) and highest (13.93±1.03 µg/g) mean concentration of Pb were recorded from Kambi Potea and Maleza, respectively (

Mercury (Hg) contamination in sediment
Mercury concentrations in lake and river sediment samples from the southern part of the lake and its catchment area showed remarkable differences between the sediment from the Rungwa and Momba rivers ( Table 3). Hg concentration in the rivers ranged from 0.01 to 1.43 μg/g ( Table 3). The concentration of Hg in the sediments from the lake ranged between 0.24 to 0.41μg/g at Kambi Potea and Maleza, respectively. The lowest (0.02±0.001 μg/g) mean concentration of Hg in the sediments was recorded from the Rungwa river mouth. The highest (0.32±0.006 μg/g) mean concentrations of Hg in the sediment samples from the rivers were recorded from the mouth of the

Discussion
Mercury and lead accumulation in seafood, water and fish products is a growing global concern that poses severe health risks to the public and ecosystem. In view of the fact that fish have many health benefits as an excellent source of protein, it is imperative to be equally aware of the risks of heavy metals in the environment and the fish we consume. While the health risks of mercury and lead poisoning may perhaps appear like a good reason to abstain from consuming fish, the benefits of eating fish may outweigh many of the dangers. This is because fish is known for its high levels of protein, low in saturated fats, and contains essential nutrients such as omega-3 fatty acids. Furthermore, Docosahexaenoic acid (DHA) is one of the vital fatty acids found in fish oil and is responsible for normal development and functioning of the human brain. 25 Therefore, given the ecological as well as economic and health importance of fish to the communities around Lake Rukwa and beyond, the present study explored various guidelines to ascertain and understand the associated health effects of consuming fish contaminated with mercury and lead, so that wellinformed decisions about the fish we consume can be made.
The present study observed Hg and Pb contamination in the muscles of C. gariepinus, O. esculentus and O. rukwaensis. Levels of contamination varied between species as well as sampling locations. The mean concentrations of heavy metals in C. gariepinus muscles were higher than in O. esculentus and O. rukwaensis samples. Higher levels of lead and mercury in the Luika and Songwe rivers indicate that gold mining in the catchment area may contribute to the higher concentration of heavy metals in fish muscles. Although there is no mining activity in the catchments areas of the Momba and Rungwa Rivers, the results indicate that the small quantities of these heavy metals in fish tissues could be associated with water and sediment transport as well as fish movement. Also, this may also be evidence that heavy metals entering the lake through the Luika and Songwe rivers are distributed to the northern part of the lake through hydrological processes. The present study is similar to that of Jezierska and Witesta which found that various heavy metals accumulate in different amounts in fish. 26 The concentration of Hg in the muscles of C. gariepinus was different from that reported by Ahmed et al. at Assiut Governorate, Egypt. 27 The present study did not provide evidence for seasonal differences in the muscle tissues of fish. Conversely, Ahmed et al. reported seasonal variations in the levels of Hg and Pb, with the highest levels being recorded in summer. 27 Higher levels of mercury and lead in the muscles of C. gariepinus and O. esculentus from rivers may be attributed by the fact that fish in the river habitat are at the top of the food chain, thus tending to accumulate metal from water. 28

Research
The concentration of lead in the muscles of fish samples were below WHO permissible limits for all fish species, indicating that they are safe for human consumption. 19 On the contrary, levels of Hg in the muscles of O. esculentus from the Maleza and Luika rivers and the Songwe River mouth were beyond the WHO permissible limits of 0.14 µg/g. 19 Furthermore, values of mercury from muscles of the same species obtained from Kambi Potea and at the middle site of the Songwe River were very close to the WHO permissible limits 0.14 µg/g, which is an indication that O. esculentus from the 2 areas are not safe for human consumption. 19 Because local communities depend greatly on fish from Lake Rukwa as a source of animal protein, the study assessed the safety of fish using both WHO permissible limits for fish and seafood as well as other international standards such as the FAO/WHO Codex. 19