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THE REPUBLIC OF UGANDA

MINISTRY OF WATER AND ENVIRONMENT

STATEMENT ON THE CAUSES OF THE BROWNISH CLOUD, FOUL

ODOR AND OILY FILM AND WHITE CREAM ON LAKE BUNYONYI AND PROPOSED ACTIONS

02ND SEPTEMBER 2025

1. Background

Lake Bunyonyi is located in southwestern Uganda, situated between the Districts of Kisoro and Rubanda in the Ruhezamyenda Catchment. It has an average depth of 39m and a reported maximum depth of 900m. With 29 islands and terraced hills surrounding it, the lake is a unique ecological and tourism destination. However, the growing environmental pressures resulting in siltation and pollution exacerbated by climate change threaten the lake’s fragile ecosystem. Over the last few weeks, waters of Lake Bunyonyi have suddenly turned brownish and begun emitting a strong foul odour, with an oily film and white cream floating on the water. These changes occurring on the lake have prompted the Ministry of Water and Environment working with its Agencies to issue a statement about the causes of the brownish cloud, foul odour, oily film, and white cream on the lakes, as well as the required actions to address these issues.

1. Field observations and interviews with the people

The Ministry of Water and Environment working with its Agencies, undertook a rapid assessment involving field observations and key informant interviews, and water quality sampling and analysis to get an understanding of the causes of the problem and actions to be taken to address the problem.

According to the locals, Lake Bunyonyi has been experiencing a change in the color of the water whenever it rains. This would typically take a maximum of 4 days, but the situation would eventually return to normal. However, in the last three weeks, there have been heavy rains in the area that led to a change in the colour of the lake water, which has persisted for over three weeks.

Most households in the area, including those in Kabale town and its surrounding trading centres, rely on Lake Bunyonyi for their domestic water needs. The persistent brown colour in Lake Bunyonyi has affected water treatment to the extent that National Water and Sewerage Corporation (NWSC) could not achieve the required standard for colour.. Whereas the final water from NWSC complied with the bacteriological standard, the failure to meet the colour standard i.e. aesthetic quality made the public to perceive the

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water to be unfit for human consumption. Some of the consumers of the NWSC supply have now resorted to finding water from other sources, which are not easily accessible.

Investigations revealed that 46 hotels operate on the shores of the lake. Although they have septic tanks and manage their wastes, there could be possibilities of some of them discharging wastewater and sewage into the lake.

Similarly, within the surroundings of the lake, there is the Harutindo Landing Centre market, which operates twice a week, attracting many people. The absence of adequate sanitation facilities at this location could be leading to waste disposal into the lake.

Based on interviews with the community and field observations, there was no evidence to suggest that there was volcanic activity at the bottom of the lake that could have led to the brown colour as earlier reported in the media. A volcanic activity would have caused tremors which would have been felt by the people and would have also caused some water turbulence and possible flooding around the lake.

3.0 Water quality analysis results

Water quality sampling was undertaken around the lake. In-situ analysis was conducted for several parameters, including Dissolved Oxygen, Temperature, and Electrical Conductivity. Samples were also collected for detailed laboratory analysis of other parameters of concern, such as Biological Oxygen Demand (BOD), Chemical Oxygen Demand (COD), Total Nitrates (TN), and Total Phosphorus (TP), among others.

The mean Dissolved Oxygen (DO) level of 1.60mg/L is alarmingly low and indicates a high risk of eutrophication, where nutrient enrichment leads to excessive plant growth and subsequent oxygen depletion. Levels this low are considered hypoxic, where most fish species cannot survive. All the samples collected have DO levels well below the recommended minimum values.

The average turbidity of 16.75 NTU and colour of 57.25 Pt-Co of the lake water exceed the normal values recorded. The high turbidity suggests an influx of suspended solids, possibly resulting from soil erosion caused by human activities in the catchment area.

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The brown colour could also be caused by dissolved organic matter from decaying vegetation, especially water hyacinth. The combined effect of high turbidity and colour reduces light penetration, which in turn affects photosynthesis and the entire aquatic food web. The turbidity levels are particularly high at the lake's deepest point (32NTU) and at Leprosy Island (19NTU).

The average pH of 6.51 falls within the optimal range of 6.0 to 8.0, suggesting a healthy balance between acidity and alkalinity. Temperature values for all the samples are within the acceptable range of 10−27oC, which is typical for a lake in this climate. The average Total Dissolved Solids (TDS) of 152.16 mg/L is well below the WHO guideline of 1000 mg/L, indicating a low level of dissolved inorganic salts.

According to NWSC, the color of the raw water at the abstraction point in Lake Bunyonyi normally ranges between 30 and 45PtCo during the dry season and between 80 and 100PtCo in the rainy season. With the recent changes in the lake the colour had gone up to 130 PtCo. Despite the challenges with brown color of the water which is making water treatment difficult, the NWSC is making all efforts to ensure that water pumped from Lake Bunyonyi is properly treated and it is safe for human consumption.

4.0 Causes of the brownish cloud, foul odor and oily film and white cream on the lake

Field findings so far indicate that the brownish cloud, foul odour and oily film and white cream on the lake are caused by the following factors:

a) Lakes often undergo a natural process of water turnover, where the deeper, colder water mixes with the warmer water at the surface, leading to increased turbidity. This is often ignited by heavy rainfall events that lead to sudden temperature variations. The increased temperatures in the dry season and heavy intense rains in the wet season increase the frequency of lake turnover not only in Bunyonyi but also in other lake systems, such as Victoria, Edward, Albert and George.

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2. The lake's location in the valley with steep slopes makes it susceptible to runoff from agricultural farms and numerous settlements in the surrounding bare hills. Stone quarrying around the lake as well as the ongoing iron ore mining within the catchment of the lake could also be contributing to the siltation of the lake and the brown color. This is exacerbated by the degraded shorelines of the lake, which result in direct surface runoff entering the water body. This contributes significantly to the siltation of the lake and the resultant brown colour of the water.

2. Poor waste management from the market, car washing bays and other establishments around the lake is contributing to a decline in water quality, resulting in foul odours, oily films, and white cream on the lake. Water quality measurements revealed that oxygen concentrations in the lake are low and pose serious threat to the lake's ecosystem and its potential as a source of safe water.

2. Considering that Lake Bunyonyi experiences a change in water colour whenever it rains, lasting for a few days before the situation returns to normal, the current brown colour of the water all over the lake, which has lasted for about three weeks, could be due to the mixing of sediments that have settled at the bottoms of the lake with those coming into the lake from the surrounding hills due to runoff. The brown color observed all over the lake implies that dissolved organic matter in the water has reached a critical level.

4.0 Required actions to address the problems

The Ministry of Water and Environment and its stakeholders have embarked on more detailed field investigations and assessment to further confirm the above observations and findings. The results will be available by end of September 2025 after which an updated statement will be issued. However, it is clear that several actions are needed in the short, medium and long term to address the problems.

The Ministry of Water and Environment developed the Ruhezamyenda Catchment Management  Plan  in  2015  and  has  been  implementing  remedial  catchment

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management measures on a small scale. The remedial catchment management measures, including soil and water conservation, tree growing, and providing alternative income-generating activities to the riparian communities, need to be scaled up.

The proposed short, medium and long-term actions will be undertaken by the Ministry of Water and Environment working with the National Environment Management Authority, National Water and Sewerage Corporation, various stakeholders and the relevant local government departments as follows:

1. Short term actions (within 1 year)

Enforcement of environment and water resources regulations

• Undertake a comprehensive assessment of the compliance level of the various establishments around the lake, based on which corrective actions will be taken.
• Map the buffer zone along the shoreline for protection from encroachment

Undertake regular inspections to ensure compliance to environment and water resources regulations failure of which penalties for non-compliance will be imposed.

Public Awareness Campaigns

• Educate the local community on the link between their activities (e.g., waste disposal, farming near the lake) and the health of the lake.
• Build capacity of the communities to empower them to be stewards of the lake's environment.

Improving technology for treatment of the lake water

• Given the high turbidity and color, NWSC is already undertaking improvements in their technology for treatment of the lake water. The filtration and chlorination treatment process is being enhanced to ensure that the final water meets the required standards.

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b) Medium to long term actions (1 to 5 years) Enforcement of environment and water resources regulations

• Demarcate the buffer zone along the shoreline and protect it from encroachment

Mandatory Catchment Protection

• Scale up protection of the catchment around the lake to prevent soil erosion and agricultural runoff into the lake. The actions will include among others promotion of sustainable farming practices, establishment of bench terraces, construction of soil and water harvesting structures, and tree planting, among other initiatives.
• Provide alternative sources of income to the upstream communities to give them incentives for the protection of the catchment

Improve Public Sanitation Infrastructure

• Collaborate with local governments and other stakeholders to establish proper sanitation and waste management facilities at the Harutindo Landing Center market and other community gathering points to prevent direct discharge of waste into the lake.

By implementing the above measures, the ecological balance of Lake Bunyonyi shall be restored thus ensuring its sustainability for both aquatic life and the local community.

Ministry of Water and Environment