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[HGG01-07] The Mechanism of Water Quality Formation in Lake Inle, Myanmar
Keywords:Mechanism of water quality formation, Pollutant load factor method, Water quality simulation, Carbonate-phosphorus binding and sedimentation mechanism, eutrophication, Pollution load
1. Introduction
Lake Inle is located in Shan State in eastern Myanmar and is the country’s second-largest natural lake. In recent years, it has been noted that the aquatic environment is deteriorating from soil runoff caused by forest devastation in the watershed and the excessive cultivation of floating gardens. Wastewater discharged by households in the villages around the lake and by tourists is also considered to be a factor in the deterioration of the aquatic environment.
Many surveys and studies have been conducted in Lake Inle focusing on soil runoff in the watershed, soil inflow into the lake. Although some water quality studies have addressed aquatic environment–related issues such as organic pollution and eutrophication due to increased pollution load from domestic wastewater and the like, these studies have been limited to surveys and there have been no studies that refer to the elucidation of water quality formation mechanisms.
This report introduces the results of a study aimed at clarifying the mechanism of water quality formation in Lake Inle, especially with regard to lake water clarity despite a high concentration of organic matter.
2. Calculation of Pollution Loads from the Watershed
Pollutant loads were estimated using the pollutant load factor method, and the causes of water quality pollution in the lake were evaluated macroscopically. According to the estimation, the pollution loads are 29,000 kg/day of COD, 20,100 kg/day of T-N, and 1,820 kg/day of T-P, and it is clear that non-point sources account for most of it. A comparison of the pollution loads in Lake Inle with that of Lake Biwa and other lakes, all of which are polluted lakes in Japan, shows that Lake Biwa and Lake Inle have the highest COD load in that order, while Lake Inle has the highest T-N and T-P load. The pollutant loads flowing into Lake Inle were found to be equal to or greater than the loads for lakes in Japan, which have remarkable water quality deterioration, and could be a factor in Lake Inle’s water quality deterioration.
3. Water Quality Degradation Areas and Factors
Various field surveys were conducted in Lake Inle and its watershed to elucidate the mechanism of water quality formation for the lake. The results of the lake water quality survey revealed the following points.
Deterioration of water quality was observed near the mouths of the major rivers that flow into the lake. At the mouth of Nant Latt River, almost all the water quality test categories showed high concentrations; it is assumed this is strongly influenced by domestic wastewater and tourism industry discharge (hotels and restaurants) from Nyaung Shwe township. On the other hand, the concentrations of organic matter and total nitrogen in the lake, especially in the northern part, were as high as the concentrations at the mouths of major rivers, while total phosphorus concentrations were low.
4. Mechanism of Water Quality Formation in Lake Inle
Based on the above results, the mechanism of water quality formation in Lake Inle was investigated and verified using a water quality simulation analysis model.
The water quality survey confirmed that eutrophication due to algae growth was suppressed in the northern part of the lake while the organic matter concentration was high. According to previous studies, carbonate concentrations in groundwater and lake water have a negative correlation with phosphorus concentrations, suggesting the existence of a carbonate-phosphorus binding and sedimentation mechanism in which carbonic acid supplied through groundwater binds to inorganic phosphorus in the lake and sedimentation occurs. The water quality survey conducted in this study suggests that this mechanism lowers the phosphorus concentration, limits the growth of phytoplankton, and maintains the clarity of the lake.
Furthermore, the analogy for the above mechanism was verified with a water quality model simulation. Specifically, it was found that lake water quality could not be successfully simulated unless the model set an appropriate sedimentation rate for inorganic phosphorus, which represents the carbonate-phosphorus binding and sedimentation mechanism. The results using water quality simulation analysis indicate that the carbonic acid–phosphorus binding and sedimentation mechanism is contributing to the sedimentation and removal of inorganic phosphorus in the lake, thereby suppressing eutrophication.
Lake Inle is located in Shan State in eastern Myanmar and is the country’s second-largest natural lake. In recent years, it has been noted that the aquatic environment is deteriorating from soil runoff caused by forest devastation in the watershed and the excessive cultivation of floating gardens. Wastewater discharged by households in the villages around the lake and by tourists is also considered to be a factor in the deterioration of the aquatic environment.
Many surveys and studies have been conducted in Lake Inle focusing on soil runoff in the watershed, soil inflow into the lake. Although some water quality studies have addressed aquatic environment–related issues such as organic pollution and eutrophication due to increased pollution load from domestic wastewater and the like, these studies have been limited to surveys and there have been no studies that refer to the elucidation of water quality formation mechanisms.
This report introduces the results of a study aimed at clarifying the mechanism of water quality formation in Lake Inle, especially with regard to lake water clarity despite a high concentration of organic matter.
2. Calculation of Pollution Loads from the Watershed
Pollutant loads were estimated using the pollutant load factor method, and the causes of water quality pollution in the lake were evaluated macroscopically. According to the estimation, the pollution loads are 29,000 kg/day of COD, 20,100 kg/day of T-N, and 1,820 kg/day of T-P, and it is clear that non-point sources account for most of it. A comparison of the pollution loads in Lake Inle with that of Lake Biwa and other lakes, all of which are polluted lakes in Japan, shows that Lake Biwa and Lake Inle have the highest COD load in that order, while Lake Inle has the highest T-N and T-P load. The pollutant loads flowing into Lake Inle were found to be equal to or greater than the loads for lakes in Japan, which have remarkable water quality deterioration, and could be a factor in Lake Inle’s water quality deterioration.
3. Water Quality Degradation Areas and Factors
Various field surveys were conducted in Lake Inle and its watershed to elucidate the mechanism of water quality formation for the lake. The results of the lake water quality survey revealed the following points.
Deterioration of water quality was observed near the mouths of the major rivers that flow into the lake. At the mouth of Nant Latt River, almost all the water quality test categories showed high concentrations; it is assumed this is strongly influenced by domestic wastewater and tourism industry discharge (hotels and restaurants) from Nyaung Shwe township. On the other hand, the concentrations of organic matter and total nitrogen in the lake, especially in the northern part, were as high as the concentrations at the mouths of major rivers, while total phosphorus concentrations were low.
4. Mechanism of Water Quality Formation in Lake Inle
Based on the above results, the mechanism of water quality formation in Lake Inle was investigated and verified using a water quality simulation analysis model.
The water quality survey confirmed that eutrophication due to algae growth was suppressed in the northern part of the lake while the organic matter concentration was high. According to previous studies, carbonate concentrations in groundwater and lake water have a negative correlation with phosphorus concentrations, suggesting the existence of a carbonate-phosphorus binding and sedimentation mechanism in which carbonic acid supplied through groundwater binds to inorganic phosphorus in the lake and sedimentation occurs. The water quality survey conducted in this study suggests that this mechanism lowers the phosphorus concentration, limits the growth of phytoplankton, and maintains the clarity of the lake.
Furthermore, the analogy for the above mechanism was verified with a water quality model simulation. Specifically, it was found that lake water quality could not be successfully simulated unless the model set an appropriate sedimentation rate for inorganic phosphorus, which represents the carbonate-phosphorus binding and sedimentation mechanism. The results using water quality simulation analysis indicate that the carbonic acid–phosphorus binding and sedimentation mechanism is contributing to the sedimentation and removal of inorganic phosphorus in the lake, thereby suppressing eutrophication.