Water Pollution History of Teganuma Swamp - 1996 to 2020s

Water Pollution History of Teganuma Swamp - 1996 to 2020s

Teganuma is a lake that stretches across the cities of Kashiwa, Abiko, Shirai, Inzai, and Kamagaya in Chiba Prefecture, and has long been a symbol of the region. However, with the rapid urbanization and intensification of agriculture after World War II, domestic and agricultural wastewater flowed into the lake, resulting in serious water pollution.

Current Situation in 1996
According to a survey by the Environment Agency, the chemical oxygen demand (COD) of Teganuma Swamp reached 25 mg/L, which was considered the worst water quality among all lakes in Japan. This COD value is more serious than that of Inbauma (21 mg/L) and Lake Sanariko in Shizuoka Prefecture (12 mg/L). The main causes of the pollution were organic substances contained in domestic wastewater and nitrogen compounds and phosphates derived from agricultural wastewater, which caused eutrophication, resulting in the abnormal occurrence of blue-green algae.

At that time, local governments and residents in the Teganuma watershed worked to improve the sewage system and spread the use of home-use combined treatment septic tanks. In addition, Asahi Kasei and Hitachi Zosen offered advanced water purification technology and worked with local residents to find ways to improve the situation, but no drastic improvements were achieved.

Current Status and Challenges in the 2020s
The water quality of Teganuma in the 2020s has been improving, with COD decreasing to 10 mg/L. However, it has yet to achieve the environmental standard of 5 mg/L or less, and is ranked fourth worst among all lakes in Japan. Based on Chiba Prefecture's "Action Plan for the Restoration of Water Circulation in Teganuma (Revised Version)," the following efforts are underway.

Sewerage improvement and rainwater infiltration facilities: Kashiwa City and Abiko City have installed new rainwater infiltration facilities and are promoting appropriate management of domestic wastewater. The installation rate of combined septic tanks is also increasing year by year.
Technical cooperation of companies: Fuji Electric and Ebara Corporation are providing water quality improvement technology, and systems to efficiently remove nitrogen and phosphorus from agricultural wastewater are being introduced.
Citizen participation: The Teganuma Water Environment Conservation Council conducts cleanup activities and environmental education for citizens. In particular, environmental education programs for elementary school students have been well received.

Specific Sources of Pollution and Their Impacts
The main cause of pollution in Teganuma is not only domestic wastewater from the cities of Kashiwa and Abiko, but also excessive use of fertilizers from agricultural activities in the watershed. Nitrogen contained in agricultural wastewater is converted to nitrate nitrogen in the watershed, causing abnormal algal blooms. Phosphate likewise becomes a nutrient for algae, leading to eutrophication.

Future Prospects
Further technological innovation and collaboration throughout the region are needed to improve the water quality of Teganuma. Chiba Prefecture is working with companies and research institutions to promote the use of equipment that increases the efficiency of nitrogen and phosphorus removal. In addition, local governments are strengthening environmental education for local residents and raising awareness to reduce domestic wastewater from households.

The revitalization of Teganuma is an initiative that will greatly contribute to the region's economy, tourism, and environmental sustainability. Going forward, it is expected that local governments, businesses, and citizens will continue to work together to eliminate pollution and restore the ecosystem.