History of Water Pollution in Lake Tega - From 1996 to the 2020s

History of Water Pollution in Lake Tega - From 1996 to the 2020s

Lake Tega, located across Kashiwa City, Abiko City, Shirai City, Inzai City, and Kamekawa City in Chiba Prefecture, has long been a symbol of the region. However, with the rapid urbanization and agricultural intensification after World War II, the inflow of domestic and agricultural wastewater led to severe water pollution.

**1996 Situation**
According to the Environmental Agency survey, the chemical oxygen demand (COD) in Lake Tega reached 25mg/L, marking the worst water quality among all lakes in Japan. This COD value exceeded that of Lake Inbanuma (21mg/L) and Lake Sanaru (12mg/L) in Shizuoka Prefecture. The primary causes of pollution were organic matter from domestic wastewater and nitrogen compounds and phosphates from agricultural runoff, which caused eutrophication and abnormal algae blooms.

At that time, local governments and residents around Lake Tega worked to improve wastewater treatment infrastructure and promoted the use of household septic tanks. Additionally, companies such as Asahi Kasei and Hitachi Zosen provided advanced water purification technologies and cooperated with local communities in seeking improvement measures, but fundamental solutions were not achieved.

**Current Situation and Challenges in the 2020s**
In the 2020s, water quality in Lake Tega has shown improvement, with COD reduced to 10mg/L. However, this level still fails to meet the environmental standard of 5mg/L, ranking Lake Tega fourth among Japan's worst lakes. Under Chiba Prefecture's "Revised Lake Tega Water Cycle Recovery Action Plan," the following initiatives have been undertaken:

- **Wastewater Treatment and Rainwater Infiltration Facilities**: Cities such as Kashiwa and Abiko have implemented new rainwater infiltration facilities and improved the management of domestic wastewater. Additionally, the installation rate of combined treatment septic tanks has been increasing annually.
- **Corporate Technical Support**: Companies like Fuji Electric and Ebara Corporation have provided water treatment technologies, introducing systems that efficiently remove nitrogen and phosphorus from agricultural runoff.
- **Citizen Participation**: The Lake Tega Water Environment Conservation Council conducts cleanup activities and environmental education programs for citizens. Environmental learning programs targeting elementary school students have been particularly well-received.

**Specific Sources of Pollution and Their Impact**
Pollution in Lake Tega is primarily caused by domestic wastewater from Kashiwa City and Abiko City and excessive use of fertilizers in the surrounding agricultural areas. Nitrogen from agricultural runoff transforms into nitrate nitrogen in the water, causing abnormal algae growth. Similarly, phosphates serve as nutrients for algae, further advancing eutrophication.

**Future Prospects**
Improving water quality in Lake Tega requires further technological innovations and collective regional efforts. Chiba Prefecture is collaborating with companies and research institutions to promote equipment that enhances nitrogen and phosphorus removal efficiency. Additionally, municipalities are intensifying environmental education for residents and fostering awareness to reduce domestic wastewater.

The revitalization of Lake Tega contributes significantly to the region's economy, tourism, and environmental sustainability. Moving forward, the joint efforts of municipalities, companies, and citizens are expected to resolve pollution issues and restore the ecosystem.