Tackling Water Pollution, a Sustainable Development Goal (SDGs)

Eutrophication: A Cause of Major Water Quality Issues

Nutrients such as nitrogen and phosphorus are essential to the survival of plants and organisms. In particular, since there is no phosphorus in the atmosphere, plants and organisms have long relied on water that contains phosphorus derived from forests, sharing these resources in a way that maintains a balanced ecosystem. However, Professor Inoue notes that in recent years, this balance has been significantly disturbed due to issues with water quality.

"Many people live on river basins, and excessive volumes of nitrogen and phosphorus flow through the rivers into enclosed bodies of water. Organic matter consumes oxygen and decomposes, and nutrients accumulate at the bottom," says Inoue. "Moreover, the consumption of oxygen results in hypoxia, which is the condition of extremely low oxygen levels. This hypoxia rises to the surface, causing blue discoloration. The high concentration of nutrients causes various problems such as red tides and algal blooms.

"In Japan, the algal bloom at Biwa Lake in the 1970s is particularly well-known. In order to address the issue and decrease the corresponding nitrogen and phosphorus, Shiga Prefecture enacted the Ordinance for the Prevention of Eutrophication of Lake Biwa in 1979, prohibiting the sale of synthetic detergents containing phosphorus along with enforcing drainage regulations on factories. The passage of this regulation led various detergent manufacturers to begin selling non-phosphate detergents, leading to the improvement of water quality nationwide. However, today, algal blooms are not just limited to Lake Biwa. Almost every year, red and blue tides occur in places like Mikawa Bay and Tokyo Bay. And it's not just Japan. Eutrophication is becoming a major environmental issue across the world," says Inoue.

Runoff from Agricultural and Urban Areas is the Cause

Ministry of the Environment regulations limiting runoff load are required in order to manage the runoff of nutrients into rivers. Runoff calculations intended for such regulations conducted in a study of Mikawa Bay revealed that the total phosphorus-generating load (t/day) in 1999 was 4.3% of domestic and industrial wastewater. In 2003, this figure dropped to 2.4%, which, along with nitrogen, marked a significant improvement.

However, measurements across multiple locations in Mikawa Bay showed that nitrogen and total phosphorus levels remain relatively unchanged, with many areas not meeting environmental standards. Why is this?

"Regulations for sewage and industrial water treatment became more stringent, and the prescribed measurement method shows the total load decreasing. However, there appears to be another factor preventing the levels from falling.

"We decided to focus on nutrient salt runoff during rain events. From 2009 to 2011, we conducted a study of the Hamada River, one of the channels flowing into Mikawa Bay. Measurements were taken at a fixed time every month and ten times after it rained. Measurement results showed that while the total phosphorus amount was 0.52% (t/km2/year) when using the prescribed measurement method, the actual value was 3.65%. In other words, phosphorus runoff was seven times the regulation load," explains Professor Inoue.

This was due to the breakdown of runoff sources. Up to that time, total load regulations assumed that approximately 90% of runoff came from discrete sources such as factory and domestic drainage. In fact, total phosphorus from diffuse sources such as agricultural and urban areas accounted for nearly 90% of runoff.

"One factor in this discrepancy is that the measurement method used for total load regulations did not alter the output levels of diffuse sources since it began in 1988. In other words, the idea that strictly regulating factory and domestic drainage would suppress eutrophication was incorrect. In particular, runoff load during rain events is believed to result in a high volume of fertilizer runoff from agricultural areas. It is also possible that livestock waste at small-scale cattle ranches flows into rivers during rain events. Runoff in urban areas is also significant. Eutrophication can only be stopped if runoff from diffuse sources can be reduced," says Inoue.

Finding out Which Particulate Phosphorus is Bioavailable

On the other hand, Professor Inoue says that while current regulations target total phosphorus, not all phosphorus is bioavailable.

"While some dissolved phosphorus in the water can be used by algae for growth, algae can use some particulate phosphorus (granular phosphorus mixed with various substances). As there is a particularly large runoff of particulate phosphorus when it rains, when it comes to eutrophication, it is critical to measure the bioavailable phosphorus. In doing so, it becomes necessary to change the way we capture it, as seen with the 3.65% measurement discovered in our previous survey of Hamada River."

Typically, particulate phosphorus is measured using a shake extraction method that takes up to 17 hours. However, Professor Inoue discovered a simple method that uses ultrasonic to divide and extract the bioavailable phosphorus from the particulate phosphorus in as little as one minute. This method was inspired by separate research, in which ultrasonic was used to extract available phosphorus in soil used by plants.

This method was used in a study of the Umeda River and the Yagyu River flowing through Toyohashi City. While total phosphorus in the Umeda River was higher, the Yagyu River had higher concentrations of bioavailable phosphorus.
"97% of the water in the Umeda River basin comes from farmland. The concentration of total phosphorus is high due to fertilizer runoff from the farmland. However, the load of dissolved phosphorus flowing into the river is not particularly high since much of the fertilizer in the soil is consumed on-site by organisms. The particulate phosphorus, coupled strongly with the iron and calcium contained in high concentrations in the soil, also cannot be used again by organisms. As such, the amount of phosphorus that can be consumed by organisms is relatively low. At the same time, the large load of particulate phosphorus that flows from forests during rain is already being used by plants and organisms, so there is little effect on eutrophication downstream.

"On the other hand, 73% of the water in the Yagyu River comes from urban areas. Since large areas are covered in asphalt, the nutrients flow straight into the rivers. This causes phytoplankton and other organic matter to grow. In other words, the concentration of bioavailable phosphorus is greater.
"It is increasingly clear that existing regulations on total phosphorus are insufficient. Moving forward, I hope to conduct a more detailed study on how organisms actually use phosphorus," says Inoue.
Professor Inoue's initiatives have shown that, despite the tendency to classify phosphorus into one category, there are significant differences in the ratio of bioavailable phosphorus depending on each region's characteristics.
Solving water problems is one of the SDGs adopted by the United Nations, and dealing with eutrophication is an urgent issue. Moving forward, it will be necessary to review the measurement methods of regulation loads and implement policies based on actual on-the-ground conditions.

Reporter's Note

Takanobu Inoue was born in Otsu, Shiga Prefecture. As a high school student, he witnessed issues with Biwa Lake's water quality. Hoping to help, he entered Hokkaido University's Department of Sanitary Engineering. Beginning by focusing on the impact to the ecosystem, he then shifted to a focus on human health, one of the original goals of sanitary engineering.

"Half of the students in the lab are foreign exchange students from Asia. Their home countries are dealing with problems such as mercury poisoning from gold mining and E. coli in drinking water. I am involved in working with the foreign exchange students on related studies.

"This year, we began a study examining the amount of plastic flowing into the ocean in Indonesia. In looking at issues such as how to limit the source for microplastics, it is clear that we have a significant mission in our society," says Inoue. I have kept the same goal since I was a high school student. I am still working every day to improve our water.