A 115 millimeter rain event, the second largest in 3 years, is being used to create a dynamic model of water flow and total phosphorus in the Tancheon River, Seoul, South Korea. A modern ecological miracle, the Tancheon was once known for its black color and high load of suspended solids due to the manufacture of charcoal (“tan”) along its banks. Urban revitalization efforts since the 2000’s have made this highly urbanized river clear again, with the only known pollutants being nutrients and fecal coliforms. A healthy, biodiverse, pervious strip of greenbelt has been established along its banks, where pavement, skyscrapers, and malls dominate the remaining urban setting into the foothills of the steep mountains surrounding the 300 km2 watershed. Because this river impacts the daily lives of millions of people, understanding how water quality and quantity change during extreme events is an important part of urban planning and ecosystem management. Using water quality data from a four month period in 2017 and historic data from the Korean Water Resources Management System, a dynamic model, STELLA, was made to understand how total phosphorus concentrations change with rainfall. The recent data are also used to identify the urban sources of nutrients.
Results/Conclusions
The 115 millimeter extreme event occurred within a four month window of water quality sampling that was conducted for multiple purposes. With the stocks, flows, and controls, the Tancheon watershed was constructed in STELLA using historic flow data to see if it would calculate the measured phosphorus levels in the Tancheon before, during, and after the event.
It did. The model output for total phosphorus closely matches the water quality data collected from the four-month sampling window. The model shows a fast rise to peak flow followed by a slow return to normal flow due to the adjacent impervious surfaces along the Tancheon and the pervious natural soils in the outer regions of the watershed. Changes in water chemistry show that rainfall seeping into the pervious natural soils is largely responsible for nutrient loads in the river. This connection is plausible as numerous crops and golf courses that are well fertilized are located in the pervious areas.
With possible sources of nutrients identified, the next step will be to coordinate with city planners a way to reduce nutrient loads to the river to keep the potential for algal blooms to a minimum and continue the two-decades old revitalization efforts.