Kombukuro Pond Park next to the Kashiwanoha Campus includes Imabukuro Pond and Benten Pond, which are supported by cold spring water, and cool-temperate vegetation such as Japanese bushberry and numagaya, which are not found in the surrounding area. However, due to the decline in recharge capacity of the recharge areas of Kombukuro-ike and Benten-ike, the amount of spring water has decreased, affecting the cool-temperate vegetation. Currently, rainwater from the urban area is channeled through a waterway laid out within the park, passed through a septic tank, and discharged into Kombukuro Pond, but the septic tank has deteriorated and is no longer functioning. Therefore, we decided to propose a purification method using a natural approach that does not rely on septic tanks.