Nutrient Loading from Roadways during Multiple Precipitation Events in Lake Inbanuma Watershed

Abstract

In Lake Inbanuma, located in the northern part of Chiba Prefecture, the ratios of nonpoint sources to the total runoff load are significant, being approximately 80% of COD, 70% of T-N, and 40% of T-P in the total runoff load originating from nonpoint sources. The Lake Water Quality Conservation Plan estimates the nonpoint source load through unit loading, but the load may vary significantly depending on precipitation volume and patterns. In this study, we focused on road runoff, a major component of urban areas, which contributes significantly to the nonpoint source load, and investigated the relationships between precipitation volume and the loads of COD_Mn, nitrogen, and phosphorus in Chiba Prefecture. On the basis of 22 rainfall surveys using an automatic water sampler and the results of 10 surveys where runoff from road drainage was sampled from the start to the end of the runoff, relationships were obtained between total precipitation and the one-rainfall loads of COD_Mn, T-N, NO₃-N, T-P, and PO₄-P. These relationships enable load estimation based on precipitation volume, suggesting that COD_Mn in the unit values for urban areas in Chiba Prefecture may be underestimated.