TEMPORAL VARIATIONS OF POLLUTANT LOADS DURING STORM EVENTS IN A SMALL RIVER BASIN

Not only to estimate the impact of storm water on receiving waters but also to deduce the origin of the pollutants, it is important to analyze the runoff characteristics of pollutants during storm events. We conducted seven surveys during storm events to clarify the runoff characteristics of various pollutants and relationships between pollutant loads and runoff patterns. In addition, origins of various ions were deduced by examining the temporal variations of ion composition. Typical first flush effects in VSS and T-BOD concentrations were observed, and it was estimated that river sediments deposited in fine days strongly affect the runoff of VSS during a storm event. NO3--N concentration decreased by the early runoff, but kept up higher level at the recession stage, while ion concentrations decreased during the rising stage and increased again at the recession stage, indicating that ions were diluted by the major runoff. Relationships between pollutant loads and runoff were examined by the empirical equation “L/A = a (Q/A)n”. Judging from the n values, ash, SS, VSS, NH4+-N and TP were easily flushed out during storm events, while TN, DN, NO3--N, K+ and DOC kept almost constant concentration during a storm event. Most ions seemed to be diluted by storm water. It was indicated that NO3-, SO42- and K+ were mainly originated from diffuse sources during storm events in the surveyed watershed, while Mg2+ and Na+ was from the same source as Cl-. Most of the nitrate and sulfate loads seemed to be drained out from the surface soil layer of river basins and carried by subsurface runoff.