Lake Hoedong, located in the south of Korea peninsula, is a small, shallow, eutrophic lake, which is an important water supply source for the city of Pusan. Although point sources are discharged after secondary treatment, there has beena steady deterioration of lake water quality. Removal of point sources alone mayproduce no measurable improvement in the lake condition because of nonpoint sources. So the objectives of this paper are to quantify the nonpoint sources and to assess theimpact to the lake. Nutrient inputs from point sources are unlikely to have been alteredsignificantly, but nonpoint nutrient inputs have great monthly variation. Contributionof nonpoint sources to the external input of total phosphorus is almost 67% in the wet season and 19% in the dry season. Assuming that a substantial nonpoint source reductionis made by the Hoedong Lake basin, we attempt to estimate the effect of the lake waterquality by mass balance equation. The nonpoint source impact to Hoedong Lake waterquality is more than 60% in the wet season.

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