Characteristics of nitrogen outflow from a high stocking density area

摘要

In recent years, eutrophication has occurred in many lakes in Japan. The excess growth of algae causes outbreaks of musty odor and the deaths of breeding fish. NO_3-N concentration of the rivers that flow into Kasumigaura lake have increased. In spite of eutrophication this lake water is used as a source of drinking water for city area. It is reported that the outflow of nitrogenous and phosphorus fertilizer is one of the causes of the eutrophication, along with animal waste. It is a serious problem for the livestock breeders and the administrative organization. In this area, upland is covered with fields, and unlined storage ponds are located in pig farms to store animal waste. In Japan, there are few reports about the water quality of rivers that flow in catchment areas with much livestock. The effect of this type of pig farm on nitrogen concentration of rivers and ground water is not clear, nor is it clear what type of treatment is most effective. The lack of a calculation method for animal waste load hinders the planning of water quality improvement of lakes. The purpose of this study is to survey the actual state of contamination of stream water by animal waste and various kinds of animal waste treatment methods. We attempt to make clear quantitatively the influence of animal waste on stream water quality and to discuss ways to decrease nitrogen outflow load. Then we surveyed the characteristics of water quality during irrigation periods and the outflow load change after a rainfall in order to calculate annual amount of nitrogen outflow due to livestock.