Abstract
This report describes our attempt to purify an irrigation pond by dredging. The pond, Kameike, locates in a rural area in Takamatsu city, and has approximate volume of 7, 000 m3. Prior to the introduction of dredging, we attempted purification by means of running the water repeatedly through paddy fields adjoining the pond ( “Direct Purification”) in 1993 and 1994. This method, however, did not have the effect as we had intended, and which implied that the sediment affected the quality of the water. Then we dredged the pond in the early spring of 1995 and implemented Direct Purification continuously in 1995. Compared to the result before the employment of dredging, there was improvement in the quality of the water in many parameters. For example, in and after August, T-N concentration reduced below 1mg/l, the standard value for the quality of irrigation water to cultivate rice. We concluded that dredging was fairly effective in purification of the pond. However, unless any other measure is taken after dredging, the sediment might shortly accumulate again and thereby the effectiveness of dredging might be diluted. Thus, we propose total control of the water quality of irrigation ponds, consisting of Direct Purification and dredging as well as primarily preventing pollutants from flowing in the ponds.
