Abstract

  Approximately 14 million people get their drinking and industrial water from the most southern part of Lake Biwa. This shallow region, however, has been eutrophicated severely from year to year by nutrients loading from both the surrounding inflowing rivers and the north basin. Sedimented solids of the surface layer of bottom sediments are stirred up very frequently by strong winds and waves in this shallow region. Moreover, the water quality has been deteriorated by the release of nutrients from the surface layer of bottom sediments in which organic matter has accumulated. For the period 1989 to 1990, the suspended solids content had a close correlation with the COD value, and the T-N and T-P concentrations in the water of inflowing rivers to the basin. T-N release rate from the surface layer of bottom sediments ranged from 6.3 to 213.8 mg m-2 d-1 and the release load was 936 kg d-1 for the entire 58 km2 of the south basin. On the other hand, T-P release rate ranted from 0.91 to 3.56 mg m-2 d-1 and the total release load was approximately 69 kg d-1 in the basin. Suction dredging of organic matter rich sediments was carried out in the basin to improve the water quality. Removed sediments were used after processing for various kinds of construction materials, such as bricks for pavement, walls of domestic houses, and ornamental materials of public buildings.

Keywords: bottom sediments, nutrient release, suction dredging, bricks, pavement