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Coastal seawaters in Osaka Bay of Japan have been contaminated by a great amount of pollutant effluent from the hinterland. The public demands for the improvement of coastal seawater qualities and the recovery of normal coastal ecosystems have rapidly been increasing with growing of particular public interest in natural environments in recent years.

Under such circumstances, we proposed "UTSURO" for improving contaminated coastal seawaters. "UTSURO" is a technical concept which consists of permeable rockfill embankments such as rubble breakwaters and an inner basin surrounded by rockfill embankments themselves. The surfaceof rock immersed in water is covered by biofilms in which numerous bacteria and protozoan live. In addition, many kinds of benthos live within rockfill embankments. These life forms the food chain and removes dissolved and particulate organic materials from contaminated seawaters. Thus seawaters in the basin surrounded by rockfill embankments are expected to be purified by repeated forward and backward-passage of seawaters through the rockfill embankments.

In order to examine the function of "UTSURO" in purifying contaminated seawaters, we chose a small tidal inlet located at an estuary, which faces Osaka Bay, with much pollutant effluent and constructed rockfill embankments at the mouth of the inlet. Since September in 1989, we have observed the purification process of contaminated brackish waters in the inlet sheltered by rockfill embankments.

After three-years observation, it was found that "USTURO" has the function of seawater purification and its function may be enhanced by the action of natural phenomena such as tides, waves, gravity, sunlight and marine life as follows:


(1) Tidal action contributes to inducing repeated forward and backward-flows in rockfill enhancing embankments.

(2) Wave action contributes to enhancing the effect of aeration due to wave breaking.

(3) Gravity action and tranquil water in the basin sheltered by rockfill embankments contribute to hastening the settling of suspended solids. It results in improving the transparency of seawaters in the basin.

(4) Sunlight can reach the sea bottom to photosynthesize. As a result, the aerobic digestion of organic sludge is promoted.

(5) Forming the food chain in the basin and rockfill embankments brings getting larger in size on marine life. It means effective fixation of organic matter and nutrients in contaminated seawaters.