Effect of countermeasures on improvement of water quality environment in the creek network area located along the lower reaches of the Chikugo River

摘要

Recently, water quality of the creek network located along the lower reaches of Japan's Chikugo River has deteriorated remarkably. Conservation of the environment and improvement of water quality are needed to maintain good sanitary conditions and preserve a comfortable living environment as well as to promote sustainable farming. This paper evaluates some countermeasures for water quality improvement in the study area bounded by the Ariake Sea and the Yabe, Okinohata, and Shiotsuka Rivers. An irrigation period and a non-irrigation period are discussed separately, because the type of farming, water use and temperature are different. The present state of water quality in the main creek can be described by the measured average concentrations of certain substances--namely 6.16 mg centre dot L~(-1) of COD, 2.37 mg centre dot L~(-1) of T-N, and 0.230 mg centre dot L~(-1) of T-P during the irrigation period. The tentative goal of water quality improvement is to reach concentrations of less than 6.0 mgcentre dot L~(-1) of COD, less than 2.0 mg centre dot L~(-1) of T-N, and less than 0.2 mg centre dot L~(-1) of T-P. The effects of countermeasures during the irrigation period can be highlighted as follows: 1) Realistic countermeasures at the farm leveldecreases COD concentration by 0.33 mg centre dot L~(-1). But significant improvement for T-N and T P is difficult to obtain. 2) Wastewater treatment projects which cover the entire study area decreases concentrations of COD, T-N, and T-P by 0.84 mg centre dot L~(-1), 0.13 mg centre dot L~(-1), and 0.029 mg centre dot L~(-1), respectively, when advanced treatment processes are applied. Application of conventional treatment processes, however, increases T-N concentration because of its low removal capacity 3) The installation of 40 direct canal purification facilities whose removal ratios are between 22 percent and 24 percent decreases COD, T-N, and T-P concentrations by 0.29 mg centre dot L~(-1), 0.10 mg centre dot L~(-1), and 0.01 mg centre dot L~(-1), respectively. 4) The inflow of newly developed water resources from the Chikugo Barrage at the rate of 1.18 m~3 centre dot s~(-1) decrease concentrations of COD, T-N, and T-P by 0.34 mg centre dot L~(-1), 0.11 mg centre dot L~(-1), and 0.015 mg centredot L~(-1), respectively. The tendency of the effects in the non-irrigation period was basically similar to that in the irrigation period. The effects of the wastewater treatment by the advanced process and of the direct canal purification were more significant in the non-irrigation period than in the irrigation period.