Construcción de bases de Gröbner

(1) Creek Network Area Located along the Lower Reaches of the Chikugo River

Recently, the water quality of the creek located along the lower reaches of the Chikugo River (Yuyama et al., 1998b and 1999) has undergone remarkable deterioration. The conservation of water quality is needed to keep good sanitary condition and a comfortable living environment as well as to make sustainable farming possible. A study to evaluate some water quality improvement measures was carried out.

The study area is bounded by the Ariake Sea and the Yabe, Okinohata and Shiotsuka Rivers. The measures set up in this area to counter flood, drought and water quality deterioration conditions have to be discussed at the same time. As of 1996, about 60 percent of a total of 3,140 ha of land were used as low-lying paddy fields. The testing of the Chikugo Barrage as a new source of water was started in 1997. Observations made during the irrigation period and the non-irrigation period are discussed separately, because different types of farming, water use and temperature occur during these periods.

At the time of the study, the water quality characteristics observed in the main creek during the irrigation period were as follows: COD, 6.16 mg/L; T-N, 2.37 mg/L; and T-P, 0.230 mg/L. With a view to create a desirable rural area environment, tentative goals for water quality improvement were set as follows: COD, less than 6.0 mg/L; T-N, less than 2.0 mg/L; and T-P, less than 0.2 mg/L.

A flow diagram analysis technique was used to create a diagnostic of the current water quality situation. The study led to the observations enumerated below.

a)The load from household discharges accounted for 30-60 percent of the total discharge load generated within the watershed of the creek.

b)The source of the water supply for irrigation was an important factor affecting the water quality of the creek.

c)The significant discharge load from paddy fields was estimated to have the following

characteristics: COD, 50-400 kg·ha-1_·yr-1_{; T-N, 0-40 kg·ha}-1_·yr-1_{; and T-P, 0.3-10 kg·ha} 1_·yr-1_.

d)The specific variation rate for the parameters observed in the creek was estimated to be 100

to 400, -150 to -30 and 2 to 60 mg·m-2_·d-1 _{for COD, T-N and T-P, respectively.}

The following measures for preserving water quality were under consideration: the promotion of farming with low discharge load; wastewater treatment; direct purification; inflow of water from the Chikugo Barrage to the creek; the unification of laver (sic) factories; and the dredging of bottom sediments in the creek. Among these measures, the first four were evaluated and their observed effects during the irrigation period were as described below.

a)Realistic measures taken at the farm level led to a decrease of the COD concentration by 0.33 mg/L, but significant improvement for the T-N and T-P parameters proved to be difficult. b)The initiation of wastewater treatment projects covering all the study area led to a decrease

of the concentration of the COD, T-N and T-P parameters, by 0.84 mg/L, 0.13 mg/Land

0.029 mg/L respectively, where an advanced treatment process was applied. In cases where a conventional treatment process was used, its low removal capacity was reflected in an increases of the T- N concentration.

c)Some forty direct canal purification facilities were installed and such action led to pollutant removal with a 22-24 percent efficacy factor and a decrease of the COD, T-N and T-P concentrations by 0.29 mg/L, 0.10 mg/L and 0.01 mg/L, respectively.

d)The inflow of water from the newly developed Chikugo Barrage at the rate of 1.18 m3_/s

contributed to a decrease of COD, T-N and T-P concentrations of by 0.34 mg/L, 0.11 mg/L and 0.015 mg/L, respectively.

The effects observed during the non-irrigation period showed basically a similar trend as that noted during the irrigation period. The positive effects of wastewater treatment with an advanced process and of direct canal purification were more significant during the non-irrigation period than those during the irrigation period.

The goal of achieving adequate water quality is not an easy one to pursue and an objective evaluation of the results obtained can be helpful to planners and decision-makers who are responsible for water quality conservation. It is necessary that the measures taken be integrated to the maximum extent possible.

(2) Kojima Lake Project

Kojima Lake was constructed artificially by closing the Kojima Bay in 1956. The purpose of the project was to eliminate drought and salt damage for 5,100 ha of coastal farmland area, to improve the internal drainage for lowland, and to create a safe dike.

The relevant characteristics of Kojima Lake are: 532 km2 _{of catchment area, 1,880 ha of}

lake area, a total volume of 26 MCM (million cubic metres), an effective storage volume of 18 MCM, a controlled water level (above sea level) varying between +0.8 m (non-irrigation period) and +0.5 m (irrigation period); a mean water depth of 24 m and a hydraulic retention time (HRT) of 0.05 year.

Since its creation, the water quality of Kojima Lake has been deteriorating due to significant urbanization in the watershed. The water quality parameters observed during the study were: COD, 10-12 mg/L; T-N, 1.35~1.8 mg/L; and T-P. 0.18~0.24 mg/L. This situation is the cause of social tensions. Improvements of the water quality will require cutbacks of both the inflow load from hinterland areas and the sources of pollution within the lake itself. The latter type of pollution is mainly due to releases from bottom sediments.

Based on the background information mentioned above, the new project to improve water quality of Kojima Lake was started from 1992 and covered an area made up of 4,480 ha of paddy fields and 20 ha of upland fields. The water quality targets set under this project were as follows, corresponding to the standard set for the irrigation water used paddy fields: COD, less than 6.0 mg/L and T-N, less than 1.0 mg/L. An outline of the actions to be taken as part of this project during the period 1992~2003 is provided below (Oda, 1997 and Terao, 1998):

a)Dredging of bottom sediment: 2,000 thousand m3_.

b)Dredging of gut (water route): 300 thousand m3_.

c)Covering with sand: 500 thousand m3 _{(depth = 50 cm).}

d)Filling up deep area.

e)Developing land with dehydrated sludge.