Fluctuations in zinc concentrations in the agricultural water of drainage canals in a paddy field block within a small catchment area

Zinc is an essential element for biological activity. On the other, zinc is also a hazardous material for the growth of aquatic life if excess amounts of zinc occur in the aquatic environment. The Ministry of the Environment in Japan has determined that a zinc concentration of less that 30 micro g/L in water should be one of the Living Environment Items of the Environmental Quality Standards for Water Pollution. This standard was decided from the view point of the conservation of aquatic life. Quantitative data on the dynamics of zinc in water environments is required in order to conserve aquatic life. This study focused on zinc dynamics in the agricultural water of drainage canals. The pattern of distribution of zinc concentrations in environment waters is basically influenced by the geological conditions, and the fluctuation in zinc concentrations is determined by the climatic conditions, such as the intensity of rainfall. To add to these natural factors, the dynamics of zinc in paddy field blocks is influenced by agricultural practices. The amount of water used for the paddy fields accounts for the largest proportion of the total amount of water use. Thus, irrigation water plays an important role in any regional hydrological cycle. In addition, the farming schedule, such as puddling, transplanting and mid-summer drainage may strongly influence the water balance and mass transfer process of materials in paddy field blocks. The characteristics of zinc dynamics in a paddy field block were studied in consideration of these agricultural practices. This paper discusses the characteristics of the fluctuation in the zinc concentrations in the discharged water from a paddy field block. Three periods of different agricultural water management, were investigated: the non-irrigated period, the puddling and transplanting period, and the normal ponding period. The results are summarized as follows. (1) The increase in zinc concentrations in the drainage water in the early Period II when the irrigation of the paddy fields started may be due to the flushing flow of irrigation water. (2) The positive correlation between zinc concentrations and turbidity in the drainage water suggests that the zinc mainly originates from suspended matter. (3) The pattern of fluctuation in the zinc concentrations during rainfall periods varied with the rainfall pattern, such as its intensity and duration. (4) The average concentration of zinc in the discharged water in normal ponding periods was at the same level as that of the irrigation water. During the periods from the starting of irrigation supply to puddling and transplanting, the average concentration of zinc in the discharged water was higher than that of irrigation water.