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[AHW16-P16] The migration of nitrate and possible impacts on groundwater of Ikuchi Island, Japan
Keywords:Nitrogen, migration , Ikuchi Island, denitrification
Ikuchi Island located in Seto Inland Sea, is one of the most famous orange and lemon production areas in Japan. Orange and lemon groves cover 42% of this island. To maintain and improve the yields, much fertilizer (~2400 kg ha-1 year-1) is applied during a whole year and nitrate contamination in this island was very serious (Onodera, et al., 2007). In order to evaluate the spatial and temporal variations of NO3-N in soil water, several observation wells with different depth (10cm, 30cm, 50cm, 70cm) were installed in one square meter of space of one orchard in Ikuchi Island. 1000ppm of KNO3-NO3 and NaCl-Cl mixtures were shed on the surface of this one square meter of space in August 20th, 2015. After that, water samples were collected from these wells every two weeks and analyzed for NO3-N, Cl.
The results showed that the highest concentrations of NO3-N in 10cm and 30cm, 50cm and 70cm were occurred in August 30th, 2015, the second water sampling time and September 15th, 2015, the third water sampling time, respectively. In addition, the peak value of concentrations of NO3-N decreased with the increase of soil depths except 10cm. This may attributed to the fact that the interval time between the first and second water sampling was 10 days, the peak value of NO3-N may have passed before we took water samples. From the relationship between the CN/CCl (the ratio of concentration of NO3-N and NaCl-Cl) and time, we found that the value of CN/CCl from 10 cm to 30cm decreased very rapidly. Moreover, it’s easier to collect water from 30cm than other depths, which may imply that place near to 30cm may be the most humid locations. Therefore, denitrification may take place in the depth from 10 to 30cm, resulting in the decline of NO3-N concentration. The migration rates of NO3-N in soil water were estimated to be about 3.0cm/day and 2.5 cm/day in the depth from 0cm to 30cm and 30cm to70cm. The groundwater level is about one meter in this area, NO3- would migrate into groundwater about 24 days later after 1000 ppm nitrate fertilizer was applied.