Dynamics of nitrate concentration in groundwater of reservoir area of Komesu subsurface dam, Okinawa, Japan
2007
Yoshimoto, S.(National Inst. of Rural Engineering, Tsukuba, Ibaraki (Japan)) | Tsuchihara, T. | Ishida, S. | Imaizumi, M.
Fluctuations of nitrate concentrations in groundwater and its major sources of nitrate in the Ryukyu limestone area, the southern part of Okinawa Island, are studied. The study area is located in Itoman City, which is one of satellite cities of Naha City. The long-term monitoring data on nitrate concentrations at 15 springs from 1990 to 2003 shows a decreasing trend from the mid-1990s, although they fluctuated greatly with precipitation. The emission factor method on the basis of agricultural statistical data is applied to estimate the nitrogen load to groundwater. The nitrogen loads from livestock manure and domestic wastewater monotonically increased from 1975, and, accordingly, the total load showed an increasing trend. However, the load from chemical fertilizer has been gradually decreasing after peaking in 1990 as the area under cultivation has decreased. The trend in the nitrate concentration corresponded only to that of the load from chemical fertilizer, leading to the hypothesis that chemical fertilizer most strongly affects the groundwater quality. Fluctuation of nitrogen loss is calculated from the data of nitrate concentration in groundwater and groundwater discharge to the sea, which is estimated by the parallel 10 tank models. The estimated nitrogen balance shows that the total nitrogen loss is nearly balanced with the sum of loads of chemical fertilizer and rainfall. The nitrogen dynamics in this area is simulated by the nitrogen flow models which consist of the parallel 10 tank models groundwater with nitrate leaching model (Kiho and Islum, 1995) on the hypothesis that chemical fertilizer is the major nitrogen source. The model result shows the approximate decreasing trend of nitrogen concentration in groundwater.
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