Fig. 6 shows an example of a calculation made in the southern and western parts of Ibaraki Prefecture. The data on wells with too high a nitrate concentration were obtained by researchers of the Ibaraki Prefectural Institute of Public Health in 1988. The data refers to 800 wells in 17 municipalities. The N load index was calculated from the agricultural census of 1975, 1980 and 1985. The correlations between the two variables were highly significant (P<0.01).
The western part of Ibaraki Prefecture is an important center of production of Chinese cabbage and Japanese pear, with a high N load. The southern part is urban, and the area of agricultural land is relatively small. In the most polluted municipality, 86% of the wells had a higher nitrate-N level than the 10 mg/mL which is the maximum level for drinking water. Fig. 6 clearly shows that excess application of N is the main cause of nitrate pollution of groundwater in rural areas.
Many municipalities have the problem of nitrate pollution of groundwater. Kakamigahara City in Gifu Prefecture uses groundwater for its public water supply. In the past, sweet potato and mulberry (for sericulture) were dominant in the city. After 1968, this changed to two crops of carrot each year.
Nitrate pollution of groundwater became apparent in 1971, and got worse each year. The city and prefectural authorities investigated the cause of the pollution and concluded that the overuse of fertilizer on carrot crops was probably the main cause. They recommended that farmers reduce fertilizer N by 30%, and replace some of the chemical fertilizer with organic fertilizers which release mineral nitrogen slowly.
This change resulted in an improvement of carrot quality without any loss of yield. The efficiency of N absorption by carrot increased from 30% to 45%, while the physiological disorders caused by excess fertilizer were reduced. The change also resulted in a reduction in the area with polluted groundwater (Fig. 7).
The N load index was also effective in explaining the problems of Kakamigahara City. Table 2 shows the N load indices of eight areas in the city calculated from the agricultural census of 1980. A high load was found in the Unuma area, where the groundwater pollution area was also concentrated. In the Unuma area, 48% of the N load originated from fertilizer applied to carrot (Table 3).
Changes in the N load in the Unuma area from 1975 to 1995 were calculated using the standard N load for various crops shown in Table 1. As Fig. 8 shows, the N load increased up to 1980, but then fell to much lower levels. This explains the reduction of the polluted zone in Fig. 7, taking into account the time lag between reducing the amount of fertilizer and an improvement in water quality.
It has been estimated that 49 - 86% of non-absorbed N applied to upland soils is leached out into the groundwater. The amount of N washed out in runoff is very small (<1%) in Japan (Ogawa 1979). Most municipalities in Japan have an annual rainfall of 1,000 - 2,800 mm (1,740 mm on average), and evapotranspiration of 600 - 1,000 mm. If non-absorbed N is 200 kg/ha, then 98 - 172 kg N may leach into groundwater, giving 9.8 - 28.7 mg of nitrate per liter of groundwater, Therefore, a drastic reduction in the level of non-absorbed N is needed, in order to improve the quality of underground water in rural parts of Japan.
